ORNL AND NUCLEAR
Nuclear criticality safetyensuring the safe storage, handling, and transportation of fissionable materialsis one of several areas of science and technology upon which ORNL has had a major international impact.
In any activity involving sizeable quantities of fissionable materials, a nuclear criticality safety program must seek to prevent an unintentional, uncontrolled fission chain reaction that results from an excess of fissionable materials (e.g., uranium-235 and plutonium-239) in close proximity during processing, storage, or transport. The aim is to protect against the consequences of an inadvertent nuclear chain reaction. The need for industrial controls at sites where fissionable materials were prepared, produced, or processed was recognized in the earliest days of the nuclear program. Early sites needing these controls included the K-25 and Y-12 plants and the facilities at Hanford and Los Alamos.
The K-25 gaseous diffusion plant was the focus for the earliest criticality studies. In the mid-1940s, Edward Teller and his colleagues reviewed the plans for this plant for potential unsafe accumulations. In late 1945, Art Snell of the Laboratory investigated the safety of "product drums" for transferring uranium hexafluoride enriched in low amounts of uranium-235. It was determined that criticality might be achieved in a drum if the enrichment were greater than 10%.
In the late 1940s, experimental results were obtained at Oak Ridge and later at Los Alamos, Hanford, and Rocky Flats to guide safe use of fissionable materials in storage and transport, chemical processes being designed and operated, and metallurgical operations including machining and disposal of scrap. Of even greater importance has been the experimental data used as benchmark information to verify and validate calculation methods that are only now reaching maturity.
In 1949 the demand for this information by the rapidly growing nuclear community resulted in expansion of the Critical Experiments Laboratory. The team that operated this Y-12 Plant facility was transferred into the ORNL organization because its chief mission was to guide new reactor designs using data from critical experiments. However, it had an opportunity to assess the effects of a criticality safety accident in its own backyard.
In June 1958 the first critical accumulation of a fissionable material in an industrial process occurred within the Y-12 Plant. The cause was a leaky valve that allowed a solution containing uranium-235 to flow into a large vessel, resulting in exposure of eight men to radiation. A study at ORNL's Critical Experiments Laboratory of the energy released by the chain reaction confirmed early medical observations that the exposures were not as severe as first feared. Prompt evacuation by the personnel from the area where the reaction persisted minimized their exposures. None suffered any ill effects.
In 1950 Dixon Callihan and Sidney Visner established the ORNL Criticality Review Committee to review and approve Laboratory operations that involve potentially critical quantities of fissionable materials. The committee was headed by Joe Thomas recently.
The Laboratory supported the effort to develop national standards within the nuclear community through the American Nuclear Society program. A committee, first chaired by Callihan in the early 1960s and subsequently by Jack McLendon and Thomas, produced the first nuclear standard that gave quantitative guidance in 1964. It is one of a family of more than 20 national standards on criticality safety prepared by this international group still administered out of ORNL.
For more than 20 years, staff members at ORNL have been developing criticality safety software. The most internationally recognized software of this type is KENO, which was developed by Elliott Whitesides and Nancy Landers. The results of ORNL's critical experiments provided the benchmark data against which the results of the computer code calculations could be checked.
John Mihalczo recently has developed a technique for determining the margin by which a quantity of fissionable material is subcritical. DOE's Nuclear Criticality Technology and Safety Project, which has been managed at ORNL, created an "apprentice program" to train future experts in criticality safety.
Related Web sites