Loss-of-Coolant Accident (LOCA) – Nuclear reactors

A loss-of-coolant accident (LOCA) is a mode of failure for a nuclear reactor. If not managed properly and effectively, the results of a LOCA could result in reactor core damage. In every nuclear reactor, a separate Emergency Core Cooling System (ECCS) exists specifically to deal with the situation like LOCA.

Nuclear reactors generate heat internally in the fuel by the fission reaction, neutron with a fissile material uranium-235. This heat is removed by a coolant system to produce steam and is converted into useful electrical power. If this coolant flow is reduced, or lost altogether, the nuclear reactor's emergency shutdown system is designed to stop the fission chain reaction automatically. However, even after reactor shut down, due to radioactive decay of the fission products, the nuclear fuel will continue to generate a significant amount of heat. This decay heat needs to be taken out through secondary cooling system to maintain integrity of the fuel. If all of the independent cooling systems of the ECCS fail to operate as designed due to some reason such as failure of the pumps, this heat can increase the fuel temperature to the point of damaging the fuel and the reactor.

Damage to the reactor containment will result in the radioactive releases from the reactors. Iodine-131 and Xe-133 have half life of 8 days and 5.2 days and hence are detected in the environment in such situations. Depending upon the wind direction and speed, the radioactive isotopes will travel in dispersed form to large distances.