Post-war improvements on the World War II atomic bombs were numerous and rapidly applied. The old Fat Man nuclear device that wiped out Nagasaki was five feet in diameter and contained 5,300 pounds of high explosive. That seemed clumsy, but by the late 1950s the bomb engineers had it down to 44 pounds of explosive in a bomb that was many times more powerful. At one point, they got it down to 15 pounds. This improvement meant lighter, smaller atomic bombs that could be put in cruise missiles, air-to-air missiles, or a rocket fired from a recoilless rifle bolted to a jeep. The antique formulas having baratol or RDX explosives were supplanted with such exotics as cyclotetramethylenetetranitromine (HMX) and triaminotrinitrobenzine (TATB).[169]
Making bombs less bulky was all well and good, but as the energy from the explosives became concentrated into smaller spaces, they got touchy, or very sensitive to being slapped. There were three accidental explosion events in the 1960s, when improper handling procedures led to detonations, but there were no deaths. All operations at the plant were carefully sequestered, with strong blast walls separating an operation from all other operations and not letting an accident become a catastrophe, setting off adjacent explosives or even setting off a nuclear event. All steps of explosive manufacture were done in the smallest possible batches.
On March 30, 1977, the luck ran out at Pantex in Building 11-14A, Bay 8. A machinist had chucked a billet of high explosive in the lathe chuck and turned it by hand to see how it would spin. It was slightly out of alignment, running a bit wobbly on the lathe spindle, and at cutting speed it would vibrate. This is a common occurrence when using a gear-chuck on a lathe. As careful as you are, the work-piece will not necessarily sit right in the chuck when you tighten it up. To remedy this, an experienced machinist will pick up his much-used wooden mallet and tap the piece into alignment, hitting it on the edge that causes the most “run-out.” The last thing the machinist saw was the mallet coming down on the edge of the explosive work-piece. He and two others died instantly in the blast.
The Energy Research and Development Administration report on the accident was issued on March 1, 1979. The sensitive PBX-9404 fast explosive was replaced by less sensitive PBX-9502, and a movement to change out all the aging, increasingly sensitive explosives in weapons on the shelf gained attention. A Department of Energy Explosive Safety Manual, DOE M 440.1-1A, was in place by the mid-1990s. Pantex is still in business, refurbishing and repairing our aging inventory of weapons, which is probably about 2,200 units.
These misfortunes in the production of nuclear devices are interesting, but none were true atomic accidents. They were industrial accidents of types that could occur anywhere in the technosphere. Authentic nuclear accidents in fuel processing, usually but not always for bomb manufacture, did occur, unlike anything in the history of technology. Some were predictable, and some were not. There have been 22 documented cases of process accidents in which an unexpected criticality occurred in the United States, Russia, Great Britain, and Japan. In these incidents, there were nine fatalities due to close exposure to radiation from self-sustained fission. Accidents occurred with the fissile material in a solution or slurry in 21 cases, and one occurred in a pile of metal ingots. No criticalities were the result of powered fissile material. No accident has occurred in the transportation of fissile material or while it was being stored. Of the many survivors of criticality accidents, three had limbs amputated due to vascular system collapse. Only one incident exposed the public to radiation. There was a clump of 17 accidents between 1957 and 1971, and only two have occurred since.
The first atomic bomb was conceived, designed, and built at the Los Alamos Scientific Laboratory in New Mexico, and after the war it was expanded to one of the largest and most versatile facilities in the galaxy of national labs. In 1958 they were still doing chemical separation of plutonium at Los Alamos, even though most of this was being carried out elsewhere. Somewhere in the above-ground portion of Los Alamos was a dreary, windowless concrete room packed neatly with 264-gallon stainless steel tanks, about three feet in diameter, each held off the floor with four stubby legs and seemingly connected together in all kinds of ways by a maze of pipes, tubes, and cables. They looked like short water heaters. There was a tall sight-glass bolted to the side, so that an observer could see the liquid in the tank and tell how full it was. On top was a push-button switch. Press the switch, and an electric motor would spin a stirring impeller at the very bottom of the tank, mixing the contents into a homogenous fluid.
The tanks were part of the chemical separation system, meant to recover plutonium from machine-shop waste, leftovers in melting crucibles, or slag from casting. The tanks typically held aqueous solutions that were about 0.1 gram of plutonium per liter, which was way below anything that could be made critical, but the tanks, which had been in daily use for the past seven years, were obsolete, and they were scheduled to be replaced soon. They were still in fine condition, but they had been made in a perfect shape for accidental criticality. They had the surface-area-minimizing shape of a soup can, and the ends were rounded. By now it was realized that this was a dangerous shape, even though the procedures were designed to absolutely prohibit there ever being enough plutonium in one tank to go critical. The replacement tanks would be 10 feet high and six inches in diameter, which would discourage anything less than solid plutonium from becoming a runaway reactor. It was 4:35 P.M. on December 30, 1958, a little before quitting time on the last shift before the New Year’s holiday. A load of 129 gallons of a murky fluid consisting of plutonium, nitric acid, water, and an organic solvent had been drained out of two other vessels and transferred to this particular tank.[170] Allowed to sit for a while, the liquid had separated into 87 gallons of water in the bottom of the tank with 42 gallons of oily solvent sitting on top of the water. This was to be expected, which is why there was an aggressive stirring mechanism built into the tank. Unknown to anyone, plutonium solids, built up from years of processing, had dissolved off the insides of the tanks upstream and landed in this tank. The water in the bottom had only 2 ounces of plutonium dissolved in it, but the thin, disc-like layer of solvent on top contained a barely subcritical 6.8 pounds of plutonium, helped along in its quest to go critical by being homogeneously mixed with a hydrocarbon liquid, an excellent neutron moderator.
Cecil Kelley had spent the last 11.5 years as a plutonium-process operator at Los Alamos, and he had almost seen it all. He stepped up on a footladder to look at the contents of the tank through a glass porthole on top, cupping his left hand to shut out ambient light. The ceiling fluorescents were illumining the surface of the liquid through another porthole on the other side. It was time to mix the water and the light solvent together. Leaning on the tank, he reached for the stir button with his right hand and pressed it. It took one second for the impeller to reach speed at 60 RPM.
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In the 1960s at the apex of nuclear weapon development, the most favored chemical explosive formulas were PBX-9404 (93 % HMX, 6.5 % nitrocellulose, 0.5 % wax) and LX-17 (92.5 % TATB, 7.5 % wax). Wax was used as a binder that would melt in heat and re-solidify when cooled. The “exploding wire” detonators used pentaerythritoltetranitrate (PETN).
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The official reports of this incident always refer to an “organic solvent” without specifying what exactly was in the mix. It was surely a 30-percent solution of tributyl phosphate in kerosene, the active ingredient in the ion-exchange process known as PUREX. Invented during the Manhattan Project at the University of Chicago, PUREX (Plutonium URanium EXtraction) was the fuel-processing method of choice through the 1970s. It was classified SECRET at the time of this incident, and report writers were careful not to divulge any information that was not necessary in explaining an accident.