One-third of the total fuel load of a reactor is removed from the core every 12 to 18 months and replaced with fresh fuel. Spent fuel rods generate intense heat and high radiation and are stored underwater in pools with depths of 20 to 40 feet. The water cools the fuel and provides radiation shielding. Spent fuel is later sent for reprocessing or dry cask storage.
Without cooling, the spent fuel pool water will heat up and boil. Exposed fuel assemblies will overheat, melt or combust. Pool level is tightly controlled and water is continuously cooled by recirculation to heat exchangers and then back to the pool to resume cooling. High and low level alarms as well as redundant continuous level indication are typically required.
Consult factory for Nuclear Qualified instruments and specific model codes.