Nuclear powered locomotives
To compete with conventional trains, a nuclear-powered locomotive must combine environmental and performance advantages. It would not have any carbon footprint, could make dozens of trips without refueling and, by the way, the transition to similar locomotives would greatly simplify the construction of intermediate equipment devices. The benefit from using such transport would be higher, the longer the railway network, especially if in some area it is necessary to relatively rarely transport very large cargo in one train (for example, when transporting cargo to a cosmodrome). In the 1950s, it was assumed that such a railway line would ideally be stretched along the rugged and inaccessible sea coast. As a test route, it was planned to try to run a train from London to Scotland, and then try to adapt the technology in the Far East (Russia and China), then in Brazil, South Africa and other countries.
Model X-12
In the 1950s, the danger of radiation pollution and man-made disasters involving radioactive materials was greatly underestimated. It is not surprising that in the same year of 1954, when the submarine USS Nautilus appeared, Dr. Lyle Borst from the University of Utah proposed a design for the X-12 locomotive, which would run on a nuclear reactor fueled with uranium-235. This reactor (very compact for its time) was designed by Babcock & Wilcox. Despite the futuristic nature of this idea, Borst approached it as a businessman, and not as a scientist, promoted it in the Association of American Railroads (AAR) and in individual railroad companies. Borst emphasized that his locomotive can run along the route for months and run on uranium sulfate-235 in an aqueous solution. A device of this type already existed, it is called “salt solution reactor” (AHR).
The X-12 locomotive was supposed to consist of two block cars. One housed a reactor, turbine, condenser and generators, and the second housed a system of radiators and fans for heat removal. In essence, the X-12 was a giant diesel-electric equivalent of a diesel locomotive, but instead of a diesel engine, it had a reactor and turbine. Further, the scheme is traditional: heat from the reactor produces steam, the steam turns the turbine, mechanical energy is converted into electrical energy (in the generator), and the generator powers the engine. The engine car was 30 meters long, the radiator car was about 20 meters long. The total weight of the X-12 was close to 330 tons, about half of which was due to the shielding. This locomotive was never the largest of its kind, but as of the 1950s it was the fifth heaviest.
To reduce the size, it was necessary to abandon the heat exchanger and the secondary cooling circuit, so the turbine was driven by steam coming directly from the reactor. Naturally, this steam turned out to be extremely radioactive, and after it the entire turbine emitted radiation. It would be mortally dangerous to carry out routine repairs of such a turbine, so ideally the turbine should have operated for at least 10 years without maintenance. Neither at the time of preparation of the project, nor even now, such turbines exist. Also, due to size limitations, it turned out that it was impossible to get by with one generator, so the reactor was connected simultaneously to four generators, each of which had to generate at least 1.3 megawatts of energy. Such powerful yet compact reactors did not exist then either. So, this project demonstrated that land-based nuclear transport is much more complex and dangerous than a nuclear submarine, a nuclear icebreaker and even a nuclear aircraft: restrictions on the width of the railway track and the overall complexity of maintenance turned out to be prohibitive. However, in normal operation such a locomotive could produce 7,000 horsepower and 10,000 peak horsepower.
It still seems that it is the salt solution reactor that can be made as compact as possible, so let’s take a closer look at its design.
Design of a locomotive reactor using salt solutions
Most modern nuclear power plants use light water reactors. They use cold or boiling water to cool a reactor operating on solid fuel elements. In turn, the fuel for the AHR reactor is a liquid solution of uranyl sulfate, which can be compared to a fuel broth. The AHR is one of the earliest models of a nuclear reactor; such a unit operated at Los Alamos back in 1945.
In a mobile version, the AHR liquid reactor outperforms other models in several respects. The design of such a reactor is very simple; it has a pronounced negative temperature and vapor coefficient – thus, as the temperature of the mixture increases, the reaction rate quickly decreases. Consequently, even if the reactor boils, the chain reaction will die out. In addition, it is the reactors of the AHR model that operate on a minimum number of free neutrons, which is what makes the reactor not only compact, but also allows for quick and relatively safe regulation/change of its power.
The most serious drawback of the AHR reactor is its rapid wear due to extreme corrosion. Uranyl sulfate is, in principle, caustic, but the solution is also under constant gamma radiation, which, together with the heat generated, catalyzes the reaction, leading to the formation of free hydrogen ions and the deposition of elemental sulfur. Thus, long-term operation of the reactor without repairs becomes even less realistic.
The volume of the reactor can be reduced by increasing the proportion of uranium-235 in the solution to almost 100%. In nuclear pellets for a reactor at a conventional nuclear power plant, the proportion of uranium-235 is about 5%. Thus, fuel for a nuclear locomotive would be very expensive. Finally, a locomotive reactor installation would require a chamber to remove xenon (one of the reaction products), since it is xenon poisoning was one of the main causes of the accident at the Chernobyl nuclear power plant.
Despite all these shortcomings, Borst's model was not the last of its kind. Miniaturizing a nuclear reactor for land transport in practice turns out to be much more difficult than adapting a nuclear reactor for a submarine, icebreaker or (potentially) spacecraft, since in the last three cases it is quite possible to increase the hull of the vessel without fear of making it too heavy. But, while the X-12 is a “nuclear diesel locomotive,” at the beginning of the 21st century, a more exotic model of a “nuclear steam locomotive” was also proposed, where it was planned to use liquid helium rather than water as a coolant.
Nuclear Eskom-like steam locomotive
At the beginning of the 21st century, the first compact nuclear reactors were already designed and tested, which theoretically could be installed on a locomotive. Mini reactor South African company Eskom by 2002 operated in the range of 110 to 120 megawatts and could be operated without a cooling pond. Moreover, in 2007, NuScale (Portland, Oregon), in collaboration with Oregon State University, introduced the first small modular reactor, which was developed until the end of 2023, but was eventually discontinued; the installation was not certified due to safety issues. More details about the development and closure of this project are described Here. However, if it were possible to construct an Eskom-like reactor, approximately ten times weaker (12 megawatts) and, accordingly, more compact than existing models, it could serve as a traction unit for a locomotive. In 2001, Harry Valentine, British enthusiast revival of steam locomotives, suggestedthat the locomotive could be powered by a mini-reactor and improve the safety of the structure if several layers of insulation were used, and instead of (heavy) water, liquid helium was used as a coolant.
The fuel for a locomotive reactor, for the reasons discussed above, must be liquid or fine-grained. Based on Eskom's technology, the reactor could be filled with graphite balls about the size of a tennis ball, soaking the balls in a solution of uranium salts or filling them with powdered uranium-235. In this case, graphite would act as a neutron moderator, as in the graphite rods in a traditional nuclear reactor.
Environmental problems and prospects
Obviously, any accident involving a nuclear locomotive would have, if not catastrophic, then difficult to eliminate consequences. Apart from the outer shielding of the composition, it would be necessary to place the reactor itself in a multi-level protective structure, arranged in the form of a nesting doll, and provide an outer layer of heavy viscous liquid that would effectively absorb neutrons, prevent the refrigerant from spilling, and would not ignite.
Paradoxically, these largely mutually exclusive conditions suggest that a nuclear locomotive is likely a viable vehicle for the exploration of Mars. If we happen to transport cargo on Mars, many of the above-described disadvantages of a nuclear locomotive will be smoothed out. Firstly, an open fire is practically impossible on Mars, since carbon dioxide and argon account for almost 97% of the atmosphere there, and oxygen – 0.13%. Secondly, due to low gravity, a locomotive, even with a miniature reactor of “earthly” power, would provide sufficient traction on Mars to quickly deliver massive or modular loads. Thirdly, there are unlikely to be accessible sources of hydrocarbon fuel on Mars (if modern biogenic oil formation concept, then they won’t be there at all), and delivering a small volume of uranium to Mars is much easier than petroleum products. Finally, the sun on Mars is too dim, so large vehicles such as a locomotive cannot be powered by solar energy. It would be advantageous to make the Martian train unmanned, since in this case there would be no need to provide life support systems on board and keep oxygen supplies. The same considerations, with slight variations, will undoubtedly be relevant during the colonization of Titan.
Therefore, I will undertake to assume that nuclear-powered locomotives are not a stillborn project during the hype of newfound nuclear energy, but a foundation for the future and, perhaps, a key transport technology in the development of nearby rocky planets.
