E N V I R O N M E N T , E A R T H A N D P L A N E T A R Y S C I E N C E S

S C I E N T I F I C H I G H L I G H T S

1 4 4 H I G H L I G H T S 2 0 2 3 I

Research into severe nuclear accidents in Pressurised Water Reactors (PWR) aims to improve current nuclear accident management procedures. In this context, much past research has tried to understand the behaviour of fission products and the quantification of the active materials released from a reactor core in case of failure. However, studying irradiated nuclear fuel samples poses many risks and difficulties for researchers.

This work investigated the behaviour of fission products during the start of a severe nuclear accident by using model materials that simulate irradiated nuclear fuels. The model materials were made of UO2 doped with 11 fission product surrogates and sintered at 1650°C under H2 for two hours (SIMFUEL). Initially, the SIMFUEL was constituted of a UO2 matrix, and mainly of a metallic phase, composed of Mo, Pd, Ru, Rh and Tc, and an oxide phase with perovskite structure depicted as (Ba, Sr)(Zr, U, Ln)O3 (Ln: lanthanides). The chemical composition of the samples was thus representative of irradiated fuels since the non-dissolved fission products were distributed between metallic precipitates (containing Mo alone or alloyed with Ru, Rh and Pd), similar to the white inclusions observed in PWR irradiated fuels in normal operating conditions, and oxide precipitates such as (Ba,Sr)(Zr, U, Ln)O3, similar to the oxide or perovskite phase usually found at high temperatures in fast breeder reactors.

The materials were thermally treated from 400°C up to 1000°C in oxidising atmospheres representative of the beginning of a severe PWR accident. The samples were characterised before and after the thermal treatments using scanning electron microscopy with energy dispersive X-ray microscopy (SEM-EDX) and X-ray absorption near-edge structure spectroscopy (XANES) at beamline BM16 and SOLEIL synchrotron (France). Measurements of the Mo K-edge, Ba L3-edge and Zr K-edge were carried out in order to specify the temperature range of the fission product interactions and to describe the associated mechanism as a function of temperature and oxygen potential. The spectra showed that between 900°C and 1000°C, Mo starts to oxidise to

Fig. 118: Experimental XANES spectra: (a) Mo K-edge spectra recorded on the SIMFUEL samples after several thermal treatments (T0: initial state; O400: oxidising conditions, 400°C, etc.); (b) comparison of the Mo K-edge spectrum recorded on the sample treated at 1000°C with the spectra of available Mo-bearing references (main compounds suspected to form according to EDX results); (c) experimental high-energy-resolution fluorescence detected-XANES spectrum obtained at the Ba L3-edge on the sample treated at 1000°C and compared with Ba compounds reference spectra.

X-rays unveil the behaviour of fission products during a severe nuclear accident

X-ray absorption spectroscopy and scanning electron microscopy were used to characterise synthesised nuclear fuel samples before and after subjecting them to thermal conditions representative of the beginning of a severe nuclear accident. The results deepen our understanding of the behaviour of fission products under such conditions.