3-D Simulation of Reactivity Transients in Cores of VVER-Reactors

Reactivity initiated accidents (RIA) have to be analyzed for safety assessment of nuclear reactors. The presented 3-dimensional core simulator DYN3D/M2 describes the space dependent effects of reactivity perturbations caused by control rod motions or local changes of coolant temperature and boron concentration. The safety margin can be determined more precisely by use of 3-D models than with simpler methods as point model or one-dimensional kinetics. A nodal expansion method (NEM) for hexagonal geometry of VVER fuel elements was developed to reduce the numerical effort of 3-dimensional neutron kinetics. The changes of thermohydraulic parameters as fuel and coolant temperatures, coolant density and poisoning are determined with the help of the thermo-hydraulic model FLOCAL included in the code. The feedback effects derived from the influence of the thermohydraulic parameters on the neutron physical constants. Applying DYN3D to safety calculations of reactivity transients, the code was validated by comparing its essential parts with benchmarks, other codes and experiments. An accident caused by the ejection of a single control rod in a VVER-440 reactor was analyzed. The results show a very high power peak in the neighbourhood of the ejected rod. Runs with time step control indicate that the computer time can be reduced in comparison to fixed time step. Simulations with a smaller number of coolant channels were investigated for coupling the core model with codes simulating the whole coolant system of a nuclear power plant.