In 1999, workers were preparing a shipment of uranium solutions from a nuclear fuel fabrication plant in Japan. The workers decided to consolidate the solutions in a precipitation vat before placing them into the shipment containers. The procedure worked well for the first six solutions, but the seventh batch contained 18.8 % 235U. While two workers were adding a seventh batch of uranium solution to the tank, a critical excursion occurred. The two workers, together with a third worker nearby, observed a blue flash and fled; simultaneously, gamma radiation detectors went off in the building and two adjacent buildings, prompting all workers to evacuate. The two workers who had been pouring were transported to the hospital and began to show signs of radiation sickness during the ambulance ride. They had absorbed fatal doses in the range of 600 to 2000 rem. Explain why the criticality occurred and how it could have been avoided.