CHARACTERIZATION OF ACTINIDES IN SIMULATED ALKALINE TANK WASTE SLUDGES AND LEACHATES [report]

Kenneth L. Nash
2008 unpublished
The Principal Investigator, K. L. Nash, moved from ANL to WSU in 2003. Currently, one postdoc and two summer undergraduate students are supported at WSU and one postdoc is supported at LBNL under this project. RESEARCH OBJECTIVES Treatment of underground tanks at Hanford with concentrated alkali to improve removal of wastelimiting components of sludges has proven less efficacious for Al and Cr removal than had been hoped. Hence, more aggressive treatments of sludges, including contact with
more » ... g contact with oxidants targeting Cr(III), have been tested in a limited number of samples and found to enhance Cr removal. Unfortunately, treatments of sludge samples with oxidative alkaline leachates produce conditions under which normally insoluble actinide ions (e.g., Am 3+ , Pu 4+ , Np 4+ ) can no longer be reliably assumed to remain in the sludge phase. Few experimental or meaningful theoretical studies of actinide chemistry in strongly alkaline, strongly oxidizing solutions have been completed. Extrapolation of acid phase thermodynamic data to these radically different conditions provides little reliable guidance for predicting actinide speciation in highly salted alkaline solutions. In this project, we are investigating the fundamental chemistry of actinides in sludge simulants and supernatants under representative oxidative leaching conditions. We are also examining the potential impact of acidic leaching with concurrent secondary separations to enhance Al removal. Our objective is to provide adequate insight into actinide behavior under these conditions to enable prudent decision making as tank waste treatment protocols develop. We expect to identify those components of sludges that are likely to be problematic in the application of oxidative leaching protocols. RESEARCH PROGRESS AND IMPLICATIONS This report summarizes work after 2 years of a 3-year project. On the basis of previous extensive studies on the leaching of actinides from simulated Hanford tank sludges, we have focused on the chemical behavior of plutonium and americium in oxidative alkaline leaching and the effect of organic complexants on the speciation of actinides in alkaline media. Pu in Oxidative Alkaline Leaching. Our studies of oxidative alkaline leaching of the simulants indicated that low concentrations of permanganate in 3.0 M NaOH (10 -5 to 10 -4 M) had minimal impact on the removal of Cr from sludge samples, that is, a similar amount of Cr was leached to that seen in 3.0 M NaOH contacts.[1] At higher concentrations of permanganate, increasing amounts of chromate were seen in the leachate (Figure 1a ). One possible implication of the existence of a threshold is that some disruption of the surface of the sludge simulant is needed to provide adequate access of the oxidant to the
doi:10.2172/941420 fatcat:m6fymnp4ljf7xck2cmqp6fade4