In 1954, however, Professor Lyle B. Borst and his colleagues at the University of Utah pursued a concept for a locomotive powered by a nuclear reactor. Babcock & Wilcox Co. co-designed the reactor in a private venture. The project was dubbed X-12 and attracted the interest of five railroad companies, nine manufacturers, and the international media [1,2]. The locomotive was projected to cost 1.2 million dollars in 1954, double the price of four contemporary diesel units coupled together to produce the same horse powers.
|Ae 8/8 near Kandersteg, Berne, Switzerland (courtesy: Klaus Kort).|
The X-12's stream-lined body emulated the diesel locomotive design of the 1950s [1,2]. The two-sectioned behemoth was to consist of a 38-meter long engine, with a cab up front and the power plant behind, plus a 20-meter long 'tender' carrying the radiators for cooling turbine steam. The assembly was conceived to rest on an articulated platform riding on a (Co'Co')(Co'Co')(4) wheelbase (UIC classification).
The power plant was to consist of the nuclear reactor, the main steam turbine for power generation as well as steam condensers and chillers. A gear box coupled the turbine shaft to four electric generators.
The unavoidable massive radiation shielding called for small components in the engine room. At a rotor shaft length of only 120 inches and a diameter of only 24 inches, the team accomplished to design powerful space-saving generators that, despite their small size, could cope with the demands of the twelve 600-horsepower electric motors driving 24 wheels.
Further accommodating the limited space available, the engine's reactor was to possess a peculiar design. The reactor core was to be filled with liquid uranium oxide dissolved in sulphuric acid, providing greater symmetry for neutron fluxes at smaller neutron loss than the other popular, less efficient designs that used solid fuel packed into rods .
Eugene Wigner and Enrico Fermi developed the original reactor, known as aqueous homogeneous reactor (AHR), in the 1940s as an intermediate step to thorium reactors (Hargraves and Moir, 2011). A circulating solution of 242 liters uranyl sulphate, a yellow-green salt, served as fuel .
The fuel core of the X-12's reactor was supposed to measure only 36 inches in diameter and 10 inches deep. The reactor pressure vessel, clad in an eight-inch steel primary shield, was made to fit into the 13 x 13 x 9 feet cavity of a secondary fluid-tight shield, also made of eight inch steel, encompassing the middle section of the engine and measuring 15 by 15 by 10 feet on the outside. The space between the two shields was to be filled with high viscosity hydrocarbon fluid to absorb internal motion on accidental impact and a hydrogenous shielding material to absorb more radiation.
The reactor consisted of a cylindrical reactor pressure vessel, shorter than wide, in which a steam-driven pump force-circulates the core's fluid, representing the primary coolant circulation for nuclear fission heat transfer. For secondary cooling, roughly 10,000 ¼-inch stainless steel tubes, traversing the core, join an inlet and an outlet chamber attached to each end of the reactor pressure vessel. The water-filled chambers, covering a large part of the reactor core surface, were thought to double as coolers and neutron reflectors, diminishing the escape of neutrons from the core. Coolant pumped through the tube and chamber system feeds steam to the main turbine. An additional coolant loop circulates through the chiller banks in the tender, cooling the turbine's exhaust steam in the main condenser.
As in any nuclear power plant, the water presumably needed to be filtered for contaminants before reuse, its chemistry needed to be balanced, and some water would be lost in the process. The engine could not do without filter beds and a make-up tank.
At full power, the engine's reactor would produce 30,000 kW thermal. Despite, the fuel in the reactor pressure vessel was not going to exceed 240 ℃ (460 ℉) at 4.5 MPa (megaPascal) gauge and, therefore, would not boil. The secondary cooling water would reach 207 ℃ (405 ℉) at 1.7 MPa gauge. These values are low compared with pressurized light water reactor (PWR) widely used in contemporary commercial nuclear power plants. That design must harness water temperatures of up to 315 ℃ (600 ℉) at 15.5 MPa.
Because of the intense radiation, however, almost half the solvent would dissociate into hydrogen and oxygen (radiolysis) within 13 minutes. Hence, the liberated hydrogen needed to be continuously recombined with the oxygen in a catalytic platinum recombiner/condenser installed above the pressure vessel. The recovered water could subsequently be returned into the core.
The liquid fuel concept allowed short-lived, neutron flux-hampering, volatile fission products to simply bubble out of the core fluid, and elegantly combined neutron moderation and efficient heat transfer. In addition, spent fuel could be exchanged without opening the reactor pressure vessel, posing a crucial handling advantage, because the reactor would have needed refueling every two to four months.
We may wonder whether the locomotive would have proved safe to operate. Borst and his colleagues hardened the reactor components to withstand collisions in an effort to prevent radiation releases and fuel spills. The reactor control rods, mounted at 60 degree angle, possessed shear points that would break on impact at accelerations equal to or above 0.2 g, lowering the rods into the core and scramming the reactor. Coolant forced through a subset of tubes traversing the core was supposed to help remove the resulting decay heat from the core in such emergency shutdown.
Professor Borst argued that ultimately the benefit versus cost of fuel compared with other sources of energy would decide the X-12's future. In order to sustain a chain reaction, most nuclear fuel used in commercial applications consists of uranium-238 enriched with the more fissile uranium-235 beyond its natural prevalence of 0.7 percent. In commercial light water nuclear power reactors burning solid uranium oxide, the enrichment is in the order of 3.5 percent. By contrast, Prof. Borst intended to run his reactor with weapon-grade highly enriched uranium, that is uranium-238 enriched with uranium-235 to more than 85 percent. In fact, his patent application called for 7 kg pure uranium-235. At today's DOE prices, one filling of fuel for the locomotive would cost 104 million dollars.
The fission products of uranium-235 mainly comprise radioactive isotopes of barium, cesium, iodine, strontium, and xenon, of which the highly volatile iodine-131, with a half-life of 8 days, and barium-140, with a half-life of 13 days, pose an immediate health hazard, when released into the environment. Accidental releases of cesium-137 and strontium-90 with half-lives of about 30 years are of long-term concern. Professor Borst mentions little about the enormous expenses to provide radiation safety during fuel handling and maintenance, and eventually the decommissioning of highly radioactive components at the end of the reactors' life span. Moreover, reprocessing of used nuclear fuel has been wrought with unresolved technological pitfalls to date, and endstorage sites where highly radioactive waste can be safely and indefinitely stored remain elusive in the U.S.
Three scores ago, the advent of nuclear power's commercial use was greeted with exuberant enthusiasm, promising a future of limitless energy and boundless applications. Despite the optimism, Prof. Borst's idea of a mobile reactor must have been met with skepticism from its conception. A prototype X-12 locomotive never gained traction as much as we know, probably because of the enormous investments already involved in the development of the engine alone.
It seems certain that cost weighed on the minds of the railroad company executives as heavily as the X-12 might have weighed on the tracks. Without substantial government subsidies, particularly for highly purified uranium-235, the railroad companies could not have run such locomotive cost-effectively. Liability insurance would have been enormous. I counted a dozen major U.S. train wrecks in 2011 alone. Moreover, safety and security of the use of nuclear-powered train engines would pose daunting challenges to homeland security today.
Confronted with what arguably must have seemed insurmountable obstacles from the project's conception, Borst and the University of Utah appeared in no particular hurry to obtain approval for a patent . The parties applied in 1955. The application was eventually approved in 1964, consuming almost a decade. At best, the X-12 may add an illustrious conversation piece to a model railroad today.
- "Auf Bahnsteig 3 - Atom-D-Zug", hobby, July 7, 1954.
- "The atomic locomotive. A physics professor's practical dream, the massive X-12 could run for months on a charge of U-235." Life, Jun 21, 1954.
- Borst LB (1964) Nuclear reactor for a railway vehicle. U.S. Patent № 3,127,321.
- Hargraves R, Moir R (2011) Liquid Fuel Nuclear Reactors. American Physical Society Forum on Physics & Society.