b'P ROVING THE P RINCIPLE1. Criticality Accident of October 16, 1959A bank of storage cylinders containing a uranium solution was air-sparged (airwas bubbled violently into the solution to mix it). The cylinders were geometrical-ly safe, but the sparging initiated a siphon that transferred 200 liters of the solu-tion to a 5,000-gallon tank containing about 600 liters of water. The resultingcriticality lasted about twenty minutes.No workers were exposed to gamma or neutron radiation, as the criticalityoccurred in a cell below ground when no one was in the vicinity. Airborne activityspread through the plant through vent lines and drain connections, triggeringalarms and an evacuation. Two people who evacuated received significant betadoses (with no detectable medical consequences) as they passed areas whereradioactive gas was being released into the room from floor drains.The incident resulted in the placement of new valves, restrictions on air-flow lineswhen sparging, installation of water traps, and other measures before the plantrestarted.2. Criticality Accident of January 25, 1961About 40 liters of uranyl nitrate solution (200 grams of uranium per liter) wasforced upward from a 5-inch-diameter section of an evaporator into a 24-inch-diameter vapor disengagement cylinder, well above normal solution level.Analysts later assumed that air entered associated lines while operators wereattempting to clear a plugged line and improve a pump. When the air bubblereached the evaporator, solution was expelled from the lower section, and amomentary criticality occurred in the upper section. Radiation triggered alarms,but no personnel received more than 100 mrem exposure. Concrete shieldingwalls surrounded the location of the criticality; the vent system prevented airborneactivity from entering work areas; and equipment design prevented a persistentexcursion. No equipment was damaged.Management thereafter restricted the use of air pressure to move liquids and clearlines. A borated steel grid was installed in the disengagement cylinder. Boron is anuclear poison that absorbs neutrons, helping prevent criticalities.3. Criticality Accident of October 17, 1978During the first solvent extraction cycle in the recovery of uranium from spentfuel, the uranium was extracted from the dissolution solution and then scrubbed,stripped, and washed in various process columns to separate the uranium from fis-sion products. The criticality occurred in the scrub column (a long narrow, verticaltank). Water had leaked into the tank where the scrub chemical, aluminum nitrate,had been made up, and reduced the aluminum nitrate concentration. But this was27 4'