The Japan Atomic Energy Agency has chosen nitride fuel as the first candidate for the transmutation of long-lived minor actinides (MA) using accelerator-driven systems (ADS). The pyrochemical method has been considered for reprocessing spent MA nitride fuels, because their decay heat should be very large for aqueous reprocessing. This study was conducted to investigate the effect of decay heat on the pyrochemical reprocessing of MA nitride fuels. On the basis of the estimated decay heats and the temperature limits of the materials that are to be handled in pyrochemical reprocessing, quantities adequate for handling in argon gas atmosphere were evaluated. From these considerations, we proposed that an electrorefiner with a diameter of 26 cm comprising 12 cadmium (Cd) cathodes with a diameter of 4 cm is suitable. On the basis of the size of the electrorefiner, the number necessary to reprocess spent MA fuels from 1 ADS in 200 days was evaluated to be 25. Furthermore, the amount of Cd–actinides (An) alloy to produce An nitrides by the nitridation–distillation combined reaction process was proposed to be about one-quarter that of Cd–An cathode material. The evaluated sizes and required numbers of equipment support the feasibility of pyrochemical reprocessing for MA nitride fuels.

Regarding the root cause analysis of reactor core melt accidents at the TEPCO Fukushima Daiichi nuclear power plant, the author has already published a paper ¹⁾ on a new approach based on his original viewpoints. In this current study, he verified the new approach called the “film boiling approach” as shown below. On the basis of this approach, film boiling (with steam) continued until the amount of decay heat decreased one or two weeks after initiating dry out or Zr-H₂O reactions in the core. Subsequently, the bottom wall of the reactor pressure vessel (RPV) melted and the melted core dropped (melt-through) into the containment vessel. On the other hand, on the basis of the conventional approach, melt-through occurred for a short time after the core melted. That is, using the film boiling approach, melt-through was predicted to occur on 24 March for Unit 1 and 21 March for Unit 3, but it occurred on 12 March for Unit 1 and 14 March for Unit 3 in accordance with the conventional approach. Recently, it has been found by a robot camera in the containment vessel of Unit 1 that concrete disappeared and naked rebars appeared in the wall around the open area of the pedestal under the RPV. From this finding, the test data of “the heated concrete was washed away with water” was verified by that in the core of Unit 1; that is, there was much water on 24 March, although there was no water on 12 March. Moreover, it has been found that the radioactivity level of debris broken into pieces by hydrogen explosion is very low around the turbine building but is very high in the reactor building (RB) in Units 1 and 3. From this finding, the level around the RB was verified to be low as melt-trough had not yet occurred at the moment of the explosion (12 March in Unit 1 and 14 March in Unit 3), but the debris in the RB was subsequently activated by radio activities after melt-through.