Introduction
With the end of the Cold War, the US has declared 38.2 metric tons of plutonium excess to national security needs. Two methods, immobilization and mixed-oxide fuel fabrication with nuclear reactor energy generation, have been identified to dispose of the surplus plutonium. Both processes require the conversion of plutonium metal into plutonium oxide. The US Department of Energy (DOE) Pantex Plant outside of Amarillo, Texas, is one of the four sites being considered for plutonium conversion and mixed-oxide fuel fabrication. As such, the Amarillo National Resource Center for Plutonium (the Center) has initiated a study to determine the impact that these missions could have on the environment and public health in the Panhandle region of Texas. The study includes public health, worker safety, waste management, and environmental effects.
Pantex is the nations only remaining facility for nuclear weapons assembly, maintenance and disassembly, with a four decade record of accomplishment. Since the end of the Cold War, Pantex has been dismantling weapons no longer needed for national security. This dismantlement has left a large amount of weapons plutonium in storage at the plant.
Weapons plutonium, in the form of a solid metal shell, or pit, must be converted to a plutonium-oxide powder as a first step in immobilizing plutonium or fabricating mixed-oxide fuel. To date, the various steps in the conversion process have been tested successfully at the laboratory scale, each step having been tested separately.
For mixed-oxide fuel fabrication the plutonium metal is converted to an oxide and blended with uranium oxide. The mixed-oxide powder blend is then formed into ceramic fuel pellets. The pellets are loaded into nuclear reactor fuel rods, which are incorporated into existing nuclear power plants to generate electricity. The isotopic composition of the weapons plutonium is permanently altered during this electric power generation stage. Mixed-oxide fuel fabrication has been safely accomplished at production-scale for the past several decades in Europe.
The Center has completed the first
phase of this study, which is a preliminary assessment of the
risks associated with producing mixed-oxide fuel at Pantex. This
paper briefly describes the methodology being used in this on-going
study to calculate the risks and the results from the preliminary
analysis.
Description of Work
The risk characterization study of the proposed missions requires consideration of all related activities and processes. All wastes and their subsequent disposal methods require identification. Both normal operating conditions and maximum credible accidents are being considered. Maximum credible accidents are those with a probability of occurrence that is greater than 1x10-6 and whose postulated effects have the greatest consequences. Potential exposure pathways include air, surface water, soil, terrestrial food chain, and subsurface water. All potential source terms and pathways are being considered, and bounding pathways are being analyzed in detail. For each process, the procedures, inputs, outputs, wastes generated, chemical forms and number of modules are being identified.
To analyze the health physics and industrial hygiene related to the proposed processes, two general procedures are being used. In the first screening procedure, risks are being estimated through comparison with similar processes at other DOE and industrial facilities. The second procedure includes a detailed process-by-process analysis to identify potential accidents and releases, and associated risks from on-site and environmental exposure.
The surrogate facility method was used for the screening phase of the project. To calculate a risk relative to current operations at Pantex for the proposed operations, risks associated with current operations at Pantex were compared to risks in nuclear facilities that have had experience in storing, processing, and handling plutonium in an oxide form. For this comparison, preference was given to DOE facilities given the regulatory oversight that occurs at these facilities.
Environmental dispersion models have been developed and are being used for credible release source terms and environmental pathways. These models include the HOTSPOT computer code, and the Pascal-Gifford equations for atmospheric dispersion.
The assessment being performed is based
on data specific to the proposed missions and to the Panhandle.
Data accrued over the operating history of similar facilities
were used in the absence of process-specific data. The most appropriate
and accepted computer models are being used in the analyses.
Dose estimates are based on an assumed level of engineered safety.
Results
A preliminary characterization of relative societal risks between existing and proposed missions at Pantex was performed. Additionally, waste streams from the proposed processes were identified and models were run to determine the impacts that releases might have on the surrounding area.
Preliminary results indicate that the environmental and health impacts of a plutonium conversion facility and a mixed-oxide fuel fabrication facility at Pantex would be comparable to the impacts of current missions. During routine operations, there would be no impact to water resources, water quality, soil, or air resources. Preliminary results of maximum credible accident modeling indicate no detrimental effects on the quality of water resources, and a predicted 130 acres could potentially be impacted above levels defined by the Environmental Protective Agency's Protective Action Guidelines.
As a result of this study, the framework
was established to conduct more detailed follow-on investigations.
These studies are currently underway at the University of Texas
and Texas A&M University.