Simulation Design Of Collimation System For Reactor Thermal Neutron Radiography

Neutron radiography technology is one of the existing important non-destructive testing technology,it has been widely used in aerospace technology,nuclear industry,military,archaeological,agriculture and other fields.Compared with X-ray radiography technology and ?-ray radiography technology,neutron radiography technology has its own unique characteristics and advantages,so as to become their important complementary technology.However,the development time of neutron radiography technology is short,there are further research and development in many ways.In the neutron radiography device,the performance of neutron collimator has a great impact on the imaging resolution and exposure time.So the research and development of neutron collimation system is an important part of the development of neutron imaging technology.In this study,MCNP program is used to study and design the neutron collimation system.According to the shortcomings of the common collimator structure,the improved structure of the collimator is proposed.Under this structure,four parts of the colimation system are researched and designed,the neutron moderation and collimation,the decrease of stray neutrons and scattering neutrons,the protection of thermal neutron during transmission process,the increase of neutron ? ratio.The optimized selection and design of the material and structural dimensions of the collimator parts are completed.In the course of the simulation study,the reactor neutron source is used.The energy distribution of the source uses the thermal neutron Maxwell fission spectrum.The neutron moderator,neutron shielding,pore lining,pore filling gas and ? ray shielding are simulated for the neutrons emitted by the reactor.In the simulation study,through the comparison of different materials and structural size,the research and simulation design of collimation system are completed.The design scheme of the collimation system is determined by comparing the neutron average flux inside the field of view,the thermal neutron ratio,the neutron gamma ratio and the neutron average flux difference inside and outside the field of view.The final collimation system design is: water is the neutron moderator,the thickness of water is 14 cm,collimator inlet diameter is 2cm,collimator length is 70 cm,collimation ratio is 35,diameter of the field of view is 12 cm.The collimator wall is composed of polyethylene,boron carbide and lead,and the collimating channel is composed of three parts: a truncated cone,a large radius cylinder and a small radius cylinder.The radius difference between the two cylinders forms a shield structure at the rear end of the channel.The thickness of the shield structure is 35 cm,where the polyethylene is 24 cm and the boron carbide is 11 cm thick.The inner wall of the channel is coated with borax and the channel is filled with helium.The collimation system achieves a maximum neutron fluence rate of 2.6cm-2·s-1 in the field of view,of which thermal neutrons accounted for 74.94%.The neutron fluence rate outside the field of view is two orders of magnitude lower than it inside the field of view.The neutron gamma ratio reaches 2.18×1012cm-2·Sv-1.The parameters above can meet the requirements of neutron radiography technology.