Current reactor power monitoring systems require the use of a single fission ionization chamber to accommodate neutron flux rate measurements in the high gamma radiation field of the reactor core,while at the same time being able to accommodate measurements at low neutron flux rate during reactor start-up in a highly sensitive pulsed mode of operation.However,the sensitivity of the detector cannot be improved directly by increasing the thickness of the fissile material coating due to the short range of the fission fragments in the fissile material coating and the strong self-absorption effect.In this paper,we propose a honeycomb-like cylindrical grid fission chamber detector,which increases the detector specific surface area to three times that of an ordinary cylindrical ionization chamber,thus dramatically increasing the effective mass of the fissile material and achieving high neutron sensitivity without increasing the thickness of the fissile material coating.Monte Carlo simulations based on Geant4 show that a thermal neutron detection sensitivity of 3 cps/nv can be achieved for a fission ionization chamber with 21 grid plates and 2.5 mg/cm2 235U coating.Simulation results of the migration and diffusion of ionized electrons within the detector using Garfield++show that the use of Ar/CO2(96/4)as the working gas balances the drift velocity and lateral diffusion of ionized electrons and achieves an electron migration efficiency of 96.5％at an high voltage of 725 V.The results in this paper show that this type of honeycomb-like fission ionization chamber has high thermal neutron sensitivity in the pulsed mode,which is promising to adapt to a wide range of power monitoring of the reactor from start-up to full power operation.
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