Three-dimensional reactor dynamics code for VVER type nuclear reactors:Dissertation

A three-dimensional reactor dynamics computer code has been developed, validated and applied for transient and accident analyses of VVER type nuclear reactors. This code, HEXTRAN, is a part of the reactor physics and dynamics calculation system of the Technical Research Centre of Finland, VTT. HEXTRAN models accurately the VVER core with hexagonal fuel assemblies. The code uses advanced mathematical methods in spatial and time discretization of neutronics, heat transfer and the two-phase flow equations of hydraulics. It includes all the experience of VTT from 20 years on the accurate three-dimensional static reactor physics as well as on the one-dimensional reactor dynamics. The dynamic coupling with the thermal hydraulic system code SMABRE also allows the VVER circuit-modelling experience to be included in the analyses. With HEXTRAN it is possible to make realistic time-dependent analyses starting from the actual core cycle conditions. Methods for making conservative accident analyses with this best-estimate code have also been developed. The usefulness of the three-dimensionality is shown particularly in accidents with asymmetric fission power distribution originating from local neutronic or thermal hydraulic disturbances in the core or cooling circuits. Complicated accidents in which there are strong interactions between neutron kinetics and thermal hydraulics can be reliably analyzed. The new hydraulics solution method PLIM developed at VTT has been applied in HEXTRAN to remove modelling restrictions and to eliminate numerical diffusion and dispersion. The use of PLIM improves accuracy and expands the applicability range of the code. HEXTRAN has been validated against different types of relevant information available, viz. measurements of a test reactor, start-up experiment and other data of a real plant, as well as with calculations of several international benchmark problems and independent code comparisons. HEXTRAN's applicability to calculate VVER-440 and VVER-1000 type reactors has been demonstrated. Extensive analyses have been carried out with HEXTRAN on both design basis and new types of accidents, e.g. RIA, ATWS or local boron dilutions, for the Finnish Loviisa and Hungarian Paks nuclear power plants as well as for the new Russian VVER-91 concept