4.225. In the event of a significant release of radioactive material into the containment (such as in a LOCA), signals for the actuation of containment systems (such as the systems for energy management, radionuclide management and the management of combustible gases) should be derived, depending on the design, from the values of parameters such as:

— High pressure and/or high radiation levels in the containment,

— Low pressure in the reactor coolant system,

— A small subcooling margin in the reactor coolant system,

— A low water level in the reactor pressure vessel.

4.226. Many of these signals are typically used in the reactor protection system to initiate automatic containment isolation or to actuate systems important to safety (such as spray systems, ventilation systems and active igniters).

4.227. Signals for the following conditions should also be used to initiate automatic isolation or for initiating isolation by operator action in the control room:

— High levels of radiation or contamination in the containment atmosphere,

— High levels of radiation in the sump water.

4.228. The lines that penetrate the containment and that are necessary for the operation of safety systems in accident conditions should not be isolated upon the automatic isolation of the containment. Other means should be used to ensure that any release of radioactive material through the containment envelope does not exceed the limits set for plant operational states and design basis accidents.

4.229. In addition to those events for which isolation of the containment is required, there are other events for which only the individual isolation of the affected lines is necessary to limit the release of radioactive material from the containment to the environment. This is the case for a break outside the containment in a pipeline for radioactive material that penetrates the containment, or for the failure of an interface between two associated systems (such as the rupture of a heat exchanger on a water line of a component cooling system) that leads to a release of radioactive material from a system inside the containment to a system outside. The actuation of the isolation devices should be derived from the values of appropriate parameters, such as:

— Levels of radiation or of airborne contamination,

— Pressure changes,

— Temperature changes.

4.230. For all lines not associated with the operation of safety systems, the following criteria should be met:

(a) Lines that penetrate the containment envelope should be automatically isolated when process parameters indicate LOCA conditions.

(b) Lines that communicate with the containment atmosphere should be automatically isolated when a specified level of radiation in the containment atmosphere is exceeded.

(c) Lines that communicate with the containment sump and penetrate the containment should be isolated when a specified level of radiation in the sump water is exceeded.

(d) Lines that are connected to the reactor coolant system via a heat exchanger (such as the main steam lines in a pressurized water reactor) should be isolated when specified radiation levels in the lines are exceeded.