Improving Operations and Long-Term Safety of the Waste Isolation Pilot Plant

AbstractWhen present, uranium is usually an element of importance in a nuclear waste repository. In the Waste Isolation Pilot Plant (WIPP), uranium is present in significant quantities, with about 647 metric tons to be placed in the repository [1]. Therefore, the chemistry of uranium, and especially its solubility, needs to be determined under WIPP-relevant conditions.Long-term experiments were performed to measure the solubility of uranium (VI) in carbonate-free ERDA-6 brine, a simulated WIPP brine, at pCH+ values between 8 and 12.5. These data, obtained from the over-saturation approach, were the first WIPP repository-relevant data for the VI actinide oxidation state. The solubility trends observed pointed towards low uranium solubility in WIPP brine and a lack of amphotericity. At the expected pCH+ in the WIPP (˜ 9.5), measured uranium solubility approached 10-7 M. The objective of these experiments was to establish a baseline solubility to further investigate the effects of carbonate complexation on uranium solubility in WIPP brines, during the ongoing research program in actinide solubility under WIPP-relevant conditions.

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Improving Operations and Long-Term Safety of the Waste Isolation Pilot Plant

10.17226/10143 ◽

2001 ◽

Keyword(s):

Pilot Plant ◽

Waste Isolation Pilot Plant

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Influence of Carbonate on Uranium Solubility in the WIPP

MRS Proceedings ◽

10.1557/opl.2012.1005 ◽

2012 ◽

Vol 1444 ◽

Cited By ~ 1

Author(s):

Jean-Francois Lucchini ◽

Sally Ballard ◽

Hnin Khaing

Keyword(s):

Performance Assessment ◽

Pilot Plant ◽

Solubility Limit ◽

Experimental Results ◽

Waste Isolation Pilot Plant ◽

Long Term Experiments ◽

Order Of Magnitude ◽

Carbonate Complexation ◽

Borate Complexation

ABSTRACTIn the performance assessment (PA) for the Waste Isolation Pilot Plant (WIPP), the solubility of uranium (VI) was conservatively set at 10-3 M for all expected WIPP conditions, including the potential and likely effects of carbonate complexation [1]. Under WIPP-relevant conditions, long-term experiments were performed to establish the uranium (VI) solubility limits in WIPP-simulated brine over a broad range of pCH+ values [7.5-12.5] and to evaluate the contribution of carbonate complexation and hydrolysis to uranium (VI) speciation. Data obtained in carbonate-free ERDA-6 brine, a simulated WIPP brine, were reported earlier [2]. In the absence of carbonate, uranium solubility approached 10-7 M at the expected pCH+ in the WIPP (~ 9.5). In the presence of a significant amount of carbonate (millimole levels), recent experimental results showed that uranium (VI) concentrations will not exceed 10-4M. This measured solubility limit is an order of magnitude lower than the uranium solubility value currently used in the WIPP PA [3]. A small effect of borate complexation was found in the pCH+ range [7.5-10]. At pCH+ ≥ 10, hydrolysis overwhelmed carbonate effects, and no amphoteric effect was observed.

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