News coming out now is that another reactor core in the same complex has triggered an emergency alert, with increasing temperatures raising concerns of another impending problem. The reactors in this case are Light Water Reactors, which use a thermal process - controlled by the presence of water. The earthquake on Friday caused the safety systems which control the flow of water to be rendered inactive, thus removing the coolant water regulation, and hence the increase of temperature to the point where the water begins to boil. When the water level drops, exposing the fuel to air, they melt (thus producing the "melt down" phenomenon). After a now incredibly high temperature disintegrating core interacts with whatever coolant remains... this can potentially cause a huge steam explosion that can eject large amounts of radioactive material into the environment.
That's the worst case scenario. The Japanese government has provided assurance that there is no real threat, however, there are already reports of three of the people working on the damaged reactor having been exposed to radioactive substances (related to the partial meltdown). The extent of this has not been identified, but is suggested as being of a low level. There is however a history with slow responses when it comes to nuclear accidents... with incidents in Tokaimura (in Inbaraki Prefecture) in 1997 and a significant incident in 1999. The current accident is of the same order of severity as the second incident - a level 4 (out of 7) emergency.
The current plan is to fill the reactor cores with sea-water (the only large quantity of water available) - though estimates are that this process will take up to 10 days, and will also effectively mean the reactor will need to be shut down permanently.
What seems a problem to me is the rate at which the now introduced seawater heats up, as it will be unlikely to be regulated in any significant way. Unless the heated seawater is purged with cooler water. This has the problem of where the now contaminated seawater will be stored (or released). There are some very tricky decisions to be made, but they have now essentially committed the reactor to a "rapid as possible" and permanent shutdown.
Despite the severity of the action, it is somewhat incongruous to me that the Australian foreign minister (Kevin Rudd) has apparently all but demanded a full briefing on the threat posed by the reactor from his Japanese counterpart. To put this into some context, whilst Australia has the largest single deposit of uranium in the world, we do not have nuclear power stations (and only one experimental "scientific and medical" reactor). The question of nuclear power has been a long divisive one here in Australia over many decades - but the tone used by Mr Rudd beggars belief in the context of the on-going tragedy. I do not want make my blog political at all, but Australia's response concentrating on the state of the reactors seems disproportionate to the very real threat that has been manifest in Japan due to the earthquake and tsunami.
The Japanese people certainly are experiencing calamity on a number of different scales - and whilst the quakes and the tsunami threat is for the moment subsiding, there remains a very real threat. Not least of all from the condition of the reactors.
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