Change in the Temperature Coefficient of Reactivity in Light-Water Reactors During Fuel Burnup

The results of computational studies of the effect of different approximations on the estimation of the temperature coefficient of reactivity are presented. The Doppler effect is evaluated for fuel elements and groups of fuel elements in a light-water reactor (VVER, PWR) fuel assembly using a modified option of theUNC program intended for calculating two-dimensional regions with complex geometry by the first-collision probabilities (FCP) method. It is shown that taking account of the spatial nonuniformity of the resonance absorption and temperature distribution along the radius of a pellet gives a correction to the temperature coefficient of reactivity (toward lower values) by approximately 10 % in PWR compared with a calculation by a conventional method, which, as a rule, neglects the factors enumerated. This is characteristic for one fuel element or a group of fuel elements.