Determination of 137Cs content in fuel assemblies of a zero power reactor by Monte Carlo based efficiency calibration

Abstract

In the Training Reactor of the Budapest University of Technology and Economics EK-10 fuel assemblies are applied and some slightly irradiated EK-10 fuel assemblies are stored as spare fuel. These fuel assemblies were used in a critical assembly in the first part of 1970s. Due to the lack of exact data related to the power history of these assemblies the long-lived fission product (137Cs) content was determined by gamma spectrometry. Some fuel assemblies were measured in a predetermined geometric arrangement using a portable Canberra GX2518 XtRa coaxial type HPGe semiconductor gamma detector. In order to determine the energy dependent detection efficiency function for this rather complex measuring geometry Monte Carlo model was constructed. For calculation of the specific activities of radioisotopes located in the fuel assemblies MCNP6 transport code was applied. The MC simulation model such as the material composition of fuel rods, cladding and the exact geometry of fuel assemblies were designed by original documents of EK-10 fuel assembly. The MC model calculation was verified by the measured intensities of the gamma lines emitted by 235U isotope and of the gamma lines of the daughter elements of 238U.