In postulated loss-of-coolant accidents in water-cooled reactors, it is possible for an increase in Zircaloy clad temperature, coupled with reactor depressurization, to give significant local clad strain, and hence reduced area for coolant flow. This paper establishes a simple method of calculating the effect of consequent impairment of local heat-removal capability. An existing flow model, due to Gambill, has been used to estimate the local reduction in the heat transfer coefficient due to clad ballooning. By formulating an energy balance for the fuel pins, temperature transient curves for the distorted cladding are derived from those for undistorted fuel.To analyze the complicated two-phase phenomena, several simplifying assumptions are contained in the flow model. Results, therefore, are given for a range of flow and blockage assumptions, and are shown to be in reasonable accord with an analysis using large and complex computer codes and with all available experimental data.The model can be applied to all types of water-cooled reactors, including pressure tube reactors, by a suitable evaluation of the resistance to bypass flow.