Temperature transients in a rod cluster fuel during certain flow and power transients

Two computer codes FLATT and COBRA III-C are described. FLATT is developed for cylindrical geometries, but it can also be used for rod cluster fuels. It calculates transient temperatures during flow and power transients. COBRA III-C carries out subchannel thermal hydraulic analysis of rod cluster fuels and can be used for both steady state and transient conditions of single phase or two phase flows. Codes have been used to predict coolant, clad and fuel temperatures in Trombay R-5 reactor -- now under construction -- during: (1) flow transient resulting from failure of power of the coolant circulating pump and (2) temperature transient caused by power transient arising from the uncontrolled pump-up of moderator into the reactor vessel. The maximum temperatures have been found to be within acceptable limits. Reasons for differences between temperatures predicted by the two codes are discussed. Computation time required for COBRA III-C is more than that for FLATT. For parametric studies of various designs, FLATT can be used and the design options narrowed down. COBRA III-C can then be used for the final analysis of the chosen design. (M.G.B.)