# Modelling of the WWER-440 Reactor for Determination of the Spatial Weight Function of Ex-Core Detectors Using MCNP-4C2 Code

16th Symposium of AER on VVER Reactor Physics and Reactor Safety (2006, Bratislava, Slovakia)

Core Surveillance and Monitoring

## Abstract

The contribution of fuel assemblies to the response of ex-core detectors does not only depend on the power, but also on the position of the given assemblies in the reactor core. The weight of the inner fuel assemblies is several orders of magnitude lower than the outer ones. Therefore, the signal of the ex-core detectors for a given reactor power is strongly influenced by the spatial power distribution and indirectly by the parameters which determine the distribution, such as load pattern, time elapsed, etc.

The aim of the work is to describe the modelling and the methodology for calculation of the spatial weight function of ex-core detectors for the WWER-440 reactor type using Monte Carlo transport code system MCNP-4C2, taking into account different operational parameters such as power, burn-up, boric acid concentration, position of the control assembly group No 6, etc. The spatial weight function provides relationship between the spatial neutron flux density distribution or the fission density distribution in the reactor core and the ex-core detectors. The weight function is possible to define in various ways in dependence on solved problem. In this case the function gives the average number of reactions occurred in the detector for one neutron born with Watt fission spectra created in the twentieth of a fuel pin in different reactor core position.

From the definition of the weight function follows that the response of the ex-core detector is given by the weight function and the fission density distribution function product and its integration over the reactor core or its part.

The spatial weight function will be used for interpretation of reloads startup rod drop measurements, assessment of the ex-core detectors response at deep subcritical reactor stage and for other applications.