4.145. Containment bypass events arise when a fault sequence allows primary coolant and any accompanying fission products to escape to the outside atmosphere without being processed by containment systems for the management of energy, radionuclides and combustible gases. In interfacing system LOCAs, valves isolating the low pressure piping fail and the piping connected to the reactor coolant system fails outside the containment. Possible paths for interfacing system LOCAs should be eliminated as far as possible, either by relocating the system in the containment or by increasing the design pressure of the low pressure system above the pressure of the reactor coolant system. For any remaining possible paths for interfacing system LOCAs, the provisions for isolation between the high pressure system and the low pressure system should be as reliable as is practicable.

4.146. In pressurized water reactors, a steam generator tube rupture is a containment bypass event that could lead to significant releases of radioactive material. Preventive design features should be installed in steam generators to reduce the frequency of such events to a very low value. The design of the plant should allow isolation of the containment bypass due to the damaged steam generator to be achieved before the authorized limits on radioactive discharges to the environment are reached [15].