Single failure effects on surge line break transients for the VVER-440 reactor

This paper describes the analysis of surge line break transients for the soviet designed, water cooled, light water moderated, power reactors referred to as VVERS. These events represent an intermediate size loss of coolant accident (LOCA) for these plants and provide a severe challenge to the safety system design. The pressurizer surge line represents the largest diameter connection to the primary system and the break results in a relatively rapid blowdown of the primary system when compared to more conventional small break LOCAs (e,g., stuck open pressurizer relief valves). The VVER unit selected for this analysis is designated as VVER-440 Model V213. This plant generates 440 Mwe and is of current interest since fifteen are now operating and additional units are in various stages of construction. In addition to a base case surge line break analysis, this paper also presents the results of several sensitivity studies related to single failures in various plant safety systems that have been included in the design to mitigate the effects of such a LOCA on the plant and fuel system performance. Examples of the safety systems selected for these sensitivity studies include the scram system, the accumulators, and the high pressure injection system.