EPA-Provided Comment: Stakeholde r comment received at the May 12, 2010 Stakeholder meeting in Albuquerque Comment: Is DOE aware of these experiment s (Plutonium nanocolloids) and have the results been taken into account (if a pplicable) in the WIPP radionuclide release scenarios? Documentation provided at the Stakeholder meeting: Argonne National Laboratory News Release: “Scientists discover how the structure of plutonium nanocluster contaminants increases risk of spreading,” 2008. DOE Response The DOE is aware of the plutonium nanocolloid experiments as outlined in the Argonne National Laboratory news release. A more extensive description of the experimental work has also been published in Soderholm et al. (2008) . The Pu nanocolloid reported in Soderholm et al. (2008) is a specific example of an intrinsic colloid of Pu. Recent information on the formation and transport of Pu(IV) intrinsic colloids is presented in Section SOTERM-3.8.1 of CRA-2009 (DOE, 2009), which notes that th e intrinsic colloids te nd to be very low in concentration and have a limited capability for radionuclide trans port in the WIPP environment. The experimental results in Soderholm et al. (2008) therefore have limited applicab ility to long-term performance under the anticipated conditions in the WIPP. Despite this, the potential for plutonium nanocolloids to be present in the WIPP is includ ed within the concentration of Pu(IV) intrinsic colloids in WIPP performance assessment. The detailed discussion of the Pu(IV) intrinsi c colloids in Section SOTERM-6.3.2.1 of the first Compliance Recertification Appli cation (CRA-2004)(DOE, 2004) is directly relevant to the nanocolloids of plutonium. As explained in Sections SOTERM-6.3.2.3 and SOTERM-6.3.2.4 of CRA-2004, Pu(IV) intrinsic colloids are included in performance assessment at a concentration of 10 -9 M, which is conservative because the 10 -9 M concentration is an upper bound based on an equipment detection limit.

Another feature of the re sults reported in Soderholm et al. (2 008) is that Pu nanocolloids passed through an anion-exchange column in strong ac idic conditions (7.5 M nitric acid) without retardation. These experimental conditions are no t relevant to the conditions expected in the WIPP repository. Pu nanoclusters reported in Soderholm et al. (2008) are highly charged (-14) particles. Such highly charged nanoclusters woul d be efficiently sorbed on mineral surfaces in the WIPP repository. In summary, the DOE is aware of the research on Pu nanocolloids and believes that this type of colloid is unlikely to form in significant c oncentrations under WIPP relevant conditions. In addition, if this type of colloid could form in the WIPP, its presence is encompassed in the 10 -9

12 M concentration for Pu(IV) in trinsic colloids assumed in th e WIPP long-term performance assessment. DOE References Soderholm, L., Almond, P.M., Skanthakumar, S., Wilson, R. E., and P.C. Burns, “The Structure of the plutonium oxide nanocluster [Pu 38 O 56 Cl 54 (H 2 O) 8 ] 14- “, Angew. Chem. Int. Ed. , 47 (2008) 298-302. U.S. Department of Energy (DOE). 2004. Title 40 CFR Part 191 Compliance Recertification Application for the Waste Isolation Pilot Plant (March). 10 vols. DOE/WIPP 2004-3231. Carlsbad, NM: Carlsbad Field Office.

U.S. DOE. 2009. Title 40 CFR Part 191 Subparts B and C Compliance Recertification Application for the Waste Isolation Pilot Plant

. DOE/WIPP 09-3424. Carlsbad, NM: U.S. Depart ment of Energy Carlsbad Field Office.