Loss of residual heat removal during mid-loop operation: BETHSY experiments

Loss of residual heat removal has occurred in several PWRs during mid-loop operation after plant shutdown, and is now recognized as an accident situation whose relevant physical phenomena require improved understanding. Several cases have been selected for investigation in the frame of the BETHSY program, with the main purpose of providing an experimental basis for the assessment of safety codes which have been so far especially involved with accident transients initiated during full-power operation. The major results from four experiments are presented in this paper, which addresses two main kinds of physical problem: mechanisms for, and rate of, primary mass inventory reduction in the case of manways opened at various locations in the primary coolant system; cooling of the plant when the primary is only half-open through vent paths and one steam generator is available to remove decay heat in reflux condenser mode with the presence of non-condensable gas.