Irradiation of Fusion Reactor Materials and Bio-Dosimeters in JSI TRIGA Reactor

The TRIGA research reactor at Jozef Stefan Institute (JSI) is used for irradiation of various samples. Recently two projects have been initiated; the development and improvement of future fusion reactor materials and the development of bio-dosimeters. They both demand a large number of irradiations under different conditions (neutron spectra and flux) and subsequent gamma spectral analyses of the samples. In order to characterize the neutron spectra and fluxes in different irradiation channels that are of vital importance for the quality of irradiation, a detailed computational model of the TRIGA Mark-II reactor with the MCNP Monte Carlo particle transport code was developed, experimentally validated and verified. The projects and the main results are presented in the paper. Irradiation of Eurofer and SiC samples in our reactor is feasible and gives reliable results for material development and optimization in fusion reactors provided that the irradiation is supported with detailed spectrum calculations to take into account also the contribution of fast neutron reactions. Detailed knowledge of gamma and neutron spectrum is vital also in experimental development and calibration of bio-dosimeters. It can be concluded that irradiation of non-standard materials requires support of detailed calculations of spectrum in irradiation facilities not only to improve the accuracy but even to make the irradiation methods feasible. Main isotopes contributing to the long-term activation of Eurofer have relatively short life-times (100 days to several years). This composition of Eurofer is suitable from the radioactive waste disposal point of view. However one should be aware that dose rates inside and around the Eurofer structures will be significant even several months of cooling. The highest contribution to the total activity (80%) in this time interval comes from 182Ta. It may be speculated that further optimization of Eurofer with respect to the activation could be achieved if the elemental composition were re-examined, in particular considering fast neutron reactions. In comparison with several SiC composite materials Eurofer looks to be the worst choice for almost the entire time span (up to 10.000 years). However for longer times the activity of SiC is even larger than that of Eurofer mainly due to 26Al, the activation product of Si when irradiated with very fast neutrons (E > 10 MeV). The short and medium time activity of the SiC can be significantly reduced by optimizing the manufacturing process by reducing the impurity content as low as possible. At lower doses the EPR signal intensity is very weak and the relative error is very large, meaning that our methods are still not good enough for non-lethal dose measurements. Results show linear relation between the measured and delivered dose for both type of dosimeters. Currently the EPR dosimetry can be used as a tool for measuring high doses, such as ones met in or near critical fissile systems. In the future we will develop very accurate methodological approach to eliminate systematic uncertainties and verify the calculations by the use of different dosimeters appropriate for measuring high neutron and photon dose rates separately