4.120. In some containments with a steel shell, heat released in the containment under accident conditions can be removed passively through the containment walls. The secondary containment is designed to remove the heat by providing a natural circulation path for air (the chimney effect) and a means for passive spraying of the outside of the primary containment. Other contain­ments introduce passive cooling condensers that transfer the heat by means of natural convection to a water pool. If such passive containment cooling is adopted, the following aspects should be considered:

(a) The area of the cooling surface should be sufficient to transfer the heat generated in the containment and to cool down the atmosphere and the structures inside the containment. The heat transfer coefficient should be conservatively determined for all operational states.

(b) The necessary natural circulation within the containment and that to the outside heat sink should be ensured for all relevant design basis accidents.

(c) The entire system should be well validated by means of tests and analyses. A thorough search should be conducted for possible harmful effects and failure modes, in order to achieve a high degree of confidence that the safety functions will be fulfilled in all design basis accidents.

MANAGEMENT OF RADIONUCLIDES