Reactor Physics Aspects of Modelling In-Core Small Loss of Coolant Accidents

Abstract

In-core small loss of coolant accidents (LOCAs) have been analyzed over a wide range of possible initial core states including that of a high moderator poison concentration. The in-core rupture of a pressure tube and its calandria tube represents a unique class of small breaks due to the possibility of damage to in-core reactivity devices such as the shutoff rod (SOR) and mechanical control absorber (MCA) guide tubes. In addition, the potential displacement of moderator poison by the unpoisoned coolant discharge would provide an additional positive reactivity source. The upper limit of moderator poison considered was that which would occur if the reactor is restarted after being shutdown, just after being overfueled by the maximum permissible amount, for a long enough period that all saturating fission products have decayed. In the case of a pre-equilibrium core analysis, the reactor is assumed to have shut down at or near the plutonium peak core state(with no overfueling prior to the shutdown)