Spent fuel pools (SFP) are storage pools for spent fuel from nuclear reactors. They are typically 40 or more feet (12 m) deep, with the bottom 14 feet (4.3 m) equipped with storage racks designed to hold fuel assemblies removed from the reactor. A reactor’s pool is specially designed for the reactor in which the fuel was used and situated at the reactor site. In many countries, the fuel assemblies, after being in the reactor for 3 to 6 years, are stored underwater for 10 to 20 years before being sent for reprocessing or dry cask storage. The water cools the fuel and provides shielding from radiation.
In the magnitude 9 earthquake which struck the Fukushima nuclear plants in March 2011, one of the spent fuel pools lost its roof and was reported to be emitting steam so was possibly boiling. According to The Nation, “Spent fuel pools at Fukushima are not equipped with backup water-circulation systems or backup generators for the water circulation system they do have.” Later, there was some disagreement among sources as to whether the pool had boiled dry.