Calculation of CPNB44 ex-core detector weighting functions for VVER using MCNP5

G. Farkas, V. Slugeň, J. Haščík, Slovak University of Technology, Slovakia

20th Symposium of AER on VVER Reactor Physics and Reactor Safety (2010, Hanasaari, Espoo, Finland)
Spectral and Core Calculations


The paper deals with the problem of weighting factor calculation and the determination of
spatial weighting functions of ex-core detectors for VVER-440 using the Monte Carlo
method. The computational results were obtained by the well-known code MCNP5 allowing
high performance three-dimensional modeling of complex geometry of the in-vessel and exvessel reactor parts. Despite the fact that adjoint methods dominate in practice, forward mode
of code running was chosen and applied to provide more accurate results contrary to the
adjoint one. The calculation was performed for a boron lined proportional counter CPNB44
installed at the 3rd unit of NPP Jaslovsk? Bohunice. The base is the calculation of ex-core
detector reaction rate induced by a neutron generated in a given volume element of a fuel pin.
All the geometrical details and arising space heterogeneities were taken into account with the
highest accuracy in the complex reactor model. Having obtained computational results, the
weighted least square method was used to fit axial weighting functions. With respect to
horizontal direction, the polyhedral approximation of closed Jordan surfaces was used to find
the proper shape of horizontal weighting factor distribution. Sensitivity and parametric
analysis was performed to evaluate the influence of various reactor operational parameters as
well as the ex-core detector positioning on the weighting function values.

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