19.2.1 SMR R&D in the 1980s and 1990s

In the present section, the Japanese status of SMR R&D is summarized for some representative conventional and advanced reactor concepts. In Japan, there was an important period for SMR R&D in the 1980s-1990s with a lot of R&D activity. Therefore, SMR R&D activities in this period are briefly overviewed in the following.

In Japan, although the first commercial reactor for electricity generation was a gas-cooled one with a small output of 166 MWe (operated in 1966-98), which was imported from England, the next commercial reactors were all light-water reactors (LWRs). The power output of the first commercial LWR in Japan was as low as 357 MWe (operation started in 1970). However, the power output was gradually increased to 600 MWe class, and then 900 MWe class, and finally over 1100 MWe, based on the ‘scale merit’ of the larger reactor. This was the same trend as that in other countries at that time, and it was because there was basically little need in Japan for small reactors for electricity generation.

After the accidents of TMI-2 in 1979 and Chernobyl Unit 4 in 1986, however, the passive safety characteristics in the reactor safety features were considered to be favourable and were used to promote the introduction of new reactors. In order to introduce the passive safety features, such as the natural circulation core cooling and the gravity feed emergency core cooling system (ECCS) effectively into the reactor design, taking into account balance with the economic aspects, the smaller size of the reactor, typically less than about 500 MWe, was considered more suitable than the larger size, such as more than 1000 MWe. This situation did not appear only in Japan, but was a worldwide trend after TMI-2 and Chernobyl Unit 4 accidents. The most typical and well-known reactor concept in this period was that named ‘PIUS (Process Inherent Ultimate Safety)’ (622 MWe/3 modules) (Hannerz et al., 1986), a Swedish integral type LWR or pressurized water reactor (PWR) design concept by ABB-ATOM. This introduced passive measures, ones not requiring operator actions or external energy supplies, to provide safe operation for the reactor shutdown and decay heat removal after a transient or accident situation. This was the integral type reactor concept without primary coolant piping preventing the LB LOCA (large — break loss-of-coolant accident). That is a kind of iPWR (integral PWR) concept. Based on and extending this concept, three PWR type concepts were developed in Japan. The ISER (inherently safe and economical reactor) (210 MWe) concept was developed by the University of Tokyo and others (Oda et al., 1986), introducing a steel reactor vessel instead of the pre-stressed concrete reactor vessel (PCRV). The, MISIR (Mitsubishi intrinsic safe integrated reactor) (300 MWe) concept was proposed (Kudo et al., 1987), and the SPWR (system-integrated PWR) (350 MWe) concept was developed by JAERI (Japan Atomic Energy Institute) (Sako, 1988). The MRX (Marine Reactor X) (30 MWe) was developed by JAERI for marine propulsion or local energy supply as an integral type PWR with passive safety features and a submerged reactor vessel (Kusunoki et al., 2000).

Extending the conventional loop type PWR concept, the MS-600 (600 MWe) was proposed by MHI (Mitsubishi Heavy Industries) (Makihara et al., 1991), based on the AP (Advanced PWR)-600 (600 MWe) developed by Westinghouse, which introduced the passive ECCS and primary containment cooling system (PCCS). Also, extending the conventional loop type boiling water reactor (BWR) concept, HSBWR — 600 (600 MWe) by Hitachi (Kataoka et al., 1988) and TOSBWR-900P (310 MWe) by Toshiba (Nagasaka et al., 1990) were proposed. They were based on the SBWR (simplified BWR) (600 MWe) concept developed by GE (General Electric), which introduced the natural circulation core cooling and the gravity-driven ECCS.

Although many SMR concepts were developed in Japan up to the 1990s as briefly described above, none of them could be realized or developed further. This was mainly because they could not overcome the ‘scale demerit’ in the economic aspect, and hence, were not attractive to the users, i. e. the electric companies. On the other hand, the large LWRs of over 1000 MWe were continuously improved, including the safety features based on their operational experiences, especially under the national projects of the ‘improvement standardization programs for LWRs’. In this way, up to around 1985, the development of ABWR (advanced BWR) and APWR (advanced PWR), which are the ‘third generation’ reactors, were already finished and they were ready to be introduced after the Chernobyl Unit 4 accident. In reality, after the construction initiation in 1991, the first ABWR plant in the world with the power output of 1356 MWe was in operation as the Kashiwazaki-Kariwa Unit 6 in 1996.