Thermal Lattices Research Articles

Resonance shielding in thermal reactor lattices

The theoretical foundations of a new methodology for the accurate treatment of resonance absorption in thermal reactor lattice analysis are presented. This methodology is based on the solution of the point-energy transport equation in its integral or integro-differential form for a heterogeneous lattice using detailed resonance cross-section profiles. The methodology is applied to LWR benchmark analysis, with emphasis on temperature dependence of resonance absorption during fuel depletion, spatial and mutual self-shielding, integral parameter analysis and treatment of cluster geometry. The capabilities of the ozma code, which implements the new methodology are discussed. These capabilities provide a means against which simpler and more rapid resonance absorption algorithms can be checked.

Resonance absorption calculations in thermal reactors

Publisher Summary The calculation of resonance absorption rates in nuclear reactor cores has long been recognized as one of the most complicated parts of reactor analysis on account of the large number of nuclides with pronounced resonances that are present in the fuel and the drastic cross-section changes that occur in each resonance, sometimes over very narrow energy ranges. The difficulties arise both from the mathematical complexity involved in the accurate description of the cross-section shapes and their changes with temperature, and from the flux changes produced by the resonance absorption. The resonance absorption depends on the fine structure of the flux that has pronounced dips near the energies of the cross-section peaks. An accurate calculation aims at evaluating the resonance reaction rates directly after the energy dependence of the flux has been determined in sufficient detail, particularly at energies in the vicinity of the resonance peaks. Such calculations are quite feasible at present on large computers, for cases in which the material composition and geometrical description of the problem under investigation are not too complicated. This chapter discusses a number of different practical methods in which the resonance absorption problem has been handled in the case of thermal reactor lattices. Lattice heterogeneity effects lead to several practical methods for the calculation of effective resonance integrals that are utilized in a number of core physics analysis codes. They are also required in codes that evaluate the resonance reaction rates directly.

Multigroup Calculations of the Fast Spectrum and Fast Fission Effect

A multigroup procedure for the calculation of the fast fission phenomena in thermal uranium-water reactors has been developed. The method essentially consists of applying the single-flight collision concept in a manner analogous to the calculation of resonance capture in thermal reactor lattices. The collision and escape probabilities are calculated by numerical integration over the actual neutron paths encountered in a reactor lattice. The multigroup equations are solved by an iterative procedure which converges rapidly. The fast neutron spectrum, δ28 and ɛ can be obtained.Results of calculations are presented in which the value of δ28 homogeneous uranium-water mixtures and for slightly-enriched uranium-water lattices are compared with Monte Carlo calculations and experiment. Very satisfactory agreement has been obtained. Fast neutron spectra in the core of a pool type reactor and in the fuel and moderator regions of a uranium-water lattice, calculated by the present method, are also shown.