The report, a joint effort of the United States and the Russian Federation, is the latest revision of a report first published in 1967 titled "A Review of Criticality Accidents." It includes significant new information about accidents that occurred in the United States, Russia, the United Kingdom, Canada, France, Argentina and Japan. The report begins by separating accidents into two distinct categories: process accidents and research reactor accidents.
Separating the two categories is important because of distinct differences between the two. In process accidents, administrative or physical controls are generally in place to prevent any sort of criticality, meaning that when criticality occurs it is wholly unwanted and unexpected. In research reactor accidents, some measure of criticality is purposely achieved, usually during experimentation, and ends up getting out of control somehow.
One of the report's principal authors, Thomas McLaughlin, of the Laboratory's Nuclear Criticality Safety group, says the report serves not only to reconstruct criticality accidents but to also offer information central to the prevention of accidents and the most effective response should an accident occur. "The most important sections of the report deal with observations and lessons learned from process criticality accidents," said McLaughlin. "All criticality accidents are dominated by design, managerial and operational failure, it's important that these issues be the focus for accident prevention."
A criticality accident occurs when the minimum amount of fissile material required to sustain a chain reaction is accidentally brought together. For example, when the nucleus of Uranium-235 disintegrates, two or three neutrons are released, and each is capable of causing another nucleus to disintegrate. However, if the total mass of the U-235 is insufficient to sustain a chain reaction, the neutrons simply escape. In most criticality accidents this chain reaction is very short lived, causing a neutron population spike and resultant radiation, heat and, in many cases, an ethereal "blue flash," a phenomenon of the air surrounding a neutron burst becoming ionized and giving off a flash of blue light.
The first-ever criticality accident resulting in a fatality occurred at Los Alamos' Omega Site on Aug. 21, 1945, and involved a 6.2-kilogram nickel-plated plutonium sphere and neutron-reflecting tungsten-carbide blocks. A critical assembly was created accidentally while the reflecting-blocks were stacked around the Pu sphere. The lone experimenter, Harry Daghlian, while moving a block in order to take a measurement had the block slip and drop onto the center of the assembly, causing a "superprompt" criticality event. Daghlian removed the dropped block by hand to stop the chain reaction and in doing so received a radiation dose estimated at 510 rem. He died 28 days later.
The vast majority of accidents do not result in death, but there have been 21 fatalities from criticality accidents since 1945. Nine of these fatalities were due to process accidents, and 12 resulted from research reactor accidents, according to the report. Of the 21 deaths, seven occurred in the United States, 10 in the Soviet Union, two in Japan, and one each in Yugoslavia and Argentina. There have been no accidents involving the transportation or storage of fissile materials. Only one accident, at the JCO Fuel Fabrication Plant in Tokimura, Japan, resulted in measurable exposures to members of the public, and these were well below allowable annual exposures to workers.
The report, originally scheduled for publication near the end of 1999, was held until now in order to add information from the process accident in Japan. "We felt it was very important to wait until we had the facts from this latest event so they could be added to this new revision," said McLaughlin. "The entire team working on this report wanted to make sure that it was a comprehensive as possible and contained the most up-to-date information."
In addition to McLaughlin, the report's authors are Shean Monahan of LANL, Norman Pruvost of Galaxy Computer Services, Inc., and Vladimir V. Frolov, Boris G. Ryazanov and Victor I. Sviridov all of the Institute of Physics and Power Engineering in Obninsk, Russia,
Los Alamos National Laboratory, a multidisciplinary research institution engaged in strategic science on behalf of national security, is operated by Los Alamos National Security, LLC, a team composed of Bechtel National, the University of California, The Babcock & Wilcox Company, and Washington Group International for the Department of Energy's National Nuclear Security Administration.
Los Alamos enhances national security by ensuring the safety and reliability of the U.S. nuclear stockpile, developing technologies to reduce threats from weapons of mass destruction, and solving problems related to energy, environment, infrastructure, health, and global security concerns.