As of Thursday, March 17, the most urgent task of Japanese authorities at the Fukushima nuclear power plant has been to restore cooling to the pools at reactors 3 and 4 that store spent nuclear fuel. Authorities have attempted to drop water onto the reactors from helicopters and spray it from water cannons in an effort to cool the fuel rods in the pools.
A previous post describes in general terms the dangers posed by spent fuel pools. This post answers questions about spent fuel pools at U.S. nuclear reactors.
Every operating nuclear power plant in the United States has a spent fuel pool on site. (See here for the location of existing U.S. nuclear plants.) The pools serve two functions – they may be used for temporary storage of fuel during refueling operations, and are also used for longer term storage of “spent” nuclear fuel.
A couple of shut-down nuclear power plants – including Illinois’ Zion nuclear power plant and the LaCrosse power plant in Wisconsin – also continue to store fuel rods in spent fuel pools. (PDF)
The Nuclear Regulatory Commission requires that spent fuel cool for five years in spent fuel pools before being transferred to more permanent forms of storage (where they exist) such as dry casks.
As is the case at Fukushima, almost all spent fuel pools at U.S. nuclear reactors are located outside the containment structure that holds the reactor pressure vessel. (The exception being General Electric Mark III reactors, which have two pools – one inside and one outside of primary containment.) This means that only the exterior walls of the reactor building or other auxiliary structure are capable of preventing radiation from escaping in the event that coolant is lost.
In some reactors – as in the General Electric Mark I reactors at Fukushima and 23 U.S. sites – the spent fuel pools are maintained within the reactor building on an upper floor. Other reactor designs, such as pressurized water reactors, typically locate spent fuel pools in an auxiliary building attached to the reactor building. In some cases, these pools are built below grade. Power plants with more than one reactor may share a spent fuel pool.
The risks of radiation releases from the loss of coolant from spent fuel pools are quite real. Indeed, the occurrence of an earthquake that exceeds the design basis of the nuclear plant has been identified as one of the most probable causes of a loss-of-coolant accident involving spent fuel.
In 2006, the U.S. National Research Council issued a detailed report on the risk posed by a terrorist attack on spent fuel pools at nuclear reactors. Among the authors’ conclusions were that “under some conditions, a terrorist attack that partially or completely drained a spent fuel pool could lead to a propagating zirconium cladding fire and the release of large quantities of radioactive materials to the environment.”
The report also cited a 2001 Nuclear Regulatory Commission study, summarizing it as follows:
“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.”
Brookhaven National Laboratory studied the consequences of a severe spent fuel pool accident at a closed U.S. reactor in a 1997 report prepared for the U.S. Nuclear Regulatory Commission. In the most severe case, if the fuel rods rapidly caught fire after water completely drained from the spent fuel pool, an accident could:
In short, the possibility of a Fukushima-like loss of coolant to spent fuel pools - and ensuing release of radiation - is quite real in the United States, whether as a result of an earthquake, terrorist attack or other disaster. The risk of such a disaster varies from plant to plant and can be mitigated in various ways, but it cannot be fully eliminated given current spent fuel pool designs.