The quest for space travel has spawned many novel conceptual engine designs. Many of these proposed designs utilize nuclear fission or fusion to power the engine. Nuclear power is almost ideal for space propulsion due to the high energy density (energy/mass ratio) of the fuel. This allows for lighter spacecraft or longer mission lengths for otherwise identical vehicles. This use of nuclear power has been shelved since 1973 when the Nuclear Engine for Rocket Vehicle Applications (NERVA) program was shut down due to anti-nuclear sentiment in the United States. Recently, nuclear power has been undergoing a renaissance. The U. S. Department of Energy has called for new nuclear power reactors to be online within 30 years (GenIV initiative).  It is not surprising that this revitalization is currently being expanded to space-based nuclear systems by the National Aeronautic and Space Administration. NASA's Project Prometheus is tasked with exploring the icy moons of Jupiter, although I would prefer a Pluto mission.

Pluto, the ninth planet and/or 1^st Kuiper-belt object, has yet to be visited by a camera-carrying spacecraft. A conventional spacecraft aided by a Jupiter fly-by would take 10 years to reach Pluto. There are very small windows during which a conventional launch is possible. The upcoming windows for a Jupiter assisted mission to Pluto occur in 2007 and 2022. The onboard fuel after this journey would allow for a mission length of 30 days after orbit insertion at Pluto. These factors make it highly unlikely that a Pluto mission will be conducted until a nuclear propulsion system is operational. A nuclear spacecraft could reach Pluto in 10 years without a fly-by, greatly increasing the operational windows for launch. The spacecraft could also function in orbit for several years using electricity from the reactor, increasing the scientific effectiveness of the mission. These features of a nuclear probe tend to suggest a higher probability of success for a Pluto mission. In fact, Los Alamos National Laboratory has recently built a prototype reactor for use in spacecraft.

What types of reactors are suitable for spacecraft use? To answer this question, it is necessary to understand the requirements of space travel. A spacecraft must be light-weight, reliable, and for the case of a reactor, non-critical during launch. Nuclear fuel has a very high energy density. A conventionally fueled spacecraft cannot compare in the area of fuel weight. For a mission to succeed, components must not fail. For example, in space repair of a reactor component is just not feasible and failure of a key component such as a core wall results in a mission failure. Finally, the reactor cannot be operational during the launch phase of the mission. This condition requires a design that moves shielding, control rods, or fuel after launch to achieve criticality.

It is our opinion that a gas-fuel reactor is well suited to spacecraft operation. A Uranium-containing gas will be the lightest fuel source; at launch, the gas can be stored in multiple non-critical vessels and redundant valves can provide protection against failure. A gas-fuel reactor is probably the easiest design to build in redundancy and it also has the fewest moving parts. As geometry of the reactor core is what leads to criticality, the materials making up the core walls must be protected against failure.

Such gas-fuel reactors are likely to use UF_x gasses as fuel. These gasses are very reactive (i.e. corrosives) so the core walls must be inert with respect to these gasses and to the ionization products of these UF_x gasses. Because the core of a nuclear reactor is subject to a high neutron flux, the core wall material must not swell or otherwise be damaged by the particles in the core for the reactor to function. We propose to identify and prepare materials that are inert under conditions of high temperature and high neutron flux. We will study chemical reactions and local structural changes with the technique of X-ray Absorption Spectroscopy.