Estimation of Equivalent Thermal Neutron Fluence Rate in the Leakage Beam of China Initiative Accelerator Driven System

The Accelerator Driven subcritical System(ADS) is driven by spallation neutrons, which is generated during the proton bombarding the spallation target, to make the subcritical reactor operate stably and continuously. It has high potential to transmute the long-lived minor-actinides and produce cleaner nuclear energy. During the coupling process of the accelerator and reactor, the beam pipeline penetrates through the top cover of the reactor, and spallation neutrons and fission neutrons in the core tend to leak through the pipeline in large quantities. In order to reduce the equipment activation and reduce the personnel radiation dose received during the maintenance, it’s necessary to carry out relevant shielding design work. In the radiation shielding design of thermal spectrum reactor, the thermal neutron fluence rate is usually limited to 10⁵ n/(cm² · s) to reduce equipment activation and ensure maintainability. For the Accelerator Driven subcritical System, the leakage neutrons from the top cover are fast neutrons with high fluence rate. It is necessary to carry out the shielding design analysis according to an appropriate equivalent thermal neutron fluence rate. This work takes the China initiative Accelerator Driven System(CiADS) as a reference to analyze the expected radiant energy release of activated nuclides generated by the leakage neutrons, and a leakage neutron equivalent fluence rate is obtained. The applicability for this thermal neutron equivalent fluence rate is evaluated. Studies have shown that leakage neutrons with an average fluence rate of 9.292×10⁵ n/(cm² · s) are equivalent to thermal neutrons with a fluence rate of 10⁵ n/(cm² · s) in the expected radiant energy release. In the radiation shielding design, the neutron fluence rate limit outside the top cover can be conservatively taken as 8×10⁵ n/(cm² · s). This research work proposes a method for selecting the equivalent neutron fluence rate in the design of the ADS external activation shield, which can be used in the fast spectrum neutron shielding analysis. The thermal neutron equivalent fluence rate obtained in this paper can provide reference for the shielding design and analysis of CiADS.