An analytical investigation of certain problems in the area of subassembly-to-subassembly failure propagation in LMFBR's is described. Existing analyses of the response of the adjacent subassembly duct to mechanical loads are reviewed and summarized, and major uncertainties are identified. Additional analyses of the response of the adjacent subassembly to certain thermal loads are presented in two parts. In the first part, the effect of an external heat flux on duct melting and thermal stresses is considered. The external heat fluxes required to produce duct melting or excessive thermal stresses are compared with the heat fluxes that might be expected from the molten fuel deposited on the duct wall. In the second part, a thermal-hydraulic study is performed to investigate the effect of the external heat flux on the coolant temperature distribution in the adjacent subassembly. Both normal subassembly geometry and distorted subassembly geometry are considered. A detailed model of the coolant region formed by the heated duct wall and the displaced fuel pins is also analyzed to determine whether there are severe temperature gradients.

Results of two failure experiments using LMFBR-type fuel during simulated unprotected transient overpower accidents are reported and analyzed. In both experiments, a single fresh fuel pin in a Mark0IIA loop was subjected to a temperature-limited, step-reactivity irradiation in the TREAT reactor. Total energy was 490 MJ in Test H2 and in Test E4.