Plant system and required water pool capacity for large scale BWRs with inherently safe technologies

The Fukushima Daiichi Nuclear Power Plant accident and its consequences have led to some rethinking about the safety technologies used in boiling water reactors (BWRs). We have been developing the following various safe technologies: a passive water-cooling system, an infinite-time air-cooling system, a hydrogen explosion prevention system, and an operation support system to better deal with reactor accidents. These technologies are referred to as “inherently safe technologies”. The passive water-cooling system and infinite-time air-cooling system achieve core cooling without electricity. These systems are intended to cope with a long-term station blackout (SBO), such as that which occurred at the Fukushima facility. Both these cooling systems remove relatively high decay heat for the initial 10 days after an accident, and then the infinite-time air-cooling system is used alone to remove attenuated decay heat. The hydrogen explosion prevention system consists of a high-temperature resistant fuel cladding made of silicon-carbide (SiC) and a passive autocatalytic recombiner (PAR). The SiC cladding generates less hydrogen gas than the currently used zircaloy fuel cladding when core damage occurs, and the leaked hydrogen gas is recombined by the PAR. When a large-scale natural disaster occurs, fast event diagnosis and recognition of damaged equipment are necessary. Therefore, the operation support system consists of event identification and progress prediction functions to reduce the occurrence of false recognitions and human errors. This paper describes the following items: the targeted plant system; evaluation results on the required water pool capacity for 10-day water-cooling; development items for the water- and the air-cooling systems, the hydrogen explosion prevention system and the operation support system.