Long term retention of core debris

Severe accidents which lead to the production of substantial amounts of core debris can be terminated with relatively low consequences to the public if the core debris reaches a state in which it can be cooled indefinitely in an intact containment either by natural processes or by engineered safety features. We describe some of the strategies available for long-term retention of debris, and the current state of understanding and modelling. Retention in particulate form in a liquid coolant is practical, and the processes associated with debris coolability are relatively well understood (this is illustrated by comparisons between model calculations and experiments in the Sandia DCC series). However there is a need for improved understanding of the circumstances in which melts will quench to form particulates; this is being investigated with the CHYMES code. The effects of inhomogeneities in debris beds is examined; there may be the need to provide additional 'wicks' for coolant to overcome the deleterious effects of stratification. Retention as a predominantly molten mass is also possible. The CORCON-MOD2 code has been extended to allow for the effects of thermal conduction into the basemat; initial calculations show a strong sensitivity to the assumed ablation temperature, and indicate that a refractory barrier may limit penetration. (author). Poster presentation. 29 refs, 3 figs