183 Application of PRA Methodology to ECCS Suction Strainers Blockage Issue (PSAM-0152)

In Japan, activities are in progress to resolve the issue of Emergency Core Cooling System (ECCS) suction strainer blockage due to LOCA generated debris, raised as safety issue from Barsebeck-2 event in 1992. The PRA considering ECCS suction strainer blockage was implemented by Japanese BWR utilities and vendors to evaluate the influence of blockage to plant risk and the effect of additional operational measures for reference plants in this connection.

In this PRA, Core Damage Frequency (CDF) has been evaluated using the Event Tree (ET) model focused on Large LOCA (LLOCA) in current PRA models. The ET considering ECCS suction strainers blockage is based on PRA models in NUREG/CR-6224 including headings such as initiator, reactivity control, early containment pressure control, reactor core cooling, and long-term containment protection. But PRA model adopted here has some features as follows.

1) LLOCA frequency as initiator was evaluated as double-ended break frequency by using probabilistic fracture mechanics for various piping size in containment.

2) Conditional probability of ECCS suction strainer blockage was evaluated with decomposition event tree (DET) modeling each process leading to loss of NPSH of ECCS pumps, such as transfer of debris from drywell to suppression pool by LOCA and adherence of debris to strainer surface.

3) Quantitative evaluation of probability of failure of operator performance to recognize ECCS suction strainer blockage and to take action to recover ECCS such as removal of debris from suction strainer by ECCS manual stopping/restarting, and switching ECCS water source to the external source as necessary. Allowed time of no injection to reactor core was evaluated by using SAFER code and probability to succeed in recovering core injection within allowed time was evaluated.

Results of total CDF for BWR-4,BWR-5 and ABWR were between 10-7/Rx-yr and 10-8/Rx-yr, and didn't make the plant risk increase drastically. From the results of PRA considering ECCS strainer blockage, it was concluded that CDF of plant is kept low enough even if ECCS suction strainer blockage had been considered, and that additional operational measures are effective to decrease the plant risk impact.