Core restraint performance of large heterogeneous reference cores

A preliminary study of the core restraint performance of heterogeneous cores was performed as part of the Large Heterogeneous Reference Core Study (LHRCS). The performance is based on satisfying four functional requirements: (1) to provide assembly alignment for control rod insertion and fuel handling; (2) to permit fuel handling within allowable insertion and withdrawal forces; (3) to maintain the core reactivity (from radial motion) within specified limits during normal and off-normal conditions; and (4) to prevent extensive duct-to-duct contact throughout the core. Six generic cases were selected to represent the range of core arrangements whose features greatly influence core restraint performance. Case 1 is a reference homogeneous core with temperature gradients only near the fuel-blanket interface. Case 2 is also a reference homogeneous core, but has ficticious opposing temperature gradients within the fuel region. Cases 3 and 4 are tightly-coupled heterogeneous cores with fuel and blanket in the center region, respectively. Cases 5 and 6 are loosely-coupled heterogeneous cores with fuel (Case 5) and blanket (Case 6) in the center region. The most important relative difference in the performance of homogeneous and heterogeneous cores is the extent of duct-to-duct contact in the core region. Three of the four heterogeneous coresmore » have substantial duct-to-duct contact while neither of the homogeneous cores do. This performance characteristic of heterogeneous cores appear to be inherent in their design and not amenable to change by core modifications, etc. The features producing duct-to-duct contact should be kept in mind in the initial stages of laying out heterogeneous cores, and detailed core restraint analysis (i.e., 3-D) should be employed to investigate their core restraint performance in preliminary design studies.« less