Monitoring method for neutron flux for a spallation target in an accelerator driven sub-critical system

TL;DR

This paper proposes a multi-point neutron flux measurement method for spallation targets in ADS systems, utilizing Monte Carlo simulations and a fission chamber calibration technique, demonstrating effective flux monitoring.

Contribution

It introduces a novel multi-point measurement approach and a calibration method for neutron flux using a fission chamber in ADS systems, validated through simulations.

Findings

Neutron flux varies significantly at different vertical locations.

The calibration method accurately relates fission rate to neutron flux.

The proposed method performs well in simulated ADS environments.

Abstract

In this paper, we study a monitoring method for neutron flux for the spallation target used in an accelerator driven sub-critical (ADS) system, where a spallation target located vertically at the centre of a sub-critical core is bombarded vertically by high-energy protons from an accelerator. First, by considering the characteristics in the spatial variation of neutron flux from the spallation target, we propose a multi-point measurement technique, i.e. the spallation neutron flux should be measured at multiple vertical locations. To explain why the flux should be measured at multiple locations, we have studied neutron production from a tungsten target bombarded by a 250 MeV-proton beam with Geant4-based Monte Carlo simulations. The simulation results indicate that the neutron flux at the central location is up to three orders of magnitude higher than the flux at lower locations. Secondly, we have developed an effective technique in order to measure the spallation neutron flux with a fission chamber (FC), by establishing the relation between the fission rate measured by FC and the spallation neutron flux. Since this relation is linear for a FC, a constant calibration factor is used to derive the neutron flux from the measured fission rate. This calibration factor can be extracted from the energy spectra of spallation neutrons. Finally, we have evaluated the proposed calibration method for a FC in the environment of an ADS system. The results indicate that the proposed method functions very well.