User:KDS4444/Cecil Kelley criticality accident

The Cecil Kelley criticality accident was a nuclear accident that took place on December 30, 1958 at the Los Alamos National Laboratory in Los Alamos, New Mexico, in the United States. It is one of only ten such events to ever occur outside of a nuclear reactor, though it was the third such event to take place in 1958 (the others having taken place on 16 June at the Y-12 Plant in Oak Ridge, Tennessee, and on 15 October at the Vinca Nuclear Institute in Vinča, Yugoslavia). The accident involved plutonium compounds dissolved in liquid chemical reagents, and it caused the swift death of one man-- Cecil Kelley, a chemical operator-- from severe radiation poisoning within 35 hours. Much of what follows is a summary the accident described in detail by.

Context of the accident
Cecil Kelley was a 38-year-old experienced chemical operator at the Los Alamos lab where one of his duties was to operate a large mixing tank containing radioactive plutonium for the purpose of purifying and concentrating it for other laboratory uses. In pure form and under normal temperature and pressure conditions, plutonium-- a mostly man-made element existing in trace amounts in nature-- is a solid silvery metal. However, it tarnishes quickly when exposed to air and readily dissolves in concentrated hydrochloric, hydroiodic and perchloric acids. On the day of the accident the mixing tank was supposed to contain what nuclear chemists call a "lean" concentration of dissolved plutonium (≤0.1g of plutonium per liter of solution). However, as a result of what were later determined to be three "improper transfers" of plutonium waste to the tank, the concentration of plutonium in the mixing tank on this particular occasion was nearly 200 times as dense, and was distributed unevenly: the upper layers of solution had especially high concentrations, and were already close to criticality before Kelley arrived. When Kelley switched on the mixing tank, the lower and more-aqueous layers within the tank soon pushed outward and upward forming a vortex "bowl" and the higher plutonium-rich layers swirled into the vortex's bottom and center.

Among other ideal characteristics, the ideal shape for any radioactive substance to become supercritical is a perfect sphere. While the solution in center of the vortex was not exactly spherical, it sufficiently approximated a sphere long enough and with sufficient density for the dissolved plutonium to reach and cross the criticality threshold. Neutrons within the mixture began to bombard the nuclei of the solution's plutonium atoms with sufficient frequency that these atoms broke apart and released other neutrons in a sustained nuclear chain reaction lasting only 200 microseconds but releasing a huge burst of neutrons and gamma radiation. Such an uncontrolled release of nuclear energy is ofter referred to as an excursion.

The excursion
Kelly had been standing on a foot ladder looking at the contents of the mixing tank through a viewing window when the excursion event occurred. Two other technicians working within the laboratory witnessed a bright flash of light followed by the sound of a thud. The power burst either caused Kelley to collapse or knocked him off the ladder, and he had fallen to the ground. He arose disoriented, and apparently switched the mixer off and then back on again before running outside of the building (a second excursion did not take place). The other technicians found him there wobbling in a state of ataxia and only able to say to them, "I'm burning up! I'm burning up!"

Because the possibility of an excursion taking place in the mixing tank had been considered virtually impossible, the technicians decided that Kelley must have somehow been been exposed to the acid bath and one of them took him to a chemical shower while the other switched off the mixer. Additional staff members arrived within minutes to find Kelley virtually unconscious, the nurse noting innocently that he had "nice pink skin" as though from a sunburn. This pinkness was actually erythema brought on by cutaneous radiation syndrome.

Any accident at Los Alamos involving a radioactive substance requires an immediate investigation by a team of radiation monitoring staff. Even before Kelley was taken to an emergency room, these staff members began an examination of the mixing room with radiation detectors capable of assessing the alpha radiation indicative of plutonium decay. This radiation should have been widespread if any of the plutonium mixture had escaped the tank. Nothing unusual was initially found. Eighteen minutes later, however, the team began searching for gamma radiation-- they unexpectedly found this kind of radiation in abundance near the mixing tank, on the order of tens of rads per hour. Such intense gamma radiation could only have been produced during an excursion; this, combined with the otherwise inexplicable flash of light reported by the other two technicians, was sufficient to identify the event as a criticality accident.

Kelley's clinical course
For the first hour and forty minutes following the accident Kelley had been incoherent, and had gone through waves of intense vomiting and retching. He then stabilized and was able to have his pulse taken and his blood drawn. The blood draw indicated that Kelley had been exposed to approximately 900 rad from fast neutrons and 2,700 rad from gamma rays, for a total of 3,600 rad. For an adult human, exposure to 200 rad from a spherical (unfocused) radiation source such as an excursion will cause radiation sickness but is not considered definitely lethal; 240-340 rad corresponds to a 50% survival rate; a dose of 500 rad is almost always deadly; Kelley had received more than seven times the adult human lethal dose. Within six hours his lymphocytes were all but gone. A bone biopsy performed 24 hours after the incident produced bone marrow that was watery and contained no red blood cells. Numerous intravenous blood transfusions had no lasting helpful effect: just over ten hours later, after intense restlessness, sweating, becoming ashen-skinned, and having an irregular pulse, Cecil Kelley died.