4.147. A double wall containment is an arrangement with the primary containment completely enclosed in a secondary containment. The purpose of the secondary containment is not to take over the functions of the primary containment should it fail but to allow for the collection of leaks in the space between the two structures and for a filtered release via the vent stack. This function is termed secondary confinement.

4.148. The systems associated with secondary confinement should be designed to collect, filter and discharge gases and liquids containing radionu­clides that have leaked from the containment in accident conditions, or to pump leaked liquids back into the containment. This is a way of reducing accidental radioactive releases (by filtering) and their impacts (by means of stack release of gases instead of releases at ground level). The merits of a complete or partial secondary confinement should be considered for new plants. A partial secondary confinement (i. e. one which does not completely enclose the primary containment) should enclose the more leakage prone areas of the primary containment (such as the penetration areas). If no secondary confinement is provided, a thorough justification for this should be made on the basis of anticipated radioactive releases or dose calculations for all relevant design basis accidents and for severe accident conditions.

4.149. To maximize the efficiency of the secondary confinement, a filtered ventilation system should be provided. This should quickly reduce the pressure in the volume between the primary and the secondary containment (the confinement volume) to a negative gauge pressure after a postulated initiating event involving a loss of coolant and should maintain it even under the assumed worst wind conditions. If a negative gauge pressure cannot be achieved and maintained in the confinement volume, account should be taken in the calculations of the radiological consequences of the unfiltered leakage to the environment that will result. The confinement volume should be kept at subatmospheric pressure in normal operation, to enable the leaktightness of the secondary containment to be monitored.

4.150. When a secondary confinement is provided, direct leaks (i. e. leak paths from the containment directly to the outside without transiting the confinement volume) should be prevented to the extent possible. Criteria should be set for the control of direct leaks and for the leaktightness of the secondary confinement envelope. It should be verified periodically by means of testing that these criteria are being met.

4.151. The following features should be incorporated into the design to limit the number of direct leaks:

— Systems that have to penetrate the primary containment should be located in the confinement volume, either entirely (if possible) or up to the isolation valves.

— Recirculation systems (e. g. safety injection systems and spray systems) should be located entirely in the confinement volume.

— Large penetrations (e. g. the containment ventilation system) should be equipped with three isolation valves (one in the containment, one in the confinement volume and one outside the containment). The space between the second and third isolation valves should be connected to the confinement volume by a small line equipped with two isolation valves in parallel that are open when the large valves are closed; this ensures that, even with a single failure of the isolation valves, leaks from the containment are collected in the confinement volume.

— Doors of the air locks penetrating both containment walls should be equipped with a double seal; the space between the seals should be connected to the confinement volume rather than to the air lock volume when the door is closed.