In-Pile Qualification of the Fast-Neutron-Detection-System

Alternative Energies and Atomic Energy Commission: CEA, DEN, DER, Instrumentation Sensors and Dosimetry Laboratory, Cadarache, 13108 Saint-Paul-Lez-Durance, France
Alternative Energies and Atomic Energy Commission: CEA, DEN, DER, Experimental Programs Laboratory, Cadarache, 13108 Saint-Paul-Lez-Durance, France
SCK∙CEN Belgian Nuclear Research Centre, Boeretang 200, B-2400 Mol, Belgium.

email: damien.fourmentel@cea.fr.

Published online: 10 January 2018

Abstract

In order to improve measurement techniques for neutron flux assessment, a unique system for online measurement of fast neutron flux has been developed and recently qualified in-pile by the French Alternative Energies and Atomic Energy Commission (CEA) in cooperation with the Belgian Nuclear Research Centre (SCK•ECEN). The Fast-Neutron-Detection-System (FNDS) has been designed to monitor accurately high-energy neutrons flux (E > 1 MeV) in typical Material Testing Reactor conditions, where overall neutron flux level can be as high as 1015 n.cm^-2.s^-1 and is generally dominated by thermal neutrons. Moreover, the neutron flux is coupled with a high gamma flux of typically a few 1015 γ.cm^-2.s^-1, which can be highly disturbing for the online measurement of neutron fluxes.

The patented FNDS system is based on two detectors, including a miniature fission chamber with a special fissile material presenting an energy threshold near 1 MeV, which can be ²⁴²Pu for MTR conditions. Fission chambers are operated in Campbelling mode for an efficient gamma rejection. FNDS also includes a specific software that processes measurements to compensate online the fissile material depletion and to adjust the sensitivity of the detectors, in order to produce a precise evaluation of both thermal and fast neutron flux even after long term irradiation.

FNDS has been validated through a two-step experimental program. A first set of tests was performed at BR2 reactor operated by SCK•CEN in Belgium. Then a second test was recently completed at ISIS reactor operated by CEA in France. FNDS proved its ability to measure online the fast neutron flux with an overall accuracy better than 5%.