Neutron transport in doubly heterogeneous media of high temperature reactors

The AGENT code methodology was extended to include the ability to simulate the neutronics of the Very High Temperature Reactor (VHTR). This involved changes to both the geometry and flux solver modules. Changes to the geometry module included the extension of the AGENT lattice mode to model hexagonal assemblies. This involved the modification of existing primitive bodies and the addition of new ones to the AGENT library of primitive bodies. Changes to the flux solver enables AGENT to treat the Double Heterogeneity problem that results from the random distribution of the fuel grains in the graphite matrix of the VHTR fuel region. The implemented Double Heterogeneity treatment uses renewal theory with a Poisson and an isotropic distributions assumed for the chord lengths in the graphite matrix and the fuel grain regions respectively. Assuming collisionless transport, the resulting set of closed renewal equations are used to derive the analytical expressions for the flux along the neutron paths in the matrix and fuel grain regions. First flight collision and escape probabilities are used to solve for the flux in the fuel grain regions and combined with the Method of Characteristics the neutron flux in the entire double heterogeneous region is solved. The resulting modifications to the AGENT code have been verified against relevant benchmarks