Fukushima: What happened?

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
Unit 4 was shut down at the time of the accident and a pool in the building was used to store spent fuel rods – of which more later. A key feature worthy of note is that  these units were more than 35 years old, and nearly at the end of their operating life.
The reactor structure was similar for each reactor. The inner pressure vessel was contained within a ‘pear-shaped’ dry containment vessel, surrounded by a torus-shaped so-called ‘wet well’. So, yes, everything was already ‘pear-shaped’ even before the accident 😉
The structure of each reactor was similar

The structure of each reactor was similar

The structure of each reactor.

The structure of each reactor. Click to enlarge.

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.

Cooling the reactor core by discharging steam to the wet well lowers the water level in the core.

Cooling the reactor core by discharging steam to the wet well lowers the water level in the core. Click to enlarge.

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.
The progressive exposure of the core

The progressive exposure of the core is the price paid for reducing pressure and temperature in the reactor. In Reactor 1 temperatures eventually exceeded 2700 C causing a partial meltdown. The reaction of the zirconium with water released hydrogen gas into the steam.

At this point the containment is successfully containing
  • steam,
  • 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
However the pressure is now over 8 atmospheres in a containment with a design pressure of 4 bar. Operators decide to release the gas and so lower the pressure. They understand that this will (a) release relatively short-lived isotopes into the atmosphere and (b) will probably result in an explosion as the hydrogen mixes with air. However, they really have no options left. And the core remains contained with very little release of the very long-lived radioactive elements in the core.
The inevitable progression towards an explosion in the superstructure of the reactor.

The inevitable progression towards an explosion in the superstructure of the reactor. Click for a larger version.

In reactor 1 the pressure release occurred at 4:00 on 12th March and the hydrogen explosion followed shortly thereafter. The superstructure of the reactor building was blown apart, but there was no damage to critical containment systems. Eventually the entire system was cooled by flooding with seawater.
Eventually - and currently - the entire containment was flooded with sea water coolant.

Eventually - and currently - the entire containment was flooded with sea water coolant.

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’

In Reactor 2 a hydrogen explosion damaged the containment.

In Reactor 2 a hydrogen explosion damaged the containment. The reasons for this are not clear, and the consequences are not clear either.

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.

The drying out of the spent storage pool above Reactor 4.

The drying out of the spent storage pool above Reactor 4. This led to direct exposure of volatile fission products to the atmosphere - possibly more serious than the reactor accidents.

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4 Responses to “Fukushima: What happened?”

  1. Fukushima: What happened? – Ask a scientist Says:

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  2. Nestor Patrikios Says:

    Nice summary Michael, thank you.

  3. askaboutfukushimanow Says:

    Michael….are you on Facebook? We’re 10 months into the global nuclear meltdown. I have a new blog…just starting up… many resources within…please check it out and find some walls that you resonate with and STRENGTHEN THEIR HANDS. Some suggest the only solution now is to either bring the sea to the reactors, or get the reactors into the sea. Your blog says you are a physicist…. you have thoughts on that?? A good email for me is: FukushimaOnFacebook@gmail.com Thanks. Linda Ayres xoxooxoox

  4. Tsunami sinks UK nuclear revival « Protons for Breakfast Blog Says:

    […] year on, the consequences of the meltdown at the Fukushima nuclear plant are becoming clearer. Aside from the distress to the displaced people who used to live around the […]

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