b'A P P E N D I X Dnot known to operators because an alarm had not been repaired and was inopera-ble. Also, operators had not sampled the scrub solution to determine whether thealuminum nitrate was above the required concentration. As a result, the solutionwas too dilute to force the uranium into the organic phase and instead extractedsmall amounts of it out on the organic phase into the aqueous phase and accumu-lated it into the large-diameter disengaging head.Over a period of a month, uranium continued to accumulate until it reached a con-centration in the aqueous phase high enough to achieve criticality. It accumulatedin a large-diameter part of the column designed to separate the organic phase fromthe aqueous phase. The criticality reaction continued for about a half an hourbefore the operators responded to the slight pressure build-up and took steps toterminate the reaction. The criticality occurred in a well-shielded location inside a process cell and result-ed in insignificant radiation exposures to personnel or damage to equipment.The operation and management failures associated with this criticality led to a sig-nificant reassessment and evaluation of Chem Plant operations. A Plant ProtectionSystem (which consisted of a variety of changes in procedures, operating limits,sampling protocols, specifications, warning systems regarding analytical samples,and others) was installed to preclude this type of accident from happening again.27 5'