HEXTRAN - a 3-D reactor dynamics code for VVER accident analysis

The reactor dynamics calculation system of VTT Energy consists of models for both reactor types in Finland - the ABB Atom type BWRs in Olkiluoto and the VVER type PWRs in Loviisa. HEXTRAN is a three-dimensional hexagonal transient and accident analysis code developed in VTT for VVERs. Now it is possible to make fully realistic accident analyses starting from accurate core conditions. The thermal hydraulics model SMABRE is dynamically coupled with HEXTRAN for the primary and secondary circuit calculations. HEXTRAN coupled with SMABRE has been extensively applied to safety analyses of the Loviisa power station, of the new Russian VVER-91 concept, and of the Hungarian VVER-440 type NPP Paks. Main applications are the calculations of asymmetric accidents in the reactor core originating from neutronic or thermal hydraulic disturbances in core or cooling circuits. The main properties of HEXTRAN, its applications and plans for the development of its hydraulics model are presented in the paper. (14 refs., 2 figs.)