The Chem Plant at the NRTS was not the only one being built in the world. Eventually Canada, England, France, the Soviet Union, and Japan would have fuel processing and reprocessing plants serving their nuclear power plants and, in a few cases, bomb manufacturing. I would like to think that not one of these plants was built having a container inside the building that could hold a critical mass and configuration of uranium in solution, but I would be wrong. Incredibly, there has never been a Chem Plant made that could not support a critical mass in at least one tank. Given time, uranium will eventually find this tank, usually by inappropriate means, and a problematic supercriticality will result. A death in a power-reactor accident is exceedingly rare, but many people have died worldwide in criticality accidents involving nuclear fuel.
On October 16, 1959, the graveyard shift was working on 34 kilograms of uranium fuel enriched to 91 % U-235 in the form of liquid uranyl nitrate diluted with water. The next step of the processing was to extract impurities by mixing the nitrate solution with sulfuric acid in three steps. The first step had been completed, and the mixture was held in two vertical “pencil tanks.” These vessels were specifically designed to defeat criticality, being too small to hold the entire fuel load in one tank and being thin, measuring 3.050 meters tall and 0.125 meters in diameter. There was a tube leading from the drain end of the pencil tanks to a 5,000-gallon waste-receiving tank, looking like a tomato can. This tank was never, of course, supposed to have any uranium in it. It was there to hold the non-fissile waste products that were being extracted from the fuel. Just to make sure that it was impossible to siphon uranium solution out of the pencils or start a gravity drain, there was a loop in the connecting tube, located way above the top of the tanks. Only deliberate sabotage, it was thought, could cause any uranium to get into the receiving vessel.
An operator, reading his instructions, turned on two valves to spray some air into the two pencil tanks to stir it up and make sure that the acid and the uranyl nitrate were thoroughly mixed. Pencil tank number one had a pressure gauge on it so that the operator could make certain that he was applying the air at the correct sparge rate. Pencil tank number two did not have a pressure gauge, so the operator just opened the valve until he was sure the thing was good and sparged. Unfortunately, the excessive blast of air forced the liquid up through the anti-siphon loop, defeating it, and causing uranyl nitrate to siphon into the tomato can at 13 liters per minute.
The tank waited until it had 800 liters of 91 % U-235 in solution before it went supercritical. Radiation alarms started going off all over the place. They were ignored. If you evacuated the building every time one of those hyper-active alarms sounded, nothing would ever get done in this place. The third time somebody hit the evacuation alarm, people started moving. Nobody took the clearly marked evacuation routes, only the well-worn paths that they took out of the building every working day. As a result, there was a log jam at the door. Fortunately it was a small staff for this shift, and everybody showed up at the guard shack alive and well.
In the initial blue flash, hidden by the stainless steel tank walls if anyone had been looking, a hundred million billion fissions occurred. There were several excursions, with the contents of the tank boiling furiously, which would dilute the moderator with steam and kill the reaction. The steam bubbles would then collapse, and the supercritical condition would start over and do it again. After about 20 minutes, half the water content of the solution had boiled away, and the unplanned reactor aborted itself.
The lessons learned from this accident were sobering. Those particular pencil tanks were seldom used, and nobody on shift that night had any experience with them, so they had to read the written instructions, which were out of date. Routinely used waste tanks had better anti-siphoning systems. This one did not. The evacuation procedure, which had never been used before, obviously needed work. It did not take a sabotage to put uranium where it was not supposed to go. It only took the turn of an unfamiliar valve.
On January 25, 1961, the Chem Plant had been down for a year of renovation and had been operating 24 hours a day for the past four days. At 9:50 in the morning, another batch of uranyl nitrate found an “unfavorable geometry” in the upper disengagement head of the H-110 product evaporator. This was thought impossible because of an overflow line in the head that was supposed to prevent it from holding enough uranium solution to be critical. Someone had cleared out a clogged line downstream using compressed air, and it managed to blow enough uranyl nitrate into the cylindrical vessel to cause another big boil-off. Nobody was hurt, and nothing was damaged, but again it called into question several philosophies and engineering practices. The general boredom of working a shift in a chemical processing facility may have had something to do with it.
Finally, on October 17, 1978, a slow leak in a water valve eventually let the uranyl nitrate concentration in the lower disengagement section of scrubbing column H-100 grow to a supercritical level. It was another non-damaging, zero-death, embarrassing reactor where there should not have been one.
The Chem plant is still there, but it is now called INTEC, the Idaho Nuclear Technology and Engineering Center. It is working on a liquid-waste-processing method as part of the Department of Energy’s Idaho Cleanup Project. The exciting days of reactor experiments are long gone, and the purpose of this federally funded effort is to erase radiological traces of the old NRTS off the desert. Now, irrigated circles of land, planted with Idaho potatoes, encroach on the property once alight with the technology of the future.
Despite all the fine development work and all the logical reasons for reprocessing, such as greatly reducing the throw-away waste product, there is no commercial fuel reprocessing plant in the United States. Our fuel would be buried whole, if it were buried. There is currently no place to bury spent fuel in this country, so it just piles up at the power-plant sites. All other countries relying on nuclear power as a base-load source of electricity have routine reprocessing and waste burial, from Great Britain to Japan.
In spite of three criticality accidents, the Idaho Chemical Processing Plant was probably the safest fuel reprocessing facility in the entire world. We will visit this topic again in glorious detail, but now let’s remain calm, go across The Pond, and see what the reserved and quiet Brits were up to.
Chapter 5:
Making Everything Else Seem Insignificant in the UK
“Will you please issue the following operating instructions to the operator engaged in controlling the Wigner Energy Release. If the highest Uranium or Graphite temperature reaches 300 °C, then Mr Fair, Mr Gausden and Mr Robertson are to be informed at once, and the PCE alerted, to be ready to insert plugs and close the chimney base.”[114]
114
I am not sure what “PCE” means. It was possibly a typographical error in the original memo, meant to be “PEC,” or Production Executive Committee.