There are two types of breeder reactor, the fast breeder and the thermal breeder. The fast breeder runs on plutonium-239. The thermal breeder runs on uranium-233. The first of the five prototype reactor programs to be funded by the AEC in June 1954 was a sodium-cooled thermal breeder named the Sodium Reactor Experiment, or the SRE. Winner of the contract for the project was the North American Aviation’s Santa Susana Field Laboratory, located in a rocky wilderness about 35 miles northwest of Los Angeles, California, called Simi Hills.
The 2,850-acre site was divided into four sections. Three of the sections were devoted to testing high-performance rocket engines and explosives, and the fourth was populated with exotic nuclear-reactor experiments. The lab was thus blessed with all the fun stuff of the era except above-ground nuclear weapon and aircraft ejection-seat tests, and it should have been located in Idaho instead of in a place that would experience a population explosion in the next few decades. As the years passed, the valley below would jam tight with Californians, some of whom would find fault with the Santa Susana lab for no good reason other than what it was doing.
Admittedly, over the decades Santa Susana saw its share of accidents. The hot lab facility at the lab was the largest in the United States at the time. Workers could take apart highly radioactive reactor fuel assemblies using robotics behind windows three feet thick, offering complete radiation protection. Every now and then a fire would break out behind the hot-cell glass, causing massive internal contamination. The cores in four experimental reactors on site strayed outside the operating envelope and melted. Highly toxic waste disposal was handled by shooting at the barrels at a safe distance with a rifle until they exploded, sending the contents high into the air and wafting away into the valley below. In July 1994 three workers were trying to test some rocket fuel catalyst and it exploded unexpectedly, killing two, seriously injuring the third, destroying a steel rocket fuel test stand, and setting a 15-acre brush fire.[136] In 2005 a wildfire swept through Simi Hills and burned everything flammable in its path.
By April 25, 1957, the SRE was up and running and soon providing 6.5 megawatts of electricity to the Moorpark community, using a generator courtesy of Southern California Edison. It was the first civilian nuclear power consumed in the United States. In November, Edward R. Murrow featured the SRE power plant on his See It Now program on CBS television.
By current standards, the SRE was an odd power-reactor design. It was to run on fissions caused by neutrons slowed down to thermal speed, and yet the coolant was to operate at atmospheric pressure. The moderator material was graphite, a stable, solid-state material. Preventing any chance of a graphite fire, the moderator was formed into columns with a hexagonal cross-section and covered with gas-tight, pure zirconium.[137] The coolant was liquid sodium. Liquid sodium, being a dense, heat-conductive metal, is a very efficient coolant, and under foreseeable operating conditions, it never boils or produces dangerous gas pressure, as could water. It does, however, absorb an occasional neutron in a non-productive way, to much the same extent as ordinary water. Although the ultimate purpose of the SRE was to begin development of a civilian thermal breeder, its first fuel loading would be metallic uranium, slightly enriched to make up for the neutron losses in the coolant. As the basic configuration was proven by experiments, thorium-232 breeding material and uranium-233 fuel would be introduced later.[138]
There was another good reason to use sodium instead of water as the coolant. In water-cooled graphite reactors, such as the plutonium production reactors at Hanford, the unintended loss of coolant in the reactor always improves the neutron population, and the reactivity of the pile increases. The power starts going up without human intention. Graphite is a near-perfect moderator material, and any action that reduces the amount of non-graphite in a reactor, including the formation of steam bubbles, is fission-favorable. There was no such worry if sodium, with a boiling point of 1,621°F, were used instead of water. With an intended coolant outlet temperature of 650°F at full power, bubble formation in the reactor was hardly a concern, and the reactor could operate at an ideal temperature for external steam production while running under no pressure at all.
It was early in the history of power reactor development, and there were few successful plans to draw on, so there were novelties in the SRE embodiment. At least one aspect of the plan, the sodium pumps, seemed sub-optimal. The EBR-I sodium-cooled breeder reactor in Idaho had been built back in 1951 using exceedingly clever magnetic induction pumps for the coolant. A sodium pump in this system was simply a modified section of pipe, having a copper electrode on either side of the inner channel. A direct current was applied to the electrodes with a static magnetic field running vertically through the pipe, and the electrically conductive liquid metal was dragged along through the pipe by the same induced force that turns the starter motor on a car. The pump had no moving parts. EBR-I reported no problems using induction pumps, but the EBR-I was producing a scant 200 kilowatts of electricity.
For the SRE, hot oil pumps used in gasoline refineries were used to push the sodium.[139] A large electric motor, capable of moving molten sodium at 1,480 gallons per minute up a vertical pipe 60 feet high, turned a long steel shaft, ending in a turbo impeller in a tightly sealed metal case. A single, liquid-cooled ball bearing supported the working end of the shaft. The problem of keeping liquid sodium from leaking past the impeller and into the bearing was solved by modifying the end of the pump. The shaft was sealed with a ring of sodium, frozen solid in place by a separate cooling system. The coolant to be pumped into the seal could not, of course, be water, which would react enthusiastically with the sodium. It had to be a liquid that had zero trouble being next to sodium. Tetralin was chosen.
1,2,3,4-tetradhydronapthalene, or “tetralin,” is a solvent, similar to paint thinner you would buy at the hardware store, first synthesized by Auguste George Darzens in 1926. Its molecule is ten carbon atoms and a dozen hydrogens, looking like two benzene rings stuck together. It has no particular problem with sodium, and it evaporates at about 403°F. The tetralin was circulated through the sodium seal, keeping it solidified, in a continuous loop using two parallel evaporation coolers to shed the heat from the seal. Electrically driven pumps kept it moving, with a gasoline engine for a backup in case of an electrical failure. It was a complicated sub-system in a complicated power plant, requiring pipes, valves, pumps, wiring, instrumentation, tanks, and coolers, just to keep the sodium off the pump bearings. The fact that it had to be backed up was ominous. It is always a better system when if everything fails, the wreckage reduces to an inert, safe condition.
The cooling system used three closed loops. The primary loop was liquid sodium running through the reactor. The natural sodium was constantly being activated into radioactive sodium-24 by contact with the neutrons in the reactor. To eliminate the potential accident of radioactive sodium leaking into the steam system, a second sodium loop took the heat from the first loop and took it outside the reactor building, where it was used to generate steam for the turbo-generator in a water loop. For low-power experiments in which electrical power was not generated, the water loop was diverted into an air-blast heat exchanger, dumping all the power into the atmosphere. There was no danger of broken or melted fuel leaking radioactive fission products into the surroundings, as the second sodium loop was a well-designed buffer. No expensive containment structure was needed for the reactor, because there was no chance of a radiation-scattering steam explosion in the building. The steam was generated out in the yard, and it was not connected directly to the reactor.
136
Otto K. Helney, a 53-year-old engineer, Larry A. Pugh, a 51-year-old physicist, and Lee Wells, a 62-year-old assistant, seemed old enough to know better when they mixed 10 pounds of gycidal azide polymer and nitrocellulose (gun cotton) together in an aluminum pan, hoping to measure the shock wave when the two chemicals ignited. Helney and Pugh died instantly, and Wells was blown against vertical terrain and burned over 20 % of his body. Six other men died at Santa Susana in various incidents in the early 1960s. It was an exciting place to work.
137
Zirconium was an excellent choice. It is transparent to neutrons and able to withstand very high temperatures, but it was expensive. For graphite moderator units out on the periphery of the reactor core where the temperature would never be very high, stainless steel was substituted for the zirconium.
138
From whose stockpile would this U-233 come? U-233 does not occur in nature, and it must be made by production reactor from Th-232. Experimental U-233 reactor fuel probably came from a stockpile at Los Alamos. In 1955 a composite Pu-239/U-233 atomic bomb named MET was built and tested in Operation Teapot. The U-233 was made using the B Reactor at the Hanford site, and this material was used in several thermal breeder experiments in the late 1950s and early 1960s. The stockpile wound up at Oak Ridge, and a $511M contract from the DOE to dispose of this material is currently under way.
139
Electromagnetic sodium pumps were not unknown to the SRE design group. In fact, one was used to load the cold trap, designed to remove oxidized sodium and any other contaminants from the primary cooling loop. This pump failed about 60 days into operation at power in late June 1957. Similar electromagnetic devices, called “eddy current brakes,” were used on the primary and secondary coolant loops to stop the coolant from moving after a reactor scram.