GEANT4Simulation For The Nondestructive Assay Of Uranium And Plutonium Content In Spent Fuel

The majority of uranium and plutonium are stored in commercial spent fuel assemblies. For the purpose of preventing nuclear proliferation, it’s crucial that the uranium and plutonium content in spent fuel assemblies should be nondestructively measured before and after the stabilization process. The IAEA has been committing to developing equipment and techniques to quantify the content of uranium and plutonium in spent fuel assemblies. Under the circumstances, nondestructive Assay techniques have become powerful tools for quantitative determination of spent fuel, and played an important role in nuclear material control and accounting, also in inspection of safeguards for nuclear materials. As a result, a variety of nondestructive assay methods have been proposed to determine the content of uranium and plutonium in spent fuel. Moreover, the measurement of prompt and delayed neutron has been reported in the literature, however there is less work focused on combining these two things together to establish simultaneous equations to quantify the content of235U and239Pu in spent fuel. Therefore we propose a new nondestructive assay method which is to separately measure the prompt and delayed neutron generated by neutron irradiation through3He neutron detector. Then according to the measurement of prompt and delayed neutron, the simultaneous equations were established to quantify the content of uranium and plutonium in spent fuel.In this paper, the Monte Carlo soft package of GEANT4was carried out simulation work to verify the feasibility of the new nondestructive assay. The following three aspects are simulated with the GEANT4. Firstly, according to the experimental scene, the moderation of14MeV D-T neutron source is simulated. The appropriate moderator and the optimal thickness of the moderator are chosen so as to get the moderated neutron source. The moderated neutron energies below10eV neutrons eventually are chosen to be the induced fission neutron source; Secondly, by means of the addition of the cross section of nuclide of239Pu and the modification of the corresponding source code on GEANT4, the neutron induced fission of235U and239Pu in spent fuel is simulated. The fission neutron spectrum and the delayed neutron time distribution spectrum are obtained; Thirdly, the detection of fission neutron by3He neutron detector is simulated. The neutron detection efficiency with the incident neutron energy and the thickness of moderator are simulated. Then according to the measurement of prompt and delayed neutron, the simultaneous equations were established to quantify the content of uranium and plutonium in spent fuel. In this work, simulations have been performed with GEANT4code to verify the feasibility of the new nondestructive assay method. The simulation results showed that the content of235U and239Pu in spent fuel is in good agreement with the actual content.