Анализ подходов к оценке работоспособности оболочек стержневых твэлов энергетических ядерных реакторов с учетом ползучести

Abstract

Выполнен анализ существующих распространенных подходов к обоснованию работоспособности оболочек твэлов с учетом ползучести, основанных на рассмотрении деформирования сегментов оболочки внутренним и наружным давлениями и разностью температуры по толщине стенки. Показано, что что расчетная схема, учитывающая действие на оболочку только внутреннего и наружного давлений и перепада температуры по толщине не может давать надежных оценок работоспособности твэлов с учетом ползучести.

The creepage is one of the factors that considerably limit the working capacity of the shells of fuel elements of nuclear reactors. At the present time, to study a mechanical behavior of the shell of fuel elements it is subdivided into axial segments and consideration is given to the strain of each axial segment caused by internal and external pressures taking into consideration the temperature difference of outer and inner surfaces. Based on the damageability concept we suggested the mathematical formulation of the problem on the creepage of the axial segment of the shell of fuel element taking into consideration the action of internal and external pressures on it and the temperature difference of outer and inner surfaces that is given in the form of differential equations with boundary and initial conditions and the microscopic defect formation condition. The problem formulated in this manner was solved using the method of semi-descritization with spatial coordinate finite differences and stepwise time integration using the Merson method with the automatic selection of an integration step. It is shown that the formation time of microscopic defect in the shell of fuel element at a prescribed temperature due to the creepage is defined by the internal and external pressure difference module. It is explained by the dependence of the creepage rate and the damageability on the intensity of strains. Since the values of internal and external pressures on fuel element shells are comparable for contemporary nuclear reactors, we can draw a conclusion that a possible flexure of the shells of fuel elements should be taken into consideration in the heat carrier flow to substantiate the resource taking into consideration the creepage.