History of liquid fuel reactors

ThorCon is a liquid fuel reactor. The nuclear fuel is dissolved in a molten fluoride salt. The reactor operates at high temperature (700C) and near ambient pressure.

The liquid fuel concept goes back to the earliest days of nuclear power. After World War II, there was a debate between the Argonne Lab led by Enrico Fermi and the Oak Ridge National Lab (ORNL) led by Alvin Weinberg on how best to use nuclear power in peace. Both men were thoughtful geniuses. At the time, both thought that the world’s supply of uranium was very limited, and both knew that only 0.7% of this uranium, the isotope U-235 was fissile, that is, could be made to fission. This meant that for every ton of U-235 produced about 140 tons of uranium were required; and, in order to be useful in a reactor, that uranium had to be put through an expensive enrichment process in which the U-235 was separated from the much more common, but not fissile isotope U-238.

In the early 1950’s, the only non-experimental power reactor was the Navy’s pressurized water reactor, which had been invented by Weinberg, and developed in a crash program for submarine propulsion. This reactor used highly enriched U-235 in solid form. The working fluid was water at very high pressure (160 bar) but at a rather low temperature (330C). Both Fermi and Weinberg believed that, if we tried to use this system for civilian power, we would very quickly run out of uranium.

Fermi argued for sticking with solid fuel based on U-235, and the enrichment technology developed during the war. His solution to the fuel problem was to bombard non-fissile U-238 which makes up more than 99% of naturally occurring uranium, with high energy neutrons in the reactor. This converts some of the U-238 to plutonium which then can be fissioned. This became known as the fast breeder reactor. Most of the designs based on this concept use a liquid metal — often sodium — as the coolant.

Senator John Kennedy and Senator Gore (senior) flank Alvin Weinberg at ORNL

Weinberg, following his mentor, the uber-genius Eugene Wigner, argued for a completely different approach. His idea was a liquid fuel reactor based on converting thorium to the fissile uranium isotope, U-233. Thorium is 500 times more abundant than U-235, much more easily mined, and requires no enrichment. The reactor is made up of a core of molten salt in which the U-233 is dissolved, surrounded by a blanket, also a molten salt in which the thorium is dissolved. The blanket gets bombarded with some of the core neutrons converting the thorium to U-233. Both fluids are continuously circulated. The core salt is run through a heat exchanger, and some processing to remove fission products and add some new U-233 from the blanket. The blanket fluid is processed to remove the U-233 which is sent to the core, and replaced with new thorium.

It turned out that there was far more uranium on the planet than Fermi or Weinberg thought. The industry fastened on the pressurized water reactor (PWR) that the Navy had developed. It was the quickest way to deploy civilian reactors. The PWR took advantage of the very expensive and difficult enrichment process developed for the bomb. The same companies who built the Navy’s reactors could use nearly the same skills and knowledge to build the civilian reactors. And the PWR had an attractive business model. Once you sold a reactor, the customer had to come to you for the specialized fuel elements for the life of the reactor. In some cases, companies were willing to take a loss on the construction in order to obtain the fuel element cash flow.

Weinberg kept the liquid fuel concept alive at ORNL including building a 8 MW test reactor, called the Molten Salt Reactor Experiment (MSRE) that ran successfully for four years, 1965 to 1969. But the program had almost no political support. The decision was made to focus all the nation’s reactor research effort on the fast breeder. In 1976, the Nixon administration shut down the molten salt reactor program. A few years later the fast breeder program, which was experiencing skyrocketing costs was effectively shut down as well.