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Quantities of I-131 and Cs-137 in accumulated water in the basements of reactor buildings in process of core cooling at Fukushima Daiichi Nuclear Power Plants accident and its influence on late phase source terms

Hidaka, Akihide   ; Ishikawa, Jun 

During the Fukushima Daiichi Nuclear Power Plants accident, large amount of contaminated water was accumulated in the basements of the reactor buildings at Units 1 to 4. The present study estimated the quantities of I-131 and Cs-137 in the water as of late March based on the press-opened data by NISA. The estimated ratios of I-131 and Cs-137 quantities to the core inventories are 0.51%, 0.85% at Unit 1, 74% and 38% at Unit 2, 26% and 18% at Unit 3, respectively. According to the Henry's law, certain fraction of radionuclides in the water could be released to atmosphere due to the gas-liquid partition and contribute to increase in the source terms. Iodine-131 source term evaluation has been performed so far by the MELCOR calculation or the SPEEDI reverse estimation. The SPEEDI reverse predicted significant release until around March 26 while no prediction in MELCOR. A simplified model study showed that the release from accumulated water may explain the release after Mar.17. This strongly suggests a need for improvement of the current MELCOR approach which treats the release only from breaks of drywell or S/C for several days after the core melt.

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Category:Nuclear Science & Technology

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