On March 6, 1995, the facility manager issued the final report on an event at the Hanford Plutonium Finishing Plant that occurred on August 30, 1994, when preliminary assay results indicated that the remaining quantity of plutonium in an incinerator building was greater than the 848 grams stated in the Final Safety Analysis Report (FSAR). The plant review committee determined that this constituted an unreviewed safety question (USQ). Following a criticality safety review, facility managers placed more restrictive requirements on access to the building, limited fissile material handling in the complex and assembled a team of experts from the contractor organization and other DOE sites to assist in resolving the USQ. (ORPS Report RL--WHC-PFP-1994-0043)
The incinerator was built to recover plutonium from contaminated items such as protective clothing, gloves, rags, and paper. It operated intermittently from 1962 to 1973 and was shutdown in 1977. In the 1980's, several glove-boxes were removed from the building. In 1989, an assay of plutonium holdup in the process equipment and ductwork was performed. A total quantity of 848 grams of plutonium for the incinerator building, including 20 grams in the burning hood, was reported in the FSAR. In early 1994, workers began the process of removing ductwork and cleaning glove-boxes. Most of the work was completed in June 1994. When personnel performed an assay to determine the remaining plutonium, they found 233 grams remaining in other equipment and 91 grams in the burning hood. Because this value for the burning hood was higher than the value reported in the FSAR, they performed a second assay for the burning hood and found a similar quantity of 92 grams. Facility personnel investigating this discrepancy determined that there was fire brick in the burning hood and that the upper portion of the burning hood had not been assayed in 1989 or 1994. Personnel performing the 1989 assay had been told that the fire brick and combustion chambers had been removed from the burning hood and therefore did not account for the shielding effect of the fire brick in estimating the quantity of plutonium in the burning hood.
Technical experts performed attenuation measurements on fire brick obtained from a local coal-burning power plant and used detailed information on the composition and density of insulation material to estimate the attenuation correction factors. Based on this data, further measurements resulted in an estimated upper limit value for plutonium in the building of 6.5 kilograms. In resolving the USQ, facility personnel also performed a revised accident analysis using the 6.5 kilograms of plutonium for the incinerator building and incorporated it in the FSAR. The dose consequence resulting from this analysis remained within the original acceptance guidelines.
Facility managers determined that the direct cause of this event was human error because an incorrect value for plutonium in the building was included in the FSAR. Inadequate work planning and organization was a contributing cause since the upper portion of the burning hood was not assayed. They determined that the root cause was a verbal communication problem. Incorrect information concerning the absence of fire brick and combustion chambers was given to the scientists performing the assays in 1989. In addition to the FSAR revision, corrective actions also included a structural modification to the burning hood to mitigate the potential for release while the assays and the revised accident analysis were being performed.
In OE Weekly Summary 94-47, NFS reported on a similar event that occurred on November 16, 1994. Personnel at the Savannah River Separations Equipment Development (SED) facility reported that initial results of trap assays indicated significantly less plutonium than expected based on book value inventory data. SED is an equipment development facility that has not operated since the 1970's. It is known to have a significant quantity of plutonium within the process equipment, and it was one of the first facilities at Savannah River identified for decontamination and decommissioning (D&D). Facility managers conducted an extensive investigation and learned that, in this case, plutonium inventories were based on a best estimate with all uncertainties and biases chosen in the direction that would provide the greatest conservatism for criticality safety. They performed an evaluation after completing the trap assays and determined that the safety analysis was not impacted and that operational safety requirements were not violated. (ORPS Final Report SR--WSRC-LTA-1994-0128)
These events illustrate the information uncertainties regarding inventories of special nuclear materials that can occur when D&D activities are undertaken at DOE facilities that have been shut down for an extended period. In the past, the rules, analytical methods, facility inventory records, and operations were less formalized than they are at DOE facilities today. Facility personnel planning to undertake D&D activities should consider including a review of the facility's special nuclear material inventory methods. Gaining a thorough understanding of the facility's treatment of uncertainties before preparatory assays are undertaken can minimize the potential for time- consuming evaluations which may impact D&D project schedules.