Experimental determination and modelling of gamma radiation inside the TRIGA Mark II reactor at TU Wien

Abstract

The aim of this thesis is to improve the characterization of the gamma radiation field inside the TRIGA Mark II research reactor at the Institute of Atomic and Subatomic Physics at TU Wien. For this end, experimental measurements were carried out to determine the gamma dose rates at various positions in the reactor tank. These experimental results were then compared and validated against calculations from a computational MCNP6 model of the reactor.For the experimental task of this thesis, thermoluminescence dosimeters (TLDs) were irradiated in the reactor tank at different positions and at different reactor powers. Since the goal of this work is to examine only the gamma radiation part of the mixed radiation field in the reactor, a TLD type was selected that is, to a large extent, only gamma-sensitive. After irradiation in the reactor tank, the TLDs were heated in a read-out device and the light emitted during the heating process, which is proportional to the absorbed radiation dose, could be used to determine the gamma dose rate in the reactor tank. Various measurements were carried out to determine the gamma dose rate at a total of four positions (at different radial distances from the reactor core) and for different reactor power levels. In view of the relatively great range of gamma radiation, the main focus was on the outer positions of the reactor tank.To validate the experimental results of the TLD measurements, an MCNP model of the TRIGA Mark II reactor was used to calculate gamma dose rates in the reactor tank. In order to be able to compare the computational results with the experimental ones, the same positions and the same reactor powers were assumed in the simulation as for the TLD measurements. The MCNP model of the reactor already existed prior to this work. However, as this model was primarily developed for the analysis of the neutron flux in the reactor core,the model was spatially extended and adapted for the analysis of gamma radiation in the entire reactor tank.As expected, the results for the gamma dose rates show an approximate linear correlation with the reactor power, and an exponential decrease in the gamma dose rates with increasing distance from the reactor core can be observed. When comparing the experimental and computational results, it can be found that the experimentally determined gamma dose rates are in general higher than the ones calculated through MCNP simulation. This is particularlysignificant for low reactor powers and for positions that are close to the reactor core.This discrepancy between the TLD and MCNP results can be partly explained by the incompleteness of the MCNP model. The model does not consider delayed gammas in the reactor, which account for about 30% of the gamma flux in a reactor. In addition, the model does not account for reactor background activity or temperature fluctuations. Apart from the delayed gammas, whose influence is the same for all positions and reactor powers, the non-considerations in the MCNP model just mentioned is particularly relevant for low reactor power levels or measurements positions in the reactor tank that are close to thereactor core.