Characterization of the neutron spectra in three irradiation channels of the JSI TRIGA reactor using the GRUPINT spectrum adjustment code

Abstract

Reference neutron fields in irradiation facilities are crucial for experimental research work in nuclear science and technology in general, and at the fundamental level for the measurement of neutron cross sections and validation of evaluated cross-section data. Monte Carlo particle transport techniques in conjunction with state-of-the-art nuclear data libraries are becoming the reference in the broad context of nuclear reactor analysis, and are extensively used for detailed computational characterization of irradiation facilities, including the calculation of neutron spectra. However, adjustment of the neutron spectra based on experimental data is still required.In this work we present the characterization of the neutron spectra in the three most important irradiation channels of the JSI TRIGA reactor with different spectral characteristics, as an important step in developing the experimental capabilities of the reactor for the purpose of nuclear data validation through activation dosimetry measurements. In the first part of this work we performed detailed Monte Carlo calculations of the neutron spectra, comparing the results obtained using different nuclear data libraries. In the main part of this work we employed the JSI developed GRUPINT spectrum adjustment code for the characterization of the neutron spectra, on the basis of the calculated neutron spectra and measured reaction rate ratios. A comprehensive overview of the methodology employed in the code and its abilities is given.We observe that a positive compensation of up to 10 % in the thermal part of the spectrum is needed, while no compensation is needed in the epithermal and fast parts. Absolute neutron flux levels were computed from the adjusted neutron spectra and absolute reaction rate measurements, and compared to the results of a previous analysis using only the Monte Carlo calculations. The work presented in this paper represents the basis for future experimental and analytical work, in particular extending the scope of the measured capture and threshold dosimetry nuclear reactions, to support a thorough characterization of the neutron spectra in the irradiation channels of the reactor. This effort will enable the JSI TRIGA reactor to support a broad scope of experiments for fundamental research as well as development or testing activities at the highest level.