SHIELDING DESIGN CONCEPT FOR AN MIT NUCLEAR POWERSTATION FOR THE MOON AND MARS

摘要

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