Calculation for the water chemistry condition in the spent fuel pool in Fukushima-Daiichi Nuclear Power Station

The evaluation for the possibility of the occurrence of the leakage of cooling water and fuel by the localized corrosion of the structural materials of the spent fuel pool (SFP) and the cladding tube of the fuel assembly in the SFP is important. To evaluate the localized corrosion of these materials experimentally, the water chemistry condition in the SFP is needed. In this study, the water radiolysis calculation was performed for the water chemistry condition in SFP. The oxygen (O₂) and nitrogen (N₂) gas are dissolved in the cooling water in SFP due to the exposure to the air. And the hydrazine (N₂H₄) injection is carried out to mitigate the O₂ concentration in SFP and suppress the microbiological corrosion. So, the secondary reactions for the dissolved N₂ and the N₂H₄ were installed into the water radiolysis calculation in this study. In order to confirm the suitability of the calculation, the comparisons of calculated results with experimentally measured results were performed for the species with long life time. Major results are as follows; 1) The set of the secondary reactions containing dissolved N₂ and N₂H₄ were prepared. The calculated results using the set relatively agreed with the measured results for the species with long life time. 2) The mitigation of dissolved O₂ by the injection of N₂H₄ under gamma-ray irradiation was successfully reproduced. 3) The calculated change in pH by the generation of NO₃^- by the water radiolysis for the water containing dissolved N₂ relatively agreed with measured results. 4) The dissolved O₂ concentration was calculated for the N₂H₄ injection condition in SFP. It was confirmed that the dissolved O₂ was enough mitigated near the fuel assemblies by the reaction of dissolved O₂ with the injected N₂H₄ enhanced by the irradiation form the fuel assemblies. (author)