N Gaillard

T Moncoffre

D Wiss

O Mangin

B Dieste

R Marchand

R Ducher

Dubourg

C Panetier

Yves Pipon
email: y.pipon@ipnl.in2p3.fr

C Gaillard

N Moncoffre

T Wiss

D Mangin

O Dieste

B Marchand

R Dubourg

T Epicier

L Raimbault

Thermal behaviour of caesium implanted in UO 2 : a comparative study with the xenon behaviour

Keywords: UO2, SIMS, TEM, Cs, Xe, bubbles

Thermal behaviour of caesium implanted in UO_2 : A comparative study with the xenon behaviour

Introduction

Nuclear accidents can cause a tremendous release of various radionuclides into the environment. The main volatile radionuclides of concern in such events are the rare gases (mainly xenon) but also the volatile fission products (FP), in particular iodine and caesium. One of the radionuclides with the longest half-life detected in the environment after the accident of Fukushima Daichii (2011) is caesium-137 (30.1 years of half-life) with a contamination exceeding 600 000 Bq m -2 on 600 km 2 around the Fukushima power plants [START_REF] Champion | The IRSN's earliest assessments of the Fukushima accident's consequences for the terrestrial environment in Japan[END_REF]. Caesium release and dispersion is therefore of major impact in the case of a nuclear accident due also to its biological similarity with potassium. In the UO2 nuclear fuel used in light water reactors (LWR), it is also one of the most abundant FP. Indeed, seven atoms of caesium-135 and six atoms of caesium-137 are produced per 100 235 U fissions. For a uranium oxide fuel (UOx) with a burnup of 45 GW•d•tU -1 , it represents an accumulation of 4 kg of caesium per 1 ton of uranium, which means a concentration of around 0.7 at. % [START_REF]Déchets radioactifs, des solutions en progrès continu[END_REF]. Predicting the release of FP during a nuclear reactor accident can be supported by laboratory tests on irradiated fuels including in oxidative conditions [START_REF] Konings | Predicting material release during a nuclear reactor accident[END_REF]. To that extent, several studies were dedicated to measure the FP global release from irradiated nuclear fuel in accidental conditions, such as PHEBUS, VERCORS or VEGA integral research programs [START_REF] Pontillon | Behaviour of fission products under severe PWR accident conditions The VERCORS experimental programme-Part 2: Release and transport of fission gases and volatile fission products[END_REF][START_REF] Kudo | Releases of cesium and poorly volatile elements from UO2 and MOX fuels under severe accident conditions[END_REF][START_REF] Schwarz | PHEBUS FP: A severe accident research programme for current and advanced light water reactors[END_REF]. For instance, during VERCORS tests (in reducing conditions), it was observed that the caesium release starts at relatively low temperature, i.e. 1200 °C and is complete at 2300 °C, which supports that caesium is categorized as a volatile element. Several studies have investigated the Cs release as a function of irradiation and temperature conditions. Peehs et al., in [START_REF] Peehs | Cs and I-release source terms from irradiated LWR fuel[END_REF], have measured the Cs release from LWR irradiated fuel at different burnup in a resistance-heated Knudsen cell under vacuum. They have observed that, for a burnup of 10.9 GW•d•tU -1 , the release started at 1800 °C while at higher burnup, 33 GW•d•t -1 HM, it started at 1400 °C. Inductively coupled plasma mass spectrometry measurements performed by Scherbina et al. have shown that Cs release from irradiated fuel occurred at 1450 °C and was complete at 2000 °C under reducing atmosphere [START_REF] Ns | Effect of Redox Conditions on the Fission Products Release from Irradiated Oxide Fuel[END_REF]. In their study of fission gas and volatile fission products from a very high local burnup LWR UO2 fuel i.e. 240 GW•d•t -1 HM, Hiernaut et al. [START_REF] Hiernaut | Fission product release and microstructure changes during laboratory annealing of a very high burn-up specimen[END_REF] have shown that both fission gases and caesium show a similar trend in the release from the high burnup structure using a Knudsen cell under vacuum. There is a first release around 1000 K (725 °C) followed by a substantial release of these elements at around 1400 K (1125 °C). Whereas the onset for release of the fission gases and caesium is the same in both stages, the shape of the release of the second stage is smoother for the case of the caesium. Further studies have been performed on oxidized irradiated fuel by Hiernaut et al. [START_REF] Hiernaut | Volatile fission product behavior during thermal annealing of irradiated UO2 fuel oxidized up to U3O8[END_REF]. In a 65 GW•d•t -1 HM pre-oxidised fuel sample, it was observed that caesium starts to be steadily released from 600 K (325 °C) until 1250 K (975 °C) coinciding with the start of the sample reduction from U3O8 to U4O9. The complete release was obtained for a temperature of 1850 K (1575 °C). In post irradiation experiments, Prussin et al. have measured a Cs release of 31 % of the inventory from irradiated depleted UO2 pellets annealed at 1750 °C under a flow of H2 gas containing a small impurity of H2O (~70 ppm) [START_REF] Sg. Prussin | Release of fission-products (Xe, I, Te, Cs, Mo and Tc) from polycrystalline UO2[END_REF]. From these studies, and the release kinetics trends, the Cs release appears to be very sensitive to temperature, burnup and irradiation history of the fuel. According to Walker et al., Cs being gaseous from 1200 °C, its behaviour should be similar to Xe above this temperature [START_REF] Ct | Further observations on OCOM MOX fuel: Microstructure in the vicinity of the pellet rim and fuel-cladding interaction[END_REF]. This is in accordance with data obtained during the VERDON high temperature annealing experiments [START_REF] Geiger | Modelling nuclear fuel behaviour with TAF-ID: Calculations on the VERDON-1 experiment, representative of a nuclear severe accident[END_REF], which have highlighted quite close release kinetics between Xe, Kr and Cs. However, below 1200 °C, in some external radial part of the pellets, Desgranges et al. have pointed out that caesium was trapped whereas xenon was fully released after a power transient in an irradiated fuel [START_REF] Desgranges | Evidence of tellurium iodide compounds in a power-ramped irradiated UO2 fuel rod[END_REF][START_REF] Desgranges | Experimental evidence of the formation of a new chemical phase in a power ramped UO2 nuclear fuel[END_REF]. In these papers, the authors have concluded that the caesium transport is driven by the strong temperature gradient between the centre (up to 1800 °C during power transient) and the periphery of the pellet (down to 400 °C). The immobilization of the caesium is associated by these authors with the formation of caesium uranates solid phases but this hypothesis was not proven. Being often compared to xenon, very few data are available on caesium migration under the sole effect of temperature. The few Cs diffusion coefficients available in the literature were determined with the Booth model [START_REF] Booth | A method of calculating fission gas diffusion from UO2 fuel and its application to the X-2-f loop test[END_REF] from the Cs release measurements after irradiation of UO2 pellets [START_REF] Kudo | Releases of cesium and poorly volatile elements from UO2 and MOX fuels under severe accident conditions[END_REF][START_REF] Hiernaut | Fission product release and microstructure changes during laboratory annealing of a very high burn-up specimen[END_REF]. Therefore, in spite of numerous studies about FP release in normal and accidental conditions, uncertainties remain about the state and behaviour of caesium in the nuclear fuel. The present study aims at giving insights into the mechanisms lying behind the Cs thermal behaviour in UO2 at 1600 °C by comparing results between data obtained on caesium and on xenon.

To address these objectives, Cs and Xe implanted UO2 pellets were annealed at temperatures representative of normal and accidental conditions in a LWR fuel. Secondary Ion Mass Spectrometry (SIMS) was used to measure depth profiles before and after annealing. The aim is to characterise migration mechanisms depending on the profile evolution induced by the different annealing conditions. The SIMS technique has already been successfully used to study the behaviour of elements easily ionised in UO2, such as iodine [START_REF] Wh | Migration behaviour of iodine in nuclear fuel[END_REF][START_REF] Saidy | Thermal diffusion of iodine in UO2 and UO2+x[END_REF], chlorine [START_REF] Pipon | Thermal diffusion of chlorine in uranium dioxide[END_REF] but also molybdenum in UO2 and UO2+x [START_REF] Sarrasin | Effect of the Oxygen Potential on the Mo Migration and Speciation in UO2 and UO2+x[END_REF].

The inert gases (He, Kr and Xe) are difficult to ionise and their measurements are therefore challenging by SIMS. However, the feasibility of quantitative analyses has been already demonstrated for Kr in uranium-based materials [START_REF] Portier | SIMS analysis of 83 Kr implanted UO2[END_REF][START_REF] Zweifel | Kr implantation into heavy ion irradiated monolithic UeMo/Al systems: SIMS and SEM investigations[END_REF] and for Xe in UO2 [START_REF] Marchand | Xenon migration in UO2: A SIMS study[END_REF].

The present investigation relies on the coupling of SIMS depth profiling with TEM (Transmission Electron Microscopy) characterisations. Indeed, TEM allows characterising the quantity and nature of some defects created by Cs or Xe implantation before and after annealing. Extended defects and other features such as bubbles are quantified with respect to the implantation depth and correlated to the depth profiles obtained by SIMS.

Material and methods

In this study, depleted UO2 pellets (0.2 at. % of 235 U) supplied by FRAMATOME (ex-AREVA NP) were sintered at 1750 °C under a reducing atmosphere (Ar/H2-5%) during 5 h in order to set up a stoichiometry of UO2.00 and a high bulk density value (97.5 % of the theoretical density). Each pellet is a disk with a height of (1.7 ± 0.2) mm and diameter of (8.6 ± 0.4) mm. The mean grain size distribution is centred at around 11 µm, which is the standard average grain size in LWR fuel.

The pellets were polished on one side by the PRIMEVerre company (Montpellier, France). In order to analyse the surface texture after polishing, optic interferometry measurements on a FOGALE NANOTECH device were performed at the LaMCoS laboratory, INSA Lyon, France. The mean roughness was found to be 5 nm. The samples were subsequently annealed at 1000 °C during 10 h in a PECKLY tubular furnace under vacuum (10 -7 mbar) in order to degas particles adsorbed on the surface. Then, the samples were annealed at 1600 °C for 4 h in a NABERTHERM tubular furnace under a 5 % H2/Ar gas mixture in order to anneal the polishing damage while maintaining the UO2.00 stoichiometry. This procedure removes damage at the sample surface, which was verified by performing Doppler Broadening Positron Annihilation Spectroscopy measurements [START_REF] Djourelov | Variable energy positron beam study of Xe-implanted uranium oxide[END_REF].

Caesium or xenon ions were introduced in the pellets by ion implantation. This well-known technique has been extensively used to study diffusion of fission products in oxide nuclear fuels. Afterwards, some of the as-implanted samples were annealed either at 1000 °C or at 1600 °C in the NABERTHERM furnace under reducing atmosphere (5 % H2/Ar). The caesium or xenon distributions were measured before and after annealing by SIMS and the microstructural changes were observed by TEM.

Experimental details 3.1 Caesium and xenon implantation

The samples were implanted with 800 keV 133 Cs 2+ ions or 800 keV 136 Xe 2+ ions at IP2I (Institut de Physique des deux Infinis) Lyon, France using the IMIO400 accelerator facility with a cooling device maintaining the sample temperature at 15 °C and under vacuum (< 5 × 10 -6 mbar) preventing any oxidation. An implantation fluence of 10 15 at•cm -2 was chosen as a compromise between the low sensitivity of the xenon detection by SIMS and the need to avoid, as far as possible, the creation of too many defects which could mask small clusters or small precipitates in the material. This fluence is estimated to be representative of the Cs and Xe concentration in a nuclear fuel with a low burnup value (around 4 GW•d•tU -1 ).

Simulation of the Cs or Xe implantation profiles and energy losses was performed using the SRIM 2013 software [START_REF] Jf | The Stopping and Range of Ions in Solids[END_REF] in the full cascade mode. We considered a density of 10.7 g•cm -3 and displacement energies were set to 40 eV and 20 eV for uranium atoms and oxygen atoms respectively as determined by Soullard et al. [START_REF] Soullard | High-voltage electron-microscope observations of UO2[END_REF]. The resulting Cs and Xe depth profiles are quasi identical. They are Gaussian distributions with a maximum concentration of 0.08 at.% at a projected range of 140 nm with a straggling of 65 nm. The damage distribution peak is located at around 80 nm for a maximum value of 8 dpa (displacements per atom).

SIMS analyses

In UO2 poly-crystalline samples, the sputter velocity and the secondary ion yield are known to strongly depend on the crystalline orientation of each single grain [START_REF] Desgranges | Measurement of xenon in uranium dioxide (UO2) with SIMS[END_REF], which can in turn strongly affect the SIMS depth profile measure of the studied element. For a typical SIMS raster size (200 µm x 200 µm), a large number of grains (typically 100) is analysed when the sample is constituted of small grains (around 5 µm), so the effects due to the different grain orientations are averaged out. This was highlighted by Marchand et al. [START_REF] Marchand | Xenon migration in UO2: A SIMS study[END_REF] who performed SIMS analysis on UO2 samples with two different grain sizes (7 µm and 22 µm). A methodology similar to the Relative Sensitive Factor (RSF) introduced by Wilson [START_REF] Rg | SIMS Quantification in Si, GaAs, and diamond-an update[END_REF] was successfully used to determine the depth profiles of xenon in small UO2 grains but some discrepancies were observed for samples with large grains. In the present study, the UO2 grain size is larger than 10 µm so that the SIMS analysis area covers only a few grains. We have thus carried out two different methodologies described below to offset the effect of the preferential grain sputtering on the SIMS spectra.

First, SIMS analyses of Xe implanted samples were performed on a CAMECA IMS 6f instrument at the Ecole des Mines of Paris (France). A primary beam of 15 keV O2 + ions with a current of 90 nA was used.

The raster surface size was 150 × 150 µm². Secondary ions ( 238 U 16 O + and 136 Xe + ) were collected in the raster central part (62 µm diameter) to avoid side-wall effects. They were first accelerated by a +4.5 kV electrical potential and then analysed by a mass spectrometer. Difficulties in measuring the xenon signal arise from its high ionization energy, which explains the low sensitivity of this gas by SIMS. The ionization of xenon is done above the surface when the sputtered atoms interact with the O 2+ ions primary beam [START_REF] Desgranges | Measurement of xenon in uranium dioxide (UO2) with SIMS[END_REF]. After the analyses, a homemade software called Sims Depth Profile Achievement [START_REF] Marchand | Effets de la température et de l'irradiation sur la mobilité du xénon dans UO2 : étude profilométrique et microstructurale[END_REF] was used to process the collected raw data taking into account the different sputtering velocities of the different grains. At least three measurements were performed on each sample in order to verify the depth profile reproducibility.

Secondly, SIMS analyses of Cs implanted samples were performed on a CAMECA IMS 7f facility at the Jean Lamour Institute (IJL), Nancy, France. This instrument is equipped with a eucentric rotating sample stage (see [START_REF] Peres | Dynamic SIMS for materials analysis in nuclear science[END_REF] for details) designed by CAMECA to improve the depth resolution by inhibiting the surface roughening during SIMS analyses of poly-crystalline materials. We have shown in a previous work that the use of the eucentric rotating stage with a 15 rpm velocity for UO2 samples implanted with molybdenum significantly improves the reproducibility of the measurements [START_REF] Peres | Dynamic SIMS for materials analysis in nuclear science[END_REF]. In the present study, a primary beam of 10 keV O2 + ions with a current of 140 nA was used. The raster size was 250 × 250 µm 2 on the UO2 sample surfaces and the velocity of the eucentric stage was set to 8 rpm after some calibrations. Secondary ions ( 238 U 16 O + and 133 Cs + ) were collected on the raster central part (62 µm diameter). The conversion of chronograms into depth profiles is detailed hereafter. First, the sputtering time was converted into depth by calculating the sputtering velocity. This was done by dividing the raster depth value, measured by optical interferometry at the LaMCoS laboratory, by the final acquisition time. The depth resolution obtained is about 10 nm. The intensity of the Cs signal was converted into atomic concentration thanks to the RSF, which is expressed in equation (eq. 1).

RSF = φ × I UO ̅̅̅̅̅ (∑ I i 𝑓𝑖𝑛𝑎𝑙 𝑡𝑖𝑚𝑒 𝑖=0 -(I bgd × 𝑛 𝑐𝑦𝑐𝑙𝑒 )) × t cycle × v sputter (eq. 1)
With: φ, the implantation fluence (10 15 Cs•cm -2 ) I UO ̅̅̅̅ , the averaged 254 UO + signal taken as an internal reference (in counts•s -1 ) I i , the intensity of the collected specie « i » at a specific time (in counts•s -1 )

I bgd × 𝑛 𝑐𝑦𝑐𝑙𝑒 , the intensity of the background signal for the number « n » of measured cycles

(in counts•s -1 )
t cycle , the duration of each cycle (in seconds)

v sputter , the sputtering velocity (in cm•s -1 )

A statistic error was determined by performing 11 analyses on two different Cs as-implanted samples.

We determined a RSF value of (1.6 ± 0.3) × 10 20 at•cm -3 . This value is of the same order of magnitude than the one found for Cs in silicon [START_REF] Rg | SIMS Quantification in Si, GaAs, and diamond-an update[END_REF]. This is a low value, which indicates the ease of ionizing caesium implanted in UO2 and we can therefore estimate a limit of detection around 100 ppb.

For each SIMS analysis of a given sample, at least three craters were made and consequently three depth profiles were obtained which were further averaged.

TEM analyses

Focused Ion Beam (FIB) was used to cut lamellae from the UO2 samples. Their preparation was done by SERMA Technologie at Grenoble, France for samples implanted with xenon and at the JRC (Joint Research Centre) Karlsruhe, Germany for samples implanted with caesium. The implanted samples were first covered with a 100-200 nm gold layer in order to protect the surface from the platinum deposit that in addition protect the area of interest on the sample during FIB operation. Once the Pt deposit was made, two trenches were cut with a Ga + ion beam. The part remaining between the two trenches forms the lamella, which was cut from the sample after a pre-thinning and removed with an omniprobe needle. Then, thinning and cleaning of the lamella were performed with the Ga + beam (with care not to destroy the protecting layer). The thickness of each lamella is around 70 nm as determined via Electron Energy Loss Spectroscopy by calculating the ratio between the zero loss peak and the total counts considering the total inelastic mean free path and the sample composition.

TEM examinations were performed at the JRC Karlsruhe with a 120 keV electron beam on a Tecnai G2 TEM FEI microscope [START_REF] Wiss | Recent Results of Microstructural Characterization of Irradiated Light Water Reactor Fuels using Scanning and Transmission Electron Microscopy[END_REF] for the UO2 samples implanted with caesium and at INSA Lyon (CLYM) on a 200 keV JEOL 2010F microscope for the UO2 samples implanted with xenon. Both microscopes are equipped with a HAADF detector (High-Angle Annular Dark Field) for STEM images. In the latter imaging mode where a small electron probe is scanned over the region of interest, the detected intensity varies almost proportionally to the density of atoms and to the square of their atomic number. STEM (H)AADF images can therefore provide complementary information on bubbles or voids in comparison with conventional bright field micrographs where diffraction and dynamic effects may limit and/or distort their visibility. After FIB operation, some surface defects remain on the lamellae.

These defects, however, are small in size and in concentration and do not interfere with the defects created by ion implantation. Figure 2 displays a TEM image of a virgin UO2 sample that will be thus the reference image for further comparison with implanted and annealed UO2 micrographs. In the picture of Figure 2, the residual defects left by the FIB preparation are the dark grey dots which are attributed to defect clusters or to small dislocation loops as reported by He et al. in [START_REF] He | Microstructure evolution in Xeirradiated UO2 at room temperature[END_REF] for example. Their size do not exceed 5 nm. No significant evolution of the xenon depth profile is observed after annealing at 1600 °C up to 16 hours. This means that no apparent diffusion or release of xenon occurs during these annealing.

Results

Caesium and xenon depth profiles evolution

The caesium depth profile corresponding to the sample annealed at 1000 °C has little evolved compared to the as-implanted one. Only a small decrease of the concentration and a small deviation near 220 nm can be seen. No caesium release has been measured and no significant diffusion is observed. The somewhere low Cs and Xe concentrations can be one part of explanation for the low release also observed in irradiated fuel for equivalent low burn up.

On the contrary, the shape of the Cs depth profiles after annealing at 1600 °C is very different. First, at around 50 nm from the surface, a peak representative of a caesium accumulation is observed. Then, a small shoulder located at around 220 nm is noticed. Finally, a significant diffusion tail is observed in the 1600 °C-12 h sample, which was not visible in the as-implanted profile. It is ascribed to a caesium migration towards the bulk, which is still observed up to a depth of around 1 µm (not shown in Figure 3). The caesium global release can be calculated from the comparison between the depth profile areas of the as-implanted sample with those of the annealed samples. It was estimated to: (i) 38 % after a 4 h annealing at 1600 °C and, (ii) 50 % after a 12 h annealing at 1600 °C. We observe three different zones that are delimited by dashed lines in the figure. These zone bounds are roughly the same than the ones seen on the as-implanted sample (see Figure 4). We can also observe that zone 3 seems to host a larger population of extended defects than zone 2, which is a big difference with the as-implanted sample. This could indicate a migration and/or a growth of the dislocation loops which were previously mainly observed in zone 2. The frontier between zone 2 and zone 3, i.e. around 200 nm below the surface, contains dislocation lines of larger size than the ones observed on the as-implanted sample. These defects can be correlated to the shoulder observed on the Cs SIMS profile at around 200 nm. We can therefore assume that the shoulder is consecutive to a local redistribution of Cs atoms along the dislocations. The dislocation loop concentration in zone 3 (Figure 6) has been evaluated to be around 310 22 m -3 with sizes up to 10 nm whereas this concentration drops to 410 21 m -3 in zone 2 and with much smaller loops of 1-2 nm.

UO2 microstructural changes observed by TEM

Microstructural changes induced by ion implantation

Microstructural changes induced by the annealing at 1000 °C

Microstructural changes induced by the annealing at 1600 °C

Figure 7 presents the TEM images obtained from a Cs sample annealed at 1600 °C during 12 h. In Figure 7.a, we observe that no more dislocations appear in the whole Cs implanted region. This result indicates that all the dislocation loops and lines formed by ion implantation have been annealed. Figure 7.b displays the same region but with a magnification allowing a comparison with the Cs profile. A string formed of Cs bubbles of different nature is observed at around 50 nm below the surface. This line of bubbles fits perfectly with the Cs surface peak observed by SIMS on the sample annealed at 1600 °C. We note two bubble populations according to their size and shape: numerous spherical small bubbles and some polyhedral (faceted) larger bubbles. Figure 8.a clearly shows the correlation between the bubble shape and size: spherical bubbles have a size below 5 nm with a distribution maximum located at 3 nm while faceted bubble size ranges between 6 nm and 10 nm. The density of bubbles has been determined as (6.3 ± 2.2) x 10 22 bubbles•m - 3 for spherical bubbles and (1.3 ± 0.7) x 10 22 bubbles•m -3 for faceted bubbles. It could be argued that for the smaller bubbles the spherical nature could be doubtful and is mainly due to the limitation of the out-of-focus limitations whereas for the larger ones the shape is more evident. In their paper, Snoeck et al. [START_REF] Snoeck | Characterization of helium bubble size and faceting by electron holography[END_REF] describe how electron holography can distinguish the faceting. In the discussion here it will be referred as spherical when mentioning the small bubbles and facetted for the larger ones as it is the way they were observed. For comparison with Cs, we have analysed a UO2 sample implanted with xenon and annealed under the same conditions (implantation fluence of 10 15 at•cm -2 and annealed at 1600 °C for 12 h). Figure 9 presents one STEM micrograph (Figure 9.a), one TEM image in underfocused conditions (Figure 9.b) and the Xe depth profile measured by SIMS. As for the Cs implanted sample, no dislocations are visible anymore after the 1600 °C annealing. The underfocused TEM bright field image in Figure 9.b highlights the contrast of bubbles present in the overall implanted depth, which are also detected in the STEM image. Two populations of spherical bubbles are resolved, the size of which is respectively of 1-3 nm and 5-7 nm. The density of the Xe bubble population has been determined to be in the range of 5-9 x 10 22 bubbles•m -3 , which is of the same order of magnitude than the one measured on the caesium implanted sample and annealed at 1600 °C.

Discussion

In order to help the discussion, table 2 gathers the main TEM observations of the samples implanted with Cs or Xe and subsequently annealed at 1000 °C or 1600 °C. The competition between trapping and diffusion of volatile atoms such as caesium and xenon is mainly governed by the quantity, the nature and the mobility of the defects present in UO2. It is therefore of primary importance to characterise first the ion implantation induced defects.

Defects created during ion implantation at room temperature (RT)

TEM examinations performed on Cs as-implanted samples show mostly dislocations with a size between 5 and 10 nm. These results are consistent with those reported in the literature especially the work of Sabathier et al. [START_REF] Sabathier | In situ TEM study of temperatureinduced fission product precipitation in UO2[END_REF] who observed by TEM the nucleation of Cs and Xe bubbles implanted in UO2 at low temperature (600 °C and below). They observed 7 nm dislocation loops after 300 keV Cs implantation in UO2 at a fluence of 5 × 10 14 at•cm -2 (maximum concentration of 0.4 at. %) and also reported the presence of some dislocation lines at a fluence of 10 15 at•cm -2 [START_REF] Sabathier | In situ TEM study of temperatureinduced fission product precipitation in UO2[END_REF]. For xenon implantation at a fluence of 5 × 10 14 ions•cm -2 in UO2, two studies can be benchmarked with ours. In the first one, He et al. report dislocations formation with a size of about 7 nm and a density of 2.5 × 10 22 m -3 [START_REF] He | Microstructure evolution in Xeirradiated UO2 at room temperature[END_REF]. In the second study, Onofri et al. show that a 5 × 10 14 at•cm -2 fluence for Xe implantation produces almost exclusively (1.4 × 10 22 m -3 ) of large (~11 nm) dislocation loops and some few (1.2 × 10 15 m -3 ) dislocation lines [START_REF] Onofri | Influence of exogenous xenon atoms on the evolution kinetics of extended defects in polycrystalline UO2 using in situ TEM[END_REF]. We can therefore conclude that, for comparable implantation conditions, the damage and extended defect formation are nearly the same after Cs implantation and after Xe implantation. These kind of defects created by ion implantation correspond also to the ones observed during reactor operation in a fuel irradiated at a low burnup and at RT [START_REF] Ad | Radiation damage in uranium dioxide[END_REF].

In our experiments, the dislocations are mainly observed between 85 nm and 200 nm below the surface because of the displacement cascades occurring at the end of the Cs or Xe atoms implantation range. The defect formation process can be described in atomic scale simulation by classical molecular dynamics (MD) like for example in [START_REF] Martin | Irradiation-induced heterogeneous nucleation in uranium dioxide[END_REF]. In this work, the formation of vacancy clusters was shown in the cascade displacement core whereas the formation of interstitial clusters was foreseen in the cascade periphery. This is explained by the dislocation punching due to the elastic wave produced during a thermal spike phase, the interstitials being essentially produced at the interface between the "molten region" and the solid matrix. In addition, the same authors in [START_REF] Martin | Molecular dynamics study of the bulk temperature effect on primary radiation damage in uranium dioxide[END_REF] have highlighted that an increase of temperature favours the formation of clustered interstitial defects until the formation of dislocation lines. This is in agreement with the present experimental results. Indeed dislocation lines are barely observed on the Cs as-implanted sample whereas they are clearly seen at a depth of about 200 nm in the Cs-implanted sample annealed at 1000 °C. The local atomic redistribution occurring during their formation is probably responsible for the slight increase of Cs concentration observed at the same depth on the SIMS profile.

Another feature regarding the observations made on the as-implanted sample is that no Cs bubbles were visible. A general statement is that bubbles nucleate easier with increasing temperature and increasing implantation fluence. Except the work of He et al. [START_REF] He | Microstructure evolution in Xeirradiated UO2 at room temperature[END_REF] which highlights 1.9 nm xenon bubbles for samples implanted at 5 × 10 14 ions•cm -2 at RT (but without considering a possible sample heating by the beam), a minimum temperature of 100 °C is required to observe xenon bubbles in UO2 [START_REF] Evans | Effect of temperature on bubble precipitation in uranium dioxide implanted with krypton and xenon ions[END_REF]. The presence of xenon bubbles at RT would indicate a direct nucleation at vacancy clusters or through a ballistic process that does not require thermal Xe diffusion.

Nucleation of bubbles after annealing at 1000 °C

At 1000 °C, nanometer size bubbles of caesium atoms are observed from the surface down to a depth of around 100 nm but most probably these bubbles are present all along the implantation depth, forming a trap for caesium. This explains that no Cs diffusion nor volatilization was detected at this temperature. The formation of Cs bubbles was already observed in the work of Sabathier et al. [START_REF] Sabathier | In situ TEM study of temperatureinduced fission product precipitation in UO2[END_REF].

They studied UO2 implanted with caesium at a maximum concentration of 2 at. % (fluence of 10 16 ions•cm -2 ) and annealed at 600 °C. They reported bubbles with size around 1.7 nm and a density of ( 10 Moreover, the comparison with the work of Sabathier et al. [START_REF] Sabathier | In situ TEM study of temperatureinduced fission product precipitation in UO2[END_REF] highlights the strong influence of the Cs (or Xe) concentration and of the annealing temperature on the bubble nucleation. For a maximum concentration of 2 at. %, bubble precipitation was observed at 400 °C for xenon and at 500 °C for caesium. For a maximum concentration of 0.4 at. %, xenon bubbles appear at 600 °C. In our experiment, for a maximum concentration of 0.08 at. %, we observed caesium bubbles at 1000 °C. So, at "low" temperatures (below 1000 °C), it seems that xenon and caesium implanted at the same concentration behave the same way, i.e. both elements form spherical bubbles in UO2 with similar size, density and threshold temperature for nucleation. These bubbles are also probably pressurized if we refer to the papers of Martin et al. [START_REF] Martin | XAS characterisation of xenon bubbles in uranium dioxide[END_REF][START_REF] Pm | Behavior of fission gases in nuclear fuel: XAS characterization of Kr in UO2[END_REF]. They have shown by X-Ray absorption spectroscopy that Xe nanobubbles remain highly pressurized at roughly 2.0 GPa after an 800 °C annealing of a UO2 sample implanted with Xe at a concentration of around 8 at. %. However, the concentration is one hundred times higher than in the present experiment which could modify the quantity and the nature of the defects trapping the volatile atoms.

One way to better understand the trapping of xenon and caesium into defect clusters is to perform atomic scale simulations. Indeed, bubble nucleation is associated with the formation of vacancy clusters in which atoms are incorporated as shown by MD simulations [START_REF] Martin | Irradiation-induced heterogeneous nucleation in uranium dioxide[END_REF]. Yun et al. [START_REF] Yun | Atomic diffusion mechanism of Xe in UO2[END_REF] used ab initio calculations to obtain the incorporation energy of xenon in different point defects. They have evidenced that the Schottky defect (two oxygen vacancies bound to a uranium vacancy) is the most stable position for xenon. Andersson et al. [START_REF] Andersson | U and Xe transport in UO2 +/-x: Density functional theory calculations[END_REF] have calculated a low Xe incorporation energy of 0.18 eV for this defect. Also, from a MD study, Murphy et al. [START_REF] Murphy | Free energy of Xe incorporation at point defects and in nanovoids and bubbles in UO2[END_REF] have shown that xenon is very stable in nanovoids (created from several Schottky defects) for temperatures up to 1050 K (780 °C) and that no thermal resolution occurs during the simulation time. Not so far from these insights, Moore et al. [START_REF] Moore | Molecular dynamics simulation of Xe bubble nucleation in nanocrystalline UO2 nuclear fuel[END_REF] have found by MD simulations that in the context of a vacancy-saturated model, the formation of stable Xe clusters, driven by the migration of Xe atoms by vacancy-assisted diffusion, was possible. For caesium, only a few data exists in literature. Gupta et al. [START_REF] Gupta | Ab initio study of solution energy and diffusion of caesium in uranium dioxide[END_REF] have shown that caesium can be incorporated with a similar negative energy (~-1 eV) into a uranium vacancy, a divacancy (uranium vacancy bound to an oxygen vacancy) or a Schottky defect. All these defects involving a uranium vacancy can constitute a nucleation centre from which a bubble can grow with further trapping of caesium atoms. So whereas xenon can be only incorporated into Schottky defects and has a tendency to form clusters, caesium might be incorporated in smaller defects and its incorporation energy remains favourable (even if not so much) in any uranium defect type. To sum up, literature results seem to indicate that the nature and consequently the sink strength of the caesium traps might be different from the xenon ones.

Growth of bubbles and mobility of caesium and xenon at 1600 °C

Results obtained from 1600 °C annealing of implanted UO2 pellets show a different behaviour between xenon and caesium. Our similar conditions of implantation and annealing allow a direct comparison of the two elements. This is particularly relevant as it is often assumed that caesium behaves like rare gases in UO2 above 1200 °C, the threshold temperature value to form gaseous caesium according to Walker et al. [START_REF] Ct | Further observations on OCOM MOX fuel: Microstructure in the vicinity of the pellet rim and fuel-cladding interaction[END_REF]. In our experimental conditions, no significant release or diffusion of Xe could be evidenced while a significant release of Cs was evidenced: 50 % of Cs is released after a 12 h annealing, the remaining part either accumulating at the surface or diffusing into the bulk. The difference of release rate between caesium and xenon is likely due to different growth kinetics and behaviour of bubbles. TEM characterisations give insights into the different bubble growth mechanisms for caesium and xenon. We observe, in Figure 8, the presence of several Cs spherical bubbles with a size ranging between 1.5 nm and 4.5 nm (taking into account the error bars). It seems that there is only one distribution centred at around 3 nm. Figure 9 displays two Xe spherical bubble populations with a size centred at 2 nm and 6 nm. The Xe bi-population is not a surprise and was already reported by Chkuaseli and Matzke [START_REF] Vf | Fission-gas bubble behavior in uranium dioxide[END_REF] at a xenon concentration of 0.01 at. % in UO2 and by Martin et al. [START_REF] Martin | XAS characterisation of xenon bubbles in uranium dioxide[END_REF] at a xenon concentration of 8 at. % in UO2 after annealing at 1500 °C. In most materials, bubbles growth begins noticeably when thermal vacancies are significantly available, which is the case at 1600 °C. Further growth can then occur by migration-coalescence and even more in a vacancy gradient process as it was proposed by Evans [START_REF] Evans | Bubble diffusion to grain boundaries in UO2 and metals during annealing -a new approach[END_REF]. In our experiments, we can assume that the source of the vacancy flux originates from the surface as xenon or caesium atoms are implanted in the first 300 nm below the surface. The bubble migration velocity can be estimated from the calculations of Veshchunov et al. [START_REF] Ms | An advanced model for intragranular bubble diffusivity in irradiated UO2 fuel[END_REF]. According to these authors, the mobility of Xe bubbles increases with the temperature and is maximum for bubbles with a of 2 nm. They have shown that this mobility decreases with the increase of the bubble size. Veshchunov calculated a diffusivity coefficient "D" at 1800 K (1525 °C) for a bubble radius of 1 nm (which means a size of 2 nm) of 10 -20 m 2 s -1 . We can thus estimate a diffusion length (~√𝐷 𝑡) during a four hour annealing time ("t") of 12 nm. This indicates that bubbles in a close vicinity can easily coalesce under our conditions of annealing. The Evans process can explain the growth of the spherical bubbles for caesium and for xenon in UO2 at 1600 °C. However, no bipopulation of Cs bubbles is observed and the maximum size is lower than for xenon. This indicates that the bubble growth is different and that other mechanisms occur at the same time for caesium in UO2.

For example in [START_REF] Murphy | Free energy of Xe incorporation at point defects and in nanovoids and bubbles in UO2[END_REF], Murphy et al suggested a bubble growth by displacements of oxygen near the bubble surface rather than by U vacancies absorption.

We will now highlight the two main differences noted between the behaviour of xenon and of caesium at 1600 °C.

The first one concerns the regions where the bubbles are located. While we can observe xenon bubbles in the whole implantation region, caesium bubbles are only present inside a limited band centred at around 50 nm below the surface with a width of around 40 nm. In the rest of the sample, no caesium bubbles are observed. In particular, no Cs bubbles are visible in the first 30 nm below the surface. One hypothesis to explain this result is to consider bubble diffusion towards the surface. Verma et al.

numerically addressed the migration of the fission gas bubbles in [START_REF] Verma | Modelling intragranular bubble movement and fission gas release during post-irradiation annealing of UO2 using a meso scale and spatialized approach[END_REF] and showed that the influx of vacancies from the surface towards the bulk would create a vacancy concentration gradient. The consequence is a directed diffusion of bubbles towards the surface accelerating the release of the gas.

In the present experiment, this assumption is only valid if we consider that the diffusion coefficient of Cs bubbles is higher than the Xe bubble one. Indeed, no release of xenon is measured and we can observe small Xe bubbles pinpointed really close to the surface as shown in Figure 9 whereas 50 % of caesium is released from UO2 sample surface.

Another hypothesis to explain the disappearance of caesium bubbles near the surface and in the bulk of our samples is to consider the thermal resolution of caesium atoms into the UO2 matrix. Of course, such a process is related to the atomic solubility in UO2. Xenon is considered as insoluble in UO2, so its thermal resolution is highly unlikely even at high temperature. As already pointed out in the 5.2

paragraph, an indicator of this low solubility is shown by the value of incorporation energies which remain always positive whatever the Xe location. This is also consistent with the simulations performed by Murphy et al. [START_REF] Murphy | Free energy of Xe incorporation at point defects and in nanovoids and bubbles in UO2[END_REF] already previously mentioned. Xe thermal resolution would be possible only for highly over pressurized bubbles (very high temperature and high Xe content or very quick temperature transient) which was not achieved in our experiment. Caesium is not insoluble in UO2 but has a low solubility. The Cs incorporation energy remains always slightly negative for Cs in uranium vacancies as shown by electronic structure [START_REF] Gupta | Ab initio study of solution energy and diffusion of caesium in uranium dioxide[END_REF] or even more indirectly by empirical interatomic potential simulations [START_REF] Grimes | The stability of fission products in uranium dioxide[END_REF]. In addition, Walker et al. [START_REF] Walker | Observations on the release of cesium from UO2 fuel[END_REF] have estimated a solubility below 0.06 wt % (~0.01 at. %) at 1700 °C. This value corresponds roughly to the concentration of caesium in the rear part of the as-implanted profile. Moreover, it is worth noting that the annealing of dislocations was observed in alpha-damaged (U, Pu)O2 samples annealed at 1800 K (1523 °C) [START_REF] Wiss | TEM study of alpha-damaged plutonium and americium dioxides[END_REF].

To sum up, Cs atomic thermal resolution can be enhanced by the vacancies created by the ion implantation, by the surface but also by those created by the annealing of the dislocations at 1600 °C.

Finally, the mechanism formation of the specific region with Cs bubbles can be illustrated using With: x, the depth D, the diffusion coefficient of vacancies close to the surface t, the time of annealing

The distribution was drawn at 1000 °C taking a constant for the √Dt term. The distribution at 1600 °C has been calculated with the ratio D1600°C/D1000°C thanks to the Arrhenius law (eq. 3). This allows a rough idea of the depth reached by the surface vacancies.

D 1600°C D 1000°C = exp (- E a 1873.k B + E a 1273.k B ) (eq. 3)
With: Ea, the activation energy for the cation surface vacancies. A value of 3.0 eV has been chosen from molecular dynamics calculations [START_REF] As | Investigation of cation self-diffusion mechanisms in UO2 +/-x using molecular dynamics[END_REF] kB, the Boltzmann constant (8.6210 -5 eV•K -1 )

At 1000 °C, only spherical bubbles (~2 nm) are visible down to a depth corresponding roughly to the projected range (Rp) of the Cs distribution and the surface vacancy gradient is limited. After the annealing at 1600 °C, thermal resolution occurs in the whole sample. This favours the disappearance of the small-pressurised bubbles and a Cs atomic diffusion towards the surface. At the same time, because of the vacancies influx from the surface, caesium is trapped in a region at a depth of around 50 nm (with a width of  20 nm) where two different defect gradients coexist. The bubbles are visible down to a depth corresponding roughly to the maximum of the dpa distribution (Rd) calculated by SRIM. Near the surface (the first twenty nanometers), the diffusion of small bubbles enhanced towards the surface added to the thermal resolution explain the disappearance of the Cs bubbles. The second difference between xenon and caesium behaviour at 1600 °C concerns the bubble growth evolution. While Xe bubbles remain with a quite spherical shape, we observe large (~10 nm) faceted Cs bubbles in the region described previously. One can consider that, during the 1600 °C annealing, some Cs atoms might be re-soluted from bubbles and made available for diffusion. In a second phase, they could either be released or be trapped into already existing bubbles or into nanovoids created during implantation, thus stopping their diffusion. Rest et al. [START_REF] Rest | Fission gas release from UO2 nuclear fuel: A review[END_REF] have calculated that resolution probability for 100 nm radius bubbles was 40 times lower than for 3 nm bubbles. This would indicate that the small spherical bubbles present near larger ones would dissolve the UO2 matrix at a much higher rate than the larger ones. These large bubbles will incorporate more Cs atoms and vacancies explaining their size increase and the evolution of their shape that becomes faceted. This mechanism, which implies that a system constituted of different bubble size tends to a more energetically favourable state promoted by a decrease of its interface energy, is known as the Ostwald ripening mechanism. As a summary, Figure 11 illustrates the Cs bubble nucleation and growth in UO2 during annealing. Such bubble growth was also reported in [START_REF] He | Bubble evolution in Kr-irradiated UO2 during annealing[END_REF] for the 1.5 MeV Kr implantation at the very high fluence of 5 x 10 16 ions•cm -2 . The authors have indicated the formation, after annealing at 1000 °C, of bubbles of around 1.5 nm in size. After annealing at 1300 °C (~1 hour) the bubble size distribution was no longer uniform and some large bubbles (several nm in diameter) appeared, some of them exhibiting facets that correspond to the (111) and (020) planes of UO2. They also reported an increase of the number of the faceted bubbles at 1600 °C. This means that, likely, a coarsening of the spherical bubbles occurred as well as an increase of the number of faceted bubbles.

Figure 11-Schematic illustration of the different mechanisms of the Cs bubble nucleation and growth

From all these results, we can conclude that Xe being insoluble in UO2, the thermal resolution is highly unlikely and therefore the growth of xenon bubbles through the Oswald ripening mechanism has a low probability to happen. However, solubility can be enhanced by defect concentration and temperature.

Depending on the nature of the volatile species, thermal resolution is also a process to consider which will determine the thermal behaviour of these species either on the growth of the bubbles or on the release of the gas.

Conclusions

This paper compares the thermal migration of caesium and xenon implanted in UO2 at a fluence of 10 15 Cs•cm -2 . The originality of the work was to characterise Cs and Xe behaviour after annealing at 1000 °C and 1600 °C by combining SIMS depth profiles with TEM examinations. Caesium is known to be a volatile species and its migration is often compared to the xenon migration.

1000 °C is the typical temperature at the centre of a UO2 pellet under normal reactor operation. No Cs release is observed after a 4 h annealing at this temperature. This is explained by the formation of nanometric bubbles (diameter of around 2 nm), the nucleation mechanism of which seems to be similar to the xenon one.

At high temperature (1600 °C), caesium and xenon exhibit a different behaviour, which can be explained by their respective solubility and thermal resolution in UO2. On one hand, xenon atoms remain trapped into bubbles divided into two populations with a mean size of respectively 2 and 6 nm.

On the other hand, an important release of Cs was measured (50 % after 12 h of annealing) and two kinds of bubbles (3 nm spherical bubbles and 8 nm faceted bubbles) are observed in a band located at a depth of around 50 nm from the surface.

One assumption to explain the growth of the spherical bubbles (especially Xe bubbles) is the Evans (migration coalescence) mechanism. The fraction of Cs bubbles, which are bigger and faceted, probably results from Ostwald ripening.

It can be concluded that the main parameters governing the stability of bubbles and their growth kinetics are the availability of thermal vacancies in conjunction with the threshold temperature for the thermal resolution. Other TEM examinations performed on samples annealed at intermediate temperatures (between 1000 °C and 1600 °C) are therefore planned in order to identify the threshold temperature of caesium thermal resolution and the first steps of the caesium diffusion.

School Programme and the European Institute of Technology (EIT) for the financial support to go to the JRC in Karlsruhe.

Figure 1

 1 presents three depth profiles of caesium (Figure1.a) or xenon (Figure1.b) implanted in UO2 samples and analysed with the methodology described before for each element. These profiles are compared with the depth profiles calculated with the SRIM software. There is a good reproducibility between the curves of the different rasters and a fairly good agreement with the SRIM profile with only a small deviation for the projected range (~125 nm for the SIMS analysis).

Figure 1 -

 1 Figure 1-Three depth profiles obtained with SIMS on caesium (a) and xenon (b) as-implanted samples (10 15 at•cm -2 ) compared to the depth profile calculated with the SRIM software.

Figure 2 -

 2 Figure 2-TEM bright field image of a virgin UO2 sample.

Figure 3 .

 3 Figure 3.a displays the evolution of Cs depth profiles in UO2 after annealing at 1000 °C and at 1600 °C, and of Xe depth profiles in UO2 after annealing at 1600 °C (Figure 3.b) under reducing atmosphere.

Figure 3 -

 3 Figure 3-Caesium (a) and xenon (b) depth profiles measured by SIMS in UO2 samples as a function of annealing temperature (1000 °C or 1600 °C) and compared to the as-implanted depth profile (10 15 at•cm -2 ).

Figure 4

 4 Figure4shows a bright field image obtained on the Cs as-implanted sample (fluence of 10 15 at•cm -²) on which the dpa distribution (in red) and the Cs depth profile (in white) calculated by SRIM have been superimposed. Three sharp zones can be distinguished in function of the dark contrast intensity, which indicates different concentrations of visible extended defects. A high concentration of dislocation loops with a size of about 5-10 nm is observed in a zone located between 85 nm and 200 nm (zone 2). Less damage is observed in zone 1 between the surface and a depth of about 85 nm, although this is the region where most dpa are produced by the Cs implantation according to the SRIM calculations. A few small dislocation loops (at most 5 nm in size) remain present in zone 3 with a decreasing density until a depth of around 300 nm. Beyond ~300 nm, the image is identical to the UO2 virgin sample image (see Figure2). At last, no precipitation or bubble formation is observed after the implantation of caesium for a fluence of 10 15 Cs•cm -2 . An accurate quantification has not been made at this stage but the TEM images evidence the difference in dislocation loop density especially following the Cs distribution profile. As an indication, the concentration of loops in zone 1 amounts about 1.2x10 22 m -3 and this value is about 4 times higher in zone 2 whereas it decreases to less than 10 22 m -3 in zone 3.

Figure 4 -

 4 Figure 4-TEM bright field image obtained on the as-implanted sample (fluence of 10 15 Cs•cm -2 ). The dpa distribution (in red) and the Cs depth profile (in white) as calculated by SRIM are superimposed. Three zones have been delimited with the contrast intensity, which is indicative of the amount of extended defects. White arrows point on some dislocation loops.

Figure 5

 5 Figure5displays the TEM image of a Cs implanted sample after annealing at 1000 °C, in underfocused beam conditions (Figure5.a) and overfocused beam conditions (Figure5.b). Changing the focus conditions allows revealing the presence of cavities that appear as white spots in underfocused conditions and as black spots in overfocused conditions. As no Cs release was measured by SIMS, it can be assumed that the cavities are filled with caesium atoms and these objects will be further referred in this paper as "bubbles". So, nanometric Cs bubbles with a size (referring to their diameter) of (2  0.5) nm are visible from the surface down to a depth of 150 nm. No bubble beyond this depth can be confirmed due to the presence of too many extended defects that can mask the bubbles within our diffraction conditions. In zone 1, a density of (4.8  1.3) × 10 23 bubbles•m -3 was determined by counting manually the bubbles on different images from three different areas of the sample.

Figure 5 -

 5 Figure 5-TEM bright field images of UO2 pellets implanted at a fluence of 10 15 Cs•cm -2 and annealed at 1000 °C during 4 h in underfocused beam conditions (a) and in overfocused beam conditions (b). Arrows point at typical bubbles.

Figure 6

 6 Figure 6 displays the combination of the Cs depth profile after annealing at 1000 °C together with a TEM image taken under different conditions (orientation and magnification) than those of Figure 5.

Figure 6 -

 6 Figure 6-TEM bright field image of UO2 pellets implanted at a fluence of 10 15 Cs•cm -2 and annealed at 1000 °C during 4 h with the superimposition of the corresponding SIMS depth profile.

Figure 8

 8 presents a high magnification TEM micrograph focused on the bubble line (Figure 8.b) with a graph displaying the number of bubbles as a function of their size (Figure 8.a). The histogram was determined by counting bubbles from three different TEM images and was divided by 1 nm bins. It represents a total population of 140 bubbles.

Figure 7 -

 7 Figure 7-TEM bright field images of: (a) UO2 pellets implanted at a fluence of 10 15 Cs•cm -2 and annealed at 1600 °C during 12 h, (b) the first 200 nm (zoom of the blue rectangle in a) from the surface are displayed at higher magnification, revealing the presence of Cs bubbles parallel to the surface at 50 nm (dashed yellow rectangle). This bubble distribution is correlated with the Cs accumulation at a depth of around 50 nm and visible on the profile as obtained from SIMS measurements and presented in (c).

Figure 8 -

 8 Figure 8-(a) Spherical and faceted caesium bubble size distribution (total population of 140) determined from three different TEM images taken from a UO2 pellet implanted at a fluence of 10 15 Cs•cm -2 and annealed at 1600 °C during 12 h. A TEM bright field image of Cs bubbles taken at high magnification is shown in (b).

Figure 9 -

 9 Figure 9-STEM (a) and underfocused TEM bright field (b) micrographs of the same subsurface area of a UO2 sample implanted in Xe at a 10 15 at•cm -² fluence and annealed at 1600 °C for 12 hours. Dark frames identify the same bubbles and arrows point to some of them particularly. Xe depth profile measured by SIMS is shown in (c).

   5) × 10 23 bubbles•m -3 . The size value and the density value are pretty close to ours even if the concentration of caesium is 25 times greater in the work of Sabathier et al. This could indicate that a saturation threshold for the bubble density is already reached for a fluence of 10 15 at•cm -2 . The same phenomenon is reported for the xenon implantation in UO2 in the work of Michel et al. [40]. They have varied the implantation fluence and studied bubble formation. They have shown that the bubble size was nearly the same (around 2 nm) whatever the fluence and that the bubble density was increasing with the fluence up to a threshold of around 5 x 10 14 at•cm -2 .

Figure 10 .

 10 Figure 10. This figure displays the distributions of the defects which can be involved in the formation and migration of the caesium bubbles. The quantity of defects being unknown after annealing, the distributions have been normalized to one. The dpa distribution stands for the defects created by implantation as calculated by SRIM. Even if the distribution evolves after annealing, it gives an estimate of the presence of the implantation defects. The surface vacancy distribution has been represented by the function expressed in (eq. 2). C(x, t) = erfc ( x 2√Dt )(eq. 2)

Figure 10 -

 10 Figure 10-Sketch of the formation of the specific region (grey zone) where Cs bubbles develop after annealing at 1600 °C. Rd stands for the range of the dpa distribution calculated by SRIM and Rp for the projected range of the Cs distribution

Table 1

 1 gathers all the characterisations made on the implanted (with caesium or xenon) samples used in this work.

Table 1 -list of characterisations by SIMS or TEM made on samples implanted with Cs or Xe (10 15 at•cm -2 ) and subsequently annealed at 1000 °C or 1600 °C. Annealing conditions Analysed by SIMS Analysed by TEM

 1

	No annealing	Cs or Xe	Cs
	1000 °C (4h)	Cs	Cs
	1600 °C (3h or 4h)	Cs or Xe	-
	1600 °C (12h)	Cs or Xe	Cs or Xe
	1600 °C (16h)	Xe	-

Table 2 -Main results obtained by TEM on samples implanted with Cs or Xe (10 15 at•cm -2 ) and subsequently annealed (1000 °C or 1600 °C).

 2

		Caesium	Xenon
	Annealing conditions	1000 °C (4h)	1600 °C (12h)	1600 °C (12h)
	Bubble populations	1	2	2
	Bubble shape	Spherical	Spherical and faceted	Spherical
	Bubble mean size (nm)	2 ± 0.5	3 ± 1 and 8± 2	2 ± 1 and 6± 1
	Bubble density (m -3 )	(4.8  1.3) × 10 23	(6.3 ± 2.2) x 10 22 and (1.3 ± 0.7) x 10 22	5-9 x 10 22

Acknowledgements

The authors wish to express their thanks to Anthony Duranti (IP2I Lyon, France) for the Cs implantation on the IMIO400 implanter and Dr. Philippe Sainsot for interferometry measurements (INSA Lyon, France). Special thanks are also addressed to Alessandro Benedetti (JRC Karlsruhe, Germany) for the optimum FIB lamellae preparation and for the TEM examinations. Thanks to the InnoEnergy PhD


V Rérolle
email: v.rerolle@fluidion.com

D Angelescu

A Hausot

P Ea

N Lefèvre

C Provost

M Labaste

Development of a novel hybrid pH sensor for deployment on autonomous profiling platforms

Keywords: Hybrid pH sensor, microfluidic, colorimetric, electrode, acidification. I

Ocean acidification is a direct consequence of the atmospheric CO2 increase and represents a threat for marine ecosystems, particularly in the Arctic. High-quality seawater pH measurements with good spatial and temporal coverage are required to apprehend the ocean acidification phenomena. We are working to develop a high-accuracy, high-resolution pH sensor that has the potential to allow global ocean acidification mapping through deployment on fleets of ARGO floats and other autonomous platforms already in existence. The instrument implements a novel hybrid approach, utilizing the two different and complementary measurement techniques (potentiometric and colorimetric) to generate temporally dense and highly accurate pH data. Here we present the concept and initial results obtained from a hybrid pH sensor. Results show that the potentiometric part of the sensor is capable to operate in real ocean pressure and temperature conditions, including near-freezing temperatures typical of Arctic environmental conditions. The colorimetric part provides a stable reference to perform periodic recalibrations and remove drift.

Ocean acidification, a direct consequence of CO2 increase in the atmosphere, represents a real threat for marine ecosystems [START_REF] Doney | Ocean Acidification: the other CO2 problem[END_REF], the Arctic Ocean being the most vulnerable region with the strongest pH decrease [START_REF] Orr | Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms[END_REF][START_REF] Steinhacher | Imminent ocean acidification in the Arctic projected with the NCAR global coupled carbon cycle-climate model[END_REF]. High-quality seawater pH measurements with good spatial and temporal coverage are required to understand ocean acidification phenomena. While several ocean parameters can now be measured globally thank to a network of autonomous profiling and drifting floats [START_REF] Jayne | The Argo Program: present and future[END_REF], pH is a notable exception: there are currently very few direct pH measurement options available, despite efforts to develop a continuous accurate pH measurement system by several academic and industrial groups [START_REF] Byrne | Sensors and systems for in situ observations of marine carbon dioxide system variables[END_REF][START_REF] Rérolle | Seawater-pH measurements for oceanacidification observations[END_REF][START_REF] Kumari | Fiber optic sensors in ocean observation: A comprehensive review[END_REF]. As a consequence, longterm trends in surface pH are often inferred only indirectly from other measured ocean carbon system parameters. The oceanographic community is currently in need of autonomous pH sensor technology that will affordably, accurately and efficiently measure ocean chemistry from its shallowest to its deepest waters even in remote areas with extreme conditions. Installation of sensors on profiling and drifting floats requires a small, solid and lightweight sensor capable of performing highly accurate measurements, with fast response time and without significant measurement drift for extended deployment durations. Cost is also an important consideration, since profilers stop emitting their location at the end of battery life, making very unlikely the prospect of their recovery. Traditionally, seawater pH has been measured using one of two standard methods: potentiometry (pH electrodes) and colorimetry (using pH indicators), which remains the gold standard in the field [START_REF] Lai | Spectrophotometric measurement of freshwater pH with purified meta-cresol purple and phenol red[END_REF]. A third method, using MOS field effect transistors (MOS-FET), has recently been adapted to marine waters, and was only recently released as a commercial solution for surface and potential profiling applications [START_REF] Johnson | Deep-Sea DuraFET: A pressure tolerant pH sensor designed for global sensor networks[END_REF]; this new sensor can be categorized under potentiometry.

Every type of traditional ocean pH measurement presents a set of advantages and disadvantages. Colorimetry, for example, is well adapted to many biogeochemical parameters because it enables high-quality measurements with low detection thresholds and without drift or need for periodic recalibration. It allows the acquisition of quality data in a wide range of temperature and salinity environments, with negligible deterioration in measurement quality even after long periods of use (> 1 year). The colorimetric method does however consume liquid pH indicator (m-cresol purple being routinely used in ocean monitoring), which limits the total number of measurements available as well as their frequency. Colorimetric sensors require metered reagent dosage and accurate optical measurements that are not always easy to achieve in actual field conditions or during profiling operations [START_REF] Lai | Autonomous Optofluidic Chemical Analyzers for Marine Applications: Insights from the Submersible Autonomous Moored Instruments (SAMI) for pH and pCO2[END_REF].

The potentiometric method, meanwhile, does not require use of reagent. Potentiometric sensors (electrodes) are more easily miniaturized, consume little energy and enable continuous measurements with fast response times. Therefore, this method allows vertical profiling. However, these sensors can be very sensitive to electromagnetic perturbations or to changes in temperature and salinity, with phenomena such as electrolyte diffusion and biofilm formation further leading to frequent recalibration requirements that are impossible to accomplish on a drifting float. There is consequent reluctance within the ocean acidification community to deploy stand-alone electrode-type sensors for extended periods without human intervention.

The recently developed MOSFET-based sensors show a relatively good stability but require intricate and lengthy location-dependent calibration procedures and still suffer from measurement drift after several weeks of deployment, requiring data correction [START_REF] Miller | An evaluation of the performance of Sea-Bird Scientific's SeaFET™ autonomous pH sensor: considerations for the broader oceanographic community[END_REF].

The paper presents the concept and initial results obtained from a novel hybrid pH sensor using a combination of potentiometric and colorimetric measurements to compensate for drift and generate high-frequency and accurate ocean pH data. Our aim is to develop an accurate (≤0.010 pH units) and precise (0.001 pH units) high-resolution pH sensor that has the potential to allow global ocean acidification mapping through deployment on fleets of ARGO floats and other autonomous platforms already in existence. The instrument implements a hybrid approach, utilizing the two different and complementary measurement techniques (potentiometric and colorimetric) to generate temporally dense and highly-accurate pH data. The hybrid system is ultimately aimed to be deployed on an autonomous platform and will ensure the precise re-calibration of the potentiometric electrode by the periodic colorimetric measurements performed throughout the duration of the platform deployment.

We will start by characterizing a standalone electrode before presenting an initial ferry-box version of the hybrid sensor for continuous underway measurements, and then present a new version of the sensor that is adapted to surface water monitoring on moorings (currently under test). Results show that the potentiometric part of the sensor is capable to operate in real ocean pressure and temperature conditions, while the colorimetric part provides stable reference to perform periodic recalibrations and remove drift. Future work will focus on adapting the colorimetric analyzer housing for in-situ deployments at depth, with the ultimate goal of deploying the hybrid pH setup on autonomous drifting and profiling platforms deployed in the Arctic Ocean.

II. INSTRUMENTS AND METHODS

A. Instruments 1) Potentiometric sensor

The potentiometric sensor is a combined glass-electrode and a reference electrode (Ag/AgCl) in KCl gel. The potentiometric sensor uses an instrumentation amplifier circuit -whenever referred to in text, the electrode voltage refers to the amplified signal. The potentiometric sensor can be outfitted with either a plastic or a stainless-steel casing, depending on weight constraints and on possible presence of electrical interferences, and can perform high-frequency measurements (1 Hz) from surface to 2000 m depth in seawater and brines solution with temperature ranging from -2 to 60 °C.

2) Colorimetric analyzer

The colorimetric analyzer is based on the widely used metacresol purple (mCP) indicator for seawater pH measurement [START_REF] Clayton | Spectrophotometric seawater pH measurement: total hydrogen ion concentration scale calibration of mcresol purple and at sea results[END_REF]. The pH indicator is added to the seawater and optical spectra are recorded using a combination of LED light sources and a spectrophotometric detector, absorbance measurements being performed at 434, 578 and 700 nm. A fluidic manifold integrates a pressure control block for reducing the underway supply pressure to 6 PSI, electro-fluidic valves for controlling the flow of sample and reagent streams, a micromixer manufactured using micro-electromechanical systems (MEMS) technology and implementing the flow-folding principle, as well as MEMS hydraulic resistors for controlling flow rates throughout the microfluidic geometry. The fluidic manifold enables precise control of the reagent and sample dosage and mixing prior to injection into a custom MEMS silicon/glass 25mm-long optical cell. Coupling of the optical cell to LEDs and detector is achieved via optical fibers. A dead-end glass microfiber filter (5 μm porosity) is used to retain particles that may be present in the sample, prior to injection in the fluidic manifold. The colorimetric analyzer can be programmed to perform periodic measurements at intervals ranging from 5 minutes to several hours, depending on deployment requirements.

3) Hybrid pH instrument and Ferry-box set up

The first hybrid pH instrument presented in this work is adapted to ferry-box measurements, and is a combination of the potentiometric sensor and the colorimetric analyzer. The hybrid instrument is a stand-alone unit: it can operate on battery or mains power if present, is configured via an integrated LCD screen and two push-buttons, and all data (colorimetric and electrode) are logged internally or, optionally, uploaded to the cloud if cellphone network is present. A Java interface is used for live data observation, data retrieval over USB after longterm deployment, and electrode calibration.

The hybrid sensor is installed in-line on the ship's underway water supply: seawater flows initially to the electrode via its flow-through chamber and then to the colorimetric analyzer, where a few mL are diverted for spectrophotometric measurement (see Fig. 1 -the actual measurement is performed on 1.6µL). Sample temperature is measured at two places: within the electrode flow-through chamber and at the analyzer's optical cell.

4) Surface buoy hybrid system

The ferry-box hybrid system was modified to enable its deployment on data buoys. In this configuration, the electrode is deployed directly in-situ under the buoy while the analyzer is installed on the buoy. Seawater sampling for the colorimetric measurement is performed from the electrode location using a peristaltic pump connected to a length of tubing (See Fig. 2, left). A tangential filtration system has been developed to replace the dead-end filter for long-term and turbid water deployments. The electrode is outfitted in this case with a 316 stainless steel electronics housing to avoid electromagnetic interferences from other equipment that may be present on the buoy. Data are logged internally but also communicated remotely to cloud data servers in real time via a GSM cellular link.

The hybrid pH system was deployed on the MAREL-Iroise buoy at the entrance of the Bay of Brest (France, Brittany) on the 9th of January 2019 (Fig. 2, right). MAREL-Iroise is one of the scientific buoys of the French network for monitoring of coastal environment (COAST HF). Since year 2000 this buoy has been autonomously providing seven core parameters (conductivity, temperature, dissolved oxygen, turbidity, chlorophyll fluorescence and pH) of the marine environment, measured every 20 minutes at a depth of 2.5 m.

B. Methods

1) Chemical preparation and bench-top calibration protocol

The purified 2 mM meta-Cresol purple (mCP) indicator solution is prepared in 0.7 M NaCl solution at pH ≈8.0. mCP purification is performed by flash chromatography following the protocol detailed in [START_REF] Patsavas | Purification of meta-cresol purple and cresol red by flash chromatography: Procedures for ensuring accurate spectrophotometric seawater pH measurements[END_REF]. Tris buffer solutions at different pH are prepared in artificial seawater according to the protocol described in [START_REF] Pratt | Measurement of pHT values of Tris buffers in artificial seawater at varying mole ratios of Tris:Tris•HCl[END_REF]. pH values of buffer solutions are determined by manual benchtop colorimetric measurements using the Standard Operating Procedure (SOP) [START_REF] Dickson | Guide to best practices for ocean CO2 measurements[END_REF] and applying purified mCP characterization coefficients from [START_REF] Liu | Purification and Characterization of meta-Cresol Purple for Spectrophotometric Seawater pH Measurements[END_REF]. Temperature of the solution is measured just after recording the absorbance spectra with a hand-held thermometer. The quality of the bench top protocol is verified by analyzing Tris buffer certified reference material from Dickson's Scripps laboratory (pHref=8.093 at 25.0 °C): pH bench-top measurements are consistently within 0.001-0.004 pH units from the certified value. Bench-top pH measurements are referred to as reference pH.

2) Dependence on environmental conditions

Extensive temperature, salinity and pressure tests were performed on a standalone version of the electrode prior to integration within the hybrid system, with temperature tests also performed on the Ferry-box hybrid pH system.

Standalone pH electrode (temperature, salinity and pressure)

The effect of temperature on the electrode was characterized by measuring a range of pH buffers at six different temperatures that are representative of deep ocean, or winter surface, temperatures 5°C, 2°C, 1°C, 0°C, -1°C and -1.5 °C.

The effect of salinity on the electrode response was studied by measuring buffers of various salinities at 25 °C. The range of salinity (26.5-36.5 PSU) was obtained by diluting a single buffer solution of salinity 33.2 with mili-Q water or by adding a few milligrams of NaCl. Resulting solution salinities were measured with a conductivity probe.

Pressure tests were performed by placing the electrode in a custom hyperbaric pressure chamber and repeatedly cycling the pressure in 40-Bar steps, between 20, 60 and 100 Bar. Temperature of the Tris solution inside the pressure chamber was continuously recorded using a temperature logger. The theoretical effect of pressure on the pH of the buffer solution was estimated using equations described in [START_REF] Rodriguez | The partial molal volume and compressibility of Tris and Tris-HCl in water and 0.725 m NaCl as a function of temperature[END_REF].

Ferry-box hybrid pH system (temperature)

To test the performances of the hybrid system at different temperatures, the system was long-term tested with a large batch of Tris buffer at several temperatures between 8°C and 20°C. Small volumes of buffer solution were sampled at each temperature to measure its exact pH value using the bench-top SOP described above.

3) Hybrid pH system calibration

The Ferry-box hybrid pH sensor was calibrated in the laboratory at 20-21°C with large batches of Tris buffers, precisely characterized using the bench-top SOP described above. Water from a reservoir containing sample seawater or different standard solutions was pumped continuously in a loop using a centrifugal magnetic pump. The ambient temperature inside the experimental chamber was controlled through a forced advection air-water heat exchanger connected to a thermostated bath, whereas reservoir temperatures were directly controlled by the bath to accelerate the thermal exchange. Analyzer pH values were obtained from absorbance measurements performed in the MEMS optical cell by using published mCP characterization coefficients from [START_REF] Liu | Purification and Characterization of meta-Cresol Purple for Spectrophotometric Seawater pH Measurements[END_REF]. A slight system-specific linear correction was calculated and applied to compensate for small departures from ideal behavior due to e.g., emission and absorbance bandwidths and/or system geometries [START_REF] Yang | Seawater pH measurements in the field: a DIY photometer with 0.01 unit pH accuracy[END_REF]. The analyzer measurements were used to calibrate the electrode within the flow-through chamber.

For buoy deployment the colorimetric analyzer was calibrated using Tris buffer as for the ferry-box set up, and its measurements were again used to calibrate the electrode. The difference from the Ferry-box setup was that the electrode was now placed directly in a large seawater bath, whose pH was adjusted in steps by addition of 1M HCl or, respectively, 1M NaOH solutions. Temperature was controlled with a thermostated water bath and chamber.

III. RESULTS

A. Dependence on environmental conditions

Standalone pH electrode characterization

Effect of low temperature on the electrode's calibration is grpahically shown in Fig. 3, along with the linear calibration equations obtained at different temperatures. In order to generate a universal calibration curve, the intercept and slopes of the calibration equations were modeled to first degree as linearly dependent on temperature:

pH(V,T)=(a1*T+b1)*V+(a2*T+b2) (1)
where V is the electrode voltage (after signal amplification) and T is the temperature. The linear coefficients a1, a2, b1 and b2 are obtained by regression using the calibration equations from Fig. 3. This universal calibration results in residuals of 0.010 ± 0.009 pH units, which can be considered acceptable given that the most precise reference material available is certified to 0.003 pH units.

The electrode's response is found to slightly decrease with salinity: applying a salinity correction similar to [START_REF] Easley | Spectrophotometric Calibration of pH Electrodes in Seawater Using Purified m-Cresol Purple[END_REF] results in a difference of only 0.003 ± 0.006 pH units. The effect of salinity is therefore considered negligible over the 26.5-36.5 PSU salinity range.

Effect of the pressure cycles (in 40 Bar steps) on the electrode's response is presented on Fig. 4. The change of pressure affects the final pH value through two different mechanisms [START_REF] Rodriguez | The partial molal volume and compressibility of Tris and Tris-HCl in water and 0.725 m NaCl as a function of temperature[END_REF]. On one hand, from thermodynamic considerations, a pressure increase of 40 Bar results in a Tris buffer temperature increase in the pressure cell of approximately 0.08°C, which then translates into a pH decrease of about 0.0025 units. On the other hand, an increase in pressure results in a separate direct increase in the pH of the Tris buffer solution. Finally, when combining the opposite temperature and pressure effects, the resulting pH of the Tris solution in the chamber increases by only 0.0013 pH units per 40 Bars of pressure increase. The electrode's response changed by 2.2 ± 0.9 mV per 40 Bars step, which corresponds to a relatively stable pH of 7.5060±0.0007 units throughout the pressure cycles after compensating for the temperature change using [START_REF] Doney | Ocean Acidification: the other CO2 problem[END_REF]. The pH value measured by the electrode, compared to the actual pH of the Tris subjected to the resulting temperature and pressure changes, resulted in an error of 0.0017±0.0005 pH units per 40 Bars step. This effect of pressure on the electrode's response could be taken into account by applying a linear correction. However, in the following we considered it to be within the measurements error.

Ferry-box hybrid pH system characterization

The results of the colorimetric analyzer at different temperatures compare well to reference measurement with residuals between -0.004 and 0.006, whereas electrode results show a much larger discrepancy at low temperature (residuals up to 0.039) (Fig. 5). The good performance of the colorimetric analyzer demonstrates that the use of the temperature characterization from [START_REF] Liu | Purification and Characterization of meta-Cresol Purple for Spectrophotometric Seawater pH Measurements[END_REF] for m-CP is working well for the temperature range between 8 and 20°C used in this study. For lower temperatures, characterization from [START_REF] Papadimitriou | The measurement of pH in saline and hypersaline media at sub-zero temperatures: Characterization of Tris buffers[END_REF] should be used.

The poorer relative performance of the electrode, on the other hand, seems to be due to instabilities and drift inherent to the hardware and highlights the importance of wisely selecting the frequency of the in-situ corrections using the colorimetric sensor.

B. Hybrid pH system calibration

The hybrid instrument (colorimetric analyzer and electrode) was calibrated in Tris buffer at 21 °C, using the procedure explained in the Instruments and Methods section. The corresponding graphs are shown in Fig. 6 below. The stability of the colorimetric analyzer's calibration against reagent refilling was confirmed by using a new batch of 2mM mCP indicator, prepared a week after the instrument's calibration, and checking the colorimetric analyzer's measurements on two Tris buffer solutions and one seawater sample: the resulting data was within 0.01 pH units from the reference measurement, showing that the calibration of the analyzer was stable and suitable for analysis of both Tris and seawater samples.

A comparison between the pH electrode and the analyzer was then performed in a large seawater volume at 21 °C. The pH of the seawater was adjusted in multiple steps by successively adding 0.1 M HCl and 0.1 M NaOH solutions. Electrode and analyzer datasets followed the same general pattern but an offset of 0.2 pH units was observed between the two curves (Fig. 7), with the pH electrode reading higher than the colorimetric analyzer. Reasons for such a large offset are not clear but are probably due to the nature of the media, possibly compounded with electromagnetic interferences from other laboratory instruments (e.g. water bath or pump). To compensate for this offset, the electrode was re-calibrated directly using the colorimetric analyzer's seawater data (Fig. 6, bottom), which completely removed the initial offset (Fig. 7). An important conclusion can be drawn from this observation: to avoid any type of media effects, the electrode should be calibrated directly in seawater using parallel measurements from the colorimetric analyzer, or, as a minimum, at least one reference point should be used to re-adjust the electrode to correct for interferences.

C. Buoy deployment data

Colorimetric analyzer data during the buoy deployment are shown in Fig. 8. No usable electrode pH data were obtained due a cable waterproofing issue, which has since been fixed. The colorimetric analyzer worked well during the one-week deployment at a measurement frequency of two measurements per hour. Fig 8 also shows the moving average (overlay) and moving standard deviation (bottom) obtained over a window of 12 data points (6 hours). The data demonstrates good stability, with main variations likely due to natural variability (e.g. tides). The system precision is about 0.002 pH units as observed in the plateau-like part of the dataset on the 15 th of January.

IV.

CONCLUSIONS AND FUTURE WORK Results show that the potentiometric part of the hybrid sensor is capable to operate in real ocean pressure and temperature conditions. The electrode can perform well in near-freezing temperatures and appears suitable for Arctic environmental conditions. The colorimetric part of the hybrid sensor provides a stable reference to perform periodic recalibrations and remove drift. Protocols to calibrate both colorimetric and potentiometric sensors have been improved with an initial characterization of the colorimetric analyzer performed in Tris buffers, and a subsequent characterization of the electrode performed in seawater using the colorimetric analyzer as reference. While data from short-term buoy deployment look very encouraging, additional system characterization with multiple pH and temperature steps is needed to achieve target data quality (0.010 pH units accuracy, 0.001 pH units precision). The next deployment of the hybrid system in a seawater tank facility will be used to study the colorimetric analyzer sampling frequency required to re-calibrate the electrode's data. In parallel, adaptation of the colorimetric analyzer housing for in-situ deployments at depth has started.

Fig. 1 .

 1 Fig. 1. Schematic (left)and photo (right) of th e Ferry-box h ybrid pH sensor set up. The photo is from installation on the Oden Swedish research ship, in August 2018: the box at left is the colorimetric analyzer, while th e potentiometric electrode outfitted with a flow-through chamber are visible at right, alon g with the pressure gauge.

Fig. 2 .

 2 Fig. 2. Schematic (left) and photo (right) of the surface buoy h ybrid system. The photo shows the hybrid pH system on the MAREL-Iroise buoy in January 2019. Left: M AREL-Iroise b uoy. Top right: analyzer and battery in the aluminum box on the buoy. Bottom right: pH electrode on the aluminum pole read y to be deployed.

Fi g. 4 .

 4 Electrode output in mV, after amplification (top) and corresp onding temperature (b ottom) changes during two pressure cycles. Numb ers in red indicate pressure in the chamber (Bar).

Fig. 5 .

 5 Fig. 5. Ferry-box system temperature stud y. Top panel: Hybrid pH system measurement (colorimetric analyzer: black stars and electrod e: grey dots) versus temperature. hybrid-p H measurements (colorimetric analyzer: black stars and electrod e: grey dots) of Tris buffer. Bottom panel: pH residuals (pH reference -pH system) versus temperature.

Fig. 6 .

 6 Fig. 6. Hyb rid pH sensor calib ration: Top panel: Bench top reference Tris pH versus colorimetric analyzer pH data. Bottom: pH colorimetric data versus electrode voltage output (after amplification, 1 mV ≈ 1 mpH units) in Tris (grey dots: b ench top reference pH) and in seawater (white circles: colorimetric analyzer pH).

4 .

 4 Ifremer-Centre de Brest, REM / RDT / DCM, Z.I. de la Pointe du Diable, CS 10070, 29280 Plouzané, France, Loic.Quemener@ifremer.fr, Michel.Repecaud@ifremer.fr. 5. Station Biologique de Roscoff, UPMC, UMR CNRS 7144, pl. Georges Teissier, CS90074, 29688, Roscoff, France, bozec@sb-roscoff.fr. 6. CNRS/INSU, Division Technique, 1 place Aristide Briand, 92195 Meudon, France. laurence.beaumont@cnrs.fr, antoine.guillot@cnrs.fr. COAST-HF website: https://www.ir-ilico.fr/Les-reseauxelementaires/Fiches-d-identite-des-reseauxelementaires/COAST-HF (INSU-CNRS-IFREMER-IUEM/UBO) This work is supported by the project MACAO (ANR-17-CE04-0006-01), funded by the French Agence Nationale de Recherche. The Ferry-box system development was supported by the JERICO-NEXT program from the European Union H2020 funding program, grant number 654410 (2015-2019).

Fig. 7 .

 7 Fig. 7. Hybrid pH system seawater data comparison: analyzer (black stars) and pH electrode (grey dots: with Tris calibration and white circles: after seawater re-calibration).

Fig. 8 .

 8 Fig. 8. Top: Analyzer pH data with moving mean (window: 12 datapoints) at in situ temperature during deployment on the MAREL-Iroise buoy in January 2019.Bottom: Moving standard deviation (window: 12 datapoints) of the colorimetric analyzer pH data.

Fi g.[START_REF] Steinhacher | Imminent ocean acidification in the Arctic projected with the NCAR global coupled carbon cycle-climate model[END_REF]. Study of the effect of t emperature on the electrode response: Electrode raw pot ential V (in mV, after amplification) versus tris buffer pH.

ACKNOWLEDGEMENTS

Tests of the hybrid sensor on the MAREL buoy were achieved thanks to collaborations with IUEM-IBO, IFREMER and ROSCOFF: Peggy Rimmelin-Maury 3 , Loïc Quéméner 4 , Michel Répécaud 4 , Yann Bozec 5 , Thierry Cariou 5 allowed deployment on the MAREL-Iroise buoy, provided access to laboratory facilities, helped with the installation and provided laboratory pH analysis. Paul Sonnier 2 helped with laboratory tests and buoy installation. Antoine Guillot 6 and Laurence Beaumont 6 assisted in the deployment preparation and coordination. Kai Hong 1 helped customize the data server and cloud interface to access and visualize the instrument's data.

Work on the standalone electrode was supported by funding from Climate-KIC (SME voucher, 2015(SME voucher, -2016)).