Nuclear fuel behaviour at high temperature new insights from post-test examinations on the VERDON-1 sample
Résumé
In order to reduce the uncertainties concerning Source Term assessment under Light Water Reactor
(LWR) severe accident conditions, 4 VERDON tests were recently performed. They have been defined in the
framework of the International Source Term Program and carried out at CEA Cadarache, in the LECA-STAR
facility. Among these 4 VERDON tests, VERDON-1 test was devoted to high burn-up UO$_2$ fuel behavior and FP
release under H$_2$ reducing conditions at very high temperature.
This test was performed with the Release Circuit, which is devoted to the precise characterization of the FP release,
in terms of FP release kinetics and fission products (FPs) released fraction. The main components of the circuits are
a furnace heating the fuel sample, a filter trapping the emitted aerosols and gamma spectrometry detectors for online and post-test measurements. VERDON-1 test was conducted up to about 2600°C without reaching global fuel
relocation, allowing thereby post-test micro-analyses.
After a brief description of the fuel sample nature and the experimental set-up, we present the progress of the
experimental sequence and the associated gamma spectrometry measurements. We then focus on the major results
of the VERDON-1 tests regarding the fuel global behavior. These results are obtained by two complementary
techniques. First, the fuel sight gamma station can be used either on-line to detect fuel relocation or just after the
test, as a longitudinal bench, to get preliminary information on the global shape of the fuel sample inside the
furnace. Secondly, these information are completed by post-test examinations performed on the fuel sample after
extraction from the furnace. Complete qualitative and quantitative characterizations, including EPMA and SEM
analysis, were performed for the first time at the CEA Cadarache on a sample that has been submitted to such severe
accident conditions. These results show an advanced state of sample degradation, with locally previously molten
areas, meaning that fuel global relocation was imminent by the end of thermal-hydraulic sequence. The relocation
temperature is compared to that of previous VERCORS tests and discussed regarding the atmosphere of the test and
possible UO$_2$-ZrO$_2$ interaction.
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