On October 27, 1995, at the Savannah River Site, engineers discovered that the pH for solutions in six high-activity low- level waste tanks were above the 5.1 limit specified in the criticality safety manual. The engineers reviewed pre-shipping analysis data for the tanks and found that the pH had been above the limit since December 1994. The Criticality Review Committee determined that this event was not a significant criticality concern because the concentration of plutonium in the waste was low enough to prevent a charge in its chemical composition even though the pH was too high. The event was significant because there was no procedure to implement the criticality safety manual and control pH in the tanks. (ORPS Report SR--WSRC-LTA-1995-0110)

The high-activity low-level waste tanks are located underground and receive liquid from the high-activity drain system at the Savannah River Technical Center. When liquid volume inside a tank approaches capacity, an operator transfers the liquid to a shielded high-activity tanker trailer. Before the liquid is transferred, technicians take samples to determine activity and pH. This was the only method used to monitor pH.

As part of a project to remove nuclear incident monitors near the tanks, engineers performed a criticality study including a review of the criticality safety manual. They analyzed the pre-shipping pH data for the tanks and found the pH values to range from 5.94 to 8.6. As a compensatory measure, the high-activity drains were administratively closed to prevent additions to the tanks.

Savannah River Technical Center operators discussed the following issues at the event critique.

- The operations procedure does not control storage activities at the tank farm.

- There is no specific procedure for control of pH or storage activities.

- Operations personnel have an inadequate understanding of the criticality safety manual because they did not realize that the pH needed to be controlled.

- The rise in pH was caused by rain water seeping into a sump that drains into the tanks.

The rain seepage will be eliminated by a previously planned project to repave the area above the sump. The paving project will also provide an economical benefit by reducing the volume of water that is processed as high-activity, low-level waste. Also, managers are planning to evaluate the implementing procedures for the tanks to ensure that all criticality concerns are addressed.

This event illustrates the importance of implementing procedures and operator knowledge of pH processes required for criticality control. DOE 5480.19, Conduct of Operations, chapter XIII, states that operators should be know the aspects of facility processes and safety including the concepts of pH. DOE facility managers should review their criticality control programs for compliance with this Order.