The events at Fukushima illustrate the fact that Boiling Water Reactor (BWR) nuclear power plants normally have three or four types of used nuclear fuel inventories that must be maintained in a safe state. These are (1) the reactor core, (2) the spent fuel in the "refueling pool" that sits just beside the reactor, (3) the plant's common spent fuel storage pool, and (4) spent fuel in dry cask storage.
The water level, water chemistry, and water temperature in the refueling and spent fuel pools is carefully monitored and controlled. The refueling pool is used during plant refueling operations to stage used fuel into and out of the reactor. Thus fuel in the refueling pool fuel normally has the highest decay heat generation rate of any fuel outside of the reactor. Separate from the refueling pool is the plant's common spent fuel storage pool, which contains fuel removed from the refueling pools for long-term storage. The spent fuel pool generally services multiple reactors at multi-reactor sites. The decay heat level of the fuel in the spent fuel pool is lower than that of the fuel in the refueling pool. Then finally, there is the spent fuel in dry cask storage that is generally the oldest, coolest spent fuel at the plant.
Any event that threatens the integrity of the refueling pool and spent fuel pool boundaries, leads to leaks, or threatens the pool cooling function, can lead to fuel overheating, hydrogen generation, and fission product release. Unlike BWR severe accidents, which I spent many years studying, I have not personally analyzed spent fuel pool accidents. But others have. So here is a (very) short bibliography of public documents relevant to spent fuel pool accidents – extracted from the cobwebs of my mental attic...
1. NUREG/CR-0649, "Spent Fuel Heatup Following Loss of Water During Storage,"Allan S. Benjamin, David J. McCloskey, Dana A. Powers, Stephen A. Dupree, March 1979.
2. NUREG/CR-4982, "Severe Accidents in Spent Fuel Pools in Support of Generic Safety Issue 82," V. L. Sailer, K. R. Perkins, J. R. Weeks, H. R. Connell, July 1987.
3. Robvert Alvarez, Jan Beyea, Klaus Janberg, et. al., "Reducing the Hazards from Stored Spent Power-Reactor Fuel in the United States," Science and Gobal Security 11: 1-51, 2003
4. Mihaly Kunstar, Lajos Matus, Nora Ver, et al., "Experimental investigation of the late phase of spent fuel pool accidents," Int. J. Nuclear Energy Science and Technology, Vol. 3, No. 3, 2007, page. 287-301
As one would expect, the NRC as spent a great deal of time on this issue. The following URL is a good entry point to their analysis:
Just a quick list of relevant and useful background material from the mental archives of S. R. Greene.