ABSOLUTE FISSION RATE MEASUREMENTS IN THE FAST BREEDER BLANKET FACILITY

HWAI-PWU CHOU, Purdue University

Abstract

An automatic fission track scanner has been developed for fission track counting. The system consists of a scanning optical microscope and a microcomputer to perform scanning, data acquisition, and analysis. Output from this system provides the total number of tracks, a track size distribution, the locations as well as sizes of individual tracks, and a partial scanned image. Fused quartz has been found to be the most suitable recorder medium for the current system. The stability of the system has been extensively checked; a counting reproducibility of (+OR-)3% (one standard deviation) has been achieved. A ('244)Cm spontaneous fission source has been used to determine the detection efficiency of the system.^ The automatic track scanner has been used for absolute fission rate measurements in the Fast Breeder Blanket Facility (FBBF). The measurements serve as integral tests of calculational methods and nuclear cross section data. Both radial and axial fission rate distributions of ('232)Th, ('235)U, ('237)Np, ('238)U, and ('239)Pu have been measured in the radial blanket region. Spatial fission distributions within fuel rods have also been determined experimentally.^ Fission rates have been calculated using the two-dimensional diffusion code 2DB. Cross sections are based on the LIB-IV library. Diffusion calculations have been performed to examine the heterogeneity effects on fission rates. The measured fission rates have been compared with the fission rates calculated using the diffusion code 2DB and the Monte Carlo transport code VIM. The present results have also been compared with experiments conducted on the ZPPR-4 and the BTF-2 blankets.^ The present study has led to the following conclusions: (1) Significant differences exist between diffusion calculations and transport calculations in the prediction of fission rates in LMFBR blankets. (2) Diffusion calculations are inadequate for predicting the ('235)U, ('238)U, and ('239)Pu fission rates and thus the power production in thick uranium blankets of fast breeder reactors. (3) The Monte Carlo transport calculation better predicts fission rates of ('232)Th, ('235)U, ('237)Np, and ('239)Pu. (4) Overprediction of ('238)U fission rates by the Monte Carlo VIM calculation is attributed to ENDF/B-IV data above 1 MeV; the current results do not suggest which nuclide is the major cause of the overprediction. (5) The ENDF/B-IV fission cross sections of ('235)U and ('239)Pu appear to be consistent. (6) The current heterogeneity correction to resonance self-shielding has a relatively small effect on fission rates. (7) The 2DB calculation predicts the axial leakage of the FBBF blanket reasonably well. (8) Neutron flux depression within fuel rods does not appear to affect fission rates in the FBBF blanket. (9) Underprediction of the gradient of fission rates at the inner-outer blanket interface by the 2DB calculation is attributed to interfacial effects. ^

Degree

Ph.D.

Subject Area

Engineering, Nuclear

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