抄録
At Fukushima Daiichi Nuclear Power Plant Unit-2 (1F2), three reactor pressure peaks occurred after the forced depressurization by SRV opening at around 18:00 on March 14, 2011. Based on previous investigations, it is estimated that during the first pressure peak from around 20:00 to 21:20 on March 14, the core temperature increased and hydrogen was generated by zirconium-water reaction.
To estimate the chemical form of the released radioactive materials from the fuel and the core structure damage behavior, the temperature and the hydrogen/water vapor ratio in the core during this period was studied. The factors that affect the temperature and the hydrogen/water vapor ratio in the core were modeled. Based on this model, the distributions of temperature and hydrogen/water vapor ratio in the core were evaluated between the end of forced depressurization at 18:40 and the start of the second pressure peak at 22:40 on March 14th.
Based on the evaluation results, it was found that around the first pressure peak, zirconium-water reaction progressed mainly in the lower part of the core, which led to depletion of water vapor. On the other hand, the upper part of the core tended to become a reduced atmosphere of almost exclusively hydrogen, which condition is consistent with that for the formation of insoluble Cs particles.
Based on above discussion, new hypotheses regarding accident progression scenarios according to the observed events were proposed, such as the possibility that a leakage and blockage of the in-core instrumentation dry tubes occurred around 20:00 on the 14th, and that the neutron sources such as Cm were released from the reactor vessel to D/W through this leakage path during this period.
