b'A P P E N D I X DC r i t i cality Acc i d e n t s, Idaho Chemical Pro cessing PlantA criticality accident is an unintendedamassing of a fissionable material (like uranium) which results in the fissioning ofthe material in a chain reaction. In such an event, fission products such as heat,gamma radiation, neutrons, gases, and other emissions are released by the nuclearreaction.The designers of chemical processing, fuel fabrication, and other plants that han-dle fissionable material employ a variety of strategies to avoid accidental amass-ing of enough material to initiate a chain reaction. The examples below refer touranium, but similar principles would apply to the management of any other fis-sionable material: Geometric control: the dimensions of the containers and conveyors of uraniummake it impossible to reach a critical mass. At the Chem Plant, for example,certain dissolver vessels and storage vessels were no more than five inches indiameter. Spacing of vessels was also important, with two feet between vesselsrequired to prevent a criticality. Concentration control: Where chemical processes involve evaporation or pre-cipitation reactions which could result in the concentration of the uranium, ves-sels and containers are sized to prevent accidental accumulations of a criticalmass. Appropriate dilution may also be used to keep the solution concentrationbelow a minimum value to prevent criticalities. Mass control: In handling enriched uranium, the quantity that can be handled atany one time is limited to a specified, known-to-be-safe number of grams ofmaterial. Administrative control: Operational procedures may require two or more peo-ple to approve if a particular procedure could lead to a loss of control. Check-off points, guide limits, process alarm systems, color-coding of certain valvehandles, key-only procedures, personnel training, and other controls accompanynon-routine and many routine procedures.273'