Design Of The Monitoring System For The Temperature And Neutrons In High Power Spallation For An ADS System

An accelerator driven sub-critical system(ADS)consists of a high power proton accelerator,a heavy metal spallation target,and a subcritical reactor core.The most important idea in an ADS system is the intense external neutron source provided by the spallation target coupled to the accelerator,the intense external neutron source is used to drive the sub-critical reactor.An ADS system offers greater flexibility with respect to fuel composition,i.e.it is possible to use non-fissile fuels(e.g.Thorium)in an ADS system,without the incorporation of Uranium or Plutonium,which is required in critical fast reactors.To produce an intense external neutron source in an ADS system,one of the critical issues is to successfully build a high power spallation target.Based on the China Initiative ADS(CIADS),This thesis focuses on the monitoring of the spallation target,in particular,the monitoring of the target's temperature and spallation neutrons.In CIADS system,a tungsten target is bombarded by a 250 Me V-proton beam.The CIADS should be able to demonstrate the ADS concept at 10 MW power level.Extension of the proposed technique to an ADS system with arbitrary power level is straightforward.Prior to development of the monitoring method,we have studied the energy deposition and neutron production in spallation reaction with the Geant4-based Monte Carlo simulations.In CIADS system,a cylindrical target vessel filled with very small tungsten granulars,the so-called granular target,is located vertically in the centre ofthe reactor core.In order to build such a target in Geant4 simulation,a random packing process of granular is used to locate granulars inside the target vessel.During the packing process of granulars with radius of 2mm,3mm,and 5mm,we have observed that the packing density increases with increasing target length shorter than 50 mm and the density tends to be saturated when the length increases over 50 mm.The packing density almost keeps constant when the radius of the cylindrical target changes.In addition,the packing density changes very small with the radius of granular.Energy depositions in either a cylindrical solid target or a granular target along both axial and radial directions have been studied with the Geant4-based simulations.The simulation results indicate that the curve of energy depositions in a granular target along axial direction becomes flatter in comparison with that in a solid target.The position of Bragg peak in a granular target is deeper than that in a solid target.For both the granular target and solid target,energy depositions decease very quickly along radial direction.The energy deposition at the centre of the cylindrical target is about four orders of magnitude higher than that at the peripheral area close to the target vessel.This means that the energy mainly deposits in the centre of the cylinder and the temperature at target vessel keeps almost unchanged.However,when the proton beam is shifted from the centre of the target,the energy deposition and the temperature at the target vessel will increase dramatically.Thus,by installing 4 thermocouples at the target vessel,we can observe whether the beam is shifted from the centre of the target.Neutron production and neutron leakages from a granular target have been compared with that from a cylindrical solid target by performing the Geant4-based simulations.Neutrons leaking from a granular tungsten target increases dramatically in comparison with that from a solid tungsten target.Thus,more neutrons can be used to drive the subcritical reactor when using a granular target.For a given granular target,neutron production along the direction of the target length has also been analyzed.The neutron flux varies dramatically with z value,because of the different distance between the detection point and the target.The difference of neutron flux between the central location,i.e.z = 0mm,and the lower location,e.g.z = 1000 mm,reaches up to three orders of magnitude.Thus,we suggest that several detectors should be installed at the upper,central and lower locations to measure the distribution of neutron flux along the vertical direction.In order to observe whether the beam is shifted from the centre of the target,we also propose to put 4 neutron detectors around the target vessel.The simulation results indicate that the output current from the detector which is close to the beam area is three fold higher than that from the far one when the beam is shifted.Finally we summarized the proposed monitoring methods as follows.In order to observe whether the beam is shifted from the centre of the target,four thermocouples are installed around the target vessel,while four fission chambers are installed outside the target vessel.Another four thermocouples are installed around the cone below the cylindrical target to observe whether the tungsten granular is blocked in the target.In addition,eighteen self-powered neutron detectors are put outside the target to measure both spallation neutrons and fission neutrons in the upper,central and lower locations.The integration of the proposed monitoring system into the control system for CIADS has also been discussed.