The ability of the probabilistic safety assessment code MELCOR 1.8.2 to deal with station blackout accidents, characterized by prolonged in-vessel and primary system vapor natural circulation, is analyzed. Results of the analysis recommended a modification of the gravitational term in the momentum equation and the inclusion of the convective term to capture in-vessel natural circulation. Moreover, certain guidelines to build the thermal-hydraulic and core degradation numerical meshes must be respected. A model is proposed that has been applied to simulate the Three Mile Island Unit 2 phase 2 accident, for which natural circulation flows were supposed to take place. The compatibility of the establishment of natural circulation flow with accident measurements and estimations is observed. Furthermore, core degradation results seem reasonable at first sight, although improvements concerning these models are suggested.

The ability of the model to cope with a full sequence in a commercial plant is demonstrated: A station blackout for a one-loop pressurized water reactor was calculated from the initial event to the instant of primary system failure. In-vessel and ex-vessel natural circulation flows of vapor are automatically established, and heatup and fission product release rates are estimated.