Extensions to SCDAP/RELAP5-3D for Analysis of Advanced Reactors

The SCDAP/RELAP5-3D code was extended to enable the code to perform transient analyses of advanced LWRs (Light Water Reactors) and HTGRs (High Temperature Gas Reactors). The extensions for LWRs included: (1) representation of micro-heterogeneous fuel varying in composition in the radial and axial directions, (2) modeling of two-dimensional radial/axial heat conduction for more accurate calculation of fuel and cladding temperatures during the reflood period of a large break loss-of-coolant accident (LOCA), (3) modeling of fuel-cladding interface pressure and fuel-cladding gap conductance, (4) representation of radial power profiles varying in a discontinuous manner in the axial direction, and (5) addition of material properties for fuel composed of mixtures of ThO2-UO2 and ThO2-PuO2. The extensions for HTGR analyses included: (1) modeling of the transient two-dimensional temperature behavior of graphite moderated reactor cores (pebble bed and block-type), reactor vessel, and reactor containment, (2) modeling of flow losses and convective heat transfer in pebble bed reactor cores, (3) modeling of oxidation of graphite components in reactor cores due to the ingress of air and/or water, and (4) modeling of the affect of oxidation on the composition of gases in the reactor system. The applications of the extended code to LWR analyses showed that advanced fuels intended for proliferation resistance and waste reduction could also be designed to produce calculated peak cladding temperatures during a large break LOCA less than the 1477 K acceptance criterion in 10 CFR 50.46. Fuels composed of ThO2-UO2 and ThO2-PuO2 are examples of such fuels. The applications of the extended code to HTGR analyses showed that: (1) HTGRs can be designed for passive removal of all decay heat, and (2).