Sensitivity analysis in the problem of optimization of a VVÉR reactor configuration

Abstract

Analysis of the sensitivity of radiation field functionals to the parameters of the transport equation makes it possible to solve diverse problems of radiation physics. Those problems include evaluation of the shielding calculation error due to the indeterminacies of the initial parameters (radiation-matter interaction cross sections, assigned shielding characteristics, etc.), investigation of the laws of radiation propagation in the medium, determination of the input parameter that most affects the results, quantitative estimates of how small design changes affect the results, Issuance of practical recommendations for improving the quality of nuclear power plant design [1]. The last two are issues that are associated in one way or another with the process of logical optimization of the designs and radiation shielding of nuclear power plants. The functions of the relative sensitivity (in other words, the sensitivity profiles) serve as the initial information for optimization.In order to obtain the initial information for optimization of the VVI~R-440 reactor configuration, we have made a multigroup analysis of the sensitivity for the base model of reactor radiation shielding in the radial direction, using the example of the second unit of the Armyansk Nuclear Power Plant [2]. The chosen one-dimensional model of shielding in a cylindrical geometry corresponds to the reactor cross section in the horizontal plane passing through the center of the reactor core. The structure of the model and the size of the spatial zones are given in the first three columns of Table 1: the compartment is arbitrarily joined to the reactor cage.The radiation field and the conjugate functions were calculated from the one-dimensional ANISN program [3] in the PsS8 approximation of the scattering indicatrix and the radiation flux density, using the 171-group cross-section library VITAMIN-C [4]. Multigroup sensitivity coefficients were calculated from the modified program SWANLAKE-NG [5]. Some of the results are given as examples in Table 1 and Figs. I and 2. As the radiation field functionals we considered the fastneutron flux r t with an energy higher than 100 keV, which determines the radiation damage to the reactor vessel, the lowenergy neutron flux ~t/~, recorded with allv detector, in the vessel and in the first 50 cm of the structural concrete, which is responsible for the induced activity of the reactor materials. That induced activity must be known in order to solve the problems of decommissioning the reactor, the equivalent dose behind the radiation shielding being N. Table 1 also gives the coefficients of the relative sensitivity of the given functionals to the interaction cross section (to a change in the density of the materials of the plant) as well as for the individual spatial zones and for the entire configuration. The energy dependences of the functions of the relative sensitivity are given as an example in Figs. 1 and 2.The data in Table 1 indicate that for a fast-neutron flux r in the reactor vessel the sensitivity to t...