Some considerations on the melting of reactor fuel plates and rods

Abstract

Current practice generally leads to overdesigning with respect to catastrophic accidents involving gross melting of fuel elements. It is clear, however, that from both the standpoints of cost and safety it would be desirable to be able to base the design on calculations regarding the details of the melting process itself. This paper considers the gross melting of fuel plates and rods and seeks to estimate the extent of melting and the rapidity with which it occurs. Methods of analysis are presented in detail in the present exploratory study. For short times after the onset of melting, an analytical series solution is constructed by the ‘embedding technique’. The effect of the change in density which accompaies the change in phase is examined, and is found to be relatively unimportant as far as the melting front position is concerned. An exact steady-state solution is found, which makes possible a prediction of the maximum extent of melting in cases in which steady state can be expected. The actual progress of melting for all times is more difficult to calculate, and an approximate solution is therefore introduced. Its accuracy can, in some cases, be estimated by comparison with the known final melting extent. The approximate solution is applied to a class of melting problems, and the results are presented in the form of graphs depicting the progress of the melting front for a range of parameters representing various possible conditions.