Simulation Of Uranium Material Mass Measurement Based On Delayed Neutron

Facilities like nuclear fuel reprocessing plants will produce some drums of nuclear waste during production,in addition to nuclear material,drums usually contain a large amount of iron filings,polyethylene,fiberboard and other matrix materials.The traditional active neutron in-quiry technique of nuclear material mass is to measure the instantaneous fission neutrons with time correlation emitted after induced fission of nuclear material,however,the high background neutrons of the irradiation source limit the measurement accuracy of this technique.Therefore,the mass measurement device of large（208 L）waste drum is mainly based on the time charac-teristics of delayed neutrons.In order to make up the deficiency of uranium material mass measurement in large waste drum mass measurement devices,a simulation study of waste drum measurement method based on delayed neutron was carried out.Based on the simulation software Geant4,the Shuffler,a standard 208 L waste drum meas-uring device,was modeled and verified,and the corresponding delayed neutron physical transport process was studied.Geant4 has no delayed neutron information.By sampling the direction,energy,position,quantity and other information of delayed neutron generated by the written delayed neutron simulation program,the delayed neutron simulation was realized.The Geant4 high precision neutron model was used in neutron transport process.The relative devi-ation between the ratio of delayed neutron number to fission number in the simulation verifica-tion and the literature is 3.16%.The maximum deviation between the average energy value of each group obtained by fitting delayed neutron energy spectrum and the literature value is no more than 2.2%,indicating the correctness of the model and physical transport process estab-lished based on Geant4.The curve calibration of delayed neutron counting rate and ²³⁵U mass of U₃O₈ samples under different matrixes was studied by using the simulation model.In the absence of matrix,the ²³⁵U mass in the sample has a linear relationship with the counting rate,while the moderat-ing material in the matrix will lead to the change of the counting rate.The nonlinear counting rate curve of the matrix was corrected by using the ratio R of the two flux monitors as the index of the neutron moderating and absorption characteristics of the matrix material.For nuclear waste from spent fuel reprocessing plants,the variation of delayed neutron counting rate of U₃O₈ samples with 0.7%,2%and 3%enrichment degree in different matrixes was simulated,and the calibration coefficients of sample curves with three enrichment degrees were obtained respectively.The mass reconstruction effect of five U₃O₈ samples with enrichment degree of3%in different matrixes was verified,and the maximum measurement deviation was 9.91%,which proved the correctness of the curve calibration.The expression of the lower limit of net delayed neutron count was derived.When the background count rate was 10 counts/s,the min-imum detectable mass of the device was 302 mg,it was proved that it had the ability of meas-uring small mass uranium material.Using the model and method,the influences of the energy spectrum of different irradiation sources,the parameters of the measurement system,and the characteristics of nuclear and ma-trix materials on the results were simulated.By comparing the variation trend of delayed neu-tron count rate of U₃O₈ samples induced by ²⁵²Cf source,D-D source,D-T source,Am-Li source and Am-Be source under different matrixes,the most suitable R measurement range of the five irradiation sources was obtained.The law of Shuffler in ³He layout,pressure,radius and tube distance were studied,and the corresponding measurement optimization design was developed to increase the detection efficiency of the system from 32.43%to 34.53%.For the arrangement of nuclear materials,with the increase of nuclear material mass,both the multipli-cation effect and the self-shielding effect will make the counting rate and mass measurement curve deviate from linear.The influence of sample dispersion on delayed neutron counting rate was studied for powdery samples in the drums or nuclear materials contaminated in the decon-tamination process,the counting rate measured by Shuffler increased first and then decreased with the increase of the dispersion range of samples.