Ullage Temperatures In ‘Wet’ Spent Fuel Transport Flasks

Abstract Calculation techniques for predicting the thermal and hydrodynamic conditions in spent nuclear fuel transport flasks have recently been extended to include computational fluid dynamic (CFD) modelling. This paper outlines the modelling methods used for predicting the thermodynamic conditions in the ullage space of wet transport flasks during normal transport and in a simulated fire event. The calculations include conjugate heat transfer, that is thermal transport through substantive solid volumes enclosing fluid spaces, radiative transport within and without the fuel element arrays and convective transport with phase change. Fuel pin temperatures have been shown by this method to be lower than previously calculated by other methods. In the worst case considered, representing the highest licensed thermal load in the hottest-running package, pin temperatures have been shown to remain far below temperatures that could cause cladding deterioration or autoignition of radiolysis gas products.