For latest updates see Nuclear Energy Insitute website (includes schematic of BWR at Fukushima)
For latest press release from Fukushima plant operators see TEPCO English Website
Or see NISA - Nuclear and Industrial Safety Agency of Japan
It is reactor 4 that is causing the greatest concern. It seems fuel rods stored in cooling ponds (during the shutdown period) may have been exposed to air. It is these cooling ponds that the helicopters and fire appliances are trying to refill and/or keep cool.
If this is not achieved in the worst case senario harmful radiation could be released into the atmosphere as the ponds are not within the containment building of the reactor, other than within the second containment building which was damaged by the explosions.
For the proposed European Pressurised Reactor (EPR) one of the designs being considered for Wylfa B, according to a Nuclear Directorate report – Step 3 Reactor Chemistry assessment of the EDF and AREVA UK Division 6 assessment report No AR09/036:
For more information about the UK EPR see Areva EDF website.
The other reactor design being considered for Wylfa B is the Westinghouse AP1000, this design also has a In-Containment Refuelling Water Storage Tank. See HSE Report Step 2 Fault Analysis Assessment of the Westinghouse Submission for the AP1000:
"The AP1000 provides for in-vessel retention with features that promote external cooling of the reactor vessel:
- The reliable multi-stage reactor coolant system depressurization system results in low stresses on the vessel wall after the pressure is reduced.
- The vessel lower head has no vessel penetrations to provide a failure mode for the vessel other than creep failure of the wall itself.
- The floodable reactor cavity can submerge the vessel above the coolant loop elevation with water intentionally drained from the in-containment refuelling water storage tank. "
Udpated 21:40 Additional information added.