A Method Using Nuclear Emulsions for Measuring the Anisotropy of Fast Neutron Fluxes due to Arrangement of Fast Core Cell

Abstract

Measurements using nuclear emulsions have been made on the neutron spectra and on the fine structure of neutron fluxes in the cell of the I.4 core of the fast critical assembly at the Japan Atomic Energy Research Institute. The 1-4 core is a graphite-diluted fast core with 3:1 volume ratio of 20% enriched metallic uranium and graphite. The nuclear emulsions were irradiated in two typical patterns of arrangement of the cell (systems with graphite plates (a) bunched and (b) distributed among fuel plates).
For the distributed graphite plate system a marked discrepancy was found between the direct measurements and calculations based on the Monte Carlo method. This is attributed to anisotropy in the incident neutron flux due to the parallel plate arrangement of the simulated materials. It is concluded that Reines' formula requires correction to amount for such anisotropy, even when the emulsions are irradiated at the core center. A simple method for treating this anisotropy- is proposed for use in fine structure analysis. The method utilizes, in part, the calculated results.