Accumulation and head-loss characteristics of LOCA-generated debris in a fuel assembly

A loss of coolant accident (LOCA) would generate debris from thermal insulation and other materials in a pressurized-water reactor (PWR). Debris in the ECCS recirculating fluid that passes through the sump strainer may accumulate on the fuel assembly (FA), causing resistance to flow through this path and posing a threat to decay heat transfer. Double-ended hot leg (HT) break and double-ended cold leg (CL) break are two types of limiting break conditions, which could represent different effects including debris configuration and head loss in FA. In order to assess these two scenarios' discrepancy, in-vessel downstream effect tests simulating limiting conditions for a full-scale FA were performed. It is concluded the hot leg break condition at the maximum flow rate represents the most conservative test conditions and should be used for testing designed to define debris limits. Both limiting break simulations indicated that the addition of SiC particulates had negligible effect on pressure drop, while the addition of first batch of chemical precipitates immediately caused a steep increase in pressure drop. Observation after tests indicated that fiber and particulates were captured primarily on the fuel bottom nozzle in the CL break and the protective grid(P-grid) in the HL break. (author)