Comparison of SGTR accident under full-power and shutdown conditions

Abstract

The Steam Generator Tube Rupture is one of the frequently experienced accidents in nuclear power plants and it could inevitably result in radiation release by-passing the containment as well as the primary inventory. To cope with SGTR, therefore, not only maintaining the RCS inventory, but also keeping the radiation release as low as possible becomes important. A basic approach to SGTR accidents in the Emergency Operation Procedure (EOP) is firstly to use the safety injection system to make up the inventory loss and secondly to cool down the RCS, in order to lower the pressure until it becomes equal to the broken steam generator pressure. This is because maintaining the RCS inventory is the primary objective of EOP when accidents occur during full-power operation. However, SGTR can occur not only under the full-power conditions but also in the shut-down modes for which the RCS inventory loss would be less significant due to the lower power level. Examples of shut-down SGTR accidents are Doel-2 in 1979, Fort Calhoun in 1984, and most recently Hanul-4 in 2002. As the risk during shutdown modes are getting more and more attentions than ever, we revisited the SGTR accident of Hanul-4 and examined the appropriateness of applying EOP to the shutdown conditions as EOP was developed assuming the accident under full power conditions. We further analyzed SGTR under the full power conditions to make comparisons. As a result, we found that under the shutdown conditions, small break LOCA like SGTR, depressurization by the RCS cool-down through the intact steam generator should be given priority to limit the inventory loss and consequently to reduce the radiation release or contamination. Thus, this study indicates that it is worthwhile to develop a set of EOP dedicated to the shutdown conditions, in order to minimize the accident consequences.