Effects of Fuel Oxidation and Dissolution on Volatile Fission Product Release under Severe Accident Conditions

Abstract

The release of volatile fission products from high-burnup UO 2 fuel was examined in a steam atmosphere under severe accident conditions as a part of the VEGA program. The effects of fuel oxidation and dissolution were totally evaluated, by comparing the results with those from previous inert, hydrogen and steam atmosphere tests. It was shown that the oxidation of UO 2 to UO 2þx by steam generally enhances Cs and Kr release. However, the enhancement becomes smaller above the melting temperature of Zircaloy, about 2030 K, likely due to reduction of UO 2þx by molten Zircaloy. The burst release of Cs occurs above about 2300 K in the hydrogen atmosphere, while the release rate does not increase so significantly for the examined temperature range (<2800 K) in the steam atmosphere. Analysis of the hydrogen atmosphere test showed that fuel dissolution is apparently connected with the burst release and that a large fraction of Cs is quickly released from the dissolved fuel above 2300 K. It is considered that the fuel dissolution rate in the steam atmosphere is about 1/1000 of that in the hydrogen atmosphere.

Overview of experimental programs on core melt progression and fission product release behaviour

“…The results of the VEGA tests in steam are consistent with those results at other laboratories [39,76]. Here an increased stochiometric deviation in UO 2+x resulted in an enhanced release for the volatiles fission products.…”

Research Program for the Evaluation of Fission Product and Actinide Release Behaviour, Focusing on their Chemical Forms

“…The CORSOR-P model could better reproduce the test results under high pressure, such as in the PBF-SFD tests [28]. In addition, the data of release rates in the fuel melt and liquid formation by the high temperature reaction between fuel and zircaloy cladding for the VEGA tests were analyzed using the Booth model, the diffusion model by fuel oxidation and the fuel dissolution model [30]. This analysis result could reproduce the Cs release behaviour with the high temperature reactions between fuel and cladding material as well as fuel oxidation.…”

“…The VEGA test results were also analyzed by the CORSOR model from the viewpoint of the effects of atmospheric pressure on the FP release behaviour [28][29][30]. The CORSOR-M model was, thus, improved as the CORSOR-P model which implemented the dependence term of FP diffusion in open porosity on atmospheric pressure [28].…”

“…The release behaviour of Ru and Mo was promoted in the oxidative atmosphere, whereas those of Sr, Ba and Eu were enhanced in the reductive atmosphere. In the VEGA tests, effects of atmospheric pressure on the FP release behaviour were investigated [28][29][30]. The Cs release rate was suppressed by high pressure at a relatively low temperature condition.…”

“…The single-effect tests for the evaluation of FP and actinide release behaviour are ORNL HI/VI [24], HEVA/VERCORS [25][26][27] and VEGA [28][29][30] tests. Those tests for the evaluation of FP and actinide transport behaviour were also reviewed; such as FALCON [31], STORM [32], and WIND [33] tests.…”