API researchers study physics of complex reactor systems consisting of coupled fast and thermal cores.

The "coupled reactor" is the reactor system of two or more cores with neutron exchange between them. It may have different spectra in the cores. These cores may have different geometrical parameters. The cores may be critical or subcritical, with steady-state or pulse modes of operation.

To describe neutronics behavior of such a complex coupled reactor system, it is important to know:

• Neutronics parameters of parts of the system
• Parameters describing the coupling between the parts of the system

The 3D space-time distribution of fission events in such a coupled reactor is determined by the level of neutron interaction between the components of the system. The simulation of physics processes in coupled reactors is the challenging task. It can substantially differ from standard reactor physics approaches.