VERIFICATION OF BENCHMARK EXPERIMENTS BY DETERMINATION OF THE LEAKAGE NEUTRON SPECTRUM FROM THE SURFACE OF A DEPLETED URANIUM SPHERE WITH EMBEDDED RADIONUCLIDE SOURCES

Computational validation is presented for previous benchmark experiments designed to determine the energy spectra of the neutron current emanating from the surface of a uranium sphere into which small sources were arranged successively at the center. Such experiments must meet stringent reliability and error requirements, since they are intended for the fi nal check of new or adjusted systems of neutron constants recommended for the calculation of neutron fi elds. The comprehensiveness of the validation was secured by the fact that experiments with the three sources 252 Cf, Pu–Be, and Pu–B were validated and the calculations were performed using three systems of neutron constants BNAB-78, BBF, and BUGLE-96. The codes used to calculate the ionizing radiation during the design and operation of NPP and other facilities using atomic sources are now being improved in connection with the development of nuclear power and advances in computing technology. Complex three-dimensional problems can now be solved taking account of more stringent error requirements of the results. As a rule the, libraries of the interaction constants for neutrons interacting with the nuclei of different isotopes used in the calculations make an appreciable contribution to the error. For this reason, there is now a need to check them with the aid of benchmark experiments. Considering the importance of the results heightened requirements are now imposed on the reliability, ordinarily evaluated by performing verifi cation by means of comprehensive calculations. The simplest geometry is chosen for the experiments. The materials studied are fabricated in the form of spheres; small radionuclide sources are used in the experiments so that they can be modeled by point isotropic sources located at the center of a sphere. The geometry chosen for the experiments makes it possible to measure reliably the energy distribution of the neutron current from unit surface area of a sphere, which is the main result, and moreover to use simple one-dimensional codes and neutron-constants libraries which have proven themselves well in practice. The experimental data presented in the form of the energy distribution of the neutron current are informative, and when validating new or improved libraries of neutron constants they make it possible to fi nd on the energy scale appreciable discrepancies with experiment and introduce the corresponding corrections. The aim of the present work was to validate the results of previous benchmark experiments on determining the energy spectra of the neutron current emanating from the surface a uranium sphere. The choice of uranium is associated with the fact that it is one of the main materials used in nuclear technology.