Hot Water

Spent Fuel Pools at Fukushima

Union of Concerned Scientists

Because of their high radioactivity, fuel rods continue to produce very significant heat even after they are no longer useful for generating electricity and are removed from the reactor core. Such “spent fuel” rods need to be continually cooled for many years to prevent them from heating to a level where they would suffer damage.

To cool the rods after they are removed from the reactor core, they are placed on racks in a spent fuel pool that circulates cooled water around them. This water is circulated by pumps that are run using electricity from the power grid. Typically these pumps do not have backup power from deisel generators or batteries, so if power from the grid is interrupted, as it is in the case of the Japanese earthquake, they will stop operating.

Once the cooling pumps stop, the water in the spent fuel pools will begin to heat up and will eventually start to boil off. The pools are typically 45 feet deep with the fuel rods stored in the lower 15 feet of the pool, so 30 feet of water would have to boil off before exposing the rods. That could take several days, so this issue may only be appearing now.

If water cannot be added to the pool, or if the pool has been damaged and is leaking, the fuel may remain uncovered. The exposed fuel can get hot enough to melt, depending on how long it has been out of the reactor. If the fuel melts, it would release significant additional radioactivity into the air.

If mechanisms to fill the pool at Unit 4 are broken, or if there is a need to repair the pool, it will be difficult to get workers close enough to do this. If spent fuel has been in the pool for a relatively short time, even if the water level is at the top of the fuel rods, the radiation dose to someone at the railing of the pool would give them a lethal dose in well under a minute. This would explain why there have been reports of requests to use helicopters to deliver water to the pools. However, it appears that this is not a practical way of delivering water.

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