Engineers are still trying to bring the situation at the Fukushima Daiichi nuclear plant under control after Friday’s earthquake and resulting tsunami crippled several back-up measures leading to a system-wide cooling problem.
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.
Multiple back-up systems intended to cool the reactors failed after extensive damage from the tsunami, which swept away the power lines and all external power to the plant.
Diesel generators were able to provide power back-up for the cooling system for around an hour, but ultimately failed due to prior damage. Reactor Core Isolation Cooling (RCIC) pumps were also used but the battery-supplied control valves lost DC power after the prolonged use.
Engineers were forced to inject seawater into the reactor pressure vessel to continue the cooling process. As a consequence of efforts to cool the reactors, pressure built up inside the vessels, which had to be vented causing the explosions, although Prof Mike Reeks, from the School of Mechanical and Systems Engineering at Newcastle University, said this was also part of documented procedure.
‘This venting is controlled in the sense that they know they have to release some pressure from the reactor and what people haven’t pointed out is that they vent through filters, so that takes a lot of the radioactive particles and also some of the fission product gases,’ he said.
Paying tribute to the engineers at the Fukushima Daiichi plant, Reeks added: ‘This is what is known as a station blackout; it’s something that people studying severe accidents look at. They’re doing a great job under very unfortunate circumstances… they’ve followed the right procedures.’
Several countries, notably Germany, have made announcements to review their nuclear plants.
In the UK, energy secretary Chris Huhne has asked chief nuclear inspector Dr Mike Weightman for a thorough report on the implications of the situation in Japan and lessons to be learned from it.
Prof Neil Hyatt of the Department of Materials Science and Engineering at Sheffield University said: ‘The current Japanese nuclear emergency should not cast a shadow over building new nuclear stations in the UK.
‘The proposed new fleet of reactors do not require a diesel generator to keep coolant flowing through the core, which failed in the Fukushima reactors.’