Inventory Simulations Under Neutron Irradiation: Visualization Techniques as an Aid to Materials Design

The simulation of neutron irradiation induced transmutation using inventory codes is an important part of the research into materials in various nuclear environments, including fusion, fission, medical physics, nuclear security, and astrophysics. These simulations, even in their simplest form, such as the neutron irradiation of a single pure element, generate large time-dependent data sets of complex results. For each nuclide in the inventory, as a function of time, the output data will include the number of atoms and its contribution to a variety of radiological quantities including total or specific activity, gamma dose, heat output, and ingestion and inhalation hazards. A key challenge when performing inventory calculations is thus to represent the full complexity of the results in a visual and understandable format. This paper discusses two different approaches to visualizing inventory data: (a) nuclide maps, which allow the concentrations or activity contributions from all nuclides in the inventory to be displayed and also for the variation to be traced in time under a specific irradiation scenario, and (b) importance diagrams, which are a neutron spectrum independent representation of the dominant nuclides that contribute to the activity of an irradiated material. Finally, these techniques are applied in parallel to investigate how the activation response of molybdenum can be improved via isotopic tailoring (enrichment or depletion), which could make it a more viable alternative armor material in the design of fusion reactors.