ANALYSIS OF ACCIDENT PROGRESSION WITH THE SAMPSON CODE IN THE FUKUSHIMA DAI-ICHI NUCLEAR POWER PLANT CONSIDERING DEBRIS FLOW PATHS FROM THE CORE

摘要

The Great East Japan Earthquake and Tsunami on March 11, 2011 mark the start of the nuclear accident at the Fukushima Dai-ichi (1F) Nuclear Power Plant. Assessment of the core status and the location of core debris is very important for smooth execution of the 1F decommissioning project. In this paper, debris flow paths from the core to the lower plenum were first investigated by analysis of the XR2-1 experiment using molten core relocation analysis module of SAMPSON. The investigation showed that molten material in the control rod area was likely to accumulate on the velocity limiter. Furthermore a part of the molten metal in the fuel rod area fell directly through the inlet orifice to the lower plenum. MCRA noding for the system calculation was modified based on these observations and analysis of accident progression in 1F Unit 1 was performed. More than 96 wt% of the core debris relocated to the lower plenum was particle debris in the modified calculation. The fuel/metal molten phase that appeared in the previous calculation in which core debris was retained at the core plate location did not appear in the modified calculation. Since core debris especially UO_2 mainly fell from the inlet orifice to the lower plenum, thermal interaction between core debris and structures in the lower plenum would occur outside of the control rod guide tubes, etc. It is recommended that BWR-specific experiments and more detailed analysis be undertaken to resolve the uncertainties of the RPV bottom failure.