Cecil Kelly critical accident


May 29, 2022

The Cecil Kelley criticality accident occurred on December 30, 1958 at the Los Alamos National Laboratory in New Mexico, USA. It is one of 10 critical accidents outside a nuclear reactor in history. At the time of the 1958 accident, it was the third critical accident in history following the critical accident at the Y-12 National Security Area in Oak Ridge, Tennessee, USA on June 16 and the critical accident at the Bincha Nuclear Research Center in Belgrade, Yugoslavia on October 15. A critical reaction occurred in the plutonium compound dissolved in the liquid reagent, and in an accident, chemist Cecil Kelly died of acute radiation syndrome within 35 hours.

Thought Development

Cecil Kelley (October 16, 1920 – December 31, 1958) was a 38-year-old chemistry researcher with 11 years of experience, working more than half of his research career at the Los Alamos laboratory. One of Kelly's main studies was to operate a stainless steel mixing tank with a capacity of about 1,000 liters. Inside the tank were residual plutonium-239 left over from other experiments and studies, as well as aqueous solutions in which various organic solvents and acids were dissolved for the purpose of recovery for reuse. In its pure form and under ambient temperature and pressure conditions, plutonium is a hard, silvery metal. It discolors quickly when exposed to air and is easily soluble in hydrochloric acid, iodic acid, and perchloric acid. On the day of the accident, in the mixing tank, plutonium dissolved in nitric acid with strong corrosive properties and a water-soluble organic solvent for stabilization to prevent corrosion of containers (0.1 g of plutonium per 1 L of solvent) ), and nuclear chemists called this solution lean. However, plutonium waste has been "improperly injected" into the tank at least twice (although there is no evidence that this was intentional or publicly available, and there is no evidence that Cecil Kelly suspected or witnessed it), and the concentration of plutonium in the tank was In some domains it was approximately 200 times higher than the originally hypothesized condition. In addition, the solute in the solution was not mixed evenly, so the upper part of the solution contained more than 3 kg of plutonium, so the concentration was very high, and it was already close to the critical mass before Kelly did anything. Meanwhile, as Kelly activated the mixing tank, the solution began to swirl and bubble. The dense aqueous layer in the tank was pushed outward and wrapped around a bowl, while the less dense, plutonium-enriched layer swirled towards the center of the container. Among the ideal properties, the most ideal form for any fissile material to break through its critical mass is to have a spherical shape with the smallest surface area. The solution with a high concentration of plutonium was not spherical, but the vortex caused the solution to gather at the center, increasing the density per volume, and the water layer surrounding it increased the neutron reflectance, so the dissolved plutonium broke through the critical mass in about 1 second and caused a critical reaction. . The neutrons in the mixture hit the plutonium nuclei with sufficient frequency to cause fission and the nuclear chain reaction to release neutrons lasted only about 200 milliseconds (0.2 seconds), but the moment the chain reaction took place, a huge amount of neutrons and gamma rays were emitted. This uncontrolled release of nuclear energy is called a critical accident. In less than 3 seconds, the mixture layer was completely dispersed and the critical reaction stopped.


Cecil Kelly was stepping on a ladder at the time of the critical accident, looking at the contents of the mixing tank through the tank's viewing window. Two other technologists working inside the lab flashed blue lights and