According to the requirements of unmanned underwater vehicles for high reliability, high power, and long-life power, the design scheme of the megawatt heat pipe nuclear reactor silent unmanned portable reactor (UPR-s) is proposed by Xi'an Jiaotong University.

This study aims to design the shielding scheme for UPR-s to ensure the radiation safety of the cabin.

First of all, according to the UPS-s scheme applied to the underwater unmanned vehicle (UVV), the layout of the nuclear system and shielding was designed, and the source terms of the reactor core under both full power and shutdown status were calculated by using NECP-SARAX code. Then, initial shielding model was established with consideration of several alternative shielding materials. The deterministic neutron-photon shielding calculation code NECP-hydra was employed to analyze several shielding schemes: the initial model layout, composite shielding layout, and shadow shielding layout. Finally, the accumulated fast neutron fluence, photon dose and source intensity at the safety plane were analyzed, and a shielding optimization scheme meeting the requirements was proposed on the basis of the numerical analysis results.

Calculation results of shielding optimization scheme show that the maximum accumulated fast neutron fluence and photon dose of the safety plane at full power are 9.48×10¹¹ n·cm^-2 and 7.29×10⁵ rad, respectively. Under shutdown conditions, the maximum safe plane dose rate is 0.004 49 mSv·h^-1, and the total weight of core plus shielding is 296.35 kg.

The key parameters of optimized shielding scheme, including the cumulative fast neutron fluence, photon dose, and total shielding weight, satisfy the given design requirements.