Every day brings a new disaster from the Fukushima No 1 nuclear power plant. Four reactors have now experienced an explosion and/or fire and/or partial meltdown. Yesterday saw the first significant radiation release, from Reactor 4. It’s a complex situation, so I’ve tried to summarise it as it stands this morning.
Reactors 1 and 3, you’ll recall, experienced hydrogen explosions after gas that had been created, it seems, by fuel rods partially melting through their coverings and coming into contact with the water inside the reactor cores. This hydrogen, along with steam, was pumped into the outer containment building, where it exploded, ruining the buildings but, it seems, leaving the reactor cores intact. The situation with these two reactors seems to be under control. Sea water and boron have been pumped in to replace the water that had been turned to steam and dissociated. Using sea water was a desperate last ditch effort that will corrode the reactors to the point they can never be used again, which gives you some idea of the severity of the situation the operators were facing, but it seems to be working. I haven’t been able to find out what happens to the sea water after it has been used to cool the reactor – presumably it can’t be released back into the sea.
Reactor 4 had been offline for maintenance (along with reactors 5 and 6) when the earthquake and tsunami struck, so, unlike reactors 1-3, cooling off the core without power to pump the water around was no problem. But something went wrong with the storage pond where spent fuel rods are kept. It seems that the explosion at Reactor 3 may have damaged Reactor 4’s containment building, exposing the storage pond to the air. Steam was seen rising from this pond, apparently from the spent fuel rod’s heat, then there was an explosion (cause unknown) and a fire. A fire is the last thing you want around fuel rods because it can break them up and carry the debris into the atmosphere, dispersing the radioactivity.
The events at Reactor 4 sent radiation levels in the immediate vicinity to four times the level where radiation starts to affect human health. That isn’t a death sentence for the workers in the area, as long as they don’t stay there too long, but it was enough to prompt the evacuation of 750 staff leaving just 50 doing to seat water pumping operation. Elevated but safe radiation levels have been detected in Tokyo, over 200km away. Radiation levels reportedly fell soon after the initial explosion. The plan is to drop sea water from helicopters into Reactor 4’s containment building, putting out any fires and absorbing radiation from the spent fuel rods.
The one that looks like the real worry is Reactor 2. The water pressure in its suppression pool suddenly dropped after an explosion within it was heard, suggesting it has a leak. The suppression pool is (if I’ve read Wikipedia correctly) meant to take energy and radioactive elements out of the steam released from the reactor before it goes into the outer containment building. If the pool can’t do its job, the amount of radiation absorbed is less and the pressure that can be relieved is less and, instead, it builds up, potentially causing an explosion. The NHK coverage yesterday was describing the suppression pool as the ‘last line of defence’ against a radiation release in the event of a full meltdown. Reactor 2 is also having sea water pumped into it but with the suppression pool damaged, this may not work as well.
The difference between this situation and Chernobyl is the multiple layers of protection around the radioactive materials but we have seen all but one of them fail in some instance. Fortunately, none of the meltdowns seem to have been severe enough to break through the thick metal walls of the reactor core itself but given all the other failures, I won’t be breathing easy just yet.