Experimental Evaluation of Reactor Neutron Spectrum in Irradiation Field

Abstract

To evaluate the neutron energy spectra in the irradiation field of a materials testing reactor, the corresponding spectra are determined on a critical facility of identical core configulation by multi-foil activation measurements. The validity of this method is examined. In the present work, emphasis is laid on the thermal-to-intermediate transition energy region. For spectrum unfolding, the SAND-II code is utilized, with the cross section data for all the detector foils taken from ENDF/B-IV. Self-shielding corrections are successfully applied on the activation cross section data of the foils, being based on a mono-energetic model. An examination is made of the error incurred by this expedient. The propriety of the correction depends on the manner in which the scattering effect is treated. The spectrum obtained can be considered unique from the fact that the thermal-to-intermediate facing coeficient varied only by 2%, the thermal neutron temperature by 15°C, and the unit lethargy flux level of intermediate region by 5% between the data yielded by the different spectrum parameters which are chosen to result in a reasonable spectrum. The spectrum uniqueness is further ensured by evaluating the foil data and discarding those that add incongruity to the resulting spectrum, and selecting disintegration data that contribute to shaping the spectrum into a physically plausible form.