Plutonium Transport Through Lysimeters Exposed to Natural Weather Conditions for Two to Twelve Years
One of the most important factors influencing the subsurface transport of plutonium (Pu) is its oxidation state. Under similar geochemical conditions (e.g., groundwater pH) the mobility of reduced Pu, Pu(IV), is two to three orders of magnitude lower than that of oxidized Pu, Pu(V) and Pu(VI). However, due to a poor understanding of Pu oxidation state transformations, transport models typically employ conservative assumptions which can lead to overly conservative and costly decisions. The objective of this study was to develop a conceptual geochemical transport model to describe Pu mobility through 52-L lysimeters established in 1980. The lysimeters contained E-Area sediment and various forms of well-characterized Pu sources of known oxidation state and were exposed to natural SRS weather conditions for up to 11 years. For this study, archived core sediments from the lysimeters were retrieved and Pu concentrations in depth-discrete samples were measured and then transport of Pu was modeled using a coupled reactive transport model. The geochemical conceptual model and input values included in the transport code were based on laboratory experiments. The single most important finding from this work was regardless of the Pu oxidation state added to SRS sediments, it quickly converted to the less mobile Pu(IV) form. In conclusion, it is expected that Pu will exist primarily in the SRS subsurface environment in the relatively less mobile Pu(IV) form, irrespective of the oxidation state that it first enters the ground. The lysimeter results provide important long-term data that support the removal of important overly conservative approaches presently used to calculate risk and performance assessment associated with groundwater Pu. These findings do not contradict previous Pu modeling efforts, including the E-Area Low-Level Waste Performance Assessment or the Special Analysis on Pu disposal in SRS trenches. Instead, the results from this work could be used in future calculations to improve accuracy and reduce uncertainty and conservatism.
- Research Organization:
- Savannah River Site (SRS), Aiken, SC (United States)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- AC09-96SR18500
- OSTI ID:
- 816624
- Report Number(s):
- WSRC-TR-2003-00300; TRN: US0305061
- Resource Relation:
- Other Information: PBD: 20 Oct 2003
- Country of Publication:
- United States
- Language:
- English
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