Abstract
Estimation of the characteristics and distribution of fuel debris in the Fukushima Daiichi Nuclear Power Station Unit 2 is of growing importance for safe decommissioning for the first retrieval. In this study, we focused boron included in the control rod as neutron absorber, and estimated the amounts of boron evaporation during the early phase of core degradation (between 18:50 to 22:50 in 14th March in 2011) and chemical forms of boron accompanied with fuel debris. First of all, possible chemical reactions under accident condition were investigated based on the review of literature and research report regarding the control rod degradation. Secondly, the influential reaction during degradation of control rod was chosen based on thermal hydraulic condition evaluated with the plant parameters. Finally, it was estimated that approximately 20% (from central region of core) and 55% (from peripheral region of it) of initial boron inventory could have been released to gas phase by the timing of core support plate failure. Chemical forms of boron accompanied with fuel debris were estimated as the metallic boride ((Fe, Cr)Bx) or the ferric borate oxide (FeBxOy).
