It’s been 6 weeks since the Fukushima nuclear accident, and news reports of the incident vary from the facile
to the mundanely exagerated
However, I have yet to read a clear web account of what has actually happened! The Wikipedia timeline somehow confuses rather than elucidates. A couple of weeks back I was lucky enough to receive a PowerPoint presentation by Mattias Braun from Areva. Using this and other sources I have attempted to summarise what actually happened.
The Fukushima Daiichi Site #1 consisted of 4 boiling water reactors:
- Unit 1 – GE Mark I BWR (439 MW), Operating since 1971
- Units 2-4 – GE Mark I BWR (760 MW), Operating since 1974
11th March 2011: 14:46 The Earthquake struck causing an immediate ‘scram’ and all three operating reactors shut down safely. As I have described previously, this reduced the heat generated in the core from a breathtaking 1500 MW to something close to 100 MW – still a great deal of heat. Diesel generators started to provide power to circulate water to keep the cores cool. At this point the plant was stable and the operators were perhaps feeling relieved that things had gone at least roughly according to plan.
11th March 2011: 15:41 Now the tsunami arrives. With a height of 14 metres, it overtopped the defences designed for a 7 metre tsunami. The entire plant was disconnected from mains electricity, and the diesel generators and their fuel supplies were destroyed. The cooling system continued to cool the core powered by batteries and the operators began to cope with what had happened.
Reactor#1 At 16:36 – barely two hours after shutdown – the batteries failed. The operators now had only one core-cooling option. This was to relieve the excess pressure in the reactor vessel by discharging steam into the ‘wet well’ where it can condense as long as the temperature is below 100 °C. This reduces the temperature and pressure in the reactor vessel and provides some cooling, but inevitably lowers the level of water in the reactor vessel, eventually exposing the core material.
The liquid water in the core is now a boiling mass, and the foam provides some cooling to all the core, so even at 50% exposure of the core material, the core is safe. However further loss of coolant is critical:
- When the water level falls to 33%, the temperature of the central part of the core exceeds 900 °C, and the zirconium cladding that surrounds each fuel element swells and breaks releasing volatile fission products into the cooling water.
- When the water level falls to 25%, the temperature of the central part of the core exceeds 1200 °C, and the zirconium cladding begins to burn in the steam creating zirconium oxide, and hydrogen gas. In unit 1 it is believed that more than 300 kg of hydrogen gas was created.
- The core is believed to have been exposed for 27 hours and the temperature is believed to have eventually risen to an astonishing 2700 °C, at which point the uranium and zirconium would form a molten blob in the centre of the core.
- hydrogen gas caused by the dissociation of water in reaction with the zirconium.
- volatile elements from the core – most notably the fission products, Caesium and Iodine
Reactor#2 The story for reactor 2 is similar to that for reactor 1, but the battery system operated until 13th March at 2:44. Thereafter the progression to an explosion was inevitable. However in this case the core was only left exposed for around 7 hours so the temperature probably ‘only’ reached 2500 °C, not quite enough to cause melting of the uranium-zirconium mixture. However, in this case, the hydrogen explosion at least partly took place within the containment vessel, fracturing part of the ‘wet well’
Reactor#3 The story for reactor 3 is similar to that for reactor 1, but the battery system operated until a pump failure at 14th March at 13:25. Thereafter the progression to an explosion was similar to reactor 1. However in this case the core was only left exposed for around 7 hours so the temperature probably ‘only’ reached 1800 °C.
Reactor#4 was closed down, and the storage pool was used to store spent fuel. Without additional cooling, the water began to evaporate and eventually the used fuel rods were completely exposed allowing volatile fission products to reach the atmosphere – possibly a more serious event than the reactor accidents.