It is clear that the situation at Fukushima is grave. On Tuesday (15th March) the French ASN nuclear safety authority rated the accident on the IAEA’s International Nuclear and Radiological Event scale (INES - pictured below) at level six. Six is a ‘serious accident’ and the highest level is seven or ‘major accident’ – a level used only once, in the Chernobyl accident in 1986.
Up until Tuesday it seemed that the containment systems were largely intact despite some damaging explosions at the Fukushima site and the enormous problems of managing to pump enough water into the reactors. Heat is generated by nuclear reactions in the fuel for some considerable time after shutdown has begun. Even in normal circumstances, forced cooling is necessary for several weeks – the reactions will run their course whether the fuel is physically hot or not. Nuclear fuel is fundamentally different in this way from chemical fuel, because chemical fuel ignition depends on heat and pressure and can therefore be ‘extinguished’. Even so, refinery fires can take days to bring under control as the incident at the UK’s Buncefield Oil Storage Depot demonstrated in 2005. The risk of a thermal explosion will remain until the nuclear reaction rate falls off and with that comes the potential loss of containment.
Following Tuesday’s explosion at Reactor 2 it seems possible that a crack may have developed in one part of the containment system. Local levels of radiation are high – at one location and at one time they reached 400 millisieverts (mSv) per hour, where an average annual dose is just 2.5 millisieverts in total accumulated from medical Xrays and background radiation. But local levels have dropped since that measurement was taken and it seems likely that there was a spike in radiation levels caused by a fire in the spent fuel storage ponds at Reactor 4.
It remains very unclear how much radiation has escaped, or will still escape, into the environment. Japan has so many problems to contend with in the aftermath of Friday’s earthquake and certainly there is no wish to spread greater anxiety amongst a population still deeply in shock. The events of the last few days have involved a combination of circumstances even the most earthquake-prepared nation could not have prepared for but it is inevitable that the unfolding events in Japan will provoke a worldwide debate about the wisdom of investing in nuclear energy. However, the immediate challenges faced by Japan are likely to be dominated by the humanitarian crisis and devastation caused by the tsunami rather than the serious issue of nuclear integrity.
To read more about the implications from an engineering point of view, read this article on the student website, Platform.