Creation's Tiny Mystery

For example, Ringwood (11, 12) has suggested that highly radiation-damaged minerals that have successfully retained U, Th, and Pb (13) over a significant fraction of earth history might also serve to immobilize high-level nuclear waste in synthetic rock (SYNROC) containers, which could be buried in deep granite holes. Even though zircons are not envisioned as part of Ringwood's special type of synthetic rock waste container, our results are relevant since they show that Pb, which is much more mobile in zircons than U and Th (12, 14), has been highly retained at depths (960 to 4310 m) which more than span the proposed burial depths (1000 to 3000 m) for synthetic rock containers in granite (11). The inclusion of this elevated temperature effect in our samples means that our results provide data which have heretofore been unavailable in support of nuclear waste containment in deep granite. In addition, the contamination-free method we used to analyze the zircons for radiogenic Pb may prove valuable in searching for other minerals suitable for synthetic rock waste containment.

Because it has been suggested that temperatures in the granite formation are rising (15), we do not know precisely how long the zircons have been exposed to the present temperatures. However, by using diffusion theory and the measured diffusion coefficient of Pb in zircon (16), we can estimate future loss of Pb by diffusion in synthetic rock-encapsulated zircons buried at the proposed depths of 1000 to 3000 m (11) if we assume a temperature profile similar to that in the drill holes. At a burial depth of 3000 m (~ 200°C), we calculate that it would take 5 × 10¹⁰ years for 1 percent of the Pb to diffuse out of a 50-μm crystal. At 2200 m (~ 150°C) it would take 7.4 × 10¹³ years, and at 1000 m (~ 100°C) it would take 7.7 × 10¹⁷ years for 1 percent loss to occur (16). Since all these values greatly exceed the 10⁵ to 10⁶ years estimated for waste activity to be reduced to a safe level (11), and since, as noted earlier, U and Th are bound even more tightly than Pb in zircons (12, 14), our results appear to lend considerable support to the synthetic rock concept of nuclear waste containment in deep granite holes.

	Robert V. Gentry* Thomas J. Sworski
Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830
	Henry S. McKown David H. Smith R. E. Eby W. H. Christie
Analytical Chemistry Division, Oak Ridge National Laboratory

References and Notes

1. A. W. Laughlin and A. Eddy, Los Alamos Sci. Lab. Rep. LA-6930-MS (1977). A. W. Laughlin provided the core samples used in this work.
2. R. Laney and A. W. Laughlin, Geophys. Res. Lett. 8, 501 (1981).
3. D. H. Smith, W. H. Christie, H. S. McKown, R. L. Walker, G. R. Hertel, Int. J. Mass Spectrom. Ion Phys. 10, 343 (1972).
4. W. H. Christie and A. E. Cameron, Rev. Sci Instrum. 37, 336 (1966).
5. A. E. Cameron, D. H. Smith, R. L. Walker, Anal. Chem 41, 525 (1969).
6. D. H. Smith, W. H. Christie. R. E. Eby, Int. J. Mass Spectrum, Ion Phys. 36, 301 (1980).
7. O. Kopp and H. McSween of the Department of Geological Sciences, University of Tennessee, Knoxville, provided zircons from Gjerstad, Norway; Oaxaca, Mexico; and Henderson County, North Carolina.
8. R. E. Zartman, Los Alamos Sci. Lab. Rep. LA-7923-MS (1979).
9. If the ²⁰⁴Pb in Zartman's (8) Pb isotopic abundances in his zircons is attributed to common lead, the corrected ²⁰⁶Pb/²⁰⁷Pb ratio for the zircons from 2900 m is 11.03.
10. This criterion resulted in the rejection of four single zircon analyses whose average ²⁰⁶Pb/²⁰⁷Pb ratio was 8.8 ± 1.3. These lower ratios imply that some zircons contain more initial Pb than others, as noted in some other runs.
11. A. E. Ringwood, Safe Disposal of High Level Nuclear Reactor Wastes: A New Strategy (Australian National Univ. Press, Canberra, 1978).
12. A. E. Ringwood, K. D. Reeve, J. D. Tewhey, in Scientific Basis for Nuclear Waste Management, J. G. Moore, Ed. (Plenum, New York, 1981), vol. 3, p. 147.
13. V. M. Oversby and A. E. Ringwood, J. Waste Manage., in press. See also A. E. Ringwood, Lawrence Livermore Natl. Lab. Rep. UCRL-15347 (1981).
14. R. T. Pidgeon, J. O'Neill, L. Silver, Science 154, 1538 (1966); Fortschr. Mineral. 50, 118 (1973).
15. D. G. Brookins, R. B. Forbes, D. L. Turner, A. W. Laughlin, C. W. Naeser, Los Alamos Sci. Lab. Rep. LA-6829-MS (1977).
16. In general, if R is the gas constant, T is the absolute temperature, and D and Q are, respectively, the diffusion coefficient and activation energy of a certain nuclide in a given diffusing medium, then D = D₀ e^−Q/RT where D₀ is a temperature-independent parameter. In particular, if C₀ is the initial concentration of that nuclide within a sphere of radius a, then the average nuclide concentration C within that sphere at some later time t is given by

    C/C0  =	6		∞ ∑ 1		e−(n2π2Dt/a2)
    	π2				n2

    [see L. O. Nicolaysen, Geochim. Cosmochim. Acta 11, 41(1957)]. We used measured values of D₀ 2.2 × 10⁻² and Q = 58 kcal/mole for diffusion of Pb in zircon [see Sh. A. Magomedov, Geokhimiya 2, 263 (1970)] and a computer program to calculate the times when C/C₀ = 0.99 for T = 100°, 150°, and 200°C.
17. Research sponsored by the U.S. Department of Energy, Division of Basic Energy Sciences, under contract W-7405-eng-26 with the Union Carbide Corporation.

* Visiting scientist from Columbia Union College, Takoma Park, Md. 20112.

3 November 1981; revised 22 January 1982

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