A simple conduction model with phase change has been developed for the transient analysis of a reactor fuel pin based on average properties and lumped parameter techniques. It has been shown that the transient behavior of fuel and clad can be accurately described by simple analytical expressions that agree with conventional numerical approaches for undercooling transient analysis. By assuming that the heat transfer resistance between the fuel and clad remains the same for both steady-state and transient periods, the phase-change problem for fuel and clad melting can be significantly simplified. Using the predetermined average overall heat transfer coefficient across a fuel pin in the steady-state period, the average transient fuel and clad temperatures can be formulated analytically. For loss-of-flow at constant power, the start of melting and complete melting for both the fuel and clad can be estimated with considerable accuracy. The purpose of this analysis is to provide a simple useful tool to obtain the general information about fuel and clad leading into the cooling transients. Such a simple fuel and clad thermal transient model is particularly useful to multichannel analyses where conventional conduction computer codes require considerable computing time and storage space. At present, this formulation is being employed for cladding motion and cladding blockage formation in the subassembly of a reactor core. The solution of the simple conduction model for the incipient melting time and the complete melting time of the clad in both R-5 and L-2 test fuel pins was compared with the numerical solution obtained from the SAS code, and the agreement between both solutions is excellent. For the R-5 test case, the analytical solution of the simple conduction model for the complete melting times for both clad and fuel was further compared with numerical solution calculated from THTB code at various axial position of the fuel pin. The comparison shows good agreement. The present lumped parameter model for a fuel pin has been developed to be used in an analysis o f multichannel clad motions in a loss-of-flow accident.
