# Is there a chemical form of cesium involved in the release of fission gases?

Florian Castermans, Lionel Desgranges, Yves Pontillon, Rodrigue Largenton, Gianguido Baldinozzi

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Florian Castermans, Lionel Desgranges, Yves Pontillon, Rodrigue Largenton, Gianguido Baldinozzi. Is there a chemical form of cesium involved in the release of fission gases?. JEEP2025: Journées d'Etude des Equilibres entre Phases, Mar 2025, Paris, France. 2025. hal-05304321

HAL Id: hal-05304321

https://cnrs.hal.science/hal-05304321v1

Submitted on 8 Oct 2025

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Distributed under a Creative Commons Attribution 4.0 International License
## Fission gases release measurement in LOCA type conditions

The amount of radioactive fission products (FPs) that can be released from irradiated fuel under accident conditions must be considered when designing safety systems for nuclear reactors.

* Heat treatments on irradiated nuclear fuels, representing the thermal transients (TT) to which fuels can be exposed in accident situations

<figure><img src="image_2.png" /></figure>

Loss-Of-Coolant Accident (LOCA) : thermal transient

* Observation of discontinuous release of fission gases (Xe, Kr) which are referred to as « burst » releases in high burn-up fuels

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

Experimental setup MERARG in a high activity cell at CEA Cadarache

<figure><img src="image_5.png" /></figure>

>1100°C : model of grain boundaries decohesion (GB) and tunneling [1]

« Burst » releases of <sup>85</sup>Kr during a LOCA TT type

600-800°C: no consensus over the physical mechanism leading to the grain boundaries opening

## Cesium : a possible key player

« Volatile » fission products => greatest impact in an accident scenario. Among them, Cs is the more abundant : focus on this species

* Signs of simultaneous transport of Cs with fission gases

* Release of Cs measured in a Knudsen cell before 1000K (~730°C) [2]

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

* Observation of white precipitates during TT : related to Cs bleeding/migration?

## Cesium behavior during thermal transients

### Cs distribution at grain boundaries (GB)

**Studies on irradiated fuel:**

* Fission gas diffusion in bubbles at the GB

<figure><img src="image_8.png" /></figure>

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SEM observation of intergranular bubbles in a high burnup fuel [3]

SIMS observation of Cs at the GB after a power ramp [4]

**Hypothesis of a correlated behaviour between FG and Cs at GB**

### Thermochemical state of Cs

* Interaction with other FP (I, Mo, Te...)

* Superposition of concentration profiles during TT on iodine and cesium implanted pellets

<figure><img src="image_10.png" /></figure>

Comparison of iodine and cesium concentration profiles in ion-implanted samples [6]

* Thermochemical calculation

* Calculate the state of a system at thermodynamic equilibrium from the free enthalpy of each phase

* Predict the potential phases observed in the experiments

<figure><img src="image_11.png" /></figure>

Predominance diagram for Mo-I-Cs-Te-O2 system (TAF-ID) [6]

## Conclusions & Perspectives

* ✓ The Cs distribution and the chemical state at the grain boundaries could play an important role in the release burst at 600-800°C

* Experiment to characterize the Cs distribution before and after heat treatment at the grain boundaries

Reactor
irradiation/sampling

* Sample characteristics (Burn-up)

Thermal transient in
MERARG setup

* in-line-y-metry of gas release

* Cesium filter

Determination of
released species

* SEM/SIMS on the filter

* Which species have been released with Cs?

Characterization of Cs
at the GB before and
after TT

* Microprobe, SIMS and TEM analysis

<footnote>[1] Y.Pontillon et al., 2018, New insight on volatile fission products (I and Cs) release from high burnup UO<sub>2</sub> under LOCA type conditions, TopFuel2018</footnote>

<footnote>[2] J.-P. Hiernaut et al., 2008, Fission product release and microstructure changes during laboratory annealing of a very high burn-up fuel specimen, JNM 377 p313-324</footnote>

<footnote>[3] Zacharie-Aubrun et al., 2018, Focused ion beam-scanning electron microscope examination of high burn-up UO<sub>2</sub> in the center of a pellet, NET 50, p259-267</footnote>

<footnote>[4] L.Desgranges et al., 2006, Comportement des PF volatils au cours d'une rampe</footnote>

<footnote>[5] S.Valin, 1999, Etude des mécanismes microstructuraux liés au relâchement des gaz de fission du dioxyde d'uranium</footnote>

<footnote>irradié, PhD thesis</footnote>

<footnote>[6] M.Rochedy, 2022, Caractérisation expérimentale des produits de fission corrosifs dans UO2 : étude du système (I-Cs-Te)-UO2, PhD thesis</footnote>