Abstract
In this paper, we study the technique for monitoring the incore neutron flux in an accelerator driven sub-critical (ADS) system, where a heavy metal spallation target located vertically at the centre of a sub-critical core is bombarded vertically by the high-energy protons from an accelerator. In an ADS system, the incore neutron flux is affected dramatically by the neutrons from the spallation target. We propose a multi-point measurement technique as follows. During the operation of the reactor, the incore neutron flux should be measured at multiple vertical locations. The detectors for the measurements of incore neutron flux may either be left in a fixed location or provided with a motorized drive to allow move vertically within the reactor core. To evaluate the proposed technique, we have studied the neutron production from spallation targets with the Geant4-based Monte Carlo simulations. In the simulations, two targets, lead and bismuth, have a cylindrical shape with the radius of 10 cm and various lengths. The proton beam with the energy of 250 MeV vertically impinges on the top of the cylindrical targets. The neutron detector with a length of 10 cm moves vertically from the top to the bottom within the reactor core to measure the incore neutrons at seven locations. The simulation results have indicated that the neutron flux at the central location is more than two orders of magnitude higher than the flux at the lower locations. Therefore, during the reactor startup, as the neutrons from the spallation target dominate, the incore neutron detectors should be put at the central location which is close to the target for the commissioning measurements of the proton accelerator.
