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A method for evaluating the service life of VVER-1000 fuel-element cladding in constant and variable fuel load regimes is proposed. It is shown that a comparative assessment of the service life of cladding with different core regimes and parameters is possible on the basis of the energy approach to creep and destruction of cladding.In the last 30 years, there has been a trend toward greater completeness and degree of validation of VVER-1000 elements, since the strength of the elements of a nuclear power plant plays an important role in safety assurance. The normative literature does not show sufficiently clearly the requirements for thermomechanical safety calculations of VVER-1000 -the most widely used reactor in Russia and Ukraine [1].According to experimental investigations with increasing number of loading cycles, the strength of fuel-element cladding is determined mainly by the following [2]: 1) corrosion cracking under stress in an atmosphere of corrosive fission products; and 2) damage under the action of repeated cyclic and long-term static loads. Mechanical damage to cladding as a result of corrosive cracking under stress can be eliminated by limiting the power per unit length and any abrupt change of the reactor power and is not studied in the present article.One promising approach to evaluating the damage to the cladding material under repeated cyclic and prolonged static loads is using the energy variant of creep theory. The proposed method of evaluating the service life of the cladding of fuel elements of light-water reactors in different loading regimes can be regarded as an elaboration of the method of [3], developed for evaluating the moment of fracture of fuel-element cladding during the devilment of a serious accident associated with coolant losses.In choosing the FEMAXI program (Japan) for computational analysis of the development of stress and deformation in cladding, an important advantage of this program was taken into account: simultaneous solution of the equations of thermal conductivity and mechanical deformation [4]. The FEMAXI program is suitable for analyzing the behavior of fuel during normal operation to burnup >50 MW·days/kg and makes it possible to determine the response of a single fuel element to the combined effect of changes in power, coolant parameters, and structural characteristics of a fuel element.In the model used in the FEMAXI program, a fuel element is divided into 10 axial segments and the power per unit length, proportional to the instantaneous power of the reactor, is given at the center of each segment. The power per unit length at other points of axial segments is calculated by extrapolating the values prescribed for the central points.The temperature distribution in a fuel element with a discrepancy of no more than 0.1% between the numerical and analytical solution is predicted for a one-dimensional radial geometry based on taking account of the internal heat release and gap, and the heat transfer between surface of a fuel element and the coolant. The finite-eleme...