Verification of the BONUS module integrated into the SOCRAT code

Abstract

The results of a verification of the BONUS method as regards predictions of the time dependence of the mass and activity of fission products produced in thermal reactors are presented. Different standard regimes of fuel irradiation in VVER-1000 are examined, and the results calculated using the autonomous version of the BONUS code and as a module integrated into SOCRAT code are compared with the results obtained using other codes, including precision program complexes. On the whole, the calculation shows good agreement between BONUS and alternative codes; the standard deviation variance is 6-13.5% for fission product mass and activity, which is comparable to the discrepancies between different variants of the alternative calculations themselves.It is obvious that the moduli of a well-balanced integrated code that describes the complex physicochemical processes occurring in a reactor facility in standard and emergency situations must conform to an overall targeted accuracy. Specifically, considering the simplification of the models used to describe fission-product transport in a reactor facility as well as the uncertainty of the parameters and input data, high-accuracy cumbersome computational methods can hardly be regarded as the optimal choice for describing effects associated with radioactive intertransmuting actinides and fission products. In this connection, the BONUS method was developed at the Institute of Problems in the Safe Development of Nuclear Energy (IBRAE) to perform simplified calculations of the time dependence of the concentration, activity, and energy release of fission products. The method is implemented as a separate program module to be included in computer codes, such as the MFPR code [1,2], that simulate the behavior of fuel and the yield of fission products in different operating regimes or in integrated codes, such as SOCRAT [3], that simulate processes occurring inside and outside vessels during anticipated and unanticipated accidents.The BONUS method describes in the one-group approximation the change of the concentration of actinides under neutron irradiation as well as the concentration of fission products taking account of their production as a result of fission, γ and β decay, and radiative capture of slow neutrons. Of course, the evolution of the radionuclide composition of the fuel after the reactor is stopped is also simulated. Actually, the BONUS module incorporated in an external code is used to calculate the production of actinides and fission products in an individual spatial cell. It is supposed that the time dependence of the specific energy release in a given cell or the thermal-neutron flux density is given in at the irradiation stage. The par-

Modeling the diffusion yield of radioactive fission products from uranium dioxide fuel

“…The boundary conditions were chosen in accordance with the thermochemical model used in MFPR, according to which (8) where ξ α is a dimensionless parameter, calculated from the condition of thermochemical equilibrium between the in-and inter-grain phases [5,6]. This parameter, which is assumed in the present work to be constant and set externally, is related with the solubility of the fission products in the matrix: it is close to 1 for fission products with poor solubility and close to zero for good solubility.…”

“…The standard technique used in many works presupposes the expansion The coefficients w n are found by solving the equation (10) Equations for Determining the Concentration of Fission Products Outside a Grain. (3) and (10) as well as the model of boundary conditions (8). Indeed, often the yield is very low, so that the concentration c out = c tot −  c in outside a grain can be many times smaller than  c in .…”

“…The numerical scheme is based on a one-group method of calculating the production and radioactive mutual transmutations of fission products in combination with the standard expansion of the radial dependence of the concentration in terms of the eigenfunctions of the Laplace operator. The production of fission products and their radioactive mutual transmutations are calculated by the BONUS procedure [7], which gives an acceptable error (~10%) of the calculation of the production of fission products and their activity and residual energy release with the lowest computing costs [8].Model Particulars of the Base MFPR. In the first theoretical works, it was supposed that the escape of fission products is associated with, first and foremost, near-surface mechanisms [1] in which fission products participate directly.…”