(552b) Monte Carlo Simulation of Tritium Generation Signals In the Utah TRIGA Reactor for Nuclear Forensics

Todd Sherman, Haori Yang, Dong-OK Choe and Tatjana Jevremovic

Nuclear Engineering Program, The University of Utah

Abstract

The University of Utah Nuclear Engineering Program houses a Mark I TRIGA Reactor,  UUTR (University of Utah TRIGA Reactor). This reactor is a pool type reactor with water being used as both coolant and neutron moderator. Regular city water is filtered and then dispensed into the tank. UUTR uses low enriched uranium fuel.

Good neutron moderators are the materials that have large scattering cross sections and small absorption cross sections. This means that the probability that neutron will collide and impart some of its energy in the target nuclei is much higher than the probability that the neutron will be captured into the target nuclei. Elements like boron and cadmium are poor moderators because they have large absorption cross sections of 767 b and 2,520 b, respectively. Instead they are used for controlling the net neutron populations in reactors by capturing neutrons. Hydrogen, oxygen and carbon are better suited for neutron moderation.

The UUTR utilizes heavy water to both moderate and reflect neutrons that find their way to the outer ring of the core. These heavy-water elements are filled with 68% D₂O and 32% H₂O. The Thermal Irradiation Facility is also filled with the same percentage of heavy water. These are the primary sources of tritium production in the UUTR. Hydrogen captures a neutron to become deuterium and deuterium captures a neutron to become tritium. Tritium has a smaller scattering cross section than the other hydrogen isotopes but has no known absorption cross section which could make it a valuable moderator. Tritium, however, is unstable and will undergo a beta decay to become a stable helium-3 nucleus. The half-life of tritium is 12.33 years.

The tritium generation in the heavy water elements and the Thermal Irradiation Facility was evaluated based on the MCNP5 model of the UUTR core. Key parameters (e.g. neutron flux inside Central Irradiation facility) were experimentally measured and used to benchmark the MCNP5 model. The MCNP5 estimates were compared with the analytically obtained values. Both, simulated and analytically estimated concentrations of tritium in UUTR’s heavy water elements will provide a set of nuclear signatures of interest to nuclear forensics.

Key words: nuclear forensics, TRIGA reactor, tritium, neutron flux, MCNP5