CONSIDERATIONS OF THE THIRD LINE OF ASSURANCE - POST-ACCIDENT HEAT REMOVAL AND CORE RETENTION IN CONTAINMENT.

LMFBR safety through defense in depth is provided by four lines of assurance (LOA 1 to 4): LOA1: prevention of accidents; LOA2: limitation of core damage; LOA3: containment of accidents in primary system cells; LOA4: attenuation of radiological materials. Primary emphasis in design is on LOA-1 or prevention of accidents, but the LMFBR can also be shown to possess inherent safety characteristics in the second, third and fourth lines. The main concern of this paper is with LOA3. Functional requirements and pertinent considerations of core retention and decay heat removal are given. The scenario of a postulated accident is described from the stage of core debris dispersion to the final stage of debris settling in either the reactor vessel or at the bottom of the reactor cell.