The Army Corps of Engineers was assigned, on April 9, 1954, to develop a range of small reactors to provide electrical power and space heating for remote conditions in the Arctic and the Antarctic. The U.S. Army Engineer Reactors Group, headquartered in Ft. Belvoir, Virginia, was in charge, and their observers had been very impressed with Untermyer’s BORAX–I experiments.
The list of specifications for the army reactors was interesting, and the boiling-water reactors being developed at NRTS were closer to meeting the requirements than were Rickover’s submarine plant or Zinn’s breeder. The reactors had to be small, relatively inexpensive, and as simple as possible, with a minimum number of moving parts. Nuclear reactors at this young stage were notorious for needing nuclear physicists, highly specialized engineers, and all manner of Ph.D. scientists on hand for normal operation. These reactors would need no such specialists at the plant site. Operators would be given a few months of training and a couple of weeks experience on a training reactor at Ft. Belvoir, but keeping a staff of thoughtful academics alive at a remote army base was out of the question. Moreover, the entire plant would have to be able to be pulled on a sled over ice, lowered into place, and hooked up with a minimum of effort. It could be broken down into sections, but they would have to screw together easily under hard conditions. Nothing could be welded together on site, and certainly no concrete, the basis of all nuclear construction, could be poured in temperatures far below freezing. There would be no calling the factory for replacement parts. They would have to be foolproof, such that an inexperienced operator or one deprived of sleep for three days would not be capable of pushing the wrong button and destroying anything.
The reactor design shop at Argonne took the challenge eagerly, and they came up with the Argonne Low Power Reactor. The name was changed to SL-1, meaning Stationary Low-power reactor, version one, and the small demonstration plant was built in a remote spot at the NRTS in Idaho. Construction and management of the project was transferred to Combustion Engineering in Stamford, Connecticut.
It was everything that the Army had asked for. It was a small boiling-water reactor, making three megawatts of heat. Of this, 200 kilowatts would be converted into electricity using a combined steam turbine-generator, 400 kilowatts would be hot air to be piped into living and working quarters, and the rest would be exhausted into the environment using an air-cooled heat exchanger. All the machinery was housed in a simple, cylindrical building made of quarter-inch steel, 38 feet in diameter and 48 feet high. In the ceiling was the “fan room,” containing the heat exchanger and electrically powered blowers. The lower section was filled in with a combination of loose gravel and steel punchings, with the reactor buried in the middle and flush with the floor. The punchings, left over from making rivet holes in bridge girders, and gravel acted as a bio-shield to absorb all radiation made during and after fission in the reactor vessel. The mid-section of the building was an open space containing the turbogenerator, transformer, tanks, valves, and equipment that could be accessed by workers.
The reactor vessel was 14 feet 6 inches tall, filled with water to the 9-foot level, leaving 5 feet 6 inches of clear space at the top where the steam could collect.
A covered walkway led to the sheet-metal control building sitting on the ground next to the reactor building. Security was provided by a high chain-link fence surrounding the site, with a guard stationed during the day shift. It was functional and as simple as it could be, and the entire facility was built for about $1.5 million. As an Arctic power source, it would cost less to buy and run than the shipping charges alone for the fuel that it was to replace.
The SL-1 plant was started up on August 11, 1958. Its Argonne design had been slightly modified to extend the operating time between fuelings to five years. The fuel assemblies had been lengthened by a few inches so that more highly enriched uranium could be included. This increased the reactivity of the core considerably, and to bring it down to a controllable level some metallic boron was added to the lower ends of the flat, aluminum-covered fuel strips. The boron would soak up neutrons and negate the extended reactivity, but as the fuel in the upper core fissioned away, the boron’s ability to absorb neutrons would also wane. Every time a boron-11 nucleus soaked up a neutron and took it out of the fission process, it would decay in multiple steps to carbon-14, which had no affinity for neutrons. Gradually, over five years, the active region of the reactor core would migrate down while maintaining the designed power level. There were better ways to do this, but for the SL-1 thin sheets of boron were riveted to the ends of the fuel assemblies.
The controls, like everything else in the plant, were designed to be as simple and foolproof as it was possible to make them. There was one main control, made as crossed cadmium-filled blades that would fit between the fuel assemblies, inserted down through the middle of the core. It was moved up and down in the reactor to control the neutron population by a rack-and-pinion mechanism, driven by a DC electric motor and gearbox bolted to the floor off to the side. The control rack penetrated the top of the reactor through a watertight seal called the “shield plug.”
Having one big, master control was fine, but for the fuel to last for more than a few years the flux profile would have to be flattened. In theory, the number of fission neutrons in the reactor at a given time would tend to peak at the center of the core. A plot of neutron population across the diameter of the reactor, or the “flux profile,” would be bell-shaped, and for this reason the fuel would fission unevenly, with center fuel burning out first. That one big control distorted the profile, actually improving it by discouraging the peak in the center. To further enhance the profile, there were eight minor controls spaced around the periphery of the core. They were shorter than the main control, with lesser loading of neutron-absorbing cadmium. The purpose of these controls was to flatten the flux, and they could be adjusted by moving them in and out with motors, just like the main control. As the fuel fissioned away over the years, the operators were expected to have to inch these controls out of the core to allow more fuel to be exposed to neutrons while trying to maintain a flat profile. Under all conditions the master control had sufficient heft to take the reactor from cold shutdown to supercriticality without any help from the peripheral devices.
There were no active control-room instruments that could evaluate the flatness of the flux profile. That was accomplished for this extended experiment using flux wires. Flux wires are aluminum with cobalt-59 pellets imbedded at one-inch intervals. Station flux wires around the core in specified locations, and the cobalt-59 was to be activated into radioactive cobalt-60 to an extent dependent on the density of neutrons. After having stayed in the core a while, these wires are retrieved and cut into one-inch lengths. The gamma rays emitted by each section of each wire are then counted, and from these measurements a map of the neutron flux in the reactor is drawn, evaluating the flux-flattening efforts. Unfortunately, to retrieve the flux wires and put in a new set, you have to take the reactor apart. It takes at least three men per shift working for a couple of days, and the reactor, of course, has to be completely shut down.