Assessing pathways to tritium production and its detailed spatial distribution throughout the VHTR

The content of this work focused on calculating tritium production in the active core region as well as the surrounding components of the Very High Temperature Reactor (VHTR) using detailed Monte Carlo (MC) simulations. This is one of VHTR operational issues that need to be addressed. Permeation models of tritium in the VHTR plant have high levels of uncertainty associated with the initial tritium source from different pathways. In the past, the sources were generally derived from simple neutronics calculations in one dimension and one group. While providing a good estimate for integral pathways such as ternary fission, quantifying system-wide production via impurities in surrounding components may be largely inaccurate. To reduce this inaccuracy, the MAVRIC sequence of the SCALE 6.1 code package was used to calculate tritium production rates using a highly detailed Monte-Carlo model for neutron transport simulations covering the whole volume inside the reactor pressure vessel. It was found that assumptions about impurity concentrations in the graphite reflector and helium coolant could lead to larger tritium production rates than previously assumed from more simplified neutronics models. Previous studies showed that tritium permeation to secondary systems already exceeded EPA standards. Using a more detailed neutronics/shielding model in this study, even higher production rates were calculated than before. Based on these results, more work needs to be done to reduce leakage to secondary systems by improving helium purification systems and reducing impurities in structural components. Sophisticated transport theory simulations are necessary to support such analyses. The knowledge obtained in this study will also be used in tritium production studies related to liquid salt cooled reactors (LSCRs). Finally, it will inform design and selection of appropriate dosimetry needed to validate simulations. (authors)