A time-dependent neutron transport code coupled with the thermal-hydraulics code athlet

We introduce a new neutron transport code for transient analyses of nuclear systems. The code system is based on the well-known Discrete Ordinates code DORT, which solves the steady-state transport equation for an arbitrary number of energy groups and standard 2D geometries. To minimise the errors due to temporal discretisation, an unconditionally stable, fully implicit time integration scheme has been employed. This requires various modifications to the transport code, the extensive use of elaborated acceleration techniques and substantially tightened convergence criteria for fluxes and fission densities. To perform coupled accident analyses, an interface to the thermal hydraulic system code ATHLET has been developed. Nodal power densities from the transport code are passed to ATHLET to calculate thermal-hydraulic system parameters, e.g. fuel and coolant temperatures. These are in turn used to generate DORT-compatible nuclear cross sections by interpolation from pre-calculated data sets for each time step. Finally, to demonstrate the transient capabilities of the coupled code system, the research reactor FRM-II has been analysed. Several design basis accidents were modelled, like the loss of offsite power and the unintended control rod withdrawl.