"Twenty-seven state attorneys general recently joined top experts in warning Congress that densely packed spent fuel pools at each U.S. plant are of more concern as targets than are reactors. Federal studies confirm that loss of pool water would lead to an uncontrollable fire, with radiation released into the atmosphere causing thousands of fatalities and costing hundreds of billions of dollars in offsite economic damage. Such pools are among the largest risks to U.S. security, what Dr. Gordon Thompson referred to as pre-deployed radiological weapons waiting for activation by an enemy. These pools are also subject to accidental loss of water and subsequent fire." - from NC WARN's Reducing Risks from Storage of High-level Waste at Nuclear Power Facilities
Pilgrim's irradiated fuel pool was originally designed to hold 880 assemblies; it now holds 2,278 assemblies and by the end of its license in 2012, it will hold 3,859 assemblies: 4 times the original capacity in the same space.
Over-crowded storage pools are dangerous. If the pool water is drained
- and there are many scenarios by which this can happen - convective air cooling would be relatively ineffective and there will be a fire and there will be a release of high level radioactivity, many times worse than Chernobyl
, enough to contaminate an area three times the size of Massachusetts.
Nearly 25 years of research support this conclusion. A 1979 study  done for the NRC by the Sandia National Laboratory showed that, in a sudden loss of all the water in the pool, a dense-packed pool, even a year after discharge, would likely heat up to the point where its zircaloy cladding would burst and catch fire. More recently, NRC's NUREG-1738, 2001 looked at loss of pool water and concluded that a "zirconium fire cannot be dismissed even many years after final reactor shutdown."
Moving from densely packed spent fuel pools to low density pools and hardened dry cask on-site storage (as an interim solution until all waste can be moved off-site) would greatly reduce the risk of such a scenario. This issue is discussed in Alvarez, Beyea, Janberg, Kang, Lyman, Macfarlane, Thompson, von Hippel, Reducing the Hazards from Stored Spent Power-Reactor Fuel in the United States
, Science and Global Security, 11:1-51, 2003.
1. Spent Fuel Heat up Following Loss of Water During Storage by Allan S. Benjamin et al. (Sandia National Laboratory, NUREG/CR_0649, SAND77-1371, 1979).