4.124. An effective way to restrict radioactive releases to the environment is to maintain the leak rate below conservative specified limits throughout the plant’s operating lifetime[9]. As a minimum, leak rates should be small enough to ensure that the relevant dose limits are not exceeded during normal operations or in accident conditions.

4.125. At the design stage, a target leak rate should be set that is well below the safety limit leak rate, i. e. well below the leak rate assumed in the assessment of possible radioactive releases arising from accidents. This margin is useful to reduce the likelihood that unforeseen modifications made at the stage of design or construction cause an actual leak rate to approach the safety limit leak rate.

4.126. To limit the number of leak paths, the number of penetrations should be kept as low as possible. The external extensions of the penetrations should be installed in a confined building, at least until the first isolation valve, in order to collect and filter any leaks before a radioactive release occurs.

4.127. Leak rates of isolation devices, air locks and penetrations should be specified with account taken of their importance to safety and the integral leaktightness of the containment.

4.128. A reliable actuation system for containment isolation should be incor­porated, as described in paras 4.169-4.183 and 4.225-4.230, to ensure the leaktightness of the containment in the event of an accident.

4.129. Additional measures to eliminate possible leakage paths should be considered if necessary. For example, some designs use a pressurization system that injects a fluid (water or nitrogen) between isolation valves in series (in which case at least three valves are necessary to cope with a single failure).