System code analysis of accumulator nitrogen discharge during LOCA experiment at PWR PACTEL test facility

Abstract

The hydroaccumulators in pressurized water reactors can inject nitrogen into the reactor system. In the primary system, nitrogen affects core cooling and accident management, both adversely and beneficially. The PWR PACTEL experiment NCG-13 have shown that during a hot leg SB LOCA, nitrogen in the primary side can block the primary to secondary heat transfer and thereby prevent the primary depressurization to the point needed for the long-term accident management. This paper presents the APROS and TRACE calculations of the PWR PACTEL NCG-13 experiment. Both codes calculate the transient progression and the timing of the main events satisfactorily, once the suitable options are selected and adjustments made. There is, however, one big discrepancy between the code simulations and the experiment: in the simulations, much more nitrogen is needed to get qualitatively the same behaviour. The difference is a factor of 2.5 by mass for stopping the depressurization and 4–6 to cause a core heat-up. This is of concern regarding the confidence in the codes, as the simulations underestimate the adverse effect of nitrogen on the core coolability.