Nuclear Power from Lunar ISRU

Thorium on the lunar surface can be transmuted into fissile uranium suitable for a controlled chain reaction to provide heat. Thorium is fertile, requiring bombardment by neutrons to become a suitable nuclear fuel. Oxides of thorium are dense and can be concentrated and beneficiated from comminuted regolith via inertial or thermal means. A neutron flux can be provided by encasing thoria within a beryllium and graphite vessel, which emits neutrons upon exposure to gamma rays or galactic cosmic rays. After a brief period at protactinium the transmuted material becomes U-233, a desirable fuel because decay product half-lives are below 100 years. When compressed into fuel pellets the uranium oxide is configured into a reactor through which a working fluid can extract thermal power. With regolith tailings as shielding such a reactor can operate safely for 30 years. A century later, the site can be harvested for specialty elements and then made available for other uses. The advent of launch-safe nuclear rockets in space greatly expands the potential for in situ resource utilization, a space-based economy, and profitable exploitation of the asteroid belt.