75 Probabilistic Safety Assessment for Low-Power and Shutdown States for LVNPP (PSAM-0092)

This paper summarizes the development and results derived from the level-1 probabilistic safety assessment for low-power and shutdown states applied to the Unit-1 BWR reactor of Laguna Verde Nuclear Power Plant (LVNPP). The purpose of this study is to identify insights related to safety during the shutdown state of the plant. It was found originally convenient to define 7 Plant Operating States (POS), just like it was done for Grand Gulf Nuclear Power Plant (GGNPP); nonetheless, slight adjustments to some POSs have been proposed and finally the whole shutdown process is represented by 6 POSs. The current advance of the study, presented in this paper, covers POS1 trough POS4.

The modification-to-the-base-model approach was used in the construction of the logical models. Event and fault trees from the base case were utilized wherever possible, but new ones were developed for the loss of SDC as well as for the alternate heat removal methods in POS3. The logical models for all the reported POSs are set up in the SAPHIRE code, which was used to quantify frequencies for accident sequences leading to core damage.

The Loss of Shutdown Cooling (SDC) as IE and its consequences came up as a source of new insights related to safety in the NPP operation. The shutdown cooling process is normally carried out by the Residual Heat Removal System (RHR). In the case of Grand Gulf, an alternate system (ADHRS) replaces the RHR in case the last one is shut down. In the case of the LVNPP the RHR doesn't have any substitute system; instead, internal procedures point out to a couple of alternate methods to cool down the reactor in the POS3 and another couple for the POS4, though insufficient procedural documentation exists on these specific alternate functions.

A total CDF of 1.94E-5 is obtained for the POS4 (annual base). Main contributor to the total CDF, as Initiating Event, continues to be the Loss of Outside Power with 77%. The Loss-of-Coolant Accident is the second in importance with 23%. Looking to the numbers, the total CDF in POS4 conditions represents almost half the total CDF for normal power operation conditions, both of them in annual base. Though, to highlight some of the possible vulnerabilities and improvement points, areas or procedures, represents without any doubt the most worthy result of this low power PSA.