Synthesis of Uranium Phosphate Phases and Potential Retardation Effects on Spent Fuel Radionuclide Migration
- PDF / 1,213,786 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 2 Downloads / 243 Views
II3.1.1
SYNTHESIS OF URANIUM PHOSPHATE PHASES AND POTENTIAL RETARDATION EFFECTS ON SPENT FUEL RADIONUCLIDE MIGRATION A.S. TURNER and D.J. WRONKIEWICZ Department of Geology and Geophysics, University of Missouri-Rolla, Rolla, MO 65409
ABSTRACT The UO2 in spent nuclear fuel is unstable in the oxidizing conditions within the volcanic tuffs at the proposed nuclear repository at Yucca Mountain, Nevada. 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 the complex structures, variable chemical compositions, and natural analogue occurrences of the uranyl phosphates suggests such a process is favorable. Current tests have focused on synthesizing such 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 90ºC for 7, 35, or 182 days. SEM analyses have confirmed that various uranium phosphate crystalline solids have formed. XRD results indicate that tests using two different calcium phosphorus source materials, Ca2P2O7 and Ca10(OH)2(PO4)6, have both created synthetic phosphuranylite, Ca(UO2)[(UO2)3(OH)2(PO4)2]2*12H2O. The formation of this phase is appears to be kinetically favored over other similar phases. Results utilizing sodium phosphates, NaH2(PO4)*H2O and Na2HPO4, have produced sodium autunite (NaUO2PO4*2H2O), but other phases are probably also present. Test results utilizing potassium phosphate, K3PO4, were inconclusive. Experiments using surrogate radionuclides are currently being 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, effectively limiting any further migration. INTRODUCTION Yucca Mountain, Nevada, the site of the proposed nuclear repository for the United States, is located in southern Nye County, approximately 100 miles northwest of Las Vegas [1]. Spent nuclear fuel, composed almost entirely of UO2 is unstable in the oxidizing conditions expected within the volcanic tuffs of Yucca Mountain [2]. Over time, the spent fuel is expected to oxidize and corrode, causing the release of uranium and contained actinides and fission products [3]. When combined with groundwater components, very soluble and mobile complexes may form, allowing these substances to be transported from the spent fuel container and eventually away from the repository [4]. The goal of this project is to test the initial feasibility of using a reactive chemical barrier to inhibit the migration of harmful actinides, fission products, and uranium from the near field waste package environment of a spent
Data Loading...
