Applicability test of plasma cutting technology for dismantling of the core internals of Fukushima Daiichi NPS

It was reported that Fukushima Daiichi Nuclear Power Station (1F) had lost the cooling function of the reactor by the Tohoku Earthquake. It is assumed that the core internals became narrow and complicated debris structure mixed with the molten fuel. In consideration of the above situations, the plasma arc cutting method, which can be used in the water, has been developed for the removal of the molten core internals through cutting tests for 3 years since FY 2012. And it was confirmed that the plasma arc method is useful for the removal of the core internals etc. The results in FY 2012 were reported in 'R and D of the fuel debris removal technologies by plasma arc cutting technology (JAEA-Technology 2013-040)' and this report summarizes the results of FY 2012, 2013 and 2014. The effects of difference of the cutting conditions, improvement of the cutting performance and applicability of methods for the composite material cutting were confirmed in the tests of the FY 2013 and 2014 based on the results of FY 2012. At first, we conducted some basic tests by using the specimen of the core internals with the molten metal. And it was confirmed that the suppression of dross is possible by optimization of cutting conditions. The cutting performance improvement method by heating the cutting objects in advance was developed for the removal of the fuel debris, which is assumed to exist on the bottom of the reactor pressure vessel, with unknown thickness. Moreover it was confirmed that the mixed material with the fuel debris and the molten metal can be cut by the combination method by the plasma arc and the plasma jet that can cut the non conductive material. According to these results, we confirmed that plasma cutting method can be applied for the removal of the fuel debris and the molten core internals. (author)