ISSUES AFFECTING ADVANCED PASSIVE LIGHT-WATER REACTOR SAFETY ANALYSIS

Next generation commercial reactor designs emphasize enhanced safety b y means of improved safety system reliability and performance. These o bjectives are achieved via safety system simplification and reliance o n immutable natural forces for system operation. Simulating the perfor mance of these safety systems will be central to analytical safety eva luation of advanced passive reactor designs. Yet, the characteristical ly small driving forces of these safety systems pose challenging compu tational problems to current thermal-hydraulic systems analysis codes. Additionally, the safety systems generally interact closely with one another, requiring accurate, integrated simulation of the nuclear stea m supply system, engineered safeguards and containment. Furthermore, n umerical safety analysis of these advanced passive reactor designs wil l necessitate simulation of long-duration, slowly-developing transient s compared with current reactor designs. The composite effects of smal l computational inaccuracies on induced system interactions and pertur bations over long periods may well lead to predicted results which are significantly different than would otherwise be expected or might act ually occur. Comparisons between the engineered safety features of com peting U.S. advanced light water reactor designs and analogous present day reactor designs arc examined relative to the adequacy of existing thermal-hydraulic safety codes in predicting the mechanisms of passiv e safety. Areas where existing codes may require modification, extensi on or assessment relative to passive safety designs are identified. Co nclusions concerning the applicability of these codes to advanced pass ive light water reactor safety analysis are presented.