A HOST PHASE FOR THE DISPOSAL OF WEAPONS PLUTONIUM
Research was conducted into the possible use of zircon (ZrSiO{sub 4}) as a host phase for storage or disposal of excess weapons plutonium. Zircon is one of the most chemically durable minerals. Its structure can accommodate a variety of elements, including plutonium and uranium. Natural zircon contains uranium and thorium together in different quantities, usually in the range of less than one weight percent up to several weight percent. Zircon occurs in nature as a crystalline or a partially to fully metamict mineral, depending on age and actinide element concentration, i.e., on radiation damage. These zircon samples have been studied extensively and the results are documented in the literature in terms of radiation damage to the crystal structure and related property changes, e.g., density, hardness, loss of uranium and lead, etc. Thus, a unique suite of natural analogues are available to describe the effect of decay of {sup 239}Pu on zircon's structure and how zircon's physical and chemical properties will be affected over very long periods of time. Actually, the oldest zircon samples known are over 3 billion years old. This period covers the time for decay of {sup 239}Pu (half-life 24,300 yr.) and most of its daughter {sup 235}U (half-life 700 million yr.). Because of its chemical durability, even under extreme geological conditions, zircon is the most widely used mineral for geochronological dating (7,000 publications). It is the oldest dated mineral on earth and in the universe. Zircon has already been doped with about 10 weight percent of plutonium. Pure PuSiO{sub 4} has also been synthesized and has the same crystal structure as zircon. However, use of zircon as a storage medium or waste form for plutonium requires further materials characterization. Experiments can either be conducted in laboratories where plutonium can be handled or plutonium can be simulated by other elements, and experiments can be done under less restricted conditions. The authors conducted work with zircon doped with thorium, uranium and cerium, respectively. They synthesized various zircon compositions and studied the solid solution properties of mixed (Zr,X)SiO{sub 4} [X represents Th, U, Ce, respectively]. They measured the dissolution rate of pure crystalline zircon at elevated temperatures and of an amorphous hydrated zircon. This final report together with two previous annual reports summarize the accomplishments made in two areas: (1) synthesis of zircon solid solutions with Th, U, and Ce; and (2) measurement of the chemical durability of zircon. The focus of the final report is on the measurement of zircon's dissolution rate in water and on the determination of volubility limits of Th, U, and Ce in zircon.
- Research Organization:
- Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- W-7405-ENG-36
- OSTI ID:
- 758782
- Report Number(s):
- LA-SUB-99-66; TRN: US0004612
- Resource Relation:
- Other Information: PBD: 1 Jan 1999
- Country of Publication:
- United States
- Language:
- English
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