12.76. During the regular refueling operation, about once a year, the head of the reactor vessel is open to the containment. Care must then be taken to avoid damage to the spent fuel rods that would result in the release of fission products, especially iodine vapor and noble gases present in the gap between the fuel pellets and the cladding and in the gas plenum above the pellets (§7.168). Such damage could conceivably arise from overheating caused by inadvertent criticality or by greatly impaired ability to remove decay heat. Core criticality is prevented by detailed administrative pro­cedures for removal of control elements, and critical geometry of the with­drawn fuel-rod assemblies is avoided by proper spacing in storage racks. Adequate cooling for removal of decay heat (and radiation shielding) is provided by keeping the assemblies under water at all times. Furthermore, boric acid is added to the reactor water to ensure subcritical conditions.

12.77. Damage to a spent-fuel assembly could also occur if the assembly were dropped. The use of special gripping and handling devices makes this very improbable. Nevertheless, the Safety Analysis Reports are required to include a calculation of the radioactivity that might be released to the environment if a fuel-rod assembly suffered severe mechanical damage inside the containment vessel. Damage outside the containment might result from the assembly being dropped to the floor of the spent-fuel storage area. However, such an event is extremely unlikely since fuel is transferred from the reactor to the pool under water and not elevated above the pool.