Fukushima Reactor Review

Chris -   To bring us up to date with events at the Fukushima Nuclear Power Plant, and explain how his own work fits in with this week's Nuclear Science Question and Answer theme, we're joined by Dr Ian Farnan from the Department of Earth Sciences at Cambridge University.

Ian -   Hello, Chris.

Chris -   Just to let people understand about what you do first.  Could you just give us a quick round up on what you work on?

Ian -   My main research is on the disposal of nuclear waste and in particular I'm going to head up a research consortium funded by the nuclear decommissioning authority on disposing of spent nuclear fuel.  The way that radioactivity leaks from spent nuclear fuel eventually is by its interaction with water. What's happening in Fukushima is the interaction of water with fresh fuel.

Chris -   When the tsunami struck it knocked out the backup generators that they had at the plant, which were there to pump water through the core, and disabled those generators.  What then unfolded?  What was the chain of events - if you excuse the radiation type pun - which then ensued?

Ian -   Well there was a little bit of extra leeway, they had some batteries which ran for a little while, for about 8 hours, and then they just ran out.  At that point they've got no way of pumping the water through the reactor to keep it cool. So the water in the reactor starts to boil and eventually, it comes out to what's called a pressure regulator which is below the reactor in a large pit, which is within the containment and can contain an enormous amount of water.  The dramatic thing that you saw on the TV was the problem that there must have been some interaction with the zircaloy which clads the fuel which started to get oxidised at high temperatures.  The fuel heated upand that produced some hydrogen.  So there was a mixture of hydrogen gas in this big pit below the reactor.  At some point, the pressure was getting too high and the operators realised that in order to preserve the integrity of the reactor pressure vessel, they needed to vent that pit.  When they did that, the hydrogen came out and it obviously encountered some oxygen and there was an explosion, and that's what you saw on TV.  Basically, there's a weak roof on the top of those buildings with some blowout panels, and that just blew out, a very dramatic event.

Chris -   But subsequent to that, what was then the threat, the fact that you had no way of cooling a nuclear core that was still producing quite a bit of heat?  I mean, the figures I saw was that it was still producing heat at the rate of 7 megawatts, just a shut down nuclear core as it was.

Ian -   Exactly.  If you take the Daiichi-1, I think was about 700-megawatts.  So, it immediately had shut down, but even though you stop the critical reaction at that point with the rods in, you still get 5% of the power, and that's the thermal power.  So the thermal power reactor is three times the electrical power.  That's just the efficiency of the generating process.  So for the thermal power we would have something like 2.1 gigawatts, so 5% of that is still a lot of energy.  But after about an hour, that was already down to about 1%.  So, then you've got a lot of exponential cooling, but still a very large amount of energy which you need to dissipate.  I mean, this is the point of nuclear power.  There's an enormous amount of energy there.

Chris -   Absolutely.  None of us are in any doubt about the power of nuclear energy.  What about the fact that they couldn't then restore cooling to it, so they then had to start pumping sea water in?  What's happening in terms of the products  - what's in that reactor that's getting out?  Is it still contained or are we seeing contamination?

Ian -   Well, I think almost certainly we know that the fuel has been compromised.  The cladding has been breached because what's being detected in the atmosphere are iodine and there's been some caesium.  Almost certainly there would have been other gases like krypton and xenon.  With a nuclear fuel it's mainly uranium dioxide when it starts.  When you have fission, you produce an enormous cocktail of fission products.  These are elements which have roughly half the mass of uranium each.  Some of those elements can be accommodated chemically or are solid within the fuel.  Some are gases, so iodine is effectively a gas at temperatures of operation, as are xenon and krypton, so those go into the clad of the fuel.  Caesium does not incorporate well into the fuel, so that lies on the grain boundaries of the fuel.  So the moment the fuel is breached, that material will start to be released and that tells us that the fuel cladding has been breached.  Whether it's completely melted together, that's to be seen and we may not know for maybe 4 or 5 years, whether there's been any sort of large degree of melting or whether we've just got serious compromise of the fuel clad.

Dr Farnan went on to
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