Preliminary Evaluation of Long-Term Cooling Considering Chemical Precipitation and Subsequent Impact on the Recirculating Fluid

Generic Safety Issue 191 (GSI-191) addresses a variety of concerns associated with the operation of the emergency core cooling system (ECCS) and the containment spray system (CSS) in the recirculation mode. There has been concern that following a LOCA, chemical precipitate, fibrous and particulate debris within the sump could collect on the sump screen and block the flow of cooling water into the core. The effects of the debris that passes through the sump screen has also been noted as a point of concern. This debris could be ingested into the ECCS and flow into the RCS. The Pressurized Water Reactor Owners Group (PWROG) undertook a program to analyze the effect of debris and precipitates on core cooling for PWRs when the ECCS is realigned to recirculate coolant from the containment sump. The objective of the program was to provide reasonable assurance that sufficient long term core cooling (LTCC) is achieved for PWRs, satisfying the requirements of 10CRF50.46 with respect to debris and chemical products that might be transported to the reactor vessel and core by the coolant recirculating from the containment sump. Chemicals may also deposit on the hot fuel rods and possibly insulate them and inhibit decay heat removal. PWROG developed a LOCA deposition model (LOCADM) to predict chemical deposition on fuel cladding due to the transport of debris and chemical products into the RCS and the core region by the coolant recirculated from the containment sump. Based on the LOCADM, this work performed a plant-specific LOCADM evaluation to show that the plant conditions would maintain adequate LTCC capability