Analysis of the neutron spectrum for the validation of computational methods for the optimisation of research reactor utilisation

A project was initiated at the Jozef Stefan Institute to develop methodology for the validation of the computational models for the TRIGA reactor. A detailed computational model of the TRIGA Mark-II reactor with the MCNP Monte Carlo particle transport code was developed. A special feature of the model is flexible input design that allows relative simple modeling of operational characteristics such as fuel shuffling operations, control rod positions, etc. Validation of the spatial flux distribution was done by the first campaign of measurements in which Al-Au (0.1 % Au) monitor foils were irradiated in 33 positions in the core and its periphery. The results showed a very good agreement between measured and calculated spatial flux distributions in the reactor core and are reported by Trkov et al [1]. To validate the neutron spectra in commonly used irradiation positions series of activation experiments was performed. A larger number of monitor materials were irradiated in two irradiation positions: the CC and one channel in the carrousel facility (IC40). Monitors irradiated in the first experiment included gold, aluminium, zirconium, zinc, nickel, molybdenum, tungsten and manganese. The second experiment was performed to confirm the estimated neutron spectra in the central channel (CC) and IC40. Monitors irradiated in the second experiment included gold, aluminium, uranium, thorium and tungsten. The preliminary results of the neutron spectrum analysis in different irradiation channels show good agreement of the reaction rates calculated from measured specific saturation activities and the values obtained from the cross sections and the final calculated spectra for both investigated irradiation positions. It is therefore assumed that the calculated neutron spectrum represents the true neutron spectrum in the observed irradiation positions and that it can in future be used for the validation of the nuclear cross section data. (author)