Computer Model for Steady-State and Transient Analysis of Nuclear Reactor Cores. DYN3D is a three-dimensional code for the calculation of steady states and transients in light water reactors with hexagonal or quadratic fuel assemblies. The neutron kinetics is connected with a two-phase thermohydraulic model and a fuel rod model. DYN3D is linked to the thermo-hydraulic system code ATHLET in three different ways - external coupling, internal coupling and parallel coupling. DYN3D is capable of calculating the burn-up prior to transient investigations. Presently, a new version with multiple energy groups and SP3 neutron transport option is being developed.