SHIELDING DESIGN CONCEPT FOR AN MIT NUCLEAR POWER STATION FOR THE MOON AND MARS

摘要

For human exploration of the Lunar or Martian surfaces to be feasible, a reliable, robust power source will be required. The demands of these two environments exclude the use of chemical, solar or radioisotope power sources in the long term and lead naturally to the use of fission reactor technology. For humans to live and work in proximity to such a reactor, an effective shielding system is necessary. This investigation focuses on the design of such a system for a 100kWe fast reactor intended to operate on the surface of the Moon and Mars. The objective of the shielding design is to engineer a system that will reduce the nominal radiation dose received from the reactor to humans to a reasonably achievable low level. Combining radiation dose guidelines from the Department of Energy (DOE) and the International Commission on Radiological Protection (ICRP), a maximum dose of 2.0 mrem/hr was chosen as a reasonable target. Evaluating mass requirements for the entire reactor system, an upper bound of 2 metric tons was selected for the shield. With these constraints, a two-component radial shield was required. The shield comprises a 49 cm thick boron carbide (B4C) layer placed against the reactor reflector and a 12 cm thick tungsten (W) layer placed against the B4C. The B4C is responsible for stopping neutrons, while the tungsten attenuates gamma rays. These layers are in the shape of a semi-cylindrical shell, covering eighty degrees of arc around the reactor. This eighty-degree shadow shield was necessitated by the mass constraint. Axially, the shield extends 2 cm above and below the core to reduce dose from scattered radiation. No other axial shielding was required. For flexibility, the shield comprises two identical pieces, each covering forty degrees of the reactor surface. These pieces can pivot automatically around the reactor allowing the direction of shielding to be adjusted. The shield has a total mass of 1.994 metric tons and at a distance of 7.4 m from the outer tungsten surface, the dose falls to the chosen 2 mrem/hr.