A bunch of young engineers at NASA, pursuing the next heavy lift space vehicle, made an interesting discovery recently; they found that, in order to go forward, they needed to look back – They’re now designing the next gen rocket motor by revisiting the venerable F1, the heart of the Apollo program’s Saturn V. In that same vein, nuclear engineers are reaching back in an effort to come up with a 21st century reactor. What they’ve found is know as a molten salt reactor.
When we talk salt, we mean salts in the chemical sense; these are simple, stable compounds, most of which are ionic. Table salt, or sodium chloride, is a perfect example. Separately, both sodium and chlorine are seriously reactive elements. Sodium looks for any and every opportunity to drop an electron, while chlorine wants nothing more than to lose one – Yet put them together, and you get a remarkably stable compound. Heat table salt up to about 1475° F, and you have a molten, aka liquid salt. The versions used for controlled nuclear fission reaction are able to what they do at quite high temperatures; when they cool off, they contract and solidify. That’s advantageous, to say the least.
In the aftermath of World War II, a fair share of the scientists who’d worked on the atomic bomb went in pursuit of gentler uses for nuclear energy. With the military still firmly at the reins of the research train, thoughts turned to nuclear propulsion for things like ships, submarines, and even airplanes. A team from Oak Ridge National Laboratories began looking at what became known as molten salt reactors with those airplanes in mind. While nuke powered aircraft never got off the ground, (sorry, couldn’t resist…), a test reactor called the Aircraft Reactor Experiment did. By 1954, a graphite moderated reactor using a combination of uranium, sodium, and zirconium flourides, (the molten salt fuel), with beryllium oxide as a moderator, (chosen for its unparalleled thermal conductivity and insulating attributes), ran for over a week and performed quite well. With that success in their pocket, Oak Ridge began a study in 1956 to ascertain the viability of molten salt breeder reactors. Twenty years down the pipe, they’d successfully operated an 8 megawatt molten salt reactor, finding that “the salt is stable under reactor conditions, and corrosion is very low.” And then – Research just stopped. Funding was reduced to a trickle because of light water reactors – it was a situation kinda like 8 track versus cassette, VHS and Beta, or DVD against Blu-Ray – The fact is, the best technology didn’t win, and molten salt reactors faded into the sunset. Why did that happen, if the former was a better option? The answer is… Politics, my friends.
In a nutshell, light water won out because the navy chose that version to stick into its vessels. The first General Manager for the U. S. Atomic Energy Commission, MIT professor Carroll Wilson, put it this way: “The pressurized water reactor was peculiarly suitable and necessary for a submarine power plant where limitations of space and wieght were extreme. So as interest in the civilian use of nuclear power began to grow, it was natural to consider a system that had already proven reliable in submarines. This was further encouraged by the fact that the Atomic Energy Commission provided funds to build the first civilian nuclear power plant … using essentially the same system as the submarine power plant. Thus it was that a pressurized light water system became the standard model for the world.”
Molten salt reactors uses uranium salts heated to at least 950° F, (in various combinations with other salts), as the fuel, with molten fluoride salts as the coolant, at relatively low pressures. The fissile fuel can vary, depending on reactor design. In essence, a MSR design still employs a sealed loop for the fission reaction, which is then used to generate steam in a linked loop, which then powers turbines and produces power – Pretty much the same as a light water system, with the fundamental advantages we discussed earlier.
Naturally, your next question is, why was molten salt technology better? Potentially, there are a bunch of reasons. For starters, the fuel is a liquid, not a solid, and a such, it’s significantly less prone to melt downs. Secondly, the technology is far more efficient than light water reactors; where the latter wastes over 95% of the stored energy in its solid fuel, the former utilizes roughly 96% of the energy from its liquid fuel. Third, the price of molten salt reactors should be quite a bit lower than light water. Add the facts that fuel changeover in a molten salt reactor is far less frequent than in light water technology, and that the spent byproducts are far less to is than light water fuel rods. Finally, consider that you can, in fact, use spent fuel from light water reactors to power molten salt reactors. Tally all that up, and you’ve got a pretty attractive option.
There are far more outfits than just TransAtomic working on reviving this technology, and that’s a good thing. Competition breeds innovation, and that’s what the world needs right now. nuclear power isn’t a panacea, but it is a viable, necessary element to an overarching plan to supplant fossil fuel energy production. Factor in the consideration that this is proven, known stuff, and you have a winning bridge technology.