Fukushima nuclear power plant: 1760 tons of spent nuclear fuel

[Paradoxical]

I am going to significantly alter my stance on the current nuclear disaster unfolding in Japan. It appears as though a SIGNIFICANT amount of spent fuel is stored in pools within the reactors (contrary to my prior belief).

http://criepi.denken.or.jp/result/event/seminar/2010/issf/pdf/6-1_powerpoint.pdf

As of March 2010:

3450 fuel assemblies were stored in pools inside the reactor buildings

408 fuel assemblies were stored in dry casks

6291 fuel assemblies were stored in a common storage pool.


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The dry casks are almost certainly safe as they are stored in a separate building and none of the major buildings seem to have suffered structural damage from the tsunami.

Similarly, the common storage pool also appears to be in safe condition.

HOWEVER, almost 600 tons (1.2 million pounds) worth of spent fuel are stored in pools inside the individual reactor buildings. This is a major cause for concern given the recent explosions that appear to have caused significant damage to at least two reactor buildings.

If the distribution of spent fuel is equal between the 6 reactors, that would lead us to believe that almost 200 tons of spent nuclear fuel is currently at risk of being exposed to the open air.

Safety of Spent Nuclear Storage Pools: from 2006 report requested by Congress

The study provided a probabilistic risk assessment that identified severe accident scenarios and estimated their consequences. The analysis determined, for a given set of fuel characteristics, how much time would be required to boil off enough water to allow the fuel rods to reach temperatures sufficient to initiate a zirconium cladding fire.

The analysis suggested that large earthquakes and drops of fuel casks from an overhead crane during transfer operations were the two event initiators that could lead to a loss-of-pool-coolant accident. For cases where active cooling (but not the coolant) has been lost, the thermal-hydraulic analyses suggested that operators would have about 100 hours (more than four days) to act before the fuel was uncovered sufficiently through boiling of cooling water in the pool to allow the fuel rods to ignite. This time was characterized as an “underestimate” given the simplifications assumed for the loss-of-pool-coolant scenario.

The overall conclusion of the study was that the risk of a spent fuel pool accident leading to a zirconium cladding fire was low despite the large consequences because the predicted frequency of such accidents was very low. The study also concluded, however, that the consequences of a zirconium cladding fire in a spent fuel pool could be serious and, that once the fuel was uncovered, it might take only a few hours for the most recently discharged spent fuel rods to ignite.



http://www.nap.edu/openbook.php?record_id=11263&page=44