Development of a fuel cladding or a channel box applying silicon carbide (SiC), which has high accident tolerance, in place of zircaloy (Zry) or Steel Use Stainless (SUS) composing current light water reactors, has being proceeded with after the accident of Fukushima Daiichi Nuclear Power Plant (1F). When applying SiC to core structures of a nuclear power plant such as fuel cladding, it is expected that the difference of high temperature oxidation characteristics in the severe accident (SA) conditions would mitigate progression of core damage comparing with the current Zry fuel core. This study performed SA analyses considering high temperature chemical reaction characteristics of SiC by using SA analysis code “MAAP”, and thermal hydraulics analysis code “TRAC Toshiba version (TRAC)”, and compared the difference between SiC and Zry. Both codes originally have no model of oxidation reaction for SiC. Hence, a new model for SiC in addition to the current model for Zry was incorporated into “MAAP”. On the other hand, “TRAC” adjusted reaction rate by changing oxidation reaction coefficients in the current Zry oxidation reaction models such as Baker-Just and Cathcart correlations in order to simulate SiC-water/steam reaction. In analysis using “MAAP”, seven accident sequences from representative Probabilistic Risk Assessment ones were selected to evaluate the difference of SA behavior between two materials. As a result, in the case of replacing current Zry of fuel claddings and channel boxes into SiC, an amount of hydrogen generation reduced to about 1/6 than the case of Zry. In addition to that, in the case of replacing SUS structures in the reactor core into SiC, an amount of hydrogen generation moreover reduced to about 1/6 than the above result, which means just about 2% of an amount in the original case. On the other hand, in analysis using “TRAC”, the accident sequence for unit 3 of 1F (1F3) was selected, and reaction rate in the oxidation reaction model was examined as parameter. In the case of 1.0 time of the reaction rate, which means an original reaction rate, maximum fuel cladding temperature exceeded 2000K in 50 hour after reactor scram. However, using the reaction rate below 0.01 to the original one, the fuel cladding temperature didn’t exceed 1,600K.

There is movement which is developing the small reactor for the small electricity grid in place of a big power reactor which requires the high capital cost. This paper introduces a small power reactor whose purpose is to achieve high economic competitiveness and advanced safety. In order to attain high economic competitiveness, it is designed to be small and simple and uses natural circulation and high pressure. A steam generator is integrated into the reactor pressure vessel (RPV), thus dispensing with a primary system and preventing radiation leakage from the reactor core. The small core is designed to have a high power density (100 MW/m3, almost twice that of a conventional boiling water reactor). The concept of a 300 MWt (100 MWe) core design is established by introducing a boiling heat transfer system. By boiling cooling water, the cooling-water circulating flow quantity in a reactor core is enlarged. By increasing a flow, the minimum critical power ratio is improved, which is an important core characteristic. Furthermore, using a burnable poison (Gd2O3), the excess reactivity of a reactor core is reduced and excess reactivity is controlled only by the control rod. Moreover, the maximum linear power density is improved and the critical power ratio is minimized by optimizing the burnable poison arrangement and the control rod pattern. In order to attain high safety, our small reactor has an advanced decay heat removal system that can cool the core without external support. This decay heat removal system is part of the secondary cooling system and combined with a cooling tower. As a result, the quantity of cooling water stored in the decay heat removal system is reduced, and longtime decay heat removal is possible by small equipment.

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