Engineers strive to keep Japan's reactors under control

The plant experienced three major hydrogen-based explosions in reactors 1, 3, and then 2. Most recently there has been a fire confirmed at the site of reactor 4, which although shut down for maintenance at the time of the earthquake, houses spent fuel rods.

While radiation has leaked locally, the type of radionuclides involved, the extent of coverage and what effect this may have on human health remains unclear. 

As reported previously by The Engineer, reactors 1, 2 and 3, which were operational at the time of the earthquake on Friday, were immediately shut after sensors at the plant detected ground acceleration. Reactors 4, 5 and 6 were not operational at the time, but some housed spent fuel rods.

Although the fission reaction was stopped immediately in reactors 1–3 by inserting control rods between the fuel assemblies, fuel carries on emitting thermal heat and radioactive decay — and this is where the problems have arisen.