Risks and Uncertainties Associated With High-Level Waste Tank Closure
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II1.12.1
Risks and Uncertainties Associated With High-Level Waste Tank Closure
David W. Esh, Anna H. Bradford, Kristina L. Banovac, B. Jennifer Davis U.S. Nuclear Regulatory Commission Washington, DC 20555, U.S.A.
ABSTRACT Closure of tanks containing high-level radioactive waste (HLW) is a challenging problem involving potentially competing influences from economic, societal, and technological considerations. The U.S. Department of Energy (DOE) is faced with protecting public health and the environment while making economically responsible decisions. Risk (i.e., annual dose) is becoming more prominent as DOE’s metric to evaluate the economic consequences of its decisions. Risks are assessed through modeling and calculations commonly known as performance assessment (PA). In the process of tank closure, the U.S. Nuclear Regulatory Commission (NRC) is typically consulted to perform an independent review of DOE’s PAs. The NRC staff developed a generic PA model, applicable to HLW tank closure, which NRC utilizes to complete its independent review. The model was developed using the generic simulation software, GoldSim, because of its probabilistic capabilities and its adaptability to different problems [1]. The NRC staff uses the resultant risk from the generic models to evaluate the reasonableness of performance assessment models submitted by DOE. Large differences in the estimates of risk between the generic PA model and the DOE PA would likely indicate a need for stronger technical basis for processes significantly contributing to annual dose (risk) reduction.
BACKGROUND Liquid HLW results from the reprocessing of spent nuclear reactor fuel. Reprocessing involves the dissolution of spent nuclear fuel followed by chemical separation to selectively remove fissile material for reuse. Various waste streams have been produced during reprocessing operations, depending on the technology and waste management practices utilized at DOE sites. From the Nuclear Waste Policy Act of 1982, as amended, both the supernate and sludge resulting from the reprocessing of spent nuclear fuel are classified as HLW. In other words, wastes from reprocessing are classified by their source, and not by risk to human health or the environment. In most cases, it is not technically or economically practical to remove all reprocessing wastes prior to tank closure; therefore, DOE may choose to leave some HLW in the tanks and close the tank in place. In the past, NRC has provided criteria, as guidance to DOE, for which the residual materials in the tank can be determined to be incidental, and therefore would not need to be disposed of as HLW. The criterion evaluated in this paper is: “…wastes are to be managed, pursuant to the Atomic Energy Act, so that safety requirements comparable to the performance objectives set out in 10 CFR Part 61, Subpart C are satisfied.” The performance objectives of 10 CFR Part 61 include protection of the general population, protection of an inadvertent intruder, protection of workers, and stability of the disposal site
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