Evaluation of biases and experimental uncertainties of the fission rate profile measurements at the JSI TRIGA Mark II research reactor

Abstract : The paper focuses on a validation experiment performed at the JSI TRIGA research reactor using two absolutely calibrated CEA developed miniature fission chambers. These were used to measure a set of axial fission rate profiles through the height of the reactor core which were compared to reaction rate profiles computed using the Monte Carlo method. The purpose of the paper is to describe the support calculations and methodology used in the evaluation of the experimental and computational biases and uncertainties. This includes the study of the effect of the fission chamber structure and the aluminum guide tubes used for in-core insertion on the fission rate measurements as well as a detailed description of the influence of fuel material data error and fuel burn-up on the experimental uncertainty and computational biases. It is shown that the insertion of fission chambers and guide tubes into the core of the TRIGA reactor significantly influences the local neutron spectrum and consequently the fission rate measurements one can observe an up to 15 % decrease. A similar effect was recorded when modelling the fuel burnup, where a 10 % deficit of $^{235}$U mass results in an approximately 10 % decrease in fission rates