Tech: Introduction
Nuclear reactors using liquid-halide salts for both a nuclear fission medium and for primary heat transport were first conceived at the Oak Ridge National Laboratory in Oak Ridge, Tennessee in 1947. They were a natural outgrowth of the work at the K-25 uranium enrichment facility that utilized uranium hexafluoride for isotopic enrichment. The ORNL director at the time, Dr. Alvin Weinberg, was particularly interested in the pursuit of a breeder reactor based on a thermal-neutron spectrum using abundant thorium as the basic fuel of the reactor. His mentor, Dr. Eugene Wigner, had convinced him that the most likely course of success for a thorium breeder would be a reactor using nuclear fuels in a liquid, rather than in a solid, state. Nevertheless, his course of action up to that time had been based on aqueous solutions of uranium and thorium rather than other types of fluids.
Proposing to use liquid-halide salts, rather than aqueous solutions as a basic medium, was a rather significant departure for Weinberg’s ORNL thorium study group, but it neatly solved several of the problems that had bedeviled their aqueous design. The most basic one was that there was a fluoride form of thorium (thorium tetrafluoride) that would form a true solution, whereas there was no aqueous form of thorium that was stable in reactor operating conditions and they had to rely on a thorium dioxide slurry.
Despite their early interest, the event that “jump-started” development of liquid-halide nuclear reactors was the Nuclear Aircraft Program. This Air Force program intended to design and build a manned bomber that would be powered by a compact, high-power nuclear reactor, giving the bomber the range it needed to reach the Soviet Union in order to effect a military retaliation. A reactor capable of powering a bomber essentially needed to be a drop-in replacement for the heat generated by burning light hydrocarbons in conventional gas-turbine engines. The power density of the gas-turbine combustor is truly staggering, and to replace that with a heat exchanger or directly-cooled reactor would require reactor technology to advance far, far beyond the then-current state-of-the-art.
Weinberg and the ORNL leadership proposed to develop the nuclear aircraft reactor and were awarded the mission by the Air Force in (FIND THE DATE). They then went on to develop a variety of different reactor concepts and execute trade-off studies between them. The liquid-fluoride reactor was consistently one of the top performers for a variety of reasons. It promised to have high-core power density, which could lead to a reduction in the weight of the neutron shield. It promised to be self-controlling through the expansion or contraction of its liquid salt, and it also could avoid xenon transients that would plague solid-fueled thermal-spectrum reactors at high core-power densities. Whether these theoretical advantages would exist in a real operating reactor was a question of great concern, so the Air Force directed ORNL to proceed to build and briefly operate a proof-of-concept reactor called the Aircraft Reactor Experiment (ARE).
The ARE was rather hastily modified from an earlier design that intended to use solid nuclear fuel and liquid-sodium coolant. For this reason, it was not terribly reflective of the eventual intended airborne design. Rather, it was meant to test the chemical and nuclear stability of the fluoride fuel and to assess whether the anticipated xenon-removal advantages were present.