Thursday, March 17, 2011

Some Considerations in Spent Fuel Pool Fires

This morning, I read the Brookhaven National Labs report Severe Accidents in Spent Fuel Pools in Support of Generic Safety Issue 82 from 1987.  This is an analysis of the risk of large scale radioactivity release from the spent fuel stored onsite in a water pool at a nuclear reactor, in the case that an accident caused a loss of the water in the pool.  This is something that now may have happened (for the first time) in the pool at Fukushima-Daiichi.

The document is long and complex, and there are many very significant uncertainties - to the point where I judge that no-one will be able to reliably predict what may happen in this particular accident.  However, the key points seem to me as follows:

  • The main risk is if the amount of heat released from the fuel is enough to cause the Zircaloy cladding of the fuel rods to catch fire.  In that case, much of the radioactive material in the burning fuel rods will be dispersed into the atmosphere (if the building doesn't contain it, which it might not given the existing building damage at Fukushima-Daiichi).
  • If the rods don't catch fire, they will probably just sit in place and be intensely radioactive, but not release huge amounts of radioactivity over long distances.
  • If the entire pool were to burn and be atmospherically dispersed (the absolute worst case), the Brookhaven report estimates that a couple of hundred square miles of territory would be rendered permanently uninhabitable (ie towards a Chernobyl scale event).
  • The amount of heat produced by spent fuel decays with a half life of about a year.  Thus it is the most recent fuel-rods that are the greatest risk of starting a fire (nuclear reactors have their fuel changed out about every 18 months, so that is the frequency of new additions to the storage pool).  This is the concern with the pool at reactor 4, which has just had a reactor unload event.
  • Whether a fire can start, and how far it can propagate into the older less hot fuel is very dependent on details of the fuel storage geometry that we have no information on at Fukushima.
  • If the newest fuel becomes uncovered, a fire is likely to start fairly quickly if it's going to (certainly in the first day).  Given the lack of clear reporting in the present situation, it's possible such a fire already happened and was put out.  It's also possible there is still enough water to prevent the hottest fuel having it's Zircaloy cladding from catching fire, and only hydrogen fires have occurred.
  • In the scenario in which the water is slowly boiling off, rather than having all drained at once, it would seem hard for the whole assembly to burn.
My sense is that the worst case certainly hasn't occurred at present (the entire lot burning up would be extremely visible in video of the plant), and probably isn't likely.  The most likely scenario to result in that would be a catastrophic failure of one of the pools - perhaps damaged in the (9.0!) earthquake, and then failing totally in a subsequent explosion.

It's probably worth highlighting - the fact that none of these pools drained when the plant was first hit by a 9.0 earthquake close by, and then a massive tsunami, is pretty remarkable and some kind of testament to the quality of the original engineering.


Stuart Staniford said...

Also note, in Edwin Lyman's testimony to the Senate the other day, he said:

At least two spent fuel pools at the Fukushima plant have caught fire and are releasing radiation into the atmosphere. These pools are on the upper floor of these Mark I boiling-water reactors and are now open to the air following explosions that breached the buildings around them. The U.S. has 31 boiling-water reactors with similarly situated spent fuel pools that are far more densely packed than those at Fukushima and hence could pose far higher risks if damaged. The U.S. should act quickly to remove spent fuel from these pools and place them in dry storage casks to reduce the heat load and radioactive inventories of the pools.

I'm not sure of the basis for his evaluation of the density of the stored fuel at Fukushima, but that is a key issue: the denser the storage, the higher the risk of fire in the event of pool drainage.

Belette said...

Given the catastrophic nature of a potential drying-and-fire, and the fact that the reactor is by the sea, it seems pretty odd that the spent fuel isn't stored below sea level. That way, a fairly simple nearly-passive channel to the sea could keep them under water. This might end up with radioactive steam or something, but that is an awful lot better than a fire.

Stuart Staniford said...

Euan at TOD has some commentary that seems useful too.

risa said...

I am under the impression that the indoor pools were situated directly above the core so as, at least originally, to have a place to put them right away and minimize handling under cooler for secondary transportation and that's why they are so high up. It would have been nice to have some diesel pumps that high up as well, though.

Lars-Eric Bjerke said...

Stuart, excellent post

Spent fuel pits are reinforces concrete pools with very heavy walls mainly for radiation protection with stainless steel lining on the inside. They do not have openings below water level except for the fuel transportation gates. The sensitive parts for earthquakes are the fuel storage racks with fuel elements and the cooling system. Due to the discussions about permanents storage of spent fuel, utilities around the world have installed fuel racks for denser packing of fuel elements. There is a considerable amount of water in the pools and thus there is normally sufficient time to restore the cooling after an initiating event, however obviously not in this case. The available time has in many plants been reduced by storing more fuel in the pools.

Lars-Eric Bjerke said...

Another factor that has increase the sensitivity for overheating is the increased enrichment and improved efficiency of the fuel. Today one kg of uranium dioxide fuel produces three times the amount of electricity than it did in the 70-ties and thus the corresponding amount of residual heat which of course has too be cooled.

Soylent said...

We now know that no spent fuel pools have caught fire and in particular that #4 fuel pool, the one with fresh-out-of-the-reactor fuel that everyone was worried about, has low water contamination(few if any failed fuel elements) and the amount of debris that has fallen into the pool as a result of the explosion is less than one would expect.

The hydrogen that caused the hydrogen explosion in #4 most likely came from shared ventilation with #3.

Go look at the fuel pool videos released on the 8th of may.

Soylent said...

"Today one kg of uranium dioxide fuel produces three times the amount of electricity than it did in the 70-ties and thus the corresponding amount of residual heat which of course has too be cooled."

No, it is _power density_ that matters, not _energy density_. Radioactive isotopes reach an equilibrium concentration after a few half-lives; short-lived stuff reaches an equilibrium very quickly, while the longer-lived stuff(e.g. Cs-137) doesn't. Nearly all the heat is from short-lived isotopes in the crucial few early days post-accident.

You can also not just look at power density in isolation; you have to look at how long it takes for the water to boil away and what emergency cooling mechanisms are in place. Modern reactors are drastically better at this than old clunkers from the 1960's. E.g. in an AP-1000 it takes 72 hours for the fuel to be damaged in a site blackout with no operator intervention.

Jeffrey Crunk said...

Stuart, would you mind revisiting Fukushima? Senator Wyden's visit to the nuclear plant has rejuvenated web discussions about the risk posed to Japan and the Northern Hemisphere from the spent fuel stored at unit number four. From the reporting, Wyden seems genuinely concerned about a catastrophic event. It's not hard to find audio and video of physicist Michio Kaku speak alarmingly about Fukushima, or other historically prominent figures, such as Helen Caldicott. So, what do you make of the state of risk at Fukushima to global civilization more than one year on from 4-11?

Dr. Good Heart said...

Multiple Fukushima Spent Fuel Pools Burned And Vaporized, 600 - 6000 Pounds Plutonium Released; via @AGreenRoad