The Fukushima Daiichi reactors are six boiling water reactors (BWR) of an early 1960s design supplied by GE, Toshiba, and Hitachi with power ranges from 460 MWe to 1,100 MWe. They came into commercial operation between 1971 and 1975. Four units (1-4) are of Mark-I type containment, which is the first generation of BWR design (Fig. 24.1). At the time of the accident, the units and central storage facility contained the fuel assemblies as shown in Table 24.1.

Spent fuel —————

pool

Reactor service — floor

Concrete reactor building

Reactor pressure vessel

Primary containment drywell

Suppression pond wetwell