Experience in Simulation of Startup Measurements in the Vver-440 Reactor Using the Kiko3d Code

The purpose of this work carried out by MTA KFKI Atomic Energy Research Institute in Hungary, is to develop and validate a method for predicting the ex-core detector responses in the Russian pressurized water type VVER reactors. The time and space dependent neutron flux in the core during these measurements have been calculated by the KIKO3D nodal diffusion code. For calculating the ionization chamber signals the Green function technique has been applied. The Green functions of ionization chambers have been evaluated via solving the neutron transport equation in the reflector regions with MCNP Monte Carlo code. The detector signals have been calculated and compared with measured ones using the inverse point kinetics transformation. Large number of asymmetric rod drop measurements (with one rod stuck) and some differential rod worth measurements from the Zero Power Physics Tests were provided by the Paks NPP for validation. The initial state of experiments covers different fuels (without and with enrichment zoning) and loading patterns. The intermediate range ionization chambers have been used during the scram measurements. The newly developed method provides fairly sufficient match of measured and calculated results. The time behavior of the detector readings is well modeled. Their space dependent effects observed in the measurement are described by the code in a consistent manner. The uncertainty of scram rod worth is established by statistical analysis.