Optimizing the Initial Pressure of Accumulators for the Atucha2 NPP Using an Optimization Method

Passive safety systems like hydro accumulators offer high reliability and are therefore, when a choice is possible, often preferred over active safety systems. However, their effectiveness in case of an incident or accident depends on a large number of parameters (like break size in case of a loss of coolant accident, availability of other safety systems, initial pressure in the accumulators) and is in general difficult to predict. This paper presents a study to optimize the initial pressure and the pressure drops in the accumulator line for a intermediate break loss of coolant accident for Atucha 2, a Siemens-KWU, heavy water moderated, channel type pressurized water reactor under construction. An optimization method was applied. The thermal hydraulic system code RELAP5 mod 3.3 was used for the analysis. Three cases have been analyzed. First, the initial pressure and pressure drop in the accumulator line was optimized in case of an intermediate break in cold leg two, assuming safety injection of two of the four trains of safety systems into hot and cold leg. Second, like before, but assuming safety injection into cold leg only. Third, like case two, but grouping the four accumulators in two groups, with different initial pressure and pressure drops in the accumulator lines. The results show that a slight increase of the initial accumulator pressure compared to the design value could be beneficial for the investigated initial event. Further, case three shows that different initial pressure in the accumulators could increase the effectiveness of the intervention for the investigated accident.Copyright © 2009 by ASME