Research Of The Monitoring System For Nuclear Power And Neutron Flux For An ADS Subcritical Reactor

In an accelerator driven sub-critical(ADS)system,a subcritical reactor is driven by an intense external neutron source provided by an accelerator coupled to a spallation target located vertically in the center of the reactor core.Since the nuclear-fission chain reaction in this subcritical reactor is maintained by the external spallation neutrons,the neutron spatial distribution in the ADS subcritical reactor is decided mainly by the high energy spallation neutrons.Thus,the neutronics characteristics of an ADS subcritical reactor is very different from that of commercial critical reactors where the neutron spatial distribution is mainly related to the boundary conditions.This main difference between the neutronics characteristics of critical reactors and ADS also affect the monitoring method of nuclear power and neutron flux for ADS system.This thesis focuses on the monitoring of the incore and excore neutrons for an ADS subcritical reactor.In an ADS system,power control in the sub-critical reactor is achieved through the control of the beam current.Excore neutron flux monitoring in an ADS system,not only provides indication of reactor power,but also provides important inputs to reactor protection system during startup and power operation,and thus plays a very important role in the control and protection of ADS system.This thesis presents the excore neutron flux monitoring method which uses three fission chambers(FCs)and three uncompensated ion chambers(UICs).With three operation modes,pulse mode,current mode,and mean square voltage mode,an FC can monitor reactor power over a wide range from the source range to the intermediate and power ranges.The proposed monitoring method increases the redundancy of independent monitoring channels,improves the reliability of the protection system,and provides more information on axial and radial power distribution.In a commercial critical reactor,incore neutron flux is measured usually by several movable fission chambers which are inserted from the top of the reactor.After considering the fact that the top of the reactor core has been occupied almost by the beam injection system of the accelerator and the primary coolant system of the target,incore neutron flux in the ADS system is measured by fixed self-powered neutron detectors(SPNDs).Because the difference between the spallation neutron flux at the central location and at the lower locations attains up to three orders of magnitude,this thesis reports the monitoring method for 3-dimentional incore neutron flux in an ADS system,where the radial distribution of neutron flux is observed by measuring the flux at multiple channels and the axial distribution is obtained by measuring the flux at upper,central and lower locations.Another important contribution of this thesis is that an effective calibration method for these neutron detectors has been proposed,in order to provide the exact value of neutron flux.Since the neutron-induced reaction cross section varies dramatically with neutron's energy,these neutron detectors used usually for measuring thermal neutrons in commercial critical reactors have to be re-calibrated when being used in for measuring fast neutrons in an ADS system.The proposed calibration method in this thesis includes two steps,i.e.,firstly,these neutron detectors are calibrated with a standardized neutron source,and then,a correction factor is added in the calibration by comparing the neutron energy spectrum of the neutron source with that in ADS system.Based on Geant4 simulation,the correction factors of 5,42,and 26 are extracted for FCs,UICs,and SPNDs,respectively.In summary,not only this thesis has discussed the monitoring methods of the incore and excore neutrons for an ADS subcritical reactor,but also this thesis has proposed effective calibration methods for these neutron detectors.