Nour El Sabbagh

Carine Chassain

Hélène Ratiney

Guilhem Pagès

J.-M Bonny

J.-M Bonny Spurious

Spurious phase correction in multi-shot CSI

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Spurious Phase Correction in Multi-Shot CSI

Nour EL SABBAGH a , Carine CHASSAIN a , Hélène RATINEY b , Guilhem PAGES a , Jean-Marie BONNY a a INRAE, AgroResonance, UR QuaPA, F-63122 Saint-Gènes-Champanelle, France, b University of Lyon, CNRS, Inserm, CREATIS UMR 5220, U1206, F-69621 Lyon, France

I. Introduction

However:

-Unwanted additional phase can be accumulated during ∆TEs leading to spectral distortion, hence improper images Correction is mandatory !

III. Two Proposed Solutions

1. Post-Processing Correction (pP-C). With 𝑆 the acquired signal, 𝑘 the number of shot:

𝑆 𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 𝑘, 𝑧 = 𝑆 𝑟𝑎𝑤 𝑘, 𝑧 × exp(-𝑖2𝜋 × Δ𝑓 𝒆 𝑧 × (𝑘 -1) × ∆𝑇𝐸)
2. Switching from the 𝑓 𝒆 to the 𝑓 𝒓 (dependent on the readout position and independent of 𝑧) after the excitation pulse (RF-switch) :

𝑆 𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 𝑘 = 𝑆 𝑟𝑎𝑤 𝑘 × exp(-𝑖2𝜋 × Δ𝑓 𝒓 × (𝑘 -1) × ∆𝑇𝐸)
For non-Cartesian spatial encoding, readout is always centred. Ergo Δ𝑓 𝒓 = 0.

Fig2: MRSI encoding scheme with adjusted frequency switching -RF-switch.

IV. Application

Correction methods were applied on 13 C multi-shot CSI recorded with IDEAL SPIRAL [1] at a static magnetic field of 11.7 T.

Experimental Details:

-Phantom of four syringes containing 13 C-labelled Lactate (4 M, 182.2 ppm), Alanine (1.85 M, 175.8 ppm), Pyruvic acid (16.4 M, 174.5 ppm) and Urea (16.4 M, 162.5 ppm) -Parameter optimisation through effective number of signal averages (NSA) [1] analysis : 7 shots and ∆TE = 0.86 ms -For non hyperpolarized molecules: TR = 10 s, TE = 1.69 ms and a slice thickness of 15 mm -Image resolution of (0.78 × 0.78 × 15 mm 3 ) 1. doi 10.1002/mrm.23212 Fig3: 13 C metabolite maps of 13 C-labelled molecules, for a centred and a shifted slice, superimposed on a 1 H reference image.

-

  Multi-shot CSI : CS study during the TE increments (∆TE) (refer to Fig1) -Importance of maintaining a phase coherency during ∆TE -Phase coherency's loss induces spectral distortion II. Problem Slice Selection: -Done with shifting the excitation frequency proportionally to the wanted position 𝑧: ∆𝑓 𝒆 (𝑧) = 𝑆𝑊 × 𝑧 𝐹𝑂𝑉 𝑧 . Ergo, ∆𝑓 𝒆 = 0 for centred slice (𝑧 = 0)


E L Nour

Carine Sabbagh

Hélène Chassain

Guilhem Ratiney

Jean-Marie Bonny Pages

Inrae

U R Agroresonance

F-63122 Quapa

Phase Correction in IDEAL-type Rapid Spectroscopic

In IDEAL-type spectral selective pulses are with an excitation frequency f dependent on the slice position, and the is applied with a reception frequency f . For a proper spectral study, the phase observed during evolution times of each shot (TE) has to be dependent only on the CS. A phase coherent f -f switch is applied throughout the pulse sequence. However, depending on the frequency commutation position, phase errors are disturbing CS This presentation elucidates the impact of the commutation's position and how to to resulting phase errors.

Introduction

In order to time and increase spectral bandwidth in it is useful to acquire several experiments slightly di erent echo timing, as in IDEAL-type sequences (multi-shot) [1] presented in Figure 1. Here, the chemical shift (CS) information is acquired in multi-shot, where the echo time (TE) is incremented once per shot to sample the signal nonlinearly. The excitation is done with a spectral selective pulse with an excitation frequency f , and the acquisition is with a spiral-OUT trajectory with a reception frequency f . A coherent switch between both frequencies has be applied throughout the pulse sequence. For proper CS study, the acquired during TE, at t should only depend on the CS and not be prone to the in uence of the pulse sequence. However, a spurious phase can be accumulated during the TE depending on the frequency commutation position leading to phase errors. This presentation aims to explain how the position of the frequency switch a ects the spectral study and how to overcome the resulting phase errors.

Theory

Firstly, during the excitation time, slice selective gradient is applied, making the excitation frequency f dependent on the slice position in the z-direction (f ∝ z, f =0 for a centred slice z=0 and f ≠0 for a shifted one). Secondly, the di erent echo times (TE) correspond to the evolution time where the CS's phase variation is studied through TE increment. Finally, during the acquisition time, f can be non-nil under certain circumstances explained in the presentation. If the phase's evolution during the di erent TEs must depend only on the CS for each shot, it can also depend on either f or f depending on when the emission/reception switching is performed. We present the three possible frequency-switch cases (see Figure 2): i) Acq-Switch, f -f commutation at t ; ii) RF-Switch, f -f commutation at t ; and iii) double 0-Switch, f -0 commutation at t and 0-f commutation at t . Using Acq-Switch or RF-Switch case leads to unwanted phase accumulation when f or f is non-nil, respectively. This spurious phase has to be eliminated from the acquired signal before spectroscopic image reconstruction. The double 0-Switch case leads to no phase accumulation, making the acquired signal free of sequence-dependent phase.

Method

IDEAL SPIRAL pulse sequence was applied at 11.7 T on a phantom composed of four syringes lled with either [1-C]Pyruvic acid, [1-C]Lactate, [1-C]Alanine, or [1-C]Urea xed in 10% gelatine. Seven shots were applied with an echo time increment of 0.86 ms, determined through NSA analysis [1]. The spectroscopic (CS) images were reconstructed with the least square estimation approach described in [1]. The pulse sequence was applied using the Acq-Switch case for a centred (0 mm, f =0) and a shifted slice (-4 mm, f =-533Hz), and the RF-Switch case for the shifted one. Using spiral-OUT trajectory leads to a nil f .

Results

Figure each row presents metabolite's C image, superimposed on a H reference image of the phantom. Row 1 presents the acquired images using the Acq-Switch case for the centred slice. The metabolic images are well reconstructed and correspond to the reference image. Row 2 presents the images generated for the shifted slice, where phase errors occur and the CS information is lost. Row 3 shows the same acquired images but after f phase correction. The metabolic images are, therefore, well reconstructed. Row 4 presents the images acquired using the RF-Switch case for a shifted slice. As f =0, no unwanted phase was accumulated, and the metabolic images are successfully reconstructed without any phase correction.

Conclusion

During the di erent times (TE), unwanted phase shifts are likely to be accumulated due to non-nil excitation or reception frequency (f or f ) for Acq-or RF-Switch case, respectively. These phase errors disrupt the CS information and lead to incorrect metabolic image reconstruction. Phase correction has to be applied by removing the spurious phase prone to the pulse sequence. The framework presented is general and it clari es the di erent situations and possible solutions for phase errors Figure 2: Illustration of cases of the emission/reception frequency switching during the pulse sequence timing, leading to di erent frequency dependence during the evolution times. Acq-Switch case: f -f switch before the acquisition , and the CS behaviour detection is done in reference to f RF-Switch case: f f switch before evolution time at t , hence CS in reference to f . Double 0case: f -0 switch for the evolution time at t , 0-f switch at the end of it at t .

Figure 3: Reconstruction of the C metabolite maps acquired with the IDEAL Spiral method. For a slice centred, the proper metabolic image reconstruction of the data acquired with the Acq-Switch case was performed with no correction (top row). Rows 2 3 are C metabolite maps acquired by using the same switch case, but with a slice (-4 mm) where errors occur, before and after phase correction, respectively. Row 4 RF-Switch case was used where no correction is required and proper images are directly reconstructed.

FiguresFigure 1 :

 1 Figures

Acknowledgements

No acknowledgement found.


Mariano Tarantino
email: mariano.tarantino@enea.it

Ferry Roelofs

Afaque Shams

Abdalla Batta

Vincent Moreau

Ivan Di Piazza

Antoine Gershenfeld

Philippe Planquart

SESAME project: advancements in liquid metal thermal hydraulics experiments and simulations

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Within the framework of the Strategic Energy Technology Plan (SET-Plan), civil nuclear power is envisaged to deliver safe, sustainable, competitive and essentially carbon-free energy to Europe's citizens.

ESNII, the European Sustainable Nuclear Industry Initiative, is an European framework of collaboration, led by the industry, but involving also research bodies and nuclear stakeholders, aiming at promoting the development of Gen-IV Fast Neutron Reactor technologies, together with the supporting research infrastructures, fuel facilities and R&D work [START_REF] Esnii | The European Sustainable Nuclear Industrial Initiative: Concept Paper[END_REF].

Under the ESNII umbrella, four projects are boosted in Europe, as depicted in Figure 1.

ASTRID is the SFR industrial prototype, and it represents the shorter-term option for fast nuclear reactor in Europe being based on the proven sodium technology [START_REF] Le Coz | The ASTRID Project: Status and Future Prospects[END_REF]. ALFRED is the European demonstrator of Lead cooled Fast Reactor (LFR) technology, to be constructed in Romania [START_REF] Alemberti | The European Lead Fast Reactor strategy and the Roadmap for the Demonstrator ALFRED[END_REF]. MYRRHA, under construction in Mol (Belgium) is a multipurpose fast neutron spectrum irradiation facility proposed to operate as a large research infrastructure [START_REF] Bruyn | The MYRRHA ADS project in Belgium enters the frontend engineering phase[END_REF]. MYRRHA will also demonstrate the technological feasibility of the Accelerator Driven System (ADS) operated for waste transmutation.

The last is SEALER, a small lead cooled reactor, which is currently under development by the Swedish company LeadCold. It is designed to provide reliable and safe production of power/electricity for remote sites [START_REF] Wallenius | Design of SEALER, a very small lead-cooled reactor for commercial power production in off-grid applications[END_REF]. Except for the SEALER concept, the reactors under consideration have been described in IAEA [START_REF]Status of Fast Reactor Research and Technology Development[END_REF] and the IAEA booklet on the status of fast reactor designs and concepts [START_REF]IAEA, Status of Innovative Fast Reactor Designs and Concepts[END_REF].

For the technological development of the above mentioned projects, many efforts are devoted to the development of liquid metal technologies (lead, lead-alloys, sodium), and as consequence thermal-hydraulics of liquid metal is considered one of the key scientific subjects in the design and safety analysis. Many efforts have been spent in Europe for addressing thermal-hydraulic issues as reported in [START_REF] Roelofs | Status and Future Challenges of Liquid Metal Cooled Reactor Thermalhydraulics[END_REF][START_REF] Tenchine | Some thermal hydraulic challenges in sodium cooled fast reactors[END_REF][START_REF] Akimoto | Thermalhydraulic research in JAEA; Issues and Future Directions[END_REF][START_REF] Song | Thermal-Hydraulic R&Ds for Gen-III+ and Gen-IV Reactors at KAERI: Issues and Future Directions[END_REF][START_REF] Velusamy | Overview of pool hydraulic design of Indian prototype fast breeder reactor[END_REF][START_REF] Sienicki | Thermal Hydraulic Research and Development Needs for Lead Fast Reactors[END_REF][START_REF] Todreas | Thermal Hydraulic Challenges in Fast Reactor Design[END_REF][START_REF] Roelofs | Status and Future Challenges of CFD for Liquid Metal Cooled Reactors[END_REF][START_REF] Chellapandi | Thermal hydraulic issues and challenges for current and new generation FBRs[END_REF]. To address thermal-hydraulic issues, analytical and empirical correlations are proposed and verified, system thermal hydraulics (STH) and sub-channels codes implemented and validated. In the last fifteen years, Computational Fluid Dynamics (CFD) techniques are playing a relevant role in the design and safety assessment of liquid metal cooled fast reactors.

To advance progress in this field, the collaborative Horizon 2020 thermal hydraulic Simulations and Experiments for the Safety Assessment of MEtal cooled reactors (SESAME) project, sponsored by the European Commission, was initialized in 2015 with duration of 4 years. This project ended in 2019 [START_REF] Roelofs | Liquid Metal Thermal Hydraulics, State of the Art and Beyond: The SESAME Project[END_REF].

One of the main deliverables of this international project was a textbook titled 'Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors', [START_REF] Roelofs | Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors[END_REF].

23 European institutes and US partners were involved in the project (see Fig. 1) with about 100 researchers and 916 PMs of work (Fig. 2).

Liquid metal heat transfer

One of the most relevant task in the safety analysis of liquid metal nuclear reactors consist of in the accurate prediction of turbulent heat transfer under forced, mixed and natural convection regimes.

Presently, the most adopted models to simulate turbulent heat transfer are based on the Reynolds analogy. While this approach is applicable successfully for forced convective flows with a Prandtl number of order of unity, in the case of nuclear systems cooled with liquid metal, for which Prandtl number is higher than the unity, this approach is not enough accurate for the aim. This is especially true for the simulation of large pool reactors where all flow regimes may occur simultaneously. As consequence, an improved numerical modelling for the turbulent heat transfer in liquid metal is required, applicable with any flow regimes. Improvements on modelling and simulation have been proposed and tested on different simple test cases [START_REF] Roelofs | Status and perspective of turbulence heat transfer modelling for the industrial application of liquid metal flows[END_REF]. An update of the ongoing model evaluation and development is reported in [START_REF] Shams | Status and Perspectives of Turbulent Heat Transfer Modelling in Low-Prandtl Fluids[END_REF].

The extension of the validation base for flow separation, jets, mixed convection and a rod bundle represent one of the main topics of the SESAME project. An overview of experimental and numerical activities performed, is presented in Figure 3. In [START_REF] Santis | Application of an algebraic turbulent heat flux model to a backward facing step flow at low Prandtl number[END_REF], new reference data from open literature on a backward facing step was used. It shows encouraging results for the AHFM-NRG model for turbulent heat transport coupled to an isotropic linear model for momentum. The same authors explain in [START_REF] Shams | Towards the accurate prediction of the turbulent flow and heat transfer in low-Prandtl fluids[END_REF] that they have extended their turbulent heat flux model to the use of an anisotropic non-linear model for momentum. They tested it for different scenarios like the flow between two flat plates, impinging jet case from the project and for a bare rod bundle case for which reference data was available from other projects and open literature. In [START_REF] Santis | Modelling of a planar impinging jet at unity, moderate and low Prandtl number: assessment of advanced RANS closures[END_REF], an assessment of a variety of promising models is made with respect to the impinging jet case also used in [START_REF] Shams | Towards the accurate prediction of the turbulent flow and heat transfer in low-Prandtl fluids[END_REF]. Apart from the Reynolds analogy, three different advanced models have been employed: an implicit and explicit AHFM model and the so-called Kays correlation. Limitations of the Reynolds analogy are clearly demonstrated while, all advanced models show reasonable behaviour for this forced convection case. However, they are all based on an isotropic linear model for momentum, and it is concluded that expansion to an anisotropic non-linear model (as in [START_REF] Shams | Towards the accurate prediction of the turbulent flow and heat transfer in low-Prandtl fluids[END_REF]) could clearly bring added value.

Finally, [START_REF] Shams | Status and Perspectives of Turbulent Heat Transfer Modelling in Low-Prandtl Fluids[END_REF] summarizes the latest developments with respect to advanced turbulent heat flux model developments. In the frame of the SESAME project, new reference data are assessed for a variety of advanced turbulent heat flux models, i.e. the second order TMBFeq-ATHFM model, an implicit AHFM model and the AHFM-NRG. Three different sets of reference data are assessed covering the various flow regimes. For the natural convection flow regime a Rayleigh-Bernard Convection case has been considered from literature, for the mixed convection flow regime, new data from the SESAME project has been considered and for the forced convection flow regime, again the impinging jet case has been considered. Once again, the AHFM-NRG showed good results in all flow regimes. The implicit AHFM model showed good results in the forced convection regime, while it became clear that the promising second order TMBF-eq-ATHFM will need further calibration especially for applications involving non-negligible buoyancy effects, before definite conclusions on the performance of this model can be drawn. An extensive discussion on this work, can be found in [START_REF] Shams | A Collaborative Effort Towards the Accurate Prediction of Flow and Heat transfer in Low-Prandtl Fluids[END_REF].

Core thermal hydraulics

The core thermal hydraulics work package, within the SESAME project was focused on the development and validation of numerical models for the thermal hydraulic simulation of liquid metals fast reactor cores. The developed models include sub-channel codes, reduced resolution CFD, coarse-grid-CFD and CFD models. New reference data were generated from the considered experiments, high fidelity numerical models and DNS. Experimental data is generated for wire-wrapped bundles, a bundle with spacers, the effect of blockage, and inter wrapper flow. All intended data was prepared and applied in the model development or in the validation of the used model.

In the SESAME project, a 7-pin rod bundle experiment was performed adopting water as coolant, allowing to implement a validation database for the flow field. Moroever, quasi-DNS simulation data was generated for a rod bundle with an infinite number of pins and LES data was generated for a 61-pin bundle. In [START_REF] Roelofs | Towards Validated Prediction with RANS CFD of Flow and Heat Transport in a Wire-Wrap Fuel Assembly[END_REF], the work on validating RANS CFD methods for wire-wrapped fuel assemblies is summarized. It is concluded that validation efforts up to now indicate that an accuracy within 12.5% for engineering RANS models should be feasible for all bundle sizes and all parameters checked. It is also noted that this value has to be considered as preliminary. Important steps in the validation strategy are missing, i.e. validation for large scale bundles both for the hydraulic field as well as for the thermal field. Furthermore, it is important to realize that all of the applied thermal validation simulations have used the standard Reynolds analogy with a constant turbulent Prandtl number approach and as such there is room for improvement.

Concerning grid spaced fuel assemblies, new data to support the ALFRED reactor fuel assembly design has been produced by performing experiments in a liquid metal rod bundle with and without blockages (Fig. 4). These experiments have been described in detail by [START_REF] Marinari | Temperature Profiles in the Blockage Experiment with the BFPS Test Section[END_REF]. Simulations have been performed for these experiments also. The simulations for the unblocked bundle show a good comparison with the experimental data with differences less than 10%. The simulations for the blocked bundle also show a reasonable comparison (on average in the order of 15%), except for the prediction of the wake region behind the blockage [START_REF] Mathur | Reduced Resolution RANS Approach Validation to Grid Spacer Fuel Assemblies[END_REF]. Simulations were performed using a reduced resolution RANS approach to allow scaling up to a complete ALFRED fuel assembly at reasonable computational costs. The errors involved in using a reduced resolution technique were a priori determined by comparison to RANS results and by comparing to experiments.

The interaction of turbulent flow with the fuel pins (flow induced vibrations in a fuel assembly) was experimentally investigated in a seven pin bare rod bundle using water as coolant (SEEDS-1 experimental facility). Obtained data were used to support the development and validation of numerical approaches. Simulations were based on a URANS approach with an SST k-v turbulence model and strongly coupled algorithms to account for the fluid-structure interaction. The frequency of the flow pulsations was reasonably well predicted. However, the results of the Fluid Structure Interaction (FSI) calculations deviated from the experiments in that they underpredicted the amplitude of the flow-induced vibrations and in that they over-predicted the respective frequency. Several possible reasons for the mismatch were identified, but will need future investigations to draw conclusion. In particular, the fixation and/or material properties of the transparent material, the stiffness of the rods, the modeling of the water filling of the rods, and dimensional tolerances of the components of the experimental set-up might play a role [START_REF] Bertocchi | FluidStructure Interaction of a 7rods Bundle: Benchmarking Numerical Simulations with Experimental Data[END_REF].

4 Pool thermal hydraulics SESAME work package number three, deals with HLM flows in a pool configuration at different scales (Fig. 5). Thermal stratification and mixing phenomena were Experimental data were used to validate numerical approaches developed in parallel for these facilities using CFD software. These comparisons, reported in [START_REF] Jeltsov | Validation of Star-CCM+ for liquid metal thermalhydraulics using TALL-3D experiment[END_REF][START_REF] Zwijsen | CFD Modelling of the CIRCE Facility[END_REF] show reasonable performance of the CFD models. In [START_REF] Jeltsov | Validation of Star-CCM+ for liquid metal thermalhydraulics using TALL-3D experiment[END_REF] validation of CFD was performed for the TALL facility including an elaborate sensitivity analysis. This analysis indicates that the boundary conditions (e.g. LBE mass flow rate, inlet temperature, heater power) followed by the turbulent Prandtl number and material properties (e.g. density and heat capacity of LBE) constitute the major sources of modelling uncertainty. Once the radiative heat transfer was taken into consideration, the CFD simulations reported in [START_REF] Profir | Numerical and Experimental Campains for Lead Solidification Modeling and Testing[END_REF] could reproduce with good accuracy the solidification/remelting experiments performed in the SESAME-Stand facility. The CFD models of CIRCE-ICE reported in [START_REF] Zwijsen | CFD Modelling of the CIRCE Facility[END_REF] reproduce the general flow and temperature patterns of the facility operating under nominal and transient conditions reasonably well. It was found that prediction of the stratification in the CIRCE-ICE pool is sensitive to the modelling of the conjugate heat transfer from the inner loop to the pool. Overall, modelling results of CIRCE-ICE served as valuable feedback to the experimentalists, resulting in changes made to the facility and a better data acquisition in follow-up experiments.

Finally, full CFD approaches are applied to the full scale ALFRED design [START_REF] Alemberti | The European Lead Fast Reactor strategy and the Roadmap for the Demonstrator ALFRED[END_REF], profiting from the validation efforts on the TALL and CIRCE-ICE facilities. These simulations for a full scale reactor provide designers a priori detailed insight in 3 dimensions concerning the behaviour of flow and heat transport in their design (Fig. 6).

System thermal hydraulics

In the frame of safety assessment and design of nuclear reactors, the use of system thermal-hydraulics codes is widely adopted to simulate the transient behaviour of the whole systems, i.e. primary and secondary system, including the balance of plant.

Such STH-codes have been developed mainly for PWRs and BWRs (e.g. RELAP5, CATHARE, etc..), and validated using integral test facility design specific or experimental data coming from the operation of nuclear reactors and prototypes [START_REF] Bestion | System thermalhydraulics for design basis accident analysis and simulation: Status of tools and methods and direction for future R&D[END_REF].

For the application to liquid metal fast reactor, these STH-codes need to be updated with state-of-the-art algorithms, models and correlations, and their validation extended with suitable experimental database aiming at confirming their applicability for safety analysis.

Moreover, in the case of multi-scale approaches, in which STH-codes are coupled with CFD codes to catch relevant 3D phenomena in the system simulation, the validation process has to be further extended considering the code coupling. The multi-scale approach is going to be developed both for light water [START_REF] Chanaron | Advanced multiphysics simulation for reactor safety in the framework of the NURESAFE project[END_REF] and liquid metal cooled reactors [START_REF] Bandini | Assessment of systems codes and their coupling with CFD codes in thermalhydraulic applications to innovative reactors[END_REF].

For liquid metal systems very few data set are available for the validation process, as for example the data coming from the experiments performed on TALL-3D loop.

Apart from this small scale basic experiment, validation of such multi-scale approaches has also been performed by comparing to reactor scale data from the EBR-II [START_REF] Briggs | EBR-II Passive Safety Demonstration Tests Benchmark Analyses À Phase 2[END_REF] and Phénix natural circulation tests [START_REF] Pialla | Overview of the system alone and system/ CFD coupled calculations of the PHENIX Natural Circulation Test within the THINS project[END_REF]. As these data relate to real operating reactor, the possibilities for instrumentation were limited.

One of the main goal of SESAME project was to extend the validation base of STH-codes or multi-scale approaches, providing suitable experiments for the aim (see Fig. 7).

The first level of validation data was provided by experiments performed by TALL-3D (KTH, Sweden) and NACIE-UP (ENEA, Italy) loop facilities. For scaled-up multi-scale approach, experiments on CIRCE-HERO (ENEA, Italy) have been implemented and run in the frame of the project [START_REF] Martelli | Experimental Activity for the Investigation of Mixing and Thermal Stratification Phenomena in the CIRCE Pool Facility[END_REF].

A further added value coming from the SESAME Project is the availability of experimental data (i.e. dissymmetric tests) coming from the Phénix reactor end of life tests. This data will support the validation process of multi-scale codes to a much larger extent than the natural circulation test data which were previously used [START_REF] Pialla | Overview of the system alone and system/ CFD coupled calculations of the PHENIX Natural Circulation Test within the THINS project[END_REF].

A large amount of experimental tests was performed in the TALL-3D facility [START_REF] Grishchenko | Lessons Learned from the Blind Benchmark on Natural Circulation Instability in TALL-3D[END_REF]. Specific tests were selected for blind and open benchmark with system codes or coupled multi-scale numerical approaches. The open benchmarked results from, all available simulations compared well with the experiment. The blind benchmark demonstrated a spread of the results. In fact, all possible types of transients were obtained in the simulations. An uncertainty propagation analysis was performed which provided a lot of insight. The results suggest that the current models are not capable of capturing the experimental data (even taking into account experimental uncertainties). However, the predictions are close to experimental data and do capture the character of the natural circulation instability.

The blind benchmark results on the NACIE_UP tests are reported in [START_REF] Forgione | Blind Simulations of NACIE-UP Experimental Tests by STH codes[END_REF]. The simulations showed a sufficiently good agreement among the participants regarding the general behaviour of the loop in both steady state and transient conditions. The observed discrepancies in the LBE mass flow rate were mainly related to the specific parameters adopted to set the numerical model, as the pressure loss coefficients or the gas circulation model.

With respect to CIRCE-HERO, [START_REF] Zwijsen | Multi-scale Modelling of the CIRCEHERO Facility[END_REF] reports that an interesting observation is that the two multiscale coupled models show similar overshoots in the outlet temperature of the heat exchanger. This may indicate that a particular 3D phenomenon is not captured by the STH part of the coupled model or that particular input from the experiments is missing. It is advised to investigate this further in the future. Despite the observed differences between multiscale simulations and experiments, it is concluded that multi-scale coupled techniques provide a promising methodology that deserves further investigation and qualification to be used as a tool in the design of nuclear power plants. Because of the complexity of the phenomena involved and of the size of the physical domain, the modelling of the Phénix reactor proved to be a challenging task [START_REF] Uitslag-Doolaard | Multiscale Modelling of the Phénix Dissymmetric Test Benchmark[END_REF]. The best compromise has to be found between the accuracy and the computational cost. The results reported in [START_REF] Uitslag-Doolaard | Multiscale Modelling of the Phénix Dissymmetric Test Benchmark[END_REF] show two main issues: (i) correctly computing the thermal hydraulics of the first three minutes of the dissymmetric transient and (ii) finding the correct parameters to accurately compute the remaining 27 minutes. For the first 3 minutes of the transient, it is concluded that the intermediate heat exchangers should be included in the CFD model in order to correctly compute the momentum and stratification of the sodium leaving the intermediate heat exchangers. For the remaining 27 minutes, most participants underestimate the cooling rate. A deeper investigation of the heat losses from and the thermal inertia in the Phénix reactor is therefore recommended.

Conclusions

The activities and progress in support of liquid metal cooled reactor design and safety analyses performed within the European collaborative H2020 SESAME project are described in this paper. The major outcomes are: -Turbulent heat transport in liquid metal:

• Enlargement of the reference database with new experimental and high fidelity numerical data with a focus on flow separation, jets, and rod bundle flow phenomena. • Further development and assessment of promising models like a second order heat flux model, implicit and explicit algebraic heat flux models and the application of the Kays correlation. -Core thermal hydraulics:

• Creation of new experimental and high fidelity numerical data for validation of RANS models with respect to the hydraulics of the flow in wire wrapped fuel assemblies. • New experimental data is created for the assessment of liquid metal fuel assemblies employing grid spacers including the effects of blockages. RANS modelling approaches have been validated using these data, and subsequently these validated modelling approaches have been applied to a full scale ALFRED fuel assembly. • Assessment of the influence of the inter-wrapper flow through experiments and numerical analyses which have been validated using the experimental data. • The validation base for liquid metal system thermal hydraulics has been enlarged with new experimental data ranging from a small generic scale, to intermediate scale and large scale experiments, and finally to real reactor scale. • Results of system thermal hydraulic codes and multiscale coupled simulation tools have been compared with experimental results and in general contribute to the increase in validation of the numerical tools while at the same time highlighting shortcomings on modelling as well as measurements.

Fig. 1 .

 1 Fig. 1. European liquid metal cooled reactor demonstration projects.

Fig. 2 .

 2 Fig. 2. SESAME partners.

Fig. 3 .

 3 Fig. 3. Overview of reference data referred to liquid metal heat transfer.

Fig. 5 .

 5 Fig. 5. Overview of experimental and numerical pool thermal hydraulic activities.

Fig. 4 .

 4 Fig. 4. Clad temperature distribution (a) and cross-section averaged pressure distribution along the streamwise direction (b): unperturbed case (ALFRED fuel assembly).

Fig. 6 .

 6 Fig. 6. ALFRED according to LEADER project. Geometry (a), velocity field (b) and temperature field (c).

Fig. 7 .

 7 Fig. 7. Overview of system scale experiments and simulations.

  • Creation of new experimental data and parallel model development for validation of numerical models concerning flow induced vibrations in liquid metal reactor fuel assemblies. -Pool thermal hydraulics: • Enlargement of the validation base for pool thermal hydraulics by creation of new experimental data using two important LBE facilities, i.e. TALL-3D and CIRCE. • Further development and validation of CFD tools for pool modelling. • Design (TALL-STS) and construction (SESAME-Stand) of new experimental facilities supporting development and validation of CFD models for solidification phenomena. • CFD was applied to a full scale ALFRED pool revealing some potential design improvements. -System thermal hydraulics:

The SESAME project has received funding from the Euratom research and training programme 2014-2018 under grant agreement No. 654935.

M. Tarantino et al.: EPJ Nuclear Sci. Technol.6, 18 (2020)

Author contribution statement

The corresponding author, as SESAME Project coordinator, provided for the paper writing and final review. Other authors, as work package leader of the project, provided a contribution about their own work package. They also revised the final draft before submission.


Alexis Olry De Rancourt
email: alexis.olry@univ-lorraine.fr

Julien Veytizou
email: julien.veytizou@univ-lorraine.fr

David Bertolo
email: david.bertolo@univ-lorraine.fr

Robin Vivian
email: robin.vivian@univ-lorraine.fr

J M Christian Bastien
email: christian.bastien@univ-lorraine.fr

Stéphanie Fleck
email: stephanie.fleck@univ-lorraine.fr

PrisMe: a Tangible User Interface for Group Work Regulation A Preliminary User Experience Study

Keywords: Smart classrooms, Tangible supported collaborative learning, Technology enhanced learning environments

Figure 1. PrisMe being used by a group of pupils during a test session in real school conditions.

Introduction

In line with UNESCO or OECD recommendations, K-16 educational programs stress worldwide the need to build the 21st century cross-disciplinary competencies needed by all for personal fulfillment and development, social inclusion, and active citizenship [7,[START_REF]Education and Training in Europe[END_REF][START_REF]France -Ministère de l'éducation nationale de l'enseignement supérieur et de la recherche Bulletin officiel[END_REF][START_REF]Students, Computers and Learning[END_REF]. The ambition is to enable children and teenagers to learn how to know, do, be, and live together [START_REF] Scott | The Futures of learning 2: what kind of learning for the 21st century? -UNESCO Digital Library[END_REF]. This objective, announced by Europe as a priority for the 2020s, cannot be achieved without increasing the part taken by the digital in the educational system [6]. In this context, the education system has begun a transformation towards collaborative work, autonomy, and critical thinking. Nevertheless, in order to achieve these objectives, the students must be able to regulate themselves during the collective work. Studies showing the importance of self-regulation in educational success are numerous [START_REF] Becker | Behavioral self-regulation and executive function both predict visuomotor skills and early academic achievement[END_REF][START_REF] Cole | Toward a unifying model of self-regulation: A developmental approach[END_REF][START_REF] Montroy | Social skills and problem behaviors as mediators of the relationship between behavioral self-regulation and academic achievement[END_REF][START_REF] Neuenschwander | How do different aspects of self-regulation predict successful adaptation to school[END_REF]. Supporting the implementation of reflexive and responsible positioning helps to regulate the activity. Wagener [START_REF] Wagener | L'autorégulation conjointe de la cognition et des émotions: quel impact sur les apprentissages?[END_REF], for example, highlights the need for students to develop self-regulatory skills, including thinking for themselves, setting goals to expect, and monitoring and evaluating the progress of their work against these same goals. However, our observations and the feedback from the teachers we interviewed showed that collective work generates counter-normative and harmful behaviors in the work environment. This situation leads to difficulties for teachers in classroom management.

Nault and Fijalkow [START_REF] Nault | La gestion de la classe: d'hier à demain[END_REF] describes classroom management as "the way in which interactions between a teacher and students in the classroom are regularized in a formal or informal way, even when they are unexpected situations or reactions." This definition, which is broader than just disciplinary phenomena, encompasses what Nault and Fijalkow [START_REF] Nault | La gestion de la classe: d'hier à demain[END_REF] have identified as the building blocks of classroom management. The e-TAC project aims to develop technologies that respond to real needs identified by teachers and learners, the end users. By developing technologies that offer a positive user experience, we aim to facilitate their implementation and adoption. By evaluating our technical solutions at an early stage of design, we want to validate the interactions we will develop in the next iterative loops. To support classroom management during group activities, we propose PrisMe, a set of tangible artifacts designed to encourage the regulation and autonomy of students working together.

Tangible User Interfaces and Group Work Regulation

We aim to develop a solution located close to the students and which provides direct feedback, making the consequences of their individual actions on the group perceptible to everyone [START_REF] Anastasiou | Gesture analysis in a case study with a tangible user interface for collaborative problem solving[END_REF]. In order to propose an adequate solution according to the needs identified in class, we have investigated the potential of Tangible User Interfaces (TUI). These interfaces embody interaction through task affordance, allows direct manipulation and physical representation of feedback and cues through metaphors and the embodiment of real objects and spaces [START_REF] Fishkin | A taxonomy for and analysis of tangible interfaces[END_REF][START_REF] Hornecker | Getting a grip on tangible interaction: a framework on physical space and social interaction[END_REF][START_REF] Ishii | Tangible bits: towards seamless interfaces between people, bits and atoms[END_REF]. Alrashed [START_REF] Alrashed | An observational study of usability in collaborative tangible interfaces for complex planning systems[END_REF] showed that the use of tangible interfaces forces the user to take into account the impact of his actions on those of others. We then use the term "referential anchoring" [START_REF] Resnick | Socially shared cognition[END_REF], which mediates communication. It is a "point" of support for mutual understanding. Tangible interfaces also support communication by making important information about the group's activity accessible to all (in our case, group decisions, the passing of time, noise, and the need for help that are made tangible). If these shared objects of negotiation [START_REF] Suthers | Representational guidance for collaborative learning[END_REF] are changed by an individual, he must announce, discuss or negotiate this change to the rest of the group. Tangible interactions will thus generate a common representation of work situations [START_REF] Anastasiou | Gesture analysis in a case study with a tangible user interface for collaborative problem solving[END_REF]. TUI also help limit individualistic behavior by promoting user engagement through manual activities [START_REF] Marshall | Do tangible interfaces enhance learning?[END_REF]. TUI thus has the potential to support collaborative learning [START_REF] Konkel | Envisioning tangibles and display-rich interfaces for co-located and distributed genomics collaborations[END_REF]. All those elements advocate for the use of tangible user interfaces.

Many previous works evidenced the potential of TUI to support learning or collaboration (e.g., [START_REF] Konkel | Envisioning tangibles and display-rich interfaces for co-located and distributed genomics collaborations[END_REF][START_REF] Scharf | Tangicons 3.0: an educational noncompetitive collaborative game[END_REF]). A recent study using eye tracker devices showed that participants working collaboratively on problem-solving showed more visual attention when working with tangible objects than with a more conventional mouse-screen-keyboard system [START_REF] Schneider | 3D tangibles facilitate joint visual attention in dyads[END_REF]. Kim and Maher [START_REF] Kim | The impact of tangible user interfaces on spatial cognition during collaborative design[END_REF] showed that the collaboration fostered by TUI could improve creative results compared to a Graphical User Interface (GUIs). In comparison to PrisME, a software such as classroomscreen.com, is designed to support student self-regulation while facilitating pedagogical orchestration [START_REF] Abel | Back to School: Simple Teaching Strategies from K-12 Classroom[END_REF]. It allows the remaining activity time or the class's sound level to be projected on the blackboard. This type of display is only effective if the pupils think about looking at the board regularly and if it is a whole-class work. In this case, we can even imagine that some pupils turn their backs on the blackboard. Even if some of these technologies seem to be able to help pupils' self-regulation, to our knowledge there are no tangible tools that respond specifically to the needs we have identified.

PrisMe

PrisMe is a tangible environment that might support self-regulation during the group work at school and thus facilitate the classroom management.

Gathering Needs

Our design approach relies heavily on the expertise of the teachers we work with. One of the first steps in our work has been to organize focus groups to gather students' group work experiences. We were able to identify four main difficulties of regulation among the students. These difficulties have an impact on classroom management. The first is that the pupils have difficulty to have a dialogue that allows them to make decisions collectively. There is no real group dynamic and individualistic behaviors are observed. This leads to the second difficulty, which is the over-solicitation of teachers. When a student has a question, he or she prefers to ask it directly to the teacher while the answers are often within the groups themselves. The third problem is that the pupils often find themselves rushing to get to the end in order to be on time. Teachers must constantly remind students to keep moving forward. The last challenge is that pupils are not aware of the noise they make. As time passes, the noise level increases. The teachers have to stop the activity until the noise level is low in order to resume under bearable conditions. The first recommendation we proposed is that the system should help pupils to become aware of the noise they make so that they can regulate themselves. If a tolerance level is reached, the system must require them to make less noise. Then, the tool must help students make decisions collectively by listening and respecting everyone's opinion. They must be able to vote and agree before calling the teacher or returning their work. To encourage them to ask fewer but more relevant questions, the system should limit the number of questions they can ask. Finally, our system must make them aware of the time allocated to an activity so that they can organize their work accordingly.

Design Recommendations

Early Design

This section presents all the devices' early design we have proposed. These artefacts should allow us to validate the interactions we had imagined before going through all the development phases. Based on the teachers' feedback, we imagined a set of artifacts that could support group work called PrisMe. It is composed of six elements. Each one is related to one of the issues raised by the teachers.

• Prisms (figure 3): allows students, in a group, to work together to take a decision, vote, call the teacher, or signal that the work is finished. • A lamp (figure 4): allows the teacher to know a group of students' status: everything is okay, needs help, finished work... • A Bluetooth relay (figure 4): serves as a bridge between each group member's prism and the lamp.

• An interactive sound levels meter (figure 5): allows a group to self-regulate itself concerning noise during work. • A question coupon (figure 4): allows the teacher to define the number of questions the students are allowed to ask. The teacher or the students themselves can choose this number.

• A stopwatch (figure 4): allows the teacher to set the time remaining in the activity. The teacher or the students themselves can choose the value.

Each pupil has a prism (figure 3) allowing him/her to vote on actions involving important collective decisions. To start an activity the pupils, symbolize their belonging to the group by putting their objects beside each other. They can then use them to take part in the group's decisions by voting for or against their peers' proposals. Pupils can perform four actions with their prisms using four touch zones: (1) I agree with the proposal made by a classmate (2) I do not agree with the proposal made by a classmate (3) They can also jointly call the teacher to limit individual questions (4) Finally, they can agree to declare their activity finished when it suits everyone. The second element is the Bluetooth relay (figure 4) whose role is to make the connection between all the students' prisms and the lamp. So, when the students call the teacher or declare that they have finished the activity via their prisms, the lamp lights up to signal this. The second element is a mechanical stopwatch (figure 4) with two functions. The first one is to define the time the activity will last and the second one is to define a red zone representing the time left before the end of the activity. Finally, the colored marbles (figure 4) is a question coupon that represents the number of questions the students are allowed to ask during the activity. In this way, they are encouraged to find the answers to their questions within the group.

Figure 3. A prism and its functionalities for decision making within the group

Finally, the pumpkin-shaped sound levels meter (Figure 5) allows to have color feedback according to the sound level. These ambient feedbacks should make it easier to become aware of the noise generated during the activity. If this is not enough, the pumpkin emits a sound signal to indicate that the maximum noise level tolerated by the teacher has been exceeded.

Evaluation of PrisMe

This contribution aims to assess the usability of this TUI through a preliminary study in real school context. We wanted to ensure that several PrisMe are usable by the pupils in group work, and that they are able to implement the related interactions (agree among themselves, call the teacher and declare that they have finished the activity) in an effective, efficient and satisfactory way. Before measuring its impacts in terms of learning and regulation of activities, it was necessary to ensure that students would not be blocked in using PrisMe to regulate themselves in their work.

Means and Methods

Participants. 12 pupils (5 girls, 7 boys; aged 9-11 years; Mage=9.57, SDage=0.5) from a primary school were involved in a test day. The teacher formed 4 groups of 3 pupils similar in age and grade level.

Tasks & activities. The task assigned to the students was to solve two mathematical problems dealing with concepts expected from the French curriculum. They were judged by their teacher to be within the reach of the students involved. These problems were selected in consultation with the working group on primary school of the "Institut de Recherche sur l'Enseignement des Mathématiques" in the Lorraine region in France.

Experimental protocol. Each group spent 20 minutes doing one activity, took a break, then 20 minutes doing a second activity. To counterbalance the order of activities, half of the groups carried out Activity 1 and then Activity 2 and, the other half did the reverse. In each group, identical instructions preceded the activities. After each activity with PrisMe, the short version of the AttrakDiff [START_REF] Hassenzahl | Ein Fragebogen zur Messung wahrgenommener hedonischer und pragmatischer Qualität[END_REF] and the meCUE [START_REF] Minge | meCUE-Ein modularer fragebogen zur erfassung des nutzungserlebens[END_REF] were completed by the pupils.

AttrakDiff is one of the most common for evaluating UX. For Hassenzahl [START_REF] Hassenzahl | The Thing and I (Summer of'17 Remix)[END_REF], the perceived quality will depend on the pragmatic and hedonic qualities that the user attributes to a device. The hedonic qualities are linked to the subject's evaluation of the system's capacity to provide him with pleasure and meet his needs. Pragmatic qualities mainly refer to its usefulness and usability in carrying out tasks. We have chosen the shortened version of the questionnaire with 10 items. meCUE is based on the user experience model of Thüring and Mahlke [START_REF] Thüring | Usability, aesthetics and emotions in human-technology interaction[END_REF]. In order to limit the fatigue of the pupils, we have chosen to pass only module 1 "Product perceptions" and 2 "Emotions." These dimensions complete the AttraDiff results, as the user's status is evaluated by the meCUE. It questions the impact of the use of the device on the perception that others have of the user. As PrisMe is designed to change the place of individuals within the group, these dimensions seem particularly relevant.

Results

AttrakDiff. The results show that the device is well rated in terms of hedonic and pragmatic qualities. The testers gave the device as many hedonic as pragmatic qualities. The confidence interval shows that the system can be classified as desirable (Figure 6 top). The scores for hedonic qualities (1.73), pragmatic qualities (1.56) and attractiveness (1.75) are above average and can be qualified as positive.

meCUE. The meCUE data confirm the results of the AttrakDiff. The averages for "usability," "usefulness," and "visual aesthetics" are 4.62 (1.66), 5.70 (1.22), and 5.58 (1.28) respectively (see figure 6). They are therefore all above average and confirm the positive evaluation of our system with room for improvement. A very encouraging result provided by the meCUE compared to the AttrakDiff is the "status" score, which allows us to know whether users felt that using our system changes the point of view of others about their status within the group (Figure 6 below). Indeed, the average score on the status dimension is 4.49 (1.08), indicating that children who have used PrisMe perceived their status within the group more favorably because of its use.

Conclusion and Future Work

We have put in place a design process focused on user needs for the development of PrisME. This process allowed us to make recommendations that resulted in the creation of tangible artifacts to support the regulation of group work. Preliminary results seem to indicate that pupils enjoy interacting with our artefacts. This evaluation at an early design stage allowed us to validate the interactions we proposed. However, our work should be taken in perspective since several biases were identified. The first being the desirability bias of pupils who like anything technology-related and like anything out of the ordinary at school. In addition, it is very difficult to do an evaluation in a school context. The equipment available, the size of the rooms, the habits of the pupils, makes the exercise perilous. These results allow us to positively conclude this first iteration that aimed at validating user satisfaction during a user experience study with pupils in a real use context. For now, some of the PrisME elements are interconnected which means that they cannot be used independently of one another. Other elements are not connected. This is the case for example with the marbles. Our future work will consist first in transforming these passive objects into connected ones, and second to make interconnected objects eventually independent of one other. In this case, a teacher will be able to use only one device without having to use all the environment. PrisME would thus represent a modular tangible interfaces that we make available to teachers. Moreover, PrisME would allow to record interaction data that will allow us to provide pupils' behavioral indicators for teachers. They will thus be able to know the sound level of a group during an activity, the number of questions asked, or whether they succeeded in managing their time efficiently.

Figure 2 .

 2 Figure 2. Design Recommendations

Figure 4 .

 4 Figure 4. The bluetooth relay, the lamp to call the teacher, the stopwatch and the question coupon.

Figure 5 .

 5 Figure 5. The interactive pumpkin-shaped sound levels meter gives an indication of the sound level of the group.

Figure 6 .

 6 Figure 6. Portfolio of results from AttrakDiff (top) and meCUE (below)

Acknowledgements

This study is part of the e-TAC project supported by the French Ministry of National Education and Youth within the framework of the eFRAN funding of the Future Investment Plan. Thanks to the pupils and their teacher with the support of the academic inspection and of the head of the school.