Design Of Thermal Neutron Reference Radiation Facility

Thermal Neutron Reference Radiation Facility provides thermal neutron reference radiation with standard neutron fluence rate.It is mainly used for thermal neutron-related metrology work,including providing reference radiation for the benchmark operation of thermal neutron fluence rate and the transfer of thermal neutron fluence reference value.It plays a crucial role in the calibration of thermal neutron detectors,the evaluation of neutron dose monitoring instruments and the evaluation of neutron protection materials.It also provides a research platform for scientific experiments such as thermal neutron radiation damage effects,neutron activation analysis and thermal neutron imaging,and undertakes the task of unifying domestic thermal neutron fluence rate,participating in international comparisons and realizing international mutual recognition.The original reference device was decommissioned in 1994,leading to the stagnation of domestic measurement work related to thermal neutrons.In order to restore the national benchmark for thermal neutron fluence rate,the National Institute of Metrology is preparing to build a new thermal neutron reference radiation device.The main work of this thesis is to optimize the structural design and simulation of the thermal neutron reference radiation device using Monte Carlo method,so as to improve the thermal neutron fluence rate and the uniformity of thermal neutron distribution in the radiation field as much as possible.The entire facility uses 12 241m-Be neutron sources with a total emissivity of 1.4 × 108 n/s.Based on this,a multi-layer spherical shell model was established to select the moderated materials.The simulation results show that when the thickness of graphite is 45 cm and the thickness of D2O reaches 50cm,the thermal neutron fluence rate does not increase with the increase of the thickness of the moderated material,but medium-energy neutron and fast neutron will be further reduced to increase the proportion of thermal neutrons.According to the relevant technical specifications,D2O and high-purity graphite are selected as the main moderator materials.The structural design of the thermal neutron reference radiation facility was carried out,and the corresponding design scheme was proposed:12 241Am-Be neutron sources with an emissivity deviation of 0.4%are embedded in a graphite plate according to a certain design layout and placed in the center of the device.In order to improve the efficiency of the neutron source,the whole facility is set up with two reference radiation fields,the inner field and the outer field,in two opposite directions.The inner field is generated by a 45 cm thick graphite,mainly to obtain higher thermal neutron fluence rate,and simulation calculations show that the thermal neutron fluence rate of the inner field reaches 22465 ± 38 cm-2·s-1,the thermal neutron ratio is 94.6%,and the uniformity is better than 0.1%on the reference plane with a diameter of 32 cm;the outer field is generated by a 90 cm thick D2O,and the D2O is placed in a 3 mm thick zircaloy box,mainly to obtain a thermal neutron beam with uniform distribution,and a large thermal neutron distribution area is required,The simulation results show that the thermal neutron fluence rate at the reference position of the outer field is 2524 ± 3 cm-2·s-1,and the thermal neutron ratio is 99.96%,the uniformity is better than 1.3%in the reference plane with a diameter of 84 cm.Polyethylene is used as a homogeneous lens in the front of the neutron emitting surface of the radiation field to improve the uniformity of the thermal neutron distribution,and graphite and polyethylene are used as a reflection cavity in the back to increase the thermal neutron fluence rate,and the simulation results show that the reflection cavity can increase the thermal neutron fluence rate by 3.8 times.A composite shielding structure of polyethylene,cadmium and lead is used to shield neutrons and ?-rays on the outside of the moderator.Relying on the MCNP variance reduction method,a mesh-based global variance reduction method is proposed:according to the neutron flux distribution in the phase space,a global weight window bias parameter is generated,neutron transport bias and neutron energy bias are performed,and computational resources are transferred from the high flux region to the low flux region,the weight window parameters are optimized through serial iteration,which greatly improves the calculation efficiency.Using MCNP5 to calculate the global distribution of neutron flux and dose equivalent,the neutron dose equivalent rate on the outer surface of the thermal neutron device is less than 0.29 ?Sv/h,the photon dose equivalent rate is less than 1.66 ?Sv/h,and the total dose equivalent rate is less than 1.95 ?Sv/h,which meets the design requirements of radiation protection.