A Review of Subsurface Behavior of Plutonium and Americium at the 200-PW-1/3/6 Operable Units

Abstract

(PNNL) review available information and data pertaining to Pu and americium mobility in the subsurface at Hanford, with emphasis on vadose zone contamination within the 200-PW-1/3/6 operable units, to address this inconsistency between perception and data. In response to this request, PNNL compiled, analyzed, and summarized information and data on relevant waste site structures and associated subsurface geologies, waste composition and disposal histories, Pu and americium subsurface geochemistry, and contaminant characterization underlying key operable unit waste sites. Summarized information and data were used to develop a conceptual model that attempts to explain historical Pu and americium distributions in the subsurface and their potential for future migration based on current vadose zone characteristics in the 200-PW-1/3/6 operable units. Technical data gaps and information needs for refinement of the conceptual model were identified. This report begins with a brief summary of the history and current status of 200-PW-1/3/6 operable units in Section 2. This is followed by a description of the PNNL conceptual model of Pu/Am migration at the 200-PW-1/3/6 operable units, during conditions of both past artificial recharge and current natural recharge (Section 3). Section 4 discusses data gaps and information needs. Section 5 provides recommendations for further work to address the data gaps and information needs identified in Section 4. A more detailed account of the waste disposal history, chemistry, and available characterization data for the 200-PW-1/3/6 operable units is presented in Appendix A. A summary geologic description of the 200-PW-1/3/6 operable units is presented in Appendix B. A brief discussion of the environmental geochemistry of Pu and Am is presented in Appendix C. 2.0 History and Current Status of 200-PW-1/3/6 Operable Units This section briefly summarizes the history and current status of the 200-PW-1/3/6 operable units. A more detailed account of the waste disposal history, chemistry, and summary of available characterization data for the 200-PW-1/3/6 operable units is presented in Appendix A.

Plutonium and Americium Geochemistry at Hanford: A Site Wide Review

“…Results of the analyses are tabulated in Appendix B of the remedial investigation report . Results for plutonium, americium, TBP, pH, and nitrate are discussed below (Cantrell and Riley 2008;Felmy et al 2010).…”

“…From analyses of the various waste sites within the Plutonium Finishing Plant closure zone, it was concluded that these sites can be classified into two major categories based primarily upon the type of wastes that were received by the site (Cantrell and Riley 2008). The two waste categories are low-salt near-neutral wastes and acidic high-salt waste with which organic complexants were co-disposed (Cantrell et al 2003).…”

“…Remedial investigations conducted for the 216-Z-9 Trench have shown that significant plutonium and americium have moved to depths of up to 120 ft bgs. To better understand the reasons for this observed migration of plutonium, Cantrell and Riley (2008) conducted a review of the subsurface behavior of plutonium and americium at the Hanford Site 200-PW-1/3/6 OUs. A summary of what is known regarding the history, characterization work performed, and current distribution of contaminants at the 216-Z-9 Trench is provided below.…”

“…The 216-Z-12 Crib is used as an example of a site that received low-salt near-neutral wastes. 0.01 0.10 1.00 10.00 100.00 1,000.00 10,000.00 100,000.00 Table 3.3 summarizes key features important for understanding the differences in migration behavior of plutonium and americium at these two representative sites during artificial recharge conditions (Cantrell and Riley 2008;. Prior to disposal to the 216-Z-12 Crib, waste first passed through the Z-361 settling tank.…”

Influences on Subsurface Plutonium and Americium Migration

“…Although the highest concentrations were measured in the uppermost 15 cm of the trench sediments prior to excavation (6.5 × 10 8 pCi Pu-239/g sediment, assuming a bulk density of 1.86 kg/L), 30,31 Pu has been detected up to 37 m beneath the waste discharge point in the Z-9 trench (4.4 pCi Pu-239/g sediment). 32,33 Pu and Am has not been detected in the deeper subsurface (>37 m depth) beneath the waste discharges, although Felmy et al (2010) suggested that Pu and Am may be mobilized by organics previously co-disposed, such as TBP, or their breakdown products. 29 Previous characterization of shallow (0 to ∼60 cm depth) Z-9 sediments indicated similar leaching rates when analyzed 10 years after disposal.…”