Advanced reactors can be designed to include a number of features to improve leak — tightness under severe accident conditions. Under such conditions, the containment structures will have to withstand much higher loads in terms of pressure, temperature, radiation and chemical attack than under normal operation. Leak-tightness must be assured under this harsher environment; leaks may occur from the containment structures themselves, pipe and electrical penetrations and isolation valves, hatches, locks, etc.

Leak rates from steel containments or containments with a steel liner are expected to be lower than those from concrete containments without a liner. However, in some designs, the concrete containment is surrounded by a secondary containment. There is generally a requirement to improve overall leak-tightness in advanced containment designs, which can be achieved by improved primary containment design or possibly by taking credit for a secondary containment without improvement to the leak-tightness of the primary containment. By way of example of improved leak-tightness, the design leak rate for AP600 is 0.12% per day against other current PWR rates which are in the range 0.25-0.5% per day (IAEA-TECDOC-752, 1994). Containment leak-tightness needs to be maintained under all plant states including shutdown.

There are containment bypass sequences such as interfacing LOCAs and SGTRs and leaks in these events also need to be covered. The general approach in advanced containment design is to reduce the number of penetrations. Other special measures include pressurisation systems that keep penetrations at pressures higher than the containment pressure. These systems have been proposed in plants where there is a direct release path to the environment. Suction systems have been proposed to collect the leak contents and treat it before release. Other special components such as bellow’s valves and seal welds on large equipment hatches are also considered. With all these systems, however there are issues concerning their likely performance under severe accident conditions.

There are issues such as how the leak-tightness of the containment under severe accident loads can be tested. This is particularly so if the severe accident postulated pressures are higher than the peak design pressure, so periodic testing is not possible.

Systems have also been proposed for establishing whether a large opening in the containment boundary is pre-existing when an accident occurs. This approach has been

considered for existing plants. One such system, developed by EDF gives a measure of leak-tightness by measuring the rate of increase on containment pressure cause by the usual leaks in the air supply system.