Effects of fuel oxidation and dissolution on volatile fission product release under severe accident conditions

Kudo, Tamotsu; Kida, Mitsuko; Nakamura, Takehiko; Nagase, Fumihisa; Fuketa, Toyoshi

Journal of Nuclear Science and Technology, 44(11), p.1428 - 1435, 2007/11

https://doi.org/10.3327/jnst.44.1428

Release of volatile fission product from high burnup UO fuel was examined in a steam atmosphere under severe accident conditions as a part of VEGA program. 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 oxidation of UO to UO 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 by molten Zircaloy. Quick dissolution of the fuel by molten Zircaloy and consequent burst release of Cs occur above about 2300 K in a hydrogen atmosphere, while the release rate does not increase so significantly for the examined temperature range (2800 K) in the steam atmosphere. This corresponds to decrease in fuel dissolution rate by a factor of about 1/1000.

Enhancement of cesium release from fuel due to fuel oxidation and dissolution under severe accident conditions

Kudo, Tamotsu; Nakamura, Takehiko; Kida, Mitsuko; Fuketa, Toyoshi

Proceedings of Technical Meeting on Severe Accident and Accident Management (CD-ROM), 11 Pages, 2006/03

The radionuclide release from fuel under severe accident conditions was investigated in the VEGA program. In this program, the forth test VEGA-4 was performed in the steam atmosphere using a cladded fuel. The release of cesium from the fuel in the test was slightly larger than that in an inert helium atmosphere of test VEGA-1 which used the same fuel without cladding. Oxidation of UO by steam has been known as an important mechanism, which could increase the volatile fission product release considerably. In addition, considerable amount of fuel liquefaction by the interaction with the cladding was observed in test VEGA-4. The fuel dissolution by molten zircaloy could also affect the fission product release. This report summarizes the test VEGA-4, and impacts on the cesium release from fuel by the UO oxidation and the dissolution are discussed.

Analyses of radio-nuclides release and transport in VEGA-1 and -3 tests with VICTORIA2.0 code

Hidaka, Akihide*; Kudo, Tamotsu; Kida, Mitsuko; Fuketa, Toyoshi

JAERI-Research 2005-001, 67 Pages, 2005/02

JAERI-Research-2005-001.pdf:3.38MB

In the VEGA program to investigate radionuclides release from irradiated fuel during severe accidents, the analyses are being performed with VICTORIA2.0 code for comprehensive understanding of radionuclides release and transport phenomena. The VEGA-1 and -3 tests were analyzed in the present study. The correlation for Cs diffusion coefficient in fuel grain obtained from VEGA-1 was applied to the release analysis of VEGA-3. The calculated release of Cs agreed well with the measurement. The correlation was applied to subsequent Cs transport and deposition analyses. The calculation underpredicted the total mass of Cs deposited onto the test apparatuses because nucleation of aerosol and its growth were underestimated due to the consideration of aerosol nucleation originated only from released FP in VICTORIA2.0. A sensitivity analysis with aerosol seeds for heterogeneous nucleation showed a reasonable agreement with the measured Cs distribution. It turned out that additional aerosol seeds besides the released FP be considered when the VICTORIA2.0 code is applied to the VEGA test analyses.