Before spent nuclear fuel (SNF) can be transferred into permanent storage, it must be dried. There are currently three methods of accomplishing this, i.e. drip dry, cold vacuum drying, and hot vacuum drying. The first method is used by private industry and usually can’t be applied to U.S. Department of Energy (DOE) SNF because much of the DOE fuel is damaged and will not dry properly without external assistance. Vacuum drying is expensive and time consuming. DOE has a critical need for a more efficient method.

This project for DOE’s Idaho Operations Office consisted of determining the feasibility of using anhydrous liquid ammonia to efficiently dry SNF. Anhydrous liquid ammonia and water are miscible, forming ammonium hydroxide. For drying purposes, the ammonia acts as a simple chemical transport mechanism where moisture is trapped in ammonia and transferred to a separation area where the ammonia is removed by evaporation. Ammonia then can be recovered by conventional "off-the-shelf" refrigeration techniques that are proven and commercially available. The proof of principle experiments were conducted in 1999 at the Idaho State University (ISU) Chemistry Laboratory in Pocatello, Idaho.

The SNF drying experiments were performed at +3° C (pressure of 6 atmospheres) using a porous magnesia rod as a surrogate for SNF. An extraction efficiency of 96 percent was indicated for a 1-hour ammonia soak period. A repeat determination yielded an apparent extraction efficiency of 89 percent. A third magnesia specimen was prepared at ISU (again with an hour-long extraction of water by liquid ammonia) and sent to Noah Technologies Corp. for an accurate measurement of residual moisture using EPA Method 350.2. This independent analysis by an EPA-certified laboratory found an extraction efficiency of 90 percent, confirming results obtained earlier at ISU.

Large water extraction efficiencies by anhydrous liquid ammonia over a brief period on a very hygroscopic material are encouraging for de-watering actual SNF and associated sludges. Besides the porous magnesia rod, evidence was acquired in this project that ammonia is readily adsorbed by a 3-Angstrom molecular sieve (3x10^-10 meter pore size). This suggests that the small ammonia molecule can penetrate extremely small spaces to extract moisture and is able to go anywhere in SNF that a water molecule can reach.

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