Cross-section of a typical BWR Mark I containment as used in units 1 to 5. RPV: reactor pressure vessel DW: drywell enclosing reactor pressure vessel WW: wetwell – torus-shaped all around the base enclosing steam suppression pool. Excess steam from the drywell enters the wetwell water pool via downcomer pipes. SFP: spent fuel pool area SCSW: secondary concrete shield wall The Fukushima nuclear acc