Loss of coolant accidents (LOCAs) w-hich are ana­lysed, range from very small leaks from the reactor coolant system boundary to the complete severance of the largest pipe (discharge area approximately 1 m:) in the primary coolant recirculation loop. The system response varies considerably with the size of leak. For the larger breaks, the primary coolant ra­pidly blows down and depressurises. The water coolant changes to steam and, although the reactor is shut­down (loss of moderator ensures this, even if the control rods failed to insert), the decay heat is suffi­cient to increase fuel and clad temperatures. The temperature rise is arrested by the injection of highly borated water from accumulators (storage vessels) and by pumps.

Although only some fuel clad failures are expected to occur, it is conservatively assumed that all will fail and release their fission products to the containment. The extent of the release of these to atmosphere de­pends on the leak rate from the now pressurised containment and on whether other plant failures are assumed to occur, including any which provide a di­rect or indirect leak path to atmosphere. The large. LOCA imposes increased loads on the reactor inter­nals, increased stresses on the pressure vessel and con-‘ tainment structures. Assessment of the ability of these structures to withstand the imposed temperature and pressure transients forms part of the safety case.

Small LOCAs include not only pipe breaks of lim­ited aperture but also, for example, inadvertent open­ing of relief valves and their possible failure to reseat. The accident results in a relatively slow depressuri — sation which is mitigated by the automatic operation of high pressure injection pumps. Again, although few clad failures are expected, for the purpose of radiological consequence analysis all are assumed to tail unless the leak rate is within the capacity of the make-up water supply.