Simulation Of Plutonium Mass Measurement By Passive Neutron Method

With the development of nuclear energy in China,the amount of spent fuel is also increasing,and the accounting of nuclear materials is facing great challenges.There are two types of accounting techniques for nuclear materials:destructive analysis（DA）and non-destructive analysis（NDA）.NDA does not need sampling,and the measurement speed is fast,so it has irreplaceable advantages in many occasions.Because neutron penetration is stronger than gamma ray,neutron based nondestructive analysis method is widely used in plutonium material measurement.This paper focuses on the non-destructive analysis technology based on neutron detection,and studies the factors and rules that affect the quality measurement of plutonium.The geometric models of plutonium material multiplicity counter（PSMC）and active well neutron coincidence counter（AWCC）were established by Geant4 software to simulate the neutron transport process.In the simulation,²⁵²Cf source is used as the source term to calculate the detection efficiency and attenuation time of PSMC and AWCC.The results are compared with references to verify the rationality of the model.Based on PSMC and AWCC,the factors that affect the measurement accuracy of total neutron method,neutron coincidence method and neutron multiplicity method in plutonium mass measurement are studied.（1）The influence of mass size of PSMC on the deviation of total neutron plutonium mass measurement was simulated.When the measurement time is set to 3000 s,for 0 g～100g samples,the mass deviation increases sharply with the decrease of sample mass,which is due to the large statistical error caused by the short measurement time of small mass samples.For 100 g～1000 g samples,the mass deviation increases slowly with the increase of sample mass,which is due to the sample multiplication effect.（2）AWCC was used to simulate the effect of measurement time on the deviation of plutonium mass measurement by neutron coincidence method.When the gate width is set to60μs,the mass deviation increases with the increase of the measurement time.When the measurement time reaches 3000 s,the mass deviation is basically unchanged.This is because the double counting gets enough statistics,which reduces the influence of the statistical error on the mass deviation.In order to study the measurement effect of AWCC in fast mode and thermal mode,samples with 95%and 3%enrichment were selected for simulation.The results show that for samples with 3%enrichment,the mass deviation of AWCC in fast mode is larger than that in thermal mode,and for samples with 95%enrichment,the mass deviation of AWCC in fast mode is larger than that in thermal mode.This is because the effective mass of ²⁴⁰Pu in low enrichment samples is small,and the number of neutrons produced is less,which is not conducive to fast mode detection and causes large statistical error,so the measurement effect is poor.（3）PSMC is used to simulate the influence of ³He tube radius on the deviation of plutonium mass measurement by neutron multiplicity method.When the measurement time is set to 5000 s and the gate width is set to 64μs,with the increase of ³He tube radius,the mass deviation first decreases and then increases.When the ³He tube radius is set to 1.7 cm,the simulation optimal value is reached.Because the ³He tube radius is too large or too small,the detection efficiency of the measurement system will decrease and the attenuation time will increase,resulting in a large mass deviation.