DORT-TD: A transient neutron transport code with fully implicit time integration

A new neutron transport code for time-dependent analyses of nuclear systems has been developed. The code DORT-TD is based on the well-known Discrete Ordinates code DORT, which solves the steady-state neutron transport equation in two dimensions for an arbitrary number of energy groups and standard regular geometries. For the implementation of time-dependence, a fully implicit, unconditionally stable time integration scheme was employed to minimize errors due to temporal discretization. This requires several modifications to the transport equation and the extensive use of sophisticated acceleration mechanisms. The convergence criteria for fluxes and fission densities had to be strongly tightened to ensure the reliability of results. We also allowed for cross sections varying with time to couple neutronics and thermal hydraulics calculations. The neutronics code was finally applied to a research reactor to show its capabilities for both slow and fast transients.