Investigations of the VVER-1000 coolant transient benchmark phase 1 with the coupled system code RELAP5/PARCS

As part of the reactor dynamics activities of FZK/IRS, the qualification of best-estimate coupled code systems for reactor safety evaluations is a key step toward improving their prediction capability and acceptability. The VVER-1000 coolant transient benchmark phase 1 represents an excellent opportunity to validate the simulation capability of the coupled code system RELAP5/PARCS regarding both the thermal hydraulic plant response (RELAP5) using measured data obtained during commissioning tests at the Kozloduy nuclear power plant, Unit 6, and the neutron kinetics models of PARCS for hexagonal geometries. Selected results of performed investigations will be presented and discussed in this paper. The overall trends of most plant parameters are in a reasonable agreement with the experimental data. Nevertheless multidimensional thermal hydraulic models are needed for a more realistic description of the coolant mixing phenomena within the reactor pressure vessel.