The disposal of high-level radioactive wastes generated during the reprocessing of spent fuel rods from nuclear reactors to recover the actinides is an ongoing problem for the DOE, costing billions of dollars. These wastes include defense wastes currently stored at a number of locations such as the tank farms at Hanford and the underground storage bins at the Idaho National Engineering Laboratory (INEL). Wastes generated during the reprocessing of civilian nuclear reactor spent fuel rods (before a 1977 moratorium on reprocessing initiated during President Carter's administration) are stored at the West Valley reprocessing facility (Western New York Nuclear Service Center) in West Valley, NY. These wastes represent millions of gallons of liquids and tens of thousands of cubic meters of solids; at INEL alone over 2200 m3 of unconsolidated solids remain.
Chemically and radiologically, reprocessed wastes are extremely complex. They contain fission products, residual actinides, cations from the dissolution of metal fuel rod containers, anions from acids used in the dissolution process, alkali salts employed for the purposes of neutralization, and a variety of organic sequestering agents. To reduce their volumes and to stabilize their chemistries, reprocessed commercial wastes in liquid form are often converted to solid form by drying and calcining them at temperatures below 600°C. During the calcination step, the wastes decompose into amorphous mixtures of chemically inert oxides; volatile reaction products are driven off. The solid products or calcines are characterized by moderate to high leachability and must be converted to chemically stable forms before they are disposed of.
Neither commercial nor defense high-level wastes are uniform or well-characterized. As a result, the development of waste forms that are suitable for the immobilization of reprocessed high-level calcines has continued to challenge the waste management community. In response to this challenge, researchers have proposed a variety of waste forms including noncrystalline, crystalline, and multiphase materials. Each of these materials possesses unique capabilities and deficiencies. For example, noncrystalline waste forms such as the borosilicate-based glasses are relatively insensitive to fluctuations in waste stream composition, and the process used to prepare such materials is reliable and straightforward. Industrial-scale processing of borosilicate-based waste forms is in place and has been demonstrated at the Defense Waste Processing Facility (DWPF) at the Savannah River Defense Plant. However, borosilicate-based glasses are thermodynamically unstable and are susceptible to uncontrolled crystallization under some repository conditions. Such materials lack the thermal and mechanical stability possessed by crystalline waste forms.
Unfortunately, operations at the DWPF have been repeatedly delayed because of a combination of public relations issues and technical barriers. This delay has led to renewed interest in research on alternate waste forms. Scientists in the waste manage-ment community have continued to propose, synthesize, and characterize novel waste forms. Notable among these waste forms is sodium zirconium phosphate (NaZr2(PO4)3 or NZP), a crystalline material that can accommodate the chemical complexity of high-level wastes.
Figure 4. The NZP structure is a three-dimensional network of interconnected zirconium octahedra (red polyhedra) and phosphorous tetrahedra (gray polyhedra). The structure accommodates cesium and strontium ions in large interstitial cavities occupied by sodium ions in the parent structure. Fission products and residual actinides substitute for zirconium as essential constituents of the three-dimensional network.
The three-dimensional skeletal structure of NZP, as shown in Figure 4, allows the accommodation of cations of various sizes and oxidation states on three distinct crystallographic sites; in fact, the NZP structure may accommodate all of the chemical species asso-ciated with reprocessed, commercial, high-level waste calcines. Uranium, thorium, neptunium, and plutonium occupy the zirconium site in the parent structure. Cesium and strontium are accommodated on interstitial sites typically occupied by sodium. The substitution of sodium, zirconium, or phosphorous by fission products or actinides results in a structural isotype referred to as [NZP]. A wide variety of cations have been inserted into the NZP structure. It may accommodate approximately two-thirds of the ionic species of all known elements.
The [NZP] compounds permit the incorporation of all of the ions present in reprocessed, commercial, high-level waste calcines into crystalline waste forms whose resistance to dissolution and retention of [NZP] constituents is remarkable. Cursory studies indicate that the [NZP] materials are highly resistant to radiation damage. As crystalline waste forms, [NZP] compounds offer inherently low leach rates for single phases, negligible coefficients of thermal expansion, and the ability to immobilize high concentrations of waste in high-density phases. This latter characteristic of [NZP] waste forms renders these materials promising candidates for the disposal and consequent reduction of large volumes of wastes containing significant quantities of nonradioactive constituents; plant operation, storage, and disposal costs decrease with decreasing waste volume. In this regard, crystalline waste forms, such as [NZP], offer a notable economic advantage over the less denseborosilicate-based glasses.
The ease of processing [NZP] waste forms is an additional feature that makes these materials attractive alternatives to other potential waste forms, such as SYNROC (synthetic rock), a multiphase waste form. The [NZP] materials may be synthesized by solid-state reaction of mechanically-mixed, copreci-pitated, or hydrothermally prepared powders. Such [NZP] powders may be cold-pressed and sintered in air at moderate temperatures (600-1350°C); however, modifications of these techniques would be required for the industrial-scale production of [NZP] waste forms.
Some [NZP] compounds with waste loadings as high as 40 wt % have been prepared with simulated nonradioactive, reprocessed, high-level waste calcines by the author at The Pennsylvania State University; however, the preparation of [NZP] waste forms with actual reprocessed high-level waste calcines has not been demonstrated. A methodology has been developed and tested that allows one to determine appropriate batch formulations for several waste compositions. The batch formulations take into account site and charge balance so that the final product is phase-pure, i.e., amorphous and undesired phases are absent. Future work at LANL will include the preparation of actinide-loaded samples, their characterization, and a comprehensive study of their behavior under repository conditions.
Heather Hawkins is a graduate student in the Intercollege Materials Research Program at the Pennsylvania State University. She is studying the structure and stability of actinide-doped NZP materials for the experimental portion of her Ph.D. thesis. Her mentor is Kirk Veirs of NMT-6
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