The ongoing nuclear emergency in Japan highlights a design flaw in Boiling Water Reactors (BWRs). Most Light Water Reactors (LWRs), such as the BWRs, use zirconium alloy cladding to encase the nuclear fuel, and prevent fission products mixing with the cooling water. There are basically three requirements for the cladding in an LWR: (i) it must have a low thermal neutron absorption cross-section; (ii) it must be resistant to corrosion in water; and (iii) it must have a melt/ignition temperature greater than the operating temperature of the reactor. The temperature is designed to be as high as possible to maximise the thermal efficiency of the heat engine which generates electricity from a nuclear reactor.
Now, what Fukushima has revealed is that when a complete power failure stops the flow of cooling water in a BWR, the water turns to steam, and oxidises the zirconium fuel cladding, releasing the hydrogen component of the H2O coolant. The hydrogen is then able to escape from the reactor, whence it almost inevitably causes a detonation or deflagration in the presence of atmospheric oxygen. These are the explosions suffered by Fukushima 1 on Saturday and Fukushima 3 on Monday.
Whilst these hydrogen explosions have not damaged the reactor pressure vessels themselves, it is unwise to suggest, as Malcolm Grimston, from the Energy Policy and Management Group at Imperial College, did after Saturday's detonation, that "The explosion... wasn't a terribly important event." On the contrary, such explosions are clearly highly disruptive and destructive to the efforts being made to feed cooling water into the reactor vessels and prevent a meltdown. Reports today suggest that the explosion in Fukushima 3 has damaged four of the five available pumps, and all three reactors now seem to have suffered at least a partial meltdown.
Are there alternatives to using zirconium cladding? Well, in the Magnox reactors used in the UK for some decades, the nuclear fuel was clad in magnesium oxide, which has a low neutron absorption cross-section, and which by virtue of already being an oxide, is incapable of undergoing an oxidation reaction with the carbon dioxide cooling gas used in these reactor designs. Unfortunately, magnesium oxide corrodes in water, so cannot be used in light water reactors.
There is, however, another alternative: stainless steel cladding was used in the UK's Advanced Gas-Cooled Reactors (AGRs), because the latter operate at higher temperatures for greater thermal efficiency, and magnesium oxide would become soft and potentially flammable in such circumstances. Stainless steel cladding would be resistant to oxidation in the presence of steam, but possesses a larger neutron absorption cross-section. To compensate for this the nuclear fuel would need to be subjected to further enrichment, which costs money, power and time, possibly making the whole thing uneconomical.
The nuclear industry might well suggest that the Japanese crisis has been triggered by unique tectonic circumstances irrelevant in Europe, that only a sizeable earthquake and tsunami could cause the simultaneous joint failure of the national grid and back-up diesel generators. Disturbingly, however, one can also imagine a terrorist attack which achieves exactly the same result.