The Fukushima Daiichi nuclear accident and their 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. The above mentioned 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 black out (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 after 10 days.
The hydrogen explosion prevention system consists of a high-temperature resistant fuel cladding made of silicon-carbide (SiC cladding) and a passive autocatalytic recombiner (PAR). Since the SiC cladding generates less hydrogen gas than the current zircaloy fuel cladding when core damage occurs, the risk of hydrogen leakage from a primary containment vessel (PCV) to a reactor building (R/B), such as an operating floor, can be reduced because the pressure in the PCV can be kept lower with less hydrogen gas generation. 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; the heat exchange tests conducted for both water-cooling and air-cooling systems; the air-cooling enhancing technology for air-cooling in a 4700 MW thermal power class reactor; hydrogen generation tests for SiC material; and the concept of the operation support system.