NEUTRON NOISE PATTERNS FROM COUPLED FUEL-ASSEMBLY VIBRATIONS

M Viebach,C Lange,A Hurtado,Y Bilodid,M Seidl

doi:10.1051/epjconf/202124702015

M Viebach, C Lange + Show 3 more

Open Access

https://doi.org/10.1051/epjconf/202124702015

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Abstract

The neutron flux fluctuation magnitude of KWU-built PWRs shows a hitherto unexplained correlation with the types of loaded fuel assemblies. Also, certain measured long-range neutron flux fluctuation patterns in neighboring core quadrants still lack a closed understanding of their origin. The explanation of these phenomena has recently revived a new interest in neutron noise research. The contribution at hand investigates the idea that a synchronized coolant-driven vibration of major parts of the fuel-assembly ensemble leads to these phenomena. Starting with an assumed mode of such collective vibration, the resulting effects on the time-dependent neutron-flux distribution are analyzed via a DYN3D simulation. A three-dimensional representation of the time-dependent bow of all fuel assemblies is taken into account as a nodal DYN3D feedback parameter by time-dependent variations of the fuel-assembly pitch. The impact of its variation on the cross sections is quantiﬁed using a cross-section library that is generated from the output of corresponding CASMO5 calculations. The DYN3D simulation qualitatively reproduces the measured neutron-flux fluctuation patterns. The magnitude of the fluctuations and its radial dependence are comparable to the measured details. The results imply that collective fuel-assembly vibrations are a promising candidate for being the key to understand long-known fluctuation patterns in KWU built PWRs. Further research should elaborate on possible excitation mechanisms of the assumed vibration modes.

Highlights

During the previous two decades, the neutron flux fluctuations of PWRs built by KWU (Kraftwerk Union AG) have exhibited unexplained changes of their magnitude
The simulation of the “fuel-assembly-pitch effect” reproduces main details of the described phenomenology of the neutron flux fluctuations measured in KWU PWRs, including the range and the radial dependence of the fluctuation magnitudes
A variation of the fuel-assembly parameters, e. g. corresponding to a new fuel-assembly type, have an impact on the fuel-assembly vibration amplitudes and on the neutron flux fluctuation magnitude

Summary

Introduction

During the previous two decades, the neutron flux fluctuations ( referred to as neutron noise) of PWRs built by KWU (Kraftwerk Union AG) have exhibited unexplained changes of their magnitude (cf. Ref. [1]). During the previous two decades, the neutron flux fluctuations ( referred to as neutron noise) of PWRs built by KWU (Kraftwerk Union AG) have exhibited unexplained changes of their magnitude The analysis of measured fluctuation data has drawn attention to long-known and unexplained long-range correlations [5]): in-phase behavior for axially aligned detectors and for detectors in the same core quadrant and out-of-phase behavior for detectors in opposite core regions. The axial shape is C-like (cf. Refs. [4,6])

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