Iodine release from high-burnup fuel structures: Separate-effect tests and simulated fuel pellets for better understanding of iodine behaviour in nuclear fuels

Janne Heikinheimo,Unto Tapper,Matĕj̆ Niz̆N̆Anský,Teemu Kärkelä,Mélany Gouëllo,Václav Tyrpekl

doi:10.1557/s43580-021-00175-1

Janne Heikinheimo, Unto Tapper + Show 4 more

Open Access

https://doi.org/10.1557/s43580-021-00175-1

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Abstract

Iodine release modelling of nuclear fuel pellets has major uncertainties that restrict applications in current fuel performance codes. The uncertainties origin from both the chemical behaviour of iodine in the fuel pellet and the release of different chemical species. The structure of nuclear fuel pellet evolves due to neutron and fission product irradiation, thermo-mechanical loads and fission product chemical interactions. This causes extra challenges for the fuel behaviour modelling. After sufficient amount of irradiation, a new type of structure starts forming at the cylindrical pellet outer edge. The porous structure is called high-burnup structure or rim structure. The effects of high-burnup structure on fuel behaviour become more pronounced with increasing burnup. As the phenomena in the nuclear fuel pellet are diverse, experiments with simulated fuel pellets can help in understanding and limiting the problem at hand. As fission gas or iodine release behaviour from high-burnup structure is not fully understood, the current preliminary study focuses on (i) sintering of porous fuel samples with Cs and I, (ii) measurements of released species during the annealing experiments and (iii) interpretation of the iodine release results with the scope of current fission gas release models.Graphical abstract

Highlights

High release of iodine has been reported in the annealing experiments of irradiated fuels [1,2,3]
These values were graphically calculated with the Matlab Image Viewer Distance tool for further comparison of the sample release behaviour as the sample pellets were partly cracked in the removal process from spark plasma sintering device
Sintering of the CeO2 samples with 1 mol% of CsI was done at different temperatures and with similar pressure conditions reaching 80 MPa to create different porous structures and study their implications on caesium and iodine release

Summary

Introduction

High release of iodine has been reported in the annealing experiments of irradiated fuels [1,2,3]. The release of iodine should be reduced if it appears in the fuel in less volatile chemical state, such as CsI. Despite of the results from international fission product (FP) release experiments at various temperatures [1,2,3] and mechanistic approaches for fission product transport in irradiated UO2 [5], uncertainties exist for iodine chemical state in the fuel and its release. SIMFUEL pellets containing volatile elements require a special approach to retain the volatile material inside the pellet during the sintering procedure. In the case of ionimplanted iodine, related iodine diffusion experiments have been conducted with thermal annealing [6]

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