C Sabathier

A Gallais-During
email: annelise.gallais-during@laposte.net

F Delage

S Béjaoui

S Lemehov

J Somers

D Freis

W Maschek

S Van Til

E D'agata

Outcomes of the PELGRIMM project on Am-bearing fuel in pelletized and spherepac forms

Keywords: PELGRIMM, transmutation, americium, homogeneous recycling, heterogeneous recycling, spherepac, fabrication, fuel behaviour under irradiation, experiments, simulation, MARIOS, SPHERE, MARINE, core design, safety coefficients, accidental situations

Outcomes of the PELGRIMM project progress in the development of Am-bearing fuel under pelletized and spherepac forms

INTRODUCTION

The PELGRIMM project [START_REF] Delage | Investigation of pelletized and sphere-packed oxide fuels for minor actinide transmutation in sodium fast reactors, within the FP-7 European project PELGRIMM[END_REF] stands for PELlets versus GRanulates: Irradiation, Manufacturing and Modelling. It was a FP7-European project (7 th Framework Program for funding of European research projects started between 2007 and 2013) carried out from 2012 to 2017 which was devoted to the investigation of spherepacked and pelletized fuel forms for Minor Actinide (MA) transmutation.

PELGRIMM aimed at constituting a new step in the long term process of the MA-bearing fuel testing rationale, initiated within the European projects ACSEPT (2008ACSEPT ( -2012)), F-BRIDGE (2008-2012), CP-ESFR (2008)(2009)(2010)(2011)(2012)(2013) and FAIRFUELS (2009FAIRFUELS ( -2015) ) [2]- [START_REF] Hania | FAIRFUELS : FAbrication, Irradiation and Reprocessing of FUELS and targets for transmutation[END_REF]. In addition, the PELGRIMM and ASGARD [START_REF] Ekberg | From fuel to fuel: dissolution, partitioning and manufacturing[END_REF] projects, implemented in parallel within FP-7, were able to bridge fuel developments to back-end of the fuel cycle.

Within PELGRIMM, a total of 12 partners from research laboratories, universities and industries, collaborated to share and leverage their skills, progress and achievements, covering a comprehensive set of investigations. The present paper intends to make an overview of the main technical outcomes gained within PELGRIMM, which has addressed all the key R&D items relative to fuel developments for testing, for both homogeneous and heterogeneous recycling modes and for both spherepac and pellet fuels: fuel synthesis developments, analytical (separate-effect) and semi-integral irradiations of MA-bearing fuels and their Post Irradiation Examinations (PIE), irradiation behaviour modelling and predictive code developments, preliminary safety performance assessment.

In section 2, the project context, the motivation and the technical items under consideration are firstly summarized: a brief state of the art is presented, related to the developments on MA-recycling in homogeneous and heterogeneous mode at the beginning of the PELGRIMM project; the main developments on spherepac fuel are also summarized as they highlight this type of fuel as an attractive

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3 concept for MA-bearing fuels. The extension of MA-bearing fuel preparation processes to alternative routes in order to limit secondary waste streams and simplify the steps is described in section 3. Section 4 presents the results of the PIE on SPHERE and MARIOS pins and fuels, which provide respectively a comparison between spherepacked and pelletized (U,Pu,Am)O 2 fuel performances and the very first results on helium behaviour in (U,Am)O 2 fuels. It also addresses the next step in the (U,Am)O 2 fuel safety testing rationale with a new irradiation test, MARINE, implemented in the High Flux Reactor (HFR). The progresses in the capabilities of existing models and calculation codes to describe the MA-bearing fuel behaviour under irradiation are presented in section 5. Finally, section 6 draws the main conclusions regarding a preliminary design of a Sodium-cooled Fast Reactor (SFR) core loaded with spherepacked (U,Pu,Am)O 2 fuels and the correlated preliminary safety assessment.

SUMMARY DESCRIPTION OF PROJECT CONTEXT AND OBJECTIVES

State of the art summary on MA-bearing oxide fuels developments for Gen-IV systems before PELGRIMM

High activity wastes are currently vitrified and planned to be stored in deep geological repositories.

In order to reduce the radiotoxic inventory of vitrified wastes and the footprint of deep storage [START_REF] Hoorelbeke | Impact of minor actinide transmutation options on geological disposal. The French case[END_REF], research concerning solutions that could separate the most radiotoxic and long-lived elements from spent fuel and transmute them into non-radioactive or short-lived ones in nuclear reactors is being carried out on an international level. Transmutation being only reasonably applicable for Minor Actinides (MAs), (chiefly americium, neptunium, and curium) and the best transmutation performance being obtained in fast neutron reactors, MA incorporation into the fuel has become a prerequisite for Generation IV reactors to bring benefits in the disposal requirements by reducing the MA content in the high activity wastes [START_REF] Pillon | Actinide-bearing fuels and transmutation targets[END_REF]- [START_REF] Grouiller | Transmutation in ASTRID Fast Reactors and Related Fuel Cycles: Safe Technologies and Sustainable Scenarios[END_REF]. Based on historical experience and knowledge, oxide fuels have emerged in Europe as the solution to meet the Generation IV assigned performances and reliability goals. Two main MA-recycling options have been under consideration within PELGRIMM:

• the homogeneous recycling mode, or Minor Actinide Driver Fuel (MADF) concept, where MAs are diluted in (U,Pu)O 2 standard driver fuel at a low enough content (<3%) to limit the

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MA impact on the performance of the fuel and on the core safety as well as on the fuel cycle facilities, as far as possible;

• the heterogeneous recycling mode of UO 2 fuel located in radial core blankets, or Minor Actinide Bearing Blanket (MABB) concept, where MAs are concentrated in UO 2 based fuels at a content of ~10% into the radial breeder blankets of Sodium-cooled Fast Reactors (SFRs) core in order to limit the neutron impact on the core physics; in this concept, the use of the UO 2 matrix as a support for MAs should ease developments as UO 2 behaviour under irradiation as well as UO 2 reprocessing, are well known.

Regarding the first option, national and international R&D programs have been conducted for 25 years [START_REF] Delage | Outcomes on oxide fuels developments for Minor Actinides recycling[END_REF], [START_REF] Nakashima | Current status of Global Actinide Cycle International Demonstration project[END_REF] and many issues have been addressed by previous irradiations such as SUPERFACT [START_REF] Prunier | Transmutation of Minor Actinides: behavior of americium and neptunium based fuels under irradiation[END_REF]- [START_REF] Nicolaou | Experience with fast neutron reactor fuels containing Minor Actinides: transmutation rates and radiation doses[END_REF], Am1 [START_REF] Maeda | Short-term irradiation behavior of Minor Actinides doped Uranium Plutonium Mixed Oxides fuels irradiated an experimental fast reactor[END_REF]- [START_REF] Tanaka | Microstructural evolution and Am migration behavior in Am-containing fuels at the initial stage of irradiation[END_REF] or US DOE -AFC-2C&2D [START_REF] Maclean | Irradiation of metallic and oxides fuels for actinide transmutation in the ATR[END_REF]. For the second option, operation of MABB in the reactor under very specific conditions has raised many questions and experimental data were scarce at the beginning of the PELGRIMM project, with the unique SUPERFACT irradiation [START_REF] Prunier | Some specific aspects of homogeneous Am and Np based fuel transmutation through the outcomes of the SUPERFACT experiment in PHENIX[END_REF], [START_REF] Walker | Transmutation of Neptunium and Americium in a fast neutron flux: EPMA results and KORIGEN predictions for the SUPERFACT fuels[END_REF]. A comprehensive R&D program of MABB fuel testing campaign started in 2008, including, as a first stage, two separate-effect irradiation tests: MARIOS, manufactured and irradiated within the FAIRFUELS project, and DIAMINO, implemented within the French national nuclear program [START_REF] Bonnerot | Development program on minor actinide-bearing blankets at CEA[END_REF], [START_REF] Béjaoui | AMericium-Bearing Blanket separate-effect experiments: MARIOS and DIAMINO irradiations[END_REF]. Table 1 and Table 2 give an overview of the irradiation tests already done, in progress or in preparation, related to homogeneous and heterogeneous recycling mode, at the beginning of the PELGRIMM project (i.e. in 2011).

One of the main issues still under consideration in both kinds of MA-bearing oxide fuel investigations is the high helium production during (and after) irradiation: this is a well-known specificity of fuel containing MAs since the amount of helium produced is all the more significant as the 241 Am content is high. For MADF, the helium release could induce additional fission product release, as helium is expected to be totally released at high temperatures, thus leading to enhanced Fuel Cladding Chemical Interactions (FCCI). For MABB, the high to huge helium production, combined with low temperatures of MABB fuel, could enhance the fuel gaseous swelling and correlated Fuel Cladding Mechanical Interaction (FCMI).

In addition, the impact of introducing MAs in the fuel remains a major concern for fuel plants. The

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5 high neutron emission and the high thermal power of americium and especially curium generate significant technological challenges to limit radiation exposure of staff, criticality risks, etc. Fuel preparation must be carried out in shielded cells with remote handling, which means that the processes need to be revised for simplification as well as for implementation of relatively dust-free steps in the prospect of an industrial production [START_REF] Chabert | Consideration on industrial feasibility of scenarios with the progressive deployment of Pu multirecycling in SFRs in the french nuclear power fleet[END_REF].

These two issues have led to consider the spherepac technology as an attractive alternative to classical pelletized fuel forms for MA-bearing fuels.

State of the art summary on spherepacked fuels developments before PELGRIMM

Even though pelletized fuel forms have been preferred so far, the spherepac technology, consisting of filling a pin with dense spherical fuel beads by vibro-compaction, could be attractive regarding MA-bearing fuels.

Actually, the potentially better accommodation of solid swelling of spherepac fuels (compared to pellets) through the re-arrangement of the free inter-particule areas under irradiation could ultimately lead to better safety management of the helium generated during irradiation; this point, to be demonstrated, would be a significant advantage of spherepac fuels. Additionally, the preparation process could be significantly simplified thanks to the elimination of some process steps such as milling and grinding, that involve fuel powders and dust.

From a general point of view, the properties of granulated fuel behaviour under irradiation are quite similar for spherepac fuels (spherical beads) and VIPAC fuels (angular shards) [START_REF] Pouchon | Spherepac and Vipac Fuels[END_REF], which take benefit from an extended experience as they have been operated in Russia for about forty years [START_REF] Bunk | Operation experience of the automated pilot plant for the BOR-60 vibropac fuel element and subassembly fabrication[END_REF].

Their advantages mainly are their behaviour at high temperature, which is similar to that of pellet fuels (formation of a central hole, columnar grains, etc.) after an initial stage of sintering of the areas of the fuel submitted to high temperature, as well as a good accommodation of power transients thanks to the lower cohesion of the fuel structure. Nevertheless, among the issues to be dealt with in a safety demonstration, are lower melting margins compared to pellet fuels (due to a lower thermal Within European projects frameworks, developments on granulated fuels were performed quite recently within F-BRIDGE (2008-2012) and FAIRFUELS (2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015). Within F-BRIDGE, the investigations were dedicated to the potential applicability of granulated fuels to Generation IV systems [START_REF] Valot | Basic research in support of innovative fuels design for the GEN IV systems: The F-BRIDGE project[END_REF] while within FAIRFUELS, the objective included the design, the manufacturing and the irradiation of the SPHERE experiment [START_REF]SPHERE: Irradiation of sphere-pac fuel of UPuO 2-x containing 3% Americium[END_REF].

Consequently, regarding the developments on spherepac fuels, the following points have emerged for building the PELGRIMM project: the need to perform the PIE of the SPHERE irradiation in order to better understand the behaviour under irradiation of spherepac fuel and the need to follow efforts initiated within previous initiatives regarding spherepac fuel preparation process, irradiation behaviour and performance, as well as to extend the investigation to an exploratory analysis of an SFR core loaded with spherepac Am-bearing fuels.

Overview of the PELGRIMM technical items

The key points mentioned in the previous sections were combined and used as basis for the PELGRIMM project, described in details in [START_REF] Delage | Investigation of pelletized and sphere-packed oxide fuels for minor actinide transmutation in sodium fast reactors, within the FP-7 European project PELGRIMM[END_REF]. Close connections exist within and between each set of investigations: experiments on MABB and MADF; pelletized versus spherepacked fuel behaviour; spherepac preparation through several technologies; experimental activities and modelling-simulation; and finally spherepac MADF pin behaviour under normal operating conditions and problematics of core physics under normal and off-normal conditions.

To reflect the fuel safety approach followed throughout the project, this paper gives an overview of the main outcomes gained within PELGRIMM, as follows: investigations of alternative synthesis routes for spherepac fuels, analytical (separate-effect) and semi-integral safety testing irradiations of MA-bearing fuels and their PIE, modelling and simulation of fuel behaviour under irradiation and preliminary assessment of safety performance for spherepac fuels.

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INVESTIGATIONS IN ALTERNATIVE FUEL SYNTHESIS ROUTES

Powder metallurgy flowsheets used to supply (U,Pu)O 2 or standard fuels at industrial scale can be used at lab-scale to prepare Am-bearing fuel samples, such as the MARIOS disks (see [START_REF] Prieur | Fabrication and characterisation of U 0.85 Am 0.15 O 2 discs for MARIOS irradiation program[END_REF] and section 4.2). However, the major drawback to application to Am-bearing fuels is the difficulty in managing fine powders (dust) at all stages of the process (dosing, milling, mixing, granulation, grinding, sieving, press filling, etc.). This compromises the use of metallurgical processes for industrial production of Am-bearing fuel.

Dust-free routes and simplified flowsheets are essential to scale-up the Am-bearing fuel preparation processes. In this prospect, the spherepac technology is attractive, as it would lead to a significant simplification of the preparation process through elimination of some process steps, such as milling and grinding that involve fuel powders and dust. Moreover, the resulting process would be more compact and simple than the pelletizing process.

Sol-gel processes and MARINE AmBB fuel preparation

The main advantages of sol-gel processes involve the easy shaping during the gelation stage, thanks to the fluidity of the initial solution. In particular, they allow a controlled synthesis of dense or porous microspheres depending on their subsequent use such as spherepac fuel beads or pelletized fuel made by pressing the beads. Once optimized, the technique allows spherical particles of homogeneous composition to be obtained after washing, drying and calcination resulting in condensation of the heavy metals involved [START_REF] Grandjean | TRU recycling: recent experiments and results for the conversion of minor actinides into oxide[END_REF].

Already, within FAIRFUELS, the SPHERE (U,Pu,Am 0.03 )O 2-x beads were prepared by infiltration of porous (U,Pu)O 2 precursor beads prepared by sol-gel gelation, with americium nitrate solutions (see [START_REF]SPHERE: Irradiation of sphere-pac fuel of UPuO 2-x containing 3% Americium[END_REF] and section 4.1). The MARINE irradiation [START_REF]The MARINE experiment: Irradiation of sphere-pac fuel and pellets of UO 2 for americium breading blanket concept[END_REF] is part of the second step in the testing of (U,Am)O 2 fuels (separate-effect tests like MARIOS belonging to the first step). MARINE is equivalent to SPHERE as it includes 2 mini-pins of pelletized and spherepacked fuels, but differs in fuel composition: (U 0.15 Am 0.85 )O 2-x .

For MARINE, the synthesis procedure was almost similar to SPHERE for the preparation of pellets,

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ACCEPTED MANUSCRIPT 8 but using UO 2 as precursor instead of (U,Pu)O 2 . Two preparation routes were followed to synthetise the two types of fuel (pellet and spherepac) [START_REF]The MARINE experiment: Irradiation of sphere-pac fuel and pellets of UO 2 for americium breading blanket concept[END_REF].

For pellets, the preparation flowsheet included the following steps: production of porous UO 2 beads (without americium) by the sol gel external gelation route providing beads without strict specifications on the size distribution; infiltration of the porous beads with americium solution (low acid Am nitrate solution to prevent UO 2 dissolution as much as possible) and subsequent calcination; pressing of the beads; sintering of the green pellets; control and selection. After preparation, the Am content was close to the specified 15 mol% with respect to total heavy metal. The microstructure was of good quality, with homogeneous distribution of porosity throughout the pellet (see Figure 1).

For the spherepac fuel preparation, the synthesis of the fuel was performed by group conversion from mixed nitrate solution, which was prepared by mixing U with Am nitrate solution in the required ratio. It required implementing the following steps: preparation of small size fraction by sol-gel external gelation route; preparation of large size fraction by the sol gel external gelation route to yield monodisperse sized particles; sintering of the beads; control and selection.

A detailed study using Nd prior to Am was performed to define optimal preparation conditions for large beads. Application of the process to Am showed nevertheless that the optimized conditions using acidified Nd solutions were not appropriate to provide the same good quality particles achieved with Nd surrogate solutions. Adjustments were made, leading to reasonably spherical particles (see Figure 2). The density of the large beads was estimated to be in the order of 67%, revealing that the porosity content was very high. This sol-gel process was used for the 1 st time to synthetise AmBB fuels under pellet and spherepac forms and the 2 MARINE pins were filled, one with 6 sintered pellets of (U 0.15 Am 0.85 )O 2-x , obtained from a batch of small sized beads; the other one with a stack of beads of (U 0.15 Am 0.85 )O 2-x with 2 size fractions (50µm & 800µm), packed by vibrations up to a smear density of 55.4 % of the Theoretical Density (%TD), revealing an effective packing since the particles themselves have a density of ~67%.

Variant in the sol-gel processes: Microwave internal gelation

As seen above, external gelation is a well-developed process to produce homogeneous spherical particles of fuel for the so-called spherepac concept. The process is triggered by a temperature increase, which in usual systems is induced by a conductive heat exchange with a silicon oil hot bath, leading to secondary wastes. As it is irrelevant how and from which medium the heat is introduced into the beads, a variant of the external gelation route has been studied, the so-called microwave internal gelation, using for gelation of the drops, an electromagnetic heating within a microwave cavity [START_REF] Cozzo | Methods of advanced waste conditioning by microwave internal gelation: set-up development and modelling[END_REF], [START_REF] Cabanes-Sempere | Innovative production of nuclear fuel by microwave internal gelation[END_REF] instead of a silicon oil hot bath.

The production unit was developed and the process is illustrated in Figure 4. A non-radioactive unit composed of droplet production and microwave heating equipment was built in order to investigate the preparation of spherules with a metal surrogate (cerium). Models and simulations regarding the electromagnetic heating part were also developed [START_REF] Cabanes-Sempere | Innovative production of nuclear fuel by microwave internal gelation[END_REF]. Promising results were obtained for nonradioactive surrogates (see Figure 5). The equipment was then implemented in a glove-box to permit fully remote operation and now conforms to the laboratory safety requirements. The final device is now available to test the equipment on U-type compounds.

Adaptation of the Weak Acid Resin process

The ion exchange Weak Acid Resin (WAR) process was initially used for the production of carbide kernels for High Temperature Reactor particle fuels [START_REF] Weber | Processing and Composition Control of Weak-Acid-Resin-Derived Fuel Microspheres[END_REF], [START_REF] Notz | The Preparation of HTGR Fissile Fuel Kernels by Uranium-Loading of Ion Exchange Resins[END_REF]. The WAR flowchart has been revisited and adapted to oxide fuels [START_REF]Calcined resin microsphere pelletization (CRMP): A novel process for sintered metallic oxide pellets[END_REF] up to the synthesis of (U,Am)O 2 beads and pellets [START_REF]Fabrication of uranium-americium mixed oxide pellet from microsphere precursors: application of CRMP process[END_REF].

The synthesis of (U 0.9 Am 0.1 )O 2-x microspheres was prepared by thermal treatments of ion exchange resins loaded with Am 3+ and UO 2 2+ cations. The degradation of the polymeric skeleton under air followed by reducing heat treatment led to the synthesis of spherical precursors. The reduced actinide oxide beads were thoroughly characterized by SEM, TIMS, powder-XRD and coupled µGC-TGA.

Analyses have shown that (U,Am)O 2 was produced with a reasonable amount of C residue (around

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10 1500 ppm). The morphology of the spheres is fairly good and an inspection of high-magnification images of some broken spheres has shown that the microstructure of granule is homogeneous, as can be seen in Figure 6. The diameter of the spheres is around 400µm and the apparent density is 24 %TD.

Am content versus total metal content has been assessed by the complete dissolution of the oxide and analysis of the bulk composition by TIMS: a ratio of 10.6% has been achieved, which is in fair agreement with ratio of the loading solution.

Because bulk density of microspheres was low (24%TD), densification of spherules has been investigated too. Thermal conversion tests were carried out on Ce(III) loaded resin beads, Ce being used as a surrogate of Am. Preliminary results have shown that it seems possible to reach highly dense microspheres (>90%TD) by adjusting the heating rate to a value as low as 0.1°C/min and by choosing an appropriate temperature for sintering. A modified WAR process with specific calcination conditions could then allow the synthesis of very dense actinide oxide microspheres which would meet spherepac specifications.

Moreover, oxide microspheres were suitable for pressing, and a dense pellet (density of 95%TD) has been achieved after dynamic sintering under a reducing atmosphere up to 1800°C. This pellet meets the required density specifications for Am transmutation in a UO 2 matrix in SFRs and proves the technical feasibility of this dust-free process.

OUTCOMES FROM IRRADIATION EXPERIMENTS

An irradiation campaign includes the design of the fuel, pin and device features, the preparation and assembly of the components, the implementation in a reactor and the execution of the PIE programme.

As the time required for these activities is longer than the standard duration of a European project (~4 years), irradiation campaigns on Am-bearing fuels have regularly been split in steps that were distributed in projects that followed each other.

The PELGRIMM project hosted the PIE activities for SPHERE and MARIOS irradiation tests, as well as the design, manufacturing and implementation in HFR of the MARINE irradiation test.

SPHERE

The objectives of the SPHERE semi-integral irradiation were to provide the very first results on MADF spherepac fuel behaviour under irradiation as well as a direct comparison between spherepac and pellet MADF fuel performances. To do that, the SPHERE experimental device consisted of two short pins containing (U,Pu,Am 0.03 )O 2-x fuel. One mini-pin was filled with pellets and the other with a stack of beads of 2 size fractions (50 & 800µm). The irradiation was performed in HFR during 295

Equivalent Fuel Power Days (EFPD), leading to a maximum burnup of ~5at% and a maximum linear heat rate around 300W.cm -1 . The SPHERE irradiation was designed, prepared and irradiated within the FAIRFUELS project [START_REF] Hania | FAIRFUELS : FAbrication, Irradiation and Reprocessing of FUELS and targets for transmutation[END_REF]. The details of the fuels and pins characteristics, experimental device and irradiation parameters are given in [START_REF]SPHERE: Irradiation of sphere-pac fuel of UPuO 2-x containing 3% Americium[END_REF] and summarized in Figure 3.

PELGRIMM hosted the PIE activities for the SPHERE irradiation. Optical microscopy has been performed on 6 samples: one axial and two radial cuts, for both the pellet and the spherepac fuel pins.

For each type of fuel, one of the two radial samples was cut in 4 quadrants and prepared for elemental analysis (SEM/WDS). For each sample, an estimation of the associated linear power has been deduced from post-irradiation calculations and gamma spectrometry measurements of non-volatile fission products. The main results deduced from Non Destructive Examinations (NDEs) and Destructive

Examinations (DEs) are summarized in Table 3.

From a macroscopic point of view, the behaviour of the pellet and spherepac MADF fuel is globally the same. For the two types of fuels: the cladding has not been degraded nor deformed; the fuel stacks have not been elongated (nor creeped); the non-volatile FPs ( 95 Nb, 95 Zr) distributions are consistent with axial neutron flux (peaking at the ends) and have been used to recalculate the axial power profile, used for the analysis of DEs; the puncturing results are consistent with 100% He release and ~90% Xe (and Kr) release for both spherepac and pellet fuels, despite large uncertainties due to calibration issues.

The behaviour under irradiation of pellet and spherepac fuels is also quite similar in terms of fuel

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12 microstructure. A central hole was formed, smaller (and even undetected by neutron radiography) for pelletized than for spherepacked stacks due to a lower temperature regime in pellets that can be explained by a higher thermal conductivity of the pelletized compared to spherepac fuel. Both forms of fuel exhibit a restructured region with columnar and equi-axed grains and with similar microstructure (increased/decreased porosity for pellet/spherepac respectively, with respect to the corresponding as-fabricated porosity).

The radial actinide distribution in the pellet fuel confirms the observation reported for previous comparable irradiation tests, such as SUPERFACT [START_REF] Prunier | Transmutation of Minor Actinides: behavior of americium and neptunium based fuels under irradiation[END_REF]- [START_REF] Nicolaou | Experience with fast neutron reactor fuels containing Minor Actinides: transmutation rates and radiation doses[END_REF], Am1 [START_REF] Maeda | Short-term irradiation behavior of Minor Actinides doped Uranium Plutonium Mixed Oxides fuels irradiated an experimental fast reactor[END_REF]- [START_REF] Tanaka | Microstructural evolution and Am migration behavior in Am-containing fuels at the initial stage of irradiation[END_REF] or AFC-2C&2D [START_REF] Maclean | Irradiation of metallic and oxides fuels for actinide transmutation in the ATR[END_REF]: the same tendency and magnitude have been observed. For the spherepac fuel, the americium profile shows a redistribution which seems in line with literature [START_REF] Guerin | Fuel Performance of Fast Spectrum Oxide Fuel, Comprehensive Nuclear Materials[END_REF]. The plutonium redistribution for (restructured) fuels with fuel center temperature ≥ 2000°C shows a deviation from nominally observed distributions (where Pu redistribution is more pronounced, and Pu and Am-profiles are often similar).

Finally, a significant difference between pellet and spherepac fuels lies in the presence of FCMI for the pelletized pin, which seems absent for the spherepac pin.

To complete the analysis, the pellet and spherepac fuel pins irradiated in the SPHERE experiment were simulated blindly (previously to PIE execution) with several fuel performance codes in the framework of PELGRIMM, as presented in section 5.

MARIOS

The MARIOS separate-effect test was the first irradiation of a comprehensive R&D program of MABB fuel qualification started in 2008 [START_REF] Bonnerot | Development program on minor actinide-bearing blankets at CEA[END_REF], [START_REF] Béjaoui | AMericium-Bearing Blanket separate-effect experiments: MARIOS and DIAMINO irradiations[END_REF]. It was designed to investigate helium behaviour and fuel swelling of (U 0.85 Am 0.15 )O 2-x fuel as a function of microstructure (i.e. as-fabricated porosity) and temperature. Small disks (diameter: 4.5mm and thickness: 1.5mm) of fuels with 2 different openporosity ratios (7.7% and 12.5%) were prepared by CEA and irradiated in an experimental device specifically designed in order to maintain constant flat temperature profiles in the fuel disks. Thus, four mini-pins, each containing 6 fuel disks, were irradiated in HFR during 304 EFPD at constant temperatures. The MARIOS irradiation was designed, manufactured and irradiated within the

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FAIRFUELS project [START_REF] Hania | FAIRFUELS : FAbrication, Irradiation and Reprocessing of FUELS and targets for transmutation[END_REF]. The details of the fuels and pins characteristics, experimental device and irradiation parameters are given in [START_REF] Béjaoui | AMericium-Bearing Blanket separate-effect experiments: MARIOS and DIAMINO irradiations[END_REF], [START_REF] Prieur | Fabrication and characterisation of U 0.85 Am 0.15 O 2 discs for MARIOS irradiation program[END_REF] and [START_REF]The results of the irradiation experiment MARIOS on americium transmutation[END_REF] and summarized in Figure 3.

PIE was performed within PELGRIMM. DEs focused on the effect of fuel microstructure: they were performed on the fuel disks of pin #1 and #4, with high density and low density microstructure, respectively, and which were submitted to a similar temperature of ~1000°C. DEs have included optical macro and microscopy, SEM, EPMA, SIMS and XRD. The main results deduced from NDEs and DEs are summarized in Table 4.

NDEs have revealed a clear effect of temperature regarding volatile FPs and fission gas release: Cs release has been evidenced for the pin submitted to the highest temperature (pin #2) and Xe and Kr releases are strongly temperature dependent in the investigated temperature range and rather consistent with Cs behaviour (qualitatively). On the contrary, considering that He release during cooling time is negligible, and despite large uncertainties, it has been concluded that all He produced during irradiation was released to the plenum, irrespective of differences in porosity and temperature. For this type of fuel, the threshold for high He release seems to be below 1000 °C.

The fuel disks were in a relatively good shape after irradiation (intact or in several fragments, but no powder), whatever the fuel porosity and the irradiation temperature. However, it is clear that low density fuel sample deteriorated at high temperature: the disks of pin#3 (1180°C) are in numerous small fragments whereas the disks of pin#4 (980°C) are all intact except one. For high density fuels, the effect of temperature is not so clear: the disks of pin #1 and #2 are globally in the same shape (i.e.

in several small fragments).

The effect of porosity on fuel behaviour can directly be deduced from the comparison of metrology and microanalysis results obtained on pin #1 and pin #4, as the temperature was similar for the 2 pins (990-980°C). Globally, there is no significant difference between pin #1 and pin #4 fuel behaviour.

The porosity after irradiation for both type of fuels has remained very similar to the as-fabricated porosity and no macroscopic swelling has been evidenced for pin #1 and #4 fuels, which is consistent with high He release (~100%) and low fission rate (~1.14%). Finally, the main difference between high and low density fuels lies in their mechanical behavior: most of the low density disks are intact after irradiation, which would promote the use of high porosity microstructure for such irradiation

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In addition, apparent transmutation and fission rates have also been assessed by EPMA, as well as isotopic ratios by SIMS: a very good agreement between measurements and calculations (based on neutron data and re-scaled thanks to dosimetry measurements [START_REF]The results of the irradiation experiment MARIOS on americium transmutation[END_REF]) is observed, with a transmutation rate close to 46% for a fission rate about 1.14%.

MARINE

As stated above, the MARINE irradiation [START_REF]The MARINE experiment: Irradiation of sphere-pac fuel and pellets of UO 2 for americium breading blanket concept[END_REF] is part of the second step in the testing of (U,Am)O 2 fuels (separate-effect tests like MARIOS belonging to the first step). In the MARINE experiment, the case of an AmBB pin situated close to the SFR core has been considered, with values of power, irradiation temperature and helium production situated in the upper range of those related to the various pins in the AmBB subassembly [START_REF] Valentin | Heterogeneous minor actinide transmutation on a UO 2 blanket and on (U,Pu)O 2 fuel in a SFR -preliminary design of a pin and assembly[END_REF]. This choice is consistent with the previously irradiated MARIOS experiment.

In addition, MARINE is equivalent to SPHERE as it includes 2 mini-pins of pelletized and spherepacked fuels, but differs in fuel composition: (U 0.15 Am 0.85 )O 2-x . One of its main objectives is also to study the role of microstructure and temperature on fission gas and helium release as well as on fuel swelling. To do that, an improvement of the MARINE experiment is that both mini-pins have been instrumented with pressure transducers in order to measure online the pressure to better understand the gas release behaviour during the irradiation.

The fuel preparation conditions are detailed in section 3.1 and summarized in Figure 3. The results of the calculations performed in order to design the experimental device and predict the irradiation conditions are detailed in [START_REF]The MARINE experiment: Irradiation of sphere-pac fuel and pellets of UO 2 for americium breading blanket concept[END_REF]. Neutronic analyses were performed for an effective irradiation duration of 15 cycles of the HFR. In addition, thermal analyses predicted a fuel maximum temperature ranging from 950°C to 1200°C and 920°C to 1120°C for the spherepac and pellet pins, respectively. These results were obtained assuming the absence of fuel restructuring (not expected at these temperatures).

So, outcomes from MARIOS at 980°C-1180°C could directly be used for the interpretation of MARINE PIE results. Moreover, as gas release is part of the main objectives of the MARINE

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT

15 experiment, the helium production (see Figure 7) was intended to be representative of Am recycling scenarios in SFR reactors [START_REF] Valentin | Heterogeneous minor actinide transmutation on a UO 2 blanket and on (U,Pu)O 2 fuel in a SFR -preliminary design of a pin and assembly[END_REF] and an objective of 2.7 mg.cm -3 (i.e. 336 days of irradiation in HFR) has come to be a good compromise with the timescale of the PELGRIMM project.

Finally, the irradiation of the MARINE experiment was performed in HFR during 12 reactor cycles, equivalent to 359 Full Power Days (FPD), from January 2016 up to May 2017. The on-line pressure measurements that were implemented for the first time on Am-bearing fuel pins have been available

up to the end of the 4 th cycle, for the pelletized fuel pin. From these measurements, it will be possible to deduce the kinetics of gas release as a function of irradiation time. The MARINE irradiation stage is now fully complete and PIE is expected to be performed in a near future (within another framework than PELGRIMM, that ended in June 2017) to take full benefit of this first semi-integral irradiation of pellet and spherepac MABB fuels.

MODELLING AND SIMULATION OF FUEL BEHAVIOUR UNDER IRRADIATION

Most of fuel performance codes have originally been developed, verified and validated to model the behaviour of standard fuels for Light Water Reactors and Fast Reactors systems. Some of the codes under consideration within PELGRIMM were already upgraded within previous European projects to integrate some of the specificities of Am-bearing fuels or of spherepacked fuels. These codes are MACROS [START_REF] Lemehov | MACROS Benchmark Calculations And Analysis Of Fission Gas Release In MOX With High Content Of Plutonium[END_REF], TRANSURANUS [START_REF] Lassman | TRANSURANUS: a fuel rod analysis code ready for use[END_REF], [START_REF] Van Uffelen | Extending the application range of a fuel performance code from normal operating to design basis accident conditions[END_REF], SPHERE-3 and SPACON codes [START_REF] Wallin | The sphere-pac nuclear fuel behaviour modelling at high burnup[END_REF], [START_REF] Wallin | Nuclear fuel behaviour modelling at high burn-up and its experimental support[END_REF]. In addition, the calculation code GERMINAL [START_REF] Bouineau | Toward an improved GERMINAL V2 code to model oxide fuel for sodium fast reactor[END_REF], has also been considered within PELGRIMM to describe the MABB fuel behaviour under irradiation.

The objective of the work within PELGRIMM, was to capitalize on previous investigations to address the main challenges due to specific issues of MADF and MABB fuels under spherepac and pellet forms, such as:

• low MA content in (U,Pu)O 2 support matrix and high MA content in UO 2 support matrix;

• high temperatures in SFR MADF fuels and moderate or even low temperatures (<1500°C) expected for MABB fuels in dedicated blanket assemblies near the SFR core periphery;

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 16
• high helium release ratios in MADF fuels and potentially low helium release ratios in MABB fuels, the latter potentially leading to excessive MABB fuel swelling and needing its accommodation, possibly including microstructures changes making release easier;

• transition from pellet to spherepac forms of MA-bearing fuels, leading to modification of thermal and mechanic property descriptions as well as changes in FCMI due to an expected softer mechanical behaviour of spherepac fuels.

Extensive work has been done in PELGRIMM to extend the capabilities of the codes to model and simulate the fuel behaviour under irradiation for both MADF and MABB fuels shaped as spherepac and pellet. The approach has consisted in reviewing, at the beginning of the project, the capabilities of the codes, collecting, from well-qualified experiments, data that cover material properties, cladding and design parameters, then developing models and working out recommendations. The upgrades of the codes have consisted in the implementation of new models or in the modifications of existing models and correlations for standard fuels when appropriate. The efforts have covered aspects such as: helium production and release under a fast neutron spectrum, plutonium and oxygen redistribution, melting temperatures and evolution rate of fuel restructuring, columnar grain growth and central void formation [START_REF] Ishii | An investigation of the Pu migration phenomena during irradiation in fast reactor[END_REF]- [START_REF] Talip | The dissolution of helium in La-doped UO 2 as a surrogate of hypo-stoichiometric UO 2[END_REF].

In parallel, a comparison of spherepac and pellet fuel behaviour under irradiation based on existing experimental data and a review of the principles of equivalence of the two fuel concepts have been drawn up. The aim of the study was to evaluate several analytical tools and their functional abilities and to work out recommendations for practical and adequately simplified models needed for the simulation of pelletized and spherepacked MADF fuels. For SPHERE calculations, equivalent formulas were established, between spherepac and pellets, for the most important fuel properties (thermal conductivity, irradiation-induced solid and gas swelling) and simplified semi-theoretical correlations were made to model specific power profile in the fuel pins under thermal flux.

Finally, the evaluation of the upgraded fuel performance codes (MACROS, TRANSURANUS and SPHERE-3) to simulate the behaviour of fuel pins within the SPHERE irradiation test (described in details in section 4.1) was performed (blind as the irradiation was still underway at the time of the calculations). The results of the comparison are gathered in Table 5.

M A N U S C R I P T A C C E P T E D

Whatever code was used, the results were consistent (when comparisons between codes could be made) and an initial comparison between calculated and experimental results can be made, on the basis of the PIE results of the SPHERE irradiation (see Table 3).

For the spherepacked fuel, the calculated center-line temperature is well above 1800°C during the majority of in-reactor time so that the fuel would show restructuring, sintering and density changes, with an original spherepac structure still existing within a short bound near the outer surface. This is rather consistent with the experimental observations of the neutron radiography performed after the 1 st cycle of irradiation as well as the microscopic observation performed during PIEs.

For the pelletized fuel, the calculated center-line temperature is below 1800°C, which is a threshold for columnar grain growth and central void formation. Thus, neither central hole nor fuel restructuration has been predicted by the simulation whereas a small central hole and a partially restructured fuel have been observed experimentally. However, given the low diameter of the measured central hole and the dispersion of the calculated temperatures (1400-1800°C), the calculated and measured results are not inconsistent.

Whatever the fuel shape, the major discrepancy concerns the calculated and measured released fractions for fission gas and helium. Nevertheless, the uncertainties on puncturing results were large, so the interpretation regarding gas release should be done with lot of caution. In a general way, the conclusions drawn here remain very preliminary. A refined analysis of these results has to be done (out of the PELGRIMM scope) in order to provide new insights in the modelling of MADF fuel shaped as pellets and spherepac.

SIMPLIFIED DESIGN AND SAFETY PERFORMANCE PRE-ASSESMENT OF AN ADVANCED SPHEREPAC (U,Pu,MA)O 2 SFR CORE

As spherepacked fuels are foreseen to be good candidates for MA-bearing fuel concepts, PELGRIMM started linking the investigation of spherepacked fuel synthesis and behaviour under irradiation with an assessment of core physics, design and safety performance. Among the issues to be dealt with in a safety demonstration, are lower melting margins compared to pellet fuels and an effect The study was in continuity with the former CP-ESFR project [START_REF] Fiorini | The collaborative project on European sodium fast reactor (CP-ESFR project)[END_REF] which aimed at designing and analyzing a 3600 MWth SFR loaded with a standard driver fuel (shaped as pellets) and assessing its safety behaviour and its transmutation capabilities. From CP-ESFR project recommendations, the socalled CONF-2 core, an optimized version of the 'ESFR working horse core', was chosen to start the PELGRIMM investigations [START_REF] Rineiski | ESFR core optimization and uncertainty studies[END_REF].

The CONF-2 core was firstly revisited and its safety analysis completed, analyses that were missing in the CP-ESFR project. The introduction of spherepac fuels instead of pellets as well as of (U,Pu)O 2 and (U,Pu,Am)O 2 fuel compositions, was then considered. At Begin Of Life (BOL), the essential differences between pellet and spherepac pins remain on the distribution of the free areas in the fuel columns [START_REF] Andriolo | Impact of innovative spherepac fuel on safety performance of the sodium cooled reactors[END_REF]: central hole and fuel/clad gap for pelletized fuel pins and free inter-particle spaces for spherepacked fuel pins. As a consequence, the fuel center temperature in the spherepac pins is much higher than in the pellet pins. At higher burn-up, the thermal regimes of both fuel concepts become close. Then, neutronic analyses were performed for variants of the CONF-2 core that contain up to 4% Am in the fuel. As expected, the results reveal a deterioration of the safety parameters in CONF-2 with burnup and Am content [START_REF] Andriolo | Impact of innovative spherepac fuel on safety performance of the sodium cooled reactors[END_REF]. Provided that the Am content is limited, spherepac fuel can be inserted in a SFR core instead of pellets without significantly changing the design and without having a major impact on the core layout and safety coefficients.

Finally, accidental conditions were simulated with different codes: SAS4A [53], BELLA [START_REF] Bortot | The BELLA code for point dynamic simula8ons of transients, ESNII+Workshop On Safety[END_REF] and MAT5DYN [START_REF] Darmet | Dynamical Analysis of Innovative Core Designs Facing Unprotected Transients with the MAT5DYN Code[END_REF] for the initiation phase of the accident, SIMMER-III [START_REF] Sa | SIMMER-III: A Computer Program for LMFR Core Disruptive Accident Analysis[END_REF] up to conditions of potential whole core melting and core disruption. The codes were adapted as much as possible to the specificities of a spherepac fuel, in particular to take into account the heat transfer in a bead stack and the absence of a fuel-clad gap as well as the dispersive feature of a non-restructured spherepac fuel in case of cladding failure. Two relevant accidental situations were analyzed: the unprotected loss of flow accident (ULOF) and unprotected transient over-power accident (UTOP).

For ULOF analyses, transient calculations for the CONF-2 core with the SIMMER-III code [START_REF] Andriolo | Impact of innovative spherepac fuel on safety performance of the sodium cooled reactors[END_REF]- [START_REF] Maschek | Safety Analyses for Sodium-Cooled Fast Reactors with Pelletized and sphere-pac[END_REF] were performed for both pellet and spherepac fuel; for spherepac fuel under BOL conditions, 2 cases must be considered: before and after fuel restructuring, because high temperatures could be

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT

reached by the non-restructured fuel if no starting procedure has been implemented to moderate the power level at the first power rise. Figure 8 shows the power and reactivity evolution during the ULOF for the non-and restructured spherepac fuels. The simulations of a ULOF at BOL show very mild transients [START_REF] Maschek | Safety Analyses for Sodium-Cooled Fast Reactors with Pelletized and sphere-pac[END_REF].

The same ULOF was then considered after 3 irradiation cycles. The nuclear power trace and reactivity development of the CONF-2 core is displayed in Figure 9 for both pellet and spherepac fuel.

Compared to the BOL case, a slight increase of the nuclear power peak can be observed, but the negative reactivity effects can still balance the positive contributions from core voiding. As expected from the similar thermal conductivity and macrostructure of both fuel types at this stage of irradiation, the spherepac and pellet core show a very similar behaviour under these accidental conditions. With loading of Am for transmutation and burning (2-4%) a significant deterioration of the void worth and the other safety parameters, such as the Doppler feedback, take place. Consequently the safety advantages observed in the BOL core vanish and severe transients with total core disruption and melting result, irrespective of the fuel type.

In the case of a UTOP, the use of spherepac fuel degrades the CONF-2 core safety performance, because of its low thermal conductivity, besides reducing the margin against fuel melting at steadystate, leads to larger fuel temperature gradients ensuing from positive reactivity insertions, likely causing its melting; however, the resulting transient was rather mild, with an estimated peak power level of 2-3 times the nominal one. As a result of these findings, it can be concluded that the use of spherepac fuel in the optimized CONF2 core does not lead to significant changes in the accident scenario sequence when spherepac fuel is used instead of pellet fuel. The possibility of early spalling of the spherepac particles and their release from the core might lead to an even milder accident behavior.

CONCLUSION

PELGRIMM has constituted a new step in the long term process of the MA-bearing fuel safety testing, with the investigation of a wide range of items: from pellet to spherepac fuel forms, from homogeneous to heterogeneous MA-recycling modes, from fuel preparation and characterization to

  a risk of loss of fissile granules to the coolant in case of cladding failure.

Figure 3

 3 Figure 3 summarizes the results of the MARINE fuels preparation, along with those of SPHERE and

Figure 3 gives

 3 an overview of the SPHERE, MARIOS and MARINE irradiation tests and the main outcomes M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 11 gained from each irradiation are detailed in the following sections.

  fissile granulates in the coolant in case of cladding failure.

  under irradiation, from experiments to modelling and simulation, from normal operating conditions to severe accidents.Regarding the fuel preparation aspects, alternative routes of MA-bearing fuel synthesis have been investigated to seek improvements (simplification, robustness, lower secondary waste streams…). The Am-bearing fuel for MARINE, both pellet and spherepac types, have been prepared within PELGRIMM by infiltration of porous UO 2 precursor beads, prepared by sol-gel gelation, with americium nitrate solutions. In addition, a variant of the sol gel process, based on micro-wave internal gelation was developed and a new dedicated facility is now available. In parallel, the adaptation of the WAR process to the synthesis of (U,Am)O 2 beads and pellets has started and has provided promising results with the preparation of high density microspheres and pellet. Finally, by demonstrating the feasibility of these different fuel synthesis routes, PELGRIMM has opened the path to new possibilities for Am-bearing fuel concepts.The realization of the PIE of innovative irradiation tests, such as SPHERE and MARIOS have largely contributed to improve the knowledge on Am-bearing fuel behaviour under irradiation for both MADF and MABB concepts, in spherepac and pellet forms. A first comparison between spherepacked and pelletized (U,Pu,Am)O 2 fuel performances under irradiation was made, using PIE results of the SPHERE fuels, showing that for similar irradiation conditions, despite significantly different temperatures, the behaviour of different shaped fuels was rather similar. The main difference lies in the presence of FCMI for pelletized fuels, which seems absent for spherepacked fuels. The MABB concept has reached a key step of its testing program with the first separate-effect irradiation MARIOS, which PIE has shown that whatever the fuel porosity and the irradiation temperature (in the range of 1000°C-1300°C), no significant swelling has occurred which seems consistent with low fission rate and high He helium release during irradiation. Finally, the main difference between high and low density fuels lies in a better mechanical behavior of the low density fuels in such irradiation conditions. MARINE, the first semi-integral irradiation of MABB fuels is now complete and its PIE, to be planned in a framework other than PELGRIMM, should provide complementary results to SPHERE and MARIOS PIE.For the modelling and simulation of fuel under irradiation, capabilities of the fuel performanceM A N U S C R I P T A C C E P T E DACCEPTED MANUSCRIPT 21 codes have been improved thanks to the implementation of more mechanistic models, new numerical methods, more reliable properties laws, etc. The outcome of benchmarks performed between PELGRIMM participants has been encouraging and has shown reasonably good agreements with experimental results: first attempts to simulate the fuel behaviour during SPHERE irradiation thanks to fuel performance codes have provided, for most of the cases, preliminary calculated results consistent with PIE results.In parallel, to form a coherent whole, an optimized core loaded with (U,Pu,Am)O 2 spherepac fuels was successfully assessed for safety performance, in an initial scoping assessment. Two relevant accidental situations were analyzed: the unprotected loss of flow accident (ULOF) and unprotected transient over-power accident (UTOP). Based on the current analyses, the implementation of spherepac fuel does not cause any specific design problems and the first safety analyses also indicate that spherepac fuels do not seem to cause any specific safety problems, if introduced in an SFR. Finally, the PELGRIMM project has capitalized on efforts made within previous European projects (ACSEPT, FAIRFUELS, F-BRIDGE, CP-ESFR) and has taken a new step in the development of both MA-bearing fuel options: (U,Pu,MA)O 2 and (U,MA)O 2 , related to fuel preparation processes, irradiation behaviour and core safety performance, including a comparison on fuels shaped as pellets and beads.

ACKNOWLEDGMENT

We gratefully acknowledge the PELGRIMM consortium constituted of CEA, AREVA, EDF, ENEA, ENEN, JRC, KIT, KTH, LGI, NRG, PSI, SCK CEN. This research was supported by the European Commission through the FP7 PELGRIMM project (Grant Agreement No. 295664).

DATA AVAILABILITY

The raw/processed data required to reproduce these findings cannot be shared at this time due to legal or ethical reasons.

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT

List of abbreviations

• PELGRIMM : a European Project carried out from 2012-2017 between 12 partners

• New step in development of Minor Actinide-bearing fuels for transmutation

• Spherepacked and pelletized fuels for homogeneous and heterogeneous recycling modes

• R&D on fabrication, behaviour under irradiation, modelling, simulation, neutronics


Keywords: Radioactive waste immobilisation, Borosilicate glasses, Dissolution, Solid-state NMR, Magnesium

Introduction

The majority of radioactive waste glass produced in the UK contains magnesium (Mg). This arises for historical reasons from the Mg-containing alloy cladding used on the natural uranium fuel of the first generation UK Magnox reactors. This Mg is entrained in the fission product fraction following fuel re-processing operations and becomes incorporated into the glass wasteform on vitrification. At least two studies [1,2] have shown that in long-term (up to 12.2 years), simulant UK Magnox waste glass containing Mg (and nominally no calcium (Ca)) have significantly poorer aqueous durability compared with Ca-based glasses such as the French benchmark SON68 inactive glass simulant. This paper describes a separate effect study of the influence of the substitution of Mg on the pristine structures and initial dissolution rate of simplified UK Magnox waste glasses.

The simulant Magnox waste glass with a 25 wt% waste loading (herein referred to as MW25) contains 23 oxide components (see supplementary materials), the aim of this work is to produce a simplified glass with 6-7 components that represents the main compositional features of the complex glass and allows the separate effect of Ca and Mg on glass structure to be established with all other parameters being equal. As the major structural probe will be NMR (nuclear magnetic resonance), paramagnetic lanthanides have been substituted by lanthanum (La) and iron (Fe) has been left out of the compositional mapping.

The similarity between MW25 and the simplified analogue needs to be verified for the use of simplified glass to study its full-component counterpart to be valid, especially from a Mg point of view. 25 Mg MAS NMR provides a powerful tool to investigate the local Mg environments in glasses. The major cations present in the glass are boron (B) and silicon (Si), these act as the network formers which are linked via bridging oxygens to form the network of the glass structure. Si is primarily in four-fold coordination, this is the origin of the Q n (quaternary) Si speciation. The number n, ranging from 1-4, denotes the number of bridging oxygens attached to a silicate tetrahedron. B is distributed in either three-or four-fold coordination depending on the nature and the availability of charge balancing cations. This was explored in the series of simplified glasses by 11 B MAS (Magic Angle Spinning) NMR.

Other structural changes due to Mg substitution, similarly, were examined by using 23 Na and 27 Al nuclei as NMR probes to elucidate the local coordination changes and overall charge balancing scheme.

In long-term experiments, B release is considered as the proxy for measuring overall glass dissolution in water owing to its almost complete leachability and absence from any secondary phases [1,2,3] although its slight retention in the alteration layer has been directly probed [4,5]. 11 B MAS NMR can also be used to compare the [3] B/ [4] B ratio before and after leaching experiments to investigate whether there is preferential dissolution occurring from these glasses. This in turn would indicate whether the poorer aqueous durability of Mgcontaining glasses is due to a change in B speciation. On the other hand, whether B speciation and other structural changes would fundamentally affect the aqueous durability of these glasses could be readily addressed by measuring the initial dissolution rate (r 0 ) before secondary effects, such as solution saturation and altered layer formation, start to influence the dissolution process. Comparison of r 0 with Ca/Mg ratio will help to discriminate the relative importance of structural and secondary effects on glass dissolution.

Materials and Methods

Glass preparation

The simplified glass was made according to the composition of the full-component MW25 glass (listed in Table S1). In order for the composition to represent the full-component glass as closely as possible, the molar ratios of Si, B, Na, Al and Mg were fixed at the same ratios as in the full-component glass. Meanwhile, the REE (rare-earth element) load was represented by La on a charge for charge basis except cerium which was attributed its fully oxidised charge of (4 + ). Paramagnetic transition metals such as Fe and Ni were omitted to facilitate examination of the glass structure with NMR spectroscopy. The nominal composition is listed in Table 1. Mg was then successively substituted by Ca, on an atom-for-atom basis, to produce 0, 25, 50, 75 and 100 mol% Ca-substituted simplified glasses. The glasses are denoted as MgEM, Mg75Ca25, Mg50Ca50, Mg25Ca75 and CaEM respectively. The final glass-making process consisted of a controlled heating (10 /min) of weighed amounts of the precursors (SiO 2 , Na 2 B 4 O 7 , Al 2 O 3 , Na 2 CO 3 , MgO/CaCO 3 , La 2 O 3 ) in a platinum90/rhodium10 crucible to 1500 . The melt was poured onto a steel block and allowed to cool to room temperature (RT). The glass was then broken up and re-melted using the same ramp rate to reach 1500 °C. The second melt was poured onto a heated steel block at 250 °C then left cooled to RT. Finally, the glass was annealed at 550 °C for two hours and subsequently cooled slowly in the furnace over a period of 16 hours. The compositions of these simplified glasses were analysed (Table 1) by a combination of acid digestion/solution ICP-MS and solid source LA-ICP-MS ((Laser Ablation) Inductively Coupled Plasma Mass Spectrometry) and EPMA (Electron probe microanalysis) techniques (Table S2 shows the original data presented in oxide wt%).

Leaching experiments

The glasses made as outlined in 2.1 were crushed and ground and then sieved to a size range between 75 m and 150 m. After the sieving procedure, the glass particles were washed in a controlled manner with Type I water and absolute ethanol according to the PCT-B (Product Consistency Test Type B) protocol [START_REF]ASTM C1285-14[END_REF] in order to remove any adhering fines and contaminants. The theoretical median diameter of 112.5 μm was then used for determining the specific surface area (SA) of each glass sample.

For the initial dissolution rate experiments, a sample of each glass composition ranging from 0.05 to 0.1 g was leached in a sealed PFA (Perfluoroalkoxy alkane) vessel in 100 mL 18.2 MΩ deionised water at 90 1 °C and the whole system was agitated with a magnetic stirrer. This setup results in the SA/V (surface area to volume) ratios being no greater than 22 m -1 to ensure that the initial dissolution rate (r 0 ) was measured in very dilute conditions i.e., the forward hydrolysis reactions are not hindered and the formation of effective barrier layers is precluded [START_REF] Gin | [END_REF]. This should be the maximum dissolution rate of the glass for these (temperature, DI) conditions.

The values of r 0 can be determined from the change in normalised release of Si (NL Si ) within the first 7.2 hours of leaching, with the application of the shrinking core model [3] to correct for the change in SA/V ratio over time as dissolution proceeds, as follows:

AG (t) = AG (t-1) + 10 -6
(1)

ET (t) = 10 3 (2) r 0 = = ( 3
)
(AG -altered glass fraction; C i -concentration of element i in leachate (mg/L); V -solution volume (mL); m 0 -initial mass of glass sample (g); X i -mass fraction of element i in glass;

ET -equivalent thickness of dissolved glass (nm); SA -initial specific surface area (geometric in m 2 /g); -glass density (g/cm 3 ); r 0 -initial dissolution rate (g/m 2 /d); NL inormalised release of element i (g/m 2 ); t -time interval (d)). The rationale of using Si concentration rather than that of Na is to bypass the initial ion-exchange stage and reflect the direct dissolution of the glass network.

For long-term 112-day leaching experiments, a glass sample of each composition (only 11 B NMR data of 112-day leached MgEM and MW25 is presented in this paper) of 0.22-0.23 g was leached in a PTFE linear, which was sealed inside a stainless steel vessel, in 4.0 mL 18.2

MΩ deionised water at 90 1 °C. The resulting SA/V ratio for the experiments from this setup is accordingly 1200 m -1 across all compositions.

2.3.Solid-state NMR characterisation

11 B, 27 Al and 23 Na are all non-zero spin nuclei and therefore their local environments can be detected by solid-state NMR techniques even without long-range atomic order. As presented in this paper, apart from 25 Mg, their spectra were all obtained by single-pulse excitation. ) and 23 Na in 1M NaNO 3 , 27 Al in 1M Al(NO 3 ) 3 and 25 Mg in 1M MgCl 2 as 0 ppm, respectively. 25 Mg shows very low NMR sensitivity due to its low natural abundance (10.0%), low resonance frequency (30.6 MHz at 11.7 T) and large quadrupole moment (199.4 mbarn). In addition, only 6.5 mol% of the total cations in the glass is Mg. To alleviate these difficulties, 25 Mg spectra were obtained with a DFS (double-frequency sweep) assisted rotor-synchronised spin-echo pulse sequence on the UK 850 MHz solid-state NMR facility at the University of Warwick. DFS was applied to increase the spin population difference between andenergy levels. Glass particles of MW25 (75-150 μm) and MgEM (<75 μm) were packed in 4 mm zirconia rotors and spun at 14 kHz. Spin-echo and pulses of 2.33 and 4.66 μs were applied and the recycle delays were 0.1 and 1.5 s, respectively, as the MgEM glass, unlike MW25, did not contain paramagnetic cations that enhance spin-lattice relaxation.

DMFIT software [8] was used to fit the experimental 11 B and 27 Al NMR spectra to determine the relative proportion of different species in the glass samples.

Results

3.1.Glass density

Density was determined for each simplified glass by an Archimedes principle measurement using room temperature DI as the immersion liquid. The density variation with composition is shown in Table 2. It decreases monotonically from 2.56 g/cm 3 for CaEM to 2.49 g/cm 3 for MgEM. The squared data points plotted in Figure 1 show that there is a clear systematic decrease in density as Ca is substituted for Mg on a molar basis. The solid (black) and dashed (red) lines represent possible substitution models (see Section 4.1 for further details).

3.2.Glass local structures

Boron

A stack plot of 11 B MAS NMR central transition spectra of three of the simplified glasses is shown in Figure 2 (a). Each spectrum consists of a narrow Gaussian peak centred at around 0 ppm which is attributed to B in four-fold coordination and a broadened quadrupole doublet centred at around 10 ppm which is attributed to B in three-fold coordination [START_REF] Mackenzie | Multinuclear Solid-State NMR of Inorganic Materials[END_REF]. The broader [3] B peaks arise from the asymmetric trigonal configuration of BO 3 units, which have larger electric field gradients (EFGs) compared with more symmetric BO 4 units with tetrahedral configuration. Each [3] B peak is made up of contributions from ring and non-ring BO 3 units, both of which contain a distribution of isotropic chemical shifts ( iso ) as well as EFGs. The spectra are shown normalised to [4] B intensities in order to compare the [3] B intensities. The relative intensity of the [3] B peak which is directly proportional to the [3] B fraction of total B in the glass, increases as more Mg is present. The intermediate glasses Mg75Ca25 and Mg25

Ca75 are consistent with this trend, but are omitted from the figure for clarity.

As shown, the central transition peaks of [3] B and [4] B signals are slightly overlapped.

Additionally, due to the small EFG associated with [4] B units, the first-order satellite transition sidebands are present in the full spectrum as shown in Figure 2 Table 3 shows the full fitting results reported as the relative percentage of B in three-and four-fold coordination. Excess [3] B is the excess cation mole percentage of [3] B above that found in the CaEM glass.

Sodium

Figure 4 shows the overlaid plot of 23 Na MAS NMR spectra of CaEM and MgEM glasses.

The centre peaks are coincident and centred at around -20 ppm while δ iso lies between 10~20 ppm (asterisks denote the spinning sidebands). A distribution of both isotropic chemical shifts and quadrupole interaction parameters contributes to the total linewidth, as expected in glass structures. The spectra of intermediate compositions are identical to the two endmembers shown here, but have been omitted for clarity. Despite the distributions, the identical peak position, linewidth and lineshape clearly suggest that local environments around Na are unchanged within the Ca/Mg compositional series of simplified glasses.

Aluminium

These consist of a predominant, well-defined four-coordinated ( [4] Al) and much smaller sixcoordinated ( [6] Al) peaks. The presence of five-coordinated Al ( [5] Al) is not obvious but also limited by the resolution at the magnetic field used (11.7 T). The spectra are presented with [4] Al peaks normalised to compare the minor changes in concentration of the higher coordinated [6] Al. The predominant Al speciation is [4] Al with in the range of 60~70 ppm and a tiny fraction of [6] Al with of around 5 ppm. The resonance position of [5] Al, if there is any, should lie between these two values. There is some evidence of a small coordination change for Al as Mg is added as shown in the inset of with the relative fraction of [6] Al in these three glasses is CaEM > MgEM > Mg50Ca50 and the difference between MgEM and Mg50Ca50 being almost negligible. Since the two peaks are not completely resolved with slight overlapping at the low frequency side of the [4] Al peaks, the exact relative fractions of [4] Al and [6] Al in Mg50Ca50 and two endmembers were obtained by curve fitting using a model in DMFIT program that account for the distribution in δ iso and EFGs (CzSimple). The results are summarized in Table 4. The amount of [6] Al reduces from 4.8 % of total Al or 0.2 % of total cations in the CaEM glass to 1.6 % and 0.7 % or 0.08 % and 0.03 %, respectively. An example of the fit for CaEM glass is shown in Figure 6 (see discussion) and the total fitting results are listed in Table S3.

In addition, for all three compositions, Ca/Mg substitutions almost do not cause shift in either resonance position.

Magnesium

The similarity of the Mg local environment in both simplified and full-component glasses is confirmed by the spectra shown in Figure 7 (a): within the signal-to-noise (S/N) level obtained, the overlaid 25 Mg MAS NMR spectra of full-component MW25 glass and its counterpart simulant MgEM glass assure a good representation of MW25 by the simplified glass from a Mg perspective. The δ iso is almost the same for both glasses, at around 45~50 ppm indicating a similar coordination. In addition, the lineshape of both centre peaks, i.e., a sharp edge at the high frequency edge and tails towards the low frequency end, is characteristic of a distribution in quadrupolar interaction parameters (EFG and η) with similar δ iso values [START_REF] Shimoda | [END_REF]11]. Spinning sidebands are just observable in the MgEM glass spectrum while not being observed for the MW25 glass (Figure 7 (b)).

3.3.Initial dissolution rates

As shown in Figure 3 there is no selective leaching of B associated with its coordination.

However, it is not clear whether the ratio of [3] B/ [4] B would affect the dissolution rate. To probe the effects of primary glass structure on its solubility, the initial dissolution rate (i.e. before any significant altered product has been formed on the glass surface) has been measured for all five glasses via short duration leaching experiments with a very low SA/V ratio (less than 22 m -1 ).

The full results for all five simplified glasses are illustrated in Figure 8: linear fits of the ET (effective thickness of glass dissolved) evolution with time yield the dissolution rates (see equation ( 3)). The dissolution rates based on Si concentration in the leachate all lie between 2.22~2.60 g/m 2 /d with no clear dependence on composition. The uncertainty of r 0 values measured in this way is at the level of 10% calculated by the uncertainty propagation methods as presented by Gin et.al [12] so they are effectively the same within error. However, more recent work by Fournier et al [13] suggested that the actual uncertainties are expected to be greater by taking additional uncertainties associated with the estimated glass surface area into account. This was demonstrated to be as high as 25% depending on the exact experimental conditions deployed. The complete initial dissolution data is listed in Table 5.

Discussion

4.1.Glass density and boron speciation

4.2.Glass local structures

The main purpose of making simplified glasses is to use them as surrogates for studying the full-component glasses without losing the key structural and compositional features. In comparison to other benchmark nuclear waste glasses such as SON68 and the ISG (International Simple Glass [START_REF] Gin | [END_REF]), the presence of Mg in Magnox glass is a major compositional difference and is suspected to reduce the glass aqueous durability. A fundamental requirement for studying the effect of Mg on dissolution mechanisms with a simplified glass is to ensure a similar Mg chemical environment to that in the full-component MW25 glass. Due to the difficulties associated with solid-state 25 Mg NMR spectra in general and low concentration of Mg in the MgEM glass (see Materials and Methods and Table 1), obtaining spectra of sufficient quality is very challenging. Indeed, as can be seen from Figure 7, even acquisitions carried out using a 20.0 T magnet (850 MHz 1 H) at 52.3 MHz, yield spectra that are still quite noisy after a large number of scans. In addition, spectra with sufficient S/N and resolution to distinguish between Mg environments in different simplified glasses could not be obtained (the experimental setup and length of acquisition were limited by the access time to the UK 850 MHz facility). Nevertheless, the broadly similar 25 Mg spectra of MgEM and MW25 indicate that the simplified glass provides a good representation of the Mg local environment in the full-component glass. This indicates that local structural effects, which contribute to the average values and distributions of EFGs and δ iso at the Mg site due to distributions of bond lengths and angles, are reasonably similar. This marks a good starting point for investigating the dissolution mechanisms of full-component glass. The δ iso is about 45~55 ppm in both cases (see Figure 7), which is in the four-fold coordination regime as demonstrated in other studies on well-defined Mg containing crystalline silicates and glasses [START_REF] Shimoda | [END_REF]11,16]. This suggests, at least, that some of the Mg acts as a network former in MgEM and MW25 glasses. On the other hand, some Mg may exist in higher coordination acting as a charge compensator. However, higher sensitivity and higher resolution spectra need to be obtained either at higher magnetic field or applying MQMAS (multiple-quantum MAS) pulse sequences to make the statement more convincing. Another point worth noting is the presence of spinning sidebands for the Mg site in MgEM and not in MW25, either this indicates that the average EFGs are greater at Mg sites due to higher distortion in MgEM glass than in MW25 glass or the paramagnetics in MW25 are preventing the observation of spinning sidebands. The latter explanation is more likely because the central lineshapes (as observed) appear similar, if there was a significant difference in the average EFGs then this would be apparent from the central lineshapes.

The local bonding requirements of Mg will also influence the local structure of other networkforming elements. As illustrated in Figure 2, the ratio of [3] B/ [4] B increases as the Mg content of the glass increases. This monotonic increase in [3] B is consistent with other studies on aluminoborosilicate glasses [17,18], in which B coordination is shown to be affected by the field strength of network modifiers (field strength is defined as the formal charge of a cation, Z, over the square of the average M-O distance, d, in its first coordination shell i.e. [19]): [4] B fraction decreases as the field strength of the added modifier increases. In this case, Mg has a higher field strength value than Ca because the ionic radius of Mg is smaller than Ca (e.g. 86.0 pm compared to 114.0 pm in six-fold coordination [21]).

For singly charged Na ions, one [4] B needs to be charge compensated by one Na + to achieve local charge neutrality. Such a charge compensator must be released into other structural roles, primarily as a network modifier which is linked to non-bridging oxygen (NBO), once a [4] B converts to [3] B. Both Xue and Stebbins [22] and Bunker et al. [23] have demonstrated that such a structural role change would be reflected in a noticeable shift in the 23 Na MAS NMR spectrum even for peaks profoundly broadened due to local structural distributions in glasses.

In contrast, no such local structural changes are evident from the 23 Na spectra of CaEM and MgEM glass, as shown in Figure 4 (or in intermediate compositions). In other words, Na ions are not the sole charge compensators for [4] B speciation; both Ca ions in CaEM glass and Mg in MgEM glass contribute to charge compensating for [4] B.

Despite relatively high uncertainties exist due to compositional variation and instrument error, the linear fit in Figure 9 of excess [3] B to Mg content gives a gradient of approximately 0.8. If all Ca is charge compensating [4] B and Mg was replacing Ca but not replacing its charge balancing role at all, we would expect this gradient to be 2.0 i.e. one mole of [4] B is converted to [3] B per unit of charge. Unfortunately, the exact amount of Ca that is charge balancing [4] B is unknown since there is a surfeit of charge compensating cations for the overall charge deficit (see Table 6 column 1). Nevertheless, this suggests a poorer charge balancing capability of Mg compared with Ca, which is consistent with the presence of four-coordinated Mg inferred from the 25 Mg NMR chemical shift measured in MgEM and MW25.

Samples of MgEM and MW25 leached for 112-days were examined to investigate whether the excess [3] B was responsible for the reported greater dissolution of Mg glasses compared with Ca glasses [2]. The 11 B spectra of unleached and leached glasses (as shown in Figure 3)

show that the relative proportion of [3] B and [4] B remains the same after the leaching experiments. This behaviour is consistent between the simplified glass and full-component simulant MW25 glass despite the B coordination of the latter not being well resolved. This result illustrates that coordination does not affect the leachability of B and further confirms that B release is a good proxy for evaluating the dissolution of UK MW25 radioactive waste glasses.

In inorganic Al-O environments, tetrahedral and octahedral sites of Al can be well resolved by MAS NMR (e.g., at 11.7 T) owing to the noticeable difference in δ iso as an indicator of coordination. Typically, the former locates at about 50~80 ppm and the latter at about -10~15 ppm while that of pentahedral sites are found to be between these two situations typically in the range of 30~40 ppm as found in well-defined crystalline compounds [START_REF] Mackenzie | Multinuclear Solid-State NMR of Inorganic Materials[END_REF]. Using the fitted CaEM 27 Al spectrum as an example (Figure 6), as discussed by Lacaillerie et al [24], the fact that the tentatively assigned [4] Al peak can be fitted reasonably well with only one CzSimple lineshape suggests that five-coordinated Al must either be absent in the glasses or be in very low concentrations. In fact, a slight mismatch at the high frequency end of the prominent peak might imply the existence of [5] Al species. However, curve fitting using two separate CzSimple lineshapes for the predominant peak does not improve the fit. In order to resolve a small fraction of [5] Al sites, a significantly higher magnetic field would be required. It has been reported elsewhere that, when higher field-strength modifiers are added to aluminoborosilicate glasses, a higher proportion of five-/six-coordinated Al would be present from the conversion of [4] Al [17,18,20]. The results we obtained here show a reversed trend:

CaEM glass contains a higher fraction of [6] Al compared with that in MgEM glass. However, it should be kept in mind that Al constitutes less than 5% of the total cations and the vast majority are in tetrahedral sites. Based on the analysed compositions, the fitting results (see Table 4) yield that the [6] Al in CaEM glass constitutes about 4.8% of Al compared with 1.6% in MgEM glass, these account for only 0.23% and 0.08% of the total amount of cations, respectively. Such a subtle change can easily be attributed to other factors like compositional uncertainties and thermal history. The dependence of Al coordination on T g is complex especially when compositional effects are involved at the same time [17]. The subtle changes in Al coordination observed here and its low concentration means that it does not contribute significantly to the charge compensation scheme of network formers when compared to effects observed in the 11 B MAS NMR spectra.

Table 6 lists the network formers that need to be charge compensated ( [4] B and [4] Al ) and the available charge compensators/network modifiers (Na + , Ca 2+ and La 3+ ). The network forming role of Mg in the simplified MgEM and full-component MW25 glass is important in determining the local structure of the glass because Mg in four-fold coordination ( [4] Mg) should be charge compensated due to its positive charge deficit. However, as mentioned above, despite the observation that Mg in the MgEM glass is primarily in four-fold coordination, the exact ratio of [4] Mg to higher-coordinated Mg is unknown.

The similar composition and density of the simplified glasses suggest a similar degree of network connectivity. It has been noted that an excess of network formers with positive charge deficits could be compensated by REEs acting as charge compensators [25]. Therefore, La 3+ ions are able to charge compensate in glasses when there is a lack of charge provided by usual charge compensators such as Na + and Ca 2+ . On the other hand, when not acting as charge compensators, REEs in aluminosilicate and aluminoborosilicate glasses normally have a coordination number of around 6-8 [25,26] and they have been assumed to be charge compensated by cations such as Na + and Ca 2+ through sharing of NBOs with the latter due to over-coordination [27,28]. As Table 6 indicates, La in the CaEM glass needs to be charge compensated due to a surfeit in available charge, this leads to an excessive charge of 3.8 moles per 100 moles cations. If all Mg is acting as network former in four-fold coordination, MgEM would end up with a charge deficit of 4.5 moles per 100 moles cations even with all La 3+ ions being charge compensator. Since excessive positive charge lowers glass network connectivity by creating NBOs, on balance, the contrast in charge balancing schemes between CaEM and MgEM suggests that part of Mg is charge compensating, which is in agreement with the role of Mg in partly converting [4] B to [3] B.

4.3.Initial dissolution rate

Iwalewa et al [29] measured the initial dissolution rate of full-component simulant MW25 glass in deionised water at 90 °C in SPFT (Single-Pass Flow-Through) mode and found it to be 1.84 0.35 g/m 2 /d. This agrees within measurement uncertainty to that measured on the MgEM simplified glass (see Table 5), which again suggests a good representation of the simplified glass for the full-component complex glass. As we discovered in this study, the initial dissolution rates (see Figure 8) as a function of Mg content in these glasses are the same within error. This is in contrast to the data reported in literature for experiments conducted at much longer time scales (up to 12.2 years) [2], where it was found that the longterm dissolution rates, which are generally 3-4 orders of magnitude lower than the initial dissolution rates in these glasses, differ by about one order of magnitude between a Magnox glass (similar in composition to MW25) and SON68. Despite other compositional differences between these two complex glasses, the primary difference is that Magnox glass contains Mg and no Ca and Mg is absent from SON68, which contains Ca in more or less the same proportion as MW25 contains Mg.

The residual glass dissolution rates in the long-term situation are generally believed to be due to a combination of kinetic and thermodynamic effects where transport is hindered by the presence of an altered layer of reconstructed and re-precipitated material on the glass surface, and thermodynamic driving force for dissolution is inhibited by solution chemistry (Si saturation) [START_REF] Gin | [END_REF][START_REF] Gin | [END_REF]. In experiments where fresh glass was put into Si saturated leachate i.e.

altered layer barrier removed in saturated condition, a similar dissolution rate was observed as with fresh glass in fresh (unsaturated) water [30]. This suggests that the mechanism of glass dissolution is similar in the long-term and in the initial phase. The dissolution rate differences associated with the presence or absence of Mg are determined primarily by secondary effects related to the effectiveness of the alteration layer in hindering transport to the pristine glass surface. Any fundamental change in the pristine glass, such as B coordination, induced by the presence of Mg appears to have no effect on the solubility of the glass.

Conclusion

A key question for this study was to identify the structural changes arising from the substitution of Ca for Mg in simplified UK radioactive waste glasses and determine whether this is the reason for dissolution differences observed previously for full-component radioactive waste glasses [1,2]. We conclude that the primary effect of substituting Mg is to produce a glass of lower density with more three-coordinated B. This change is not the result of competition for Na, the dominant charge balancing cation, but the result of the poorer charge balancing capability of Mg. Some Mg is in four-fold (network-forming) coordination, but the amount of [4] B to [3] B conversion and overall charge balancing scheme is consistent with Mg also being present in higher (five-/six-fold) coordination. These compositional and structural changes do not impose a noticeable change on the primary (initial) dissolution rate of the glass. Therefore, the significantly different long-term dissolution rates between Mg and Ca based radioactive waste glasses, based on this separate effects study, should be attributed to secondary effects related to the efficacy of surface layers in retarding dissolution. proportion of trigonal [3] B [3] B

Figures

[4] B [3] B

[4] B The central transition lineshape of [4] B and [3] B peaks; (b) The full spectra that manifest the sidebands of 11 B satellite transitions. The intensities of the [4] B peaks are normalised for comparison of the relative proportion of trigonal [3] B Spinning speed is 15 kHz in all cases and the spinning sidebands are denoted by asterisks. Spectra are referenced to 1M Al(NO 3 ) 3 solution Figure 6 Curve fitting of the CaEM 27 Al MAS NMR spectrum with 2 CzSimple lines: [4] Al in green and [6] Al in purple. The blue line represents the experimental spectrum while the superimposed overall fit is displayed in red [3] B and [4] B proportions in the simplified glass series: 'fraction'-the proportion of [3] B or [4] B with respect to the total amount of boron; 'mole'-the molar amount of [3] B or [4]

  (b). Quantification of either speciation therefore needs to take into account these two factors. It was achieved by curve fitting using DMFIT software by incorporating a quadrupole first-order spinning sideband manifold. One example of a fitting diagram is demonstrated in FigureS2for MgEM glass.

Figure 3

 3 Figure 3 illustrates the 11 B MAS NMR spectra of unleached and 112-day leached MgEM as

  As a macroscopic property, density variations can reflect compositional and/or structural changes in glasses. Despite the lower atomic mass of Mg compared with Ca, predicting the effect of molar substitution of Ca for Mg on glass density is not trivial. Since the glass density deceases from CaEM to MgEM monotonically, a simple isomorphous model could be expected: one Ca in the glass is substituted by one Mg without causing any structural change, which would result in a density reduction due to the lower atomic mass of Mg (24.305 g/mol) compared to Ca (40.078 g/mol). On the other hand, because the substitution is on a molar basis and glasses are made from oxide precursors, another model based on the relative molar volume of both oxides (CaO and MgO) should also be considered. In this case, MgO has a lower molar volume of 11.196 cm 3 /mol than that of CaO (16.790 cm 3 /mol)[START_REF] Rumble | CRC Handbook of Chemistry and Physics, 98[END_REF]. Overall, the density reduction would be moderated depending on relative change of mass and volume associated with the Ca/Mg substitution in the volumetric model. The prediction of glass density variations based on these two very simple models and the measured densities are depicted in Figure1. Neither of the models fully describes the decrease in glass density with Mg content, particularly for Mg75Ca25 and MgEM, which suggests that local structural changes that reduce the density, such as coordination change are taking place.From the 11 B NMR data, the observed local structural change associated with B coordination converting from four to three will influence the macroscopic density. One of the effects of converting the tetrahedral B to trigonal is a reduction in packing efficiency; trigonal B tends to form open ring-like structures i.e. boroxol rings[START_REF] Mackenzie | Multinuclear Solid-State NMR of Inorganic Materials[END_REF]. This will reduce the glass density. As the measured density decreases more rapidly with increasing Mg content than the simple models predict, the results are consistent with the fundamental structural change observed in B coordination.

Figure 1 Figure 2 11 B

 1211 Figure 1 The measured glass densities as a function of Mg content and predicted densities based on: (i) an isomorphous substitution of Ca for Mg according to the molar mass of CaO and MgO; and (ii) a volumetric substitution which also takes the molar volumes of CaO and MgO into account; see Section 4.1 for details

Figure 3 11 BFigure 4 Figure 5 27Figure 6 Figure 8 Figure 9

 1145689 Figure 3 11 B MAS NMR spectra of MgEM glass before (red) and after (black) a 112-day leaching experiments, (a) on an absolute scale and (b) normalised to make the [4] B peak intensities equal, obtained with the same experiment setup as in Figure 2; (c) 11 B MAS NMR spectra of MW25 glass before (red) and after (black) a 112-day leaching experiments normalised to make the [4] B intensities equal

Figure 1

 1 Figure1The measured glass densities as a function of Mg content and predicted densities based on: (i)

Figure 3 11 BFigure 5 27

 115 Figure 311 B MAS NMR spectra of MgEM glass before (red) and after (black) a 112-day leaching experiments, (a) on an absolute scale and (b) normalised to make the[4] B peak intensities equal, obtained with the same experiment setup as in Figure2; (c)11 B MAS NMR spectra of MW25 glass before (red) and after (black) a 112-day leaching experiments normalised to make the[4] B intensities

Figure 7 (Figure 8 Figure 9

 789 Figure 7 (a) Overlaid 25Mg MAS NMR spectra of unleached MW25 (black) and MgEM (red) glasses

Table 1 Nominal composition of the simplified MW25 glass (MgEM) and analysed compositions of the simplified glass series (cation mol%)

 1

	Tables
	Cation	Al	B	La	Mg	Ca	Na	Si
	(mol%)
	MgEM	4.87	27.58	1.72	6.54	-	15.38	43.91
	(Nominal)

Table 2 The measured glass density from CaEM to MgEM by Archimedes principle

 2

	CaEM	Mg25Ca75	Mg50Ca50	Mg75Ca25	MgEM
	Density
	2.560 0.005	2.550 0.001	2.531 0.001	2.510 0.002	2.486 0.002
	g/cm 3

Table 3

 3

B with respect to 100 moles of cations in the glass as presented in

  Erreur ! Source du renvoi introuvable. (analysed composition)

		CaEM	Mg25Ca75	Mg50Ca50	Mg75Ca25	MgEM
	[3] B fraction	48.3%	50.0%	55.0%	57.8%	61.7%
	[3] B mole	12.8	13.1	14.6	16.4	17.4
	[4] B fraction	51.7%	50.0%	45.0%	42.2%	38.3%
	[4] B mole	13.7	13.1	11.9	11.9	10.8
	Excess [3] B
		0	0.3	1.8	3.6	4.6
	mole

Table 4 The fraction of four-and/or five-coordinated aluminium and six-coordinated aluminium of MgEM, Mg50Ca50 and CaEM glasses

 4

		[4] Al	[6] Al
	CaEM	95.2%	4.8%
	MgEM	98.4%	1.6%
	Mg50Ca50	99.3%	0.7%

Table 5 Initial dissolution rate of the simplified glass series as determined from the rate of change in the effective thickness of dissolved glass based on Si concentration

 5

		CaEM	Mg25Ca75	Mg50Ca50	Mg75Ca25	MgEM
	Initial
	dissolution rate	2.33 0.23	2.60 0.26	2.54 0.25	2.42 0.24	2.22 0.22
	(g/m 2 /d)

Table 6 Total charge balancing scheme for the simplified glass series. Note: [4] Al data is only available for CaEM, MgEM and Mg50Ca50 glasses

 6

	Species (mol%)	CaEM	Mg25Ca75	Mg50Ca50	Mg75Ca25	MgEM
			Four-coordinated network former
	[4] B	13.7	13.1	11.9	11.9	10.8
	[4] Al	4.5	-	4.9	-	4.7
		Charge compensator/network modifier
	Na +	14.9	15.3	14.7	14.8	15.1
	Ca 2+	6.1	4.8	3.2	1.6	-
	La 3+	1.7	2.0	1.7	1.7	1.7
	Four-coordinated network former and Charge compensator/network modifier
	Mg	-	1.6	3.1	4.2	6.5
	Excessive positive
		3.8	-	-	-	"-4.5"*
	charges

*It assumes all the Mg in the MgEM glass is acting as network former.

Acknowledgements

R. Guo acknowledges the EPSRC and the University of Cambridge for an International Doctoral Scholarship. A portion of this work was funded by Radioactive Waste Management Limited (C.T. Brigden, S.W. Swanton and I. Farnan). The UK 850 MHz solid-state NMR Facility used in this research was funded by EPSRC and BBSRC (contract reference PR140003), as well as the University of Warwick including via part funding through Birmingham Science City Advanced Materials Projects 1 and 2 supported by Advantage West Midlands (AWM) and the European Regional Development Fund (ERDF). Collaborative assistance from the 850 MHz Facility Manager (Dinu Iuga, University of Warwick) is

acknowledged. We also appreciate the insightful comments from the diligent reviewers on the original manuscript. Their effort helped make this work more complete.