SYNTHESIS OF URANIUM PHOSPHATE PHASES AND POTENTIAL RETARDATION EFFECTS ON SPENT FUEL RADIONUCLIDES

The UO₂ in spent nuclear fuel is unstable in the oxidizing conditions within the volcanic tuffs at the proposed nuclear repository at Yucca Mountain, NV. Over time, the UO₂ will oxidize and corrode, releasing actinides and fission products to the surrounding environment. However, uranyl (U⁶⁺) phosphates (autunite, phurcalite, sodium autunite, etc.) are stable in such an oxidizing environment. The mobility of released radionuclides may be greatly retarded if they can be incorporated into these naturally stable phosphate phases, while their complex structures and variable chemical compositions highly favor such a process. Current tests have focused on synthesizing these phases by reacting uranium oxynitrate or UO₃ with a calcium, sodium, or potassium phosphate and a base (if necessary) in a Teflon reaction vessel. Excess water is added, and the solution is heated at 90^ºC for 7, 35, or 180 days. SEM analyses have confirmed that characteristic uranium phosphate crystalline solids have formed. XRD results strongly indicate that tests using two different calcium phosphates, Ca₂P₂O₇ and Ca₁₀(OH)₂(PO₄)₆, have created phosphuranylite, Ca(UO₂)[(UO₂)₃(OH)₂(PO₄)₂]₂*12H₂O (Welin, 1966), suggesting this phase is kinetically favored over other similar phases. Results utilizing sodium phosphate, NaH₂(PO₄)*H₂O, have been inconclusive as no definitive X-ray spectrum has been observed, and results using potassium phosphate, K₃PO₄, are in progress. Experiments using surrogate radionuclides will be performed in order to determine whether radionuclides, such as ²³⁹Pu, ¹³⁷Cs, and ⁹⁹Tc, released from corroded spent nuclear fuel can become incorporated into the crystalline structure of specific uranium phosphate phases formed at the base of spent fuel containers, effectively limiting any further migration.