RESONANCE SELF-SHIELDING IN FAST NEUTRON SPECTRA (FOIL, TRANSITION)
Abstract
The study of resonance self-shielding in heterogeneous media has direct applications on the design and analyses of fast neutronic problems. Integral reaction rate measurements using resonance detector foils require a self-shielding correction to account for the spatial shielding effect of the foil. Typical experimental correction technique based on extrapolating measurements for several foils of different thicknesses can lead to uncertainties in the experimental data. A new analysis method was developed to evaluate the foil correction factors of resonance detectors in fast neutron spectra. The new analysis method is based on an iterative treatment of the integral transport equation that describes the single and multiple scattering process inside the detector foil. The foil correction factor at each experimental position is related to the impinging neutron spectrum by combining the resonance self-shielding factors of individual Doppler broadened resonances with the multigroup reaction rates obtained from the results of a two dimensional diffusion theory calculation. Another important subject concerning heterogeneous resonance self-shielding occurs near a zone interface that separates regions of different material compositions. Such transition was observed experimentally in the capture data of ('238)U near the FBBF transformer-blanket interface. The short transition could not be described by routine neutronic calculation that assumed a discontinuous change in resonance self-shielding at the interface. A methodology was developed to treat the self-shielding transition near a zone interface. The method is based on an integral transport analysis of a single interface semi-infinite system. The narrow resonance approximation was applied to obtain a space and energy dependent self-shielding factor. Using the Wigner approximation for the energy dependent first collision escape probability, the resultant transitional self-shielding factor was further reduced to a more practical linear combination of various homogeneous and heterogeneous shielding factors. The analysis method was implemented in a routine cross section processing code that is based on the Bondarenko f-factor method. The result of the analysis was applied to the FBBF to improve the calculation near the transformer-blanket interface.
Degree
Ph.D.
Subject Area
Nuclear physics
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