Study Of Neutron Multiplicity Measurement Technology For Nuclear Material

Neutron multiplicity measurement technology is a kind of nuclear material mass characterization technology based on the fission neutron time correlation characteristics of nuclear material.This technology was mainly used for military control verification in the early stage and gradually used in international nuclear safeguards field by IAEA.At present,the technology is also widely used in the nuclear material accounting and control of the nuclear fuel cycle.Neutron multiplicity measurement technology is to use the time correlation of fission neutrons emitted by nuclear materials,and use mathematical tools to explain the generation,multiplication,detection and other processes of fission neutrons in nuclear materials.The concepts of probability generating function and factorial moment are used to express the parameters related to mass calculation,such as fission rate,proliferation coefficient,（α,n）and so on.The basic principle of neutron multiplicity is derived by using the above mathematical tools.According to the principle of neutron multiplicity counting,the whole simulation and experiment of neutron multiplicity are completed.Using the geent4 simulation toolkit based on Monte Carlo method to model the Uranium Neutron Coincidence Collars（UNCL）and simulate the neutron transport,and using the sorting algorithm to sort the measured neutron time series,and then using the principle of neutron multiplicity counting to process the sorted time series to get the neutron multiplicity distribution.The detection efficiency and die-away time of UNCL are calculated by ²⁵²Cf and fuel assembly respectively,and the calibration curves of ²³⁵U linear density and coincidence counting rates of fuel assembly are further simulated.Compared with the references,it found that the maximum absolute deviation of coincidence counting rate is less than 17,the maximum relative deviation is less than7%,and the average relative deviation is 0.9%.The results are basically consistent with the references,which verifies the correctness of uncl modeling,neutron transport physical process and subsequent multiplicity processing in GEANT4.On the base of ensuring the accuracy of neutron multiplicity simulation,the calibration curves of ²³⁵U linear density and coincidence rate of PWR fuel assembly are simulated by using UNCL model,and the curves are fitted by using Deming fitting method.The functional relationship between coincidence rates and ²³⁵U linear density is obtained.The calibration constants are a=9.3737 and b=0.01042.In order to evaluate the penetration ability of Am-Li neutron source to fuel assembly,the three-dimensional position information of fission neutron in the assembly was recorded.It is found that the fission neutrons are mainly concentrated in the fuel rods near the am Li neutron source,and some in the fuel rods near the polyethylene plate.However,the contribution of the fission neutrons far away from the neutron source and the plate is very small,so there may be the possibility of illegal transfer of nuclear materials in this area.The sensitivity of UNCL to the change of enrichment of nuclear materials in this region was studied by setting two kinds of enrichment fuel rods,high and low,to replace in this region,and comparing the relative deviation between the total coincidence rate after replacement and before replacement.It is found that the relative deviation of coincidence counting rate is 5.7%at the minimum and 34.5%at the maximum under the replacement of high enrichment fuel rods,while the average relative deviation is only about 2.66%under the replacement of low enrichment fuel rods.Therefore,UNCL is very sensitive to the replacement of high enrichment in this area,but not to the low enrichment or even the removal of a small amount of fuel rods.In addition,due to the appearance of gadolinium containing fuel assemblies,it is necessary to reevaluate the adaptability of uncl to gadolinium containing fuel assemblies in nuclear safeguardsapplications.By setting gadolinium rods with different gadolinium concentration and quantity,the standard fuel rods in the assembly can be replaced uniformly.By comparing the relative deviation of the total number before and after replacement,it is found that the minimum relative deviation is about 6%and the maximum is about 35%.And the sensitivity is not only affected by gadolinium concentration and quantity,but also by the replacement location.Therefore,at present,UNCL is not suitable for the measurement of uranium material mass of gadolinium fuel assembly.In this paper,²⁵²Cf and plutonium metals materials are measured by using the PSMC multiplicity counter device in the national nuclear security technology center,and the nuclear material mass is solved by the multiplicity equation.It is found that the maximum relative deviation of mass of ²⁵²Cf source with three different intensities is10%,and that of plutonium metal is 18.5%.Generally,the mass measurement error of PSMC is about 5%～10%.The reason for the large deviation of Plutonium Metal measurement result may be that the measurement time is less.By changing the coincidence gate width to measure the mass of plutonium metal,the influence of coincidence gate width on the quality measurement is studied.It is found that the coincidence gate width value is about 1.3 times of the die-away time.