INCORPORATION OF PENTAVALENT NEPTUNIUM INTO URANYL PHASES THAT MAY FORM AS ALTERATION PRODUCTS OF SPENT NUCLEAR FUEL

Technical Report · Tue Jun 21 04:00:00 EDT 2005

DOI:https://doi.org/10.2172/859179· OSTI ID:859179

NA,

Laboratory-scale simulations and studies of natural analogues have shown that alteration of spent nuclear fuel in a moist, oxidizing environment results in the formation of a variety of uranyl phases. Neptunium-237 has a half-life of 2.14 million years, and the pentavalent oxidation state is soluble in groundwater. Release of Np-237 from spent nuclear fuel in a geological repository may significantly impact the long-term performance of such a repository. Incorporation of Np, in the pentavalent oxidation state, into uranyl phases by substitution for hexavalent U is likely because of the similarity of the coordination environments of these two cations, but a charge-balance mechanism is required for substitution. A preliminary study has shown incorporation of pentavalent Np into powders of the uranyl silicate uranophane, and Na-compreignacite, a uranyl oxyhydrate [1]. Using synthesis experiments under mild hydrothermal conditions, we are examining the incorporation of pentavalent Np into selected uranyl oxyhydrates and silicates as a function of temperature and the pH of the mother solution. Analyses of powders of these uranyl phases has demonstrated both temperature and pH dependences for incorporation. Experiments are underway directed at the synthesis of single crystals of uranyl phases in the presence of 500-750 ppm pentavalent Np. The intent is to develop a basic understanding of the crystallographic and crystal chemical factors that impact incorporation of pentavalent Np into uranyl phases. Following synthesis, crystals are analyzed for Np using laser ablation ICP-MS. Preliminary results for Na-substituted metaschoepite indicate significant Np has been incorporated into the crystals. Additional phases under study include compreignacite, becquerelite, soddyite, zippeite, and (UO{sub 2}){sub 3}(PO{sub 4}){sub 2}(H{sub 2}O){sub 4}.

• Research Organization: YMP (Yucca Mountain Project, Las Vegas, Nevada)
• Sponsoring Organization: USDOE
• OSTI ID: 859179
• Report Number(s): NA; MOL.20050824.0099 DC#44906
• Country of Publication: United States
• Language: English

Related Subjects

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS
12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES
ABLATION
BECQUERELITE
CATIONS
COMPREIGNACITE
HALF-LIFE
LASERS
MONOCRYSTALS
NATURAL ANALOGUE
NEPTUNIUM
NEPTUNIUM 237
NUCLEAR FUELS
SILICATES
SODDYITE
SULFATE MINERALS
SYNTHESIS
URANIUM MINERALS
URANOPHANE
URANYL SILICATES
VALENCE