VALIDATION OF AXIAL VOID PROFILE MEASURED BY NEUTRON NOISE TECHNIQUES IN CROCUS

Mathieu Hursin,Henrik Nylén,Gregory Perret,Andreas Pautz,Pavel Frajtag,Oskari Pakari,Gabor Por,Imre Pázsit,Victor Dykin,Vincent Lamirand

doi:10.1051/epjconf/202124708004

Abstract

Recently a joint project has been carried out between the Paul Scherrer Institut, the Ecole Polytechnique Federale de Lausanne and swissnuclear, an industrial partner, in order to determine the axial void distribution in a channel installed in the reflector of the zero power research reactor CROCUS, using neutron noise techniques. The main objective of the present paper is to report on the validation of the results against an alternative measurement technique using gamma-ray attenuation and simulations with the TRACE code. For the gamma-ray attenuation experiments, the channel used in CROCUS is installed out of the core in a Plexiglass water tank. The source and detector are fixed and the channel is moved axially to keep the geometry of the source/detector arrangement untouched. This is key to measure the void effect by gamma attenuation due to the low contrast of this technique. The paper compares the experimental results obtained with both techniques, with the outcomes of simulations carried out with the TRACE code. Even though the quantitative void fraction estimations are not consistent, the trends obtained with the simulation and experimental techniques are the same. The discrepancies between the various experimental techniques and the simulation outcomes are related to the heterogeneous distribution of the water-air mixture in the radial sections of the channel.

Highlights

The development of more efficient Boiling Water Reactor (BWR) fuel assemblies raises new questions about the critical heat flux and potential dryout conditions towards the top of the core, especially since the local void fraction can only be calculated
On October 18th and 29th, additional volumetric flow rates have been investigated with flow rates ranging from 2 to 55 L.min−1 at a single axial location since the void fraction is constant throughout the channel
This paper summarized recent experimental activities carried out at Ecole Polytechnique Federale de Lausanne (EPFL) towards the validation of a theoretical method to reconstruct the void profile in a BWR channel using neutron noise measurements

Summary

Introduction

The development of more efficient Boiling Water Reactor (BWR) fuel assemblies raises new questions about the critical heat flux and potential dryout conditions towards the top of the core, especially since the local void fraction can only be calculated. One step required for the validation of the method is the measurement of the gas phase velocity in a water-air mixture by neutron noise technique. This has been carried out successfully and was reported in [2]. An independent experimental technique is used to determine the void content in the water-air mixture directly, through gamma-ray attenuation measurements Such measurement is made possible by the low flux level encountered in the reflector of CROCUS: the channel containing the water-air mixture used in the noise measurements reported in [2] can be removed from the core and characterized separately allowing for a completely independent experimental characterization of the water-air mixture. In the fourth section, the results of the two experimental methods to determine the void content in the water-air mixture are compared to each other as well as against the TRACE results

Objectives

Results

Conclusion