Design Study Of Burn-Up Depth Measurement System For The Plate-Shaped Spent Fuel Based On Compact D-D Neutron Generator

In recent years,with the rapid development of nuclear energy research,plate-shaped assembly reactors have been widely used in the field of small reactors due to the core power density advantage.Spent fuel burnup measurement is a critical part of the fuel handling system and plays an important role in ensuring fuel safety,economy,and integrity.There is an urgent need to develop burnup measurement technology for plate-shaped spent fuel with the emergence of plate-shaped fuel.Based on the compact D-D neutron generator independently developed by Lanzhou University,this proposal aims to develop burnup depth measurement technology for plate-shaped spent fuel as well as non-destructive detection technology for internal defects of spent fuel slab.Using the compact neutron generator,this work will simulate physical processes such as D-D neutron slowing,induced fission of plate-shaped spent fuel,fission neutron and transmission neutron detection to provide a theoretical basis and data support for building the measurement system of plate-shaped spent fuel in the next stage.Based on the compact D-D neutron generator developed by Lanzhou University,this thesis work has completed the physical design of the burnup depth measurement system for plate-shaped spent fuel assemblies.The structural dimensions of the neutron slowing body and the"U-shaped"³He neutron detection system have been determined through simulations.The overall structure of the depletion depth measurement system for spent fuel slab based on the compact D-D neutron generator is presented.In this work,the core structure of IAEA MTR 10MW pool reactor was selected as a reference,and the geometric model of the plate-shaped spent fuel assembly was constructed in the GEANT4 program.The ORIGEN2 program was used to simulate the trends of the main uranium,plutonium and neutron poison contents in the spent fuel assemblies with the burnup depth and to determine the key nuclide information in the plate-shaped spent fuel assembly under different burnup conditions.Simulation models of plate-shaped spent fuel assembly with different burnup depths were developed in the GEANT4 program.According to the scheme of the plate-shaped spent fuel assembly burnup depth measurement system,the"U-shaped"³He neutron detection system designed in this work shows a good linear relationship between the transmitted thermal neutrons,fission neutrons and the burnup depth of the spent fuel assembly by simultaneous measurement at the back and arm,respectively.The neutron count response model of the"U-shaped"³He neutron detection system was established by simulating the burnup depth of the spent fuel assembly.In addition,based on the principle that different fissionable nuclides induce fission to produce different numbers of transient slow-emitting neutrons,this work proposes a technique to carry out direct measurement of ²³⁵U and ²³⁹Pu contents in spent fuel assemblies based on the temporal sequence of fission transient slow-emitting neutron emission.After simulations,the maximum errors of ²³⁵U and²³⁹Pu contents measured by this technique are 3.45%and 9.81%,respectively.In this work,we propose a scheme for the measurement of burnup depth of spent fuel slab and non-destructive detection of internal defects based on thermal neutron transmission imaging technology,as well as the design structure of key components such as slowing collimator,imaging system and image conversion screen.On the one hand,the internal defects of the spent fuel slab are directly obtained by the two-dimensional distribution of the transmitted thermal neutron intensity;on the other hand,the average transmitted thermal neutron count is obtained by integrating the two-dimensional distribution of the transmitted thermal neutron intensity,and the burnup depth of the spent fuel slab is obtained.The thermal neutron transmission imaging system designed in this work has a spatial position resolution better than70μm in detecting the internal defects of the spent fuel.The transmission thermal neutron count in the imaging field of view shows a good linear response to the burnup depth of the spent fuel slab,and the response model of the transmission thermal neutron count in the imaging field of view to the burnup depth of the spent fuel slab is established by simulation.This work presents a design scheme for a burnup depth measurement system for plate-shaped spent fuel assembly based on a compact D-D neutron generator,as well as techniques for measuring the burnup depth of spent fuel slabs and non-destructive testing of internal defects.The simulation study of the burnup depth measurement technology for plate-shaped spent fuel assembly,burnup depth measurement of spent fuel slabs,and non-destructive testing of internal defects were systematically completed.A response model between the burnup depth and the transmission neutron count was established,verifying the feasibility of the scheme and laying a theoretical foundation for the development of the system based on a compact D-D neutron generator.