The melting fuel sagged and changed shape. Heat transfer between the fuel and the water was suddenly improved, and the resulting steam explosion was more energetic than expected. It completely destroyed the reactor. Contents of the reactor vessel were pounded out of shape and thrown skyward. There was no roof to carry away, but a flying periscope hit a steel roof-beam and bent it outward. Bits and pieces of reactor were scattered all over the place. There were no injuries except to a few egos, and the large bank of instruments could find no harmful release of radioactive gasses into the atmosphere. I will not say that the fine engineers at the NRTS were slow to learn, but this sort of behavior in a suddenly uncontrolled water reactor was hardly a new finding.
This was not the last reactor blown up at NRTS, but all the others were predictable, controlled, and not considered accidents. NASA planned to send up a nuclear reactor into space in the System for Nuclear Auxiliary Power program, and the SNAP-10A nuclear power generating station was scheduled to be launched into orbit in 1965.[111] There were, of course, concerns that the booster could fail before achieving orbit and a power reactor could come down in the ocean. What would be the hazards if this worst launch failure happened?
At NRTS the recently vacated ANP test facilities at Test Area North were converted to test the SNAP-10A in a simulated high-speed crash into the Pacific Ocean. It was correctly predicted that a severe nuclear power transient would result from slamming into the salt water. The reactor was naked of any shielding, and it was moderated by zirconium hydride mixed with the uranium fuel. Hitting the surface would suddenly introduce extra moderating material favorable to fission (water) and the reactor would go prompt supercritical.
A huge water tank was mounted on one of the ANP double-wide rail cars that carried a nuclear jet engine with a SNAP reactor mounted in the middle. The reactor was protected from the water with a Plexiglas shield until an explosive charge threw it out of the way and let the water crash in. The resulting fireball on April 1, 1964, threw reactor fragments far and wide.[112] To keep thermocouples and radiation instruments from being melted and lost in the explosion, the measurements were done remotely, using infrared pyrometers looking into a mirror behind a lead wall. Just to be sure, the test was run three times in experiments SNAPTRAN-1, 2, and 3, and three perfectly good SNAP reactors were destroyed.[113] They took movies.
There was also the LOFT (Loss Of Fluid Test) reactor and Semiscale, which was not really a reactor. Both experiment series were used to find what would happen if a water-cooled reactor breaks a major steam pipe. The LOFT reactor was a half-scale, fully operational pressurized water reactor, while Semiscale was a slightly safer experiment using an electrical heat source instead of fission to make the steam in a simulated power plant. These programs and the accidental experiments dating back to EBR-I were all valuable in finding how to build a safe power reactor and how not to build one. This knowledge was put to use in designing the Generation II nuclear reactors that now produce 20 percent of the electrical power in the United States. There would be commercial reactor accidents in America, but never a steam explosion, and the three men who stood on top of the SL-1 were the last people ever to die in a power reactor accident in this country.
Today, there are no ongoing reactor safety experiments, anywhere in the world.
Torturing nuclear reactors to make them give up the secrets to safe power production was not the only activity at the NRTS. Among the first building sites at the new reservation in 1948 was the Idaho Chemical Processing Plant, known to all as the “Chem Plant.” If the techniques for building civilian power plants were to be sorted out in the Idaho desert, then it made perfect sense to also work on fuel reprocessing and waste handling. A model plant was built to recycle the fuel used in reactor experiments and to develop practical methods for extracting the various components made in the fission process.
Commercial reactor fuel is uranium oxide, with two out of every three atoms in the solid material being oxygen. The uranium content in fuel is usually enriched to 3.5 % fissile uranium-235. The rest is uranium-238. After about 4.4 % of the uranium-235 has fissioned, the fuel can no longer support the self-sustained chain reaction, and it must be replaced. Approximately 20.5 % of the waste product embedded in the spent fuel is plutonium resulting from neutron activation of the U-238. The rest of the waste, 79.5 %, is fission products, 82.9 % of which are stable. That leaves 1.1 % of the spent U-235 as radioactive waste which must be either disposed of or put to use as industrial and medical isotopes. The idea of fuel reprocessing is to remove the remaining U-235 and Pu-239 from the spent fuel and return it to the energy production process. As you can see from the breakdown of the used fuel components, the waste to be buried is a tiny part of the original fuel. In 1948 when uranium was thought to be scarce, it made no sense to bury an entire used fuel-load without breaking it down and sorting the portions.
The Foster Wheeler Company of New York, experts at making oil refineries, designed the plant, the Bechtel Corporation out of San Francisco built it, and American Cyanamid ran it. The Army had wanted to operate it under military control, but the AEC wisely argued that if it was to model a commercial process, then civilians should learn how to do it. Construction took 31 months on 83 acres of flat-as-pool-table desert north of Big Southern Butte. The first shipment of fuel arrived to be processed in November 1951.
Spent fuel arrived in heavily shielded casks, strapped down to flatbed trucks. A truck would make it past the security checkpoint and roll into a special bay in the storage building. Remotely controlled mechanical hands would take off the top of the cask and gently place the spent fuel in stainless steel buckets suspended from the ceiling. Using motorized overhead tracks, the fuel rolled into the long crane building and was diverted into its appropriate place to be processed. The fuel was dissolved and then broken down into components by specific chemical reactions. The fuel solution ran through stainless steel pipes and tanks, with the routings automatically controlled by pneumatic valve sequencers behind a shield wall in the building. Pressures, temperatures, flow rates, and tank levels were monitored and recorded. Every aspect of the facility was designed to shield human workers from any contact with radiation. There were radiation detectors and alarms everywhere, and to prevent possible power outages the plant had its own motor generator.
In general, the Chem Plant was well managed and well designed, and there was never a radiation injury or an accident that damaged the equipment. There were, however, three criticality accidents in which uranium dissolved in solution managed to find itself in a critical mass and go supercritical. Enriched uranium dissolved in water or an organic solvent will become an active nuclear reactor, increasing in power, if a specific “critical mass” is accumulated. The hydrogen in the water or the solvent acts as a moderator, slowing the fission neutrons to an advantageous speed, and even a fairly low U-235 enrichment level, like 3 %, will overcome neutron losses by non-productive absorption in the moderator. This has been realized since the earliest days of reactor engineering, and those who work with uranium solutions are quite aware of the possibility.
The volume or mass of the potentially critical solution is not the only factor. Any shape that reduces the surface-area-to-volume ratio is a bad choice for a vessel that is to contain a uranium solution. In a worst-case configuration, such as a spherical tank, a minimum critical mass is needed to start a chain reaction, because the neutron leakage from the surface is minimized. Another example of a shape that is good for making an impromptu reactor is a cylindrical tank, especially one built with the “tomato can” height-to-diameter ratio. Tin cans holding food are purposely made to optimize the amount of material they can contain using the least amount of sheet metal, to lessen the manufacturing cost. A uranium solution shaped like a can is a disaster waiting to happen. If any concentration of uranium in solution is held in a tank in a reprocessing plant, then the tank has to have a seriously bad magnitude of surface area compared to its volume. It absolutely must promote neutron loss by surface leakage. The tanks are therefore vertically mounted, thin tubes. They look like thickened pipes.
111
There were actually seven SNAP-10A’s built. Three were blown up at the NRTS, one was a spare, two were used for flight system ground tests, and only one was orbited.
112
What burned, causing a “fireball?” Ask yourself, why is fire visible? Why does fire glow bright yellow? The answer is sodium contamination. Sodium atoms glow yellow when heated by an otherwise invisible flame, and the slightest trace of sodium in a fire is what makes it look like fire. In the case of SNAP-10A, it used nack, or a sodium-potassium alloy, as the coolant. There was plenty of sodium on hand to light up the event when the reactor violently overheated. The sodium and potassium reacting explosively with the water topped it off.
113
I have been unable to find a report that describes the destruction of SNAPTRAN-1. I’m not sure whether it was too insignificant to warrant a report, or too significant to be declassified.