Abstract
In July 2013, Nuclear Regulation Authority (NRA) in Japan published new regulatory requirements for the commercial light water reactors (LWR) and also prototype power generation reactors such as the sodium-cooled fast reactors (SFRs) of "Monju" in consideration of severe accidents (SAs) based on TEPCO Fukushima Daiichi nuclear power plant accident (hereinafter referred to as "1F accident") occurred in March 2011. Although the regulatory requirements for Monju will be revised by NRA with consideration for public comments, Japan Atomic Energy Agency set up "Special Committee on Monju Safety Requirements" consisting of fast breeder reactor (FBR) and safety assessment experts in order to establish original safety requirements expected for the prototype FBR "Monju" (hereinafter written as "SRs for Monju") considering SAs with knowledge from JAEA as well as scientific and technical insights from the experts. Some of the safety features of SFR results from the characteristics of coolant sodium. It does not require pressurization for power generation owing to the high heat conductivity and boiling point of coolant sodium (883 deg C at atmospheric pressure). Reactor coolant level is maintained above the reactor core by guard vessels (GVs) even if a leakage occurred. Therefore, depressurization of primary cooling system and operation of Emergency Core Cooling Systems (ECCSs) in LWRs are not required in SFRs. Liquid sodium can be used in the wide temperature range and natural circulation could be rather easily formed. In addition, multiple accident management (AM) strategies by manual operation can be applied with use of sufficient grace period (several to several ten hours) before significant core damage occurs, because temperature increase is generally gradual even under accident conditions due to large heat capacity of sodium and structures in systems for the loss of heat removal system type accidents. On the other hand, it should take into consideration that not only the possibilities of positive void reactivity and of recriticality due to fuel compaction but also high chemical activity of sodium. For establishing the safety requirements for SFRs, safety characteristics of SFRs, which are different from LWR, shall be taken into account appropriately. This paper summarizes the above mentioned SRs for Monju discussed by the committee, in order to secure the safety of people and to protect environment from accidents.
