Hostname: page-component-8448b6f56d-cfpbc Total loading time: 0 Render date: 2024-04-19T04:00:37.062Z Has data issue: false hasContentIssue false
  • English
  • Français

Thermochemical Investigations of Zirconolite, Pyrochlore and Brannerite: Candidate Materials for the Immobilization of Plutonium

Published online by Cambridge University Press:  01 February 2011

K. B. Helean ,
A. Navrotsky ,
J. Lian  and
R. C. Ewing
K. B. Helean
Affiliation:
Thermochemistry Facility, NEAT ORU, University of California at Davis, One Shields Ave., Davis CA 95616
A. Navrotsky
Affiliation:
Thermochemistry Facility, NEAT ORU, University of California at Davis, One Shields Ave., Davis CA 95616
J. Lian
Affiliation:
Department of Nuclear Engineering and Radiological Sciences, University of Michigan, 2355 Bonisteel Blvd., Ann Arbor MI 48109
R. C. Ewing
Affiliation:
Department of Nuclear Engineering and Radiological Sciences, University of Michigan, 2355 Bonisteel Blvd., Ann Arbor MI 48109
Get access

Abstract

Standard enthalpies of formation, ΔH°f (kJ/mol) at 298 K, were derived for three pyrochlore phases: Ca0.93Ce1.00Ti2.035O7.00 (−3656.0±5.6), Ca1.46U4+0.23U6+0.46Ti1.85O7.00 (−3610.6±4.1), Gd2Ti2O7 (−3822.5±4.9) and two zirconolite phases: CaZr1.03Ti1.97O7 (−3719.4±3.9) and CaHf1.02Ti1.98O7 (−3720.5±3.9). Enthalpies of formation with respect to an oxide phase assemblage, ΔH°f-ox (kJ/mol) at 298 K: CaO + MO2 + 2TiO2 = CaMTi2O7 or Gd2O3 + 2TiO2 = Gd2Ti2O7, and an oxide/perovskite phase assemblage: ΔH°f-pv+ox (kJ/mol) at 298 K: CaTiO3 + MO2 + TiO2 = CaMTi2O7, M = Ce, U, Hf, Zr were also calculated. All of the pyrochlore and zirconolite materials studied were stable in enthalpy with respect to their oxides. ΔH°f-ox: Ca0.93Ce1.00Ti2.035O7.00 (−54.1±5.2), Ca1.46U4+0.23U6+0.46Ti1.85O7.00 (−123.1±3.4), Gd2Ti2O7 (−113.4±2.8), CaZr1.03Ti1.97O7 (−89.6±2.8) and CaHf1.02Ti1.98O7 (−74.8±3.1). With respect to a perovskite plus oxide phase assemblage the hafnium zirconolite was marginally metastable in enthalpy (ΔH°f-pv+ox = +6.0±3.5 kJ/mol) while zirconolite was marginally stable in enthalpy (ΔH°f-pv+ox = −8.8±3.3 kJ/mol). The Ce-pyrochlore was not thermodynamically stable as measured by the enthalpy change relative to perovskite plus oxides: ΔH°f-pv+ox = Ca0.93Ce1.00Ti2.035O7.00 (+21.0±5.5). The U-pyrochlore sample was marginally stable in enthalpy relative to a perovskite plus oxide assemblage: ΔH°f-pv+ox = Ca1.46U4+0.23U6+0.46+Ti1.85O7.00 (−5.1±4.0). A significant field where the proposed waste form for excess weapons plutonium is not thermodynamically stable both at room temperature and at high temperature was defined.

ΔH°f (kJ/mol) at 298 K were derived for three brannerite phases: CeTi2O6 (−2948.8±4.3), U0.97Ti2.03O6 (−2977.9±3.5) and ThTi2O6 (−3096.5±4.3). Enthalpies of formation with respect to an oxide phase assemblage, ΔH°f-ox (kJ/mol) at 298 K: MO2 + 2TiO2 = MTi2O6, M = Ce, U, Th were also calculated. ΔH°f-ox: CeTi2O6 (+29.4±3.6), U0.97Ti2.03O6 (−7.7±2.8) and ThTi2O6 (+19.4±1.6). Thus, Ce and Th-brannerite are entropy stabilized and thermodynamically stable only at high temperature.

These thermochemical data are correlated to the observed trends in the radiation “susceptibility” of these phases. Thermodynamic data relevant to waste form processing and optimization are also discussed.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 807: Symposium Scientific Basis for Nuclear Waste Management XXVII , 2003 , 297
Copyright
Copyright © Materials Research Society 2004

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

REFERENCES

1. Jostsons, A., Vance, L. and Ebbinghaus, B., in: Proceedings of the International Conference on Future Nuclear Systems, Global 99, Jackson Hole, Wyoming (American Nuclear Society CDROM, 1999).Google Scholar
2. Helean, K.B., Begg, B. D., Navrotsky, A., Ebbinghaus, B., Webber, W. J., and Ewing, R. C. in Scientific Basis for Nuclear Waste Management XXIV (Mat. Res. Soc. Symp. Proc. Vol. 663, p. 157, 2001).Google Scholar
3. Helean, K.B. (2002) Dissertation. Ph.D. Materials Science and Engineering, University of California at Davis, Winter, 2002.Google Scholar
4. Helean, K.B., Navrotsky, A., Vance, E.R., Carter, M.L., Ebbinghaus, B., Krikorian, O., Lian, J., Wang, L.M. and Catalano, J.G., J. Nucl. Mater. 303, 226 (2002).Google Scholar
5. Helean, K.B., Navrotsky, A., Lumpkin, G.R., Colella, M., Lian, J., Ewing, R.C., Ebbinghaus, B. and Catalano, J.G., J. Nucl. Mater. In press (2003).Google Scholar
6. Wang, S.X., Wang, L.M., Ewing, R.C., Was, G.S., Lumpkin, G.R., Nucl. Instr. Meth. Phys. B148, 704 (1999).Google Scholar
7. Wang, S.X., Begg, B.D., Wang, L.M., Ewing, R.C., Weber, W.J., Kutty, KVG, J Mater Res., 14, 4470 (1999).Google Scholar
8. Wang, S.X., Wang, L.M., Ewing, R.C. and Govindan Kutty, K.V., Nucl. Instr. Meth. Phys. B169, 135, (2000).Google Scholar
9. Wang, S.X., Lumpkin, G.R., Wang, L.M., and Ewing, R.C., Nucl. Instr. Meth. Phys. B166–167, 293, (2000).Google Scholar
10. Subramanian, M.A., Aravamudan, G., Rao, GVS, Prog Solid State Chem., 15, 55, (1983).Google Scholar
11. Lian, J., Wang, L.M., Wang, S.X., Chen, J., Boatner, L.A., Ewing, R.C., Phys. Rev. Lett, 87, 145901, (2001).Google Scholar
12. Lian, J., Xu, X.T., Kutty, K.V.G., Chen, J., Wang, L.M., Ewing, R.C., Phys. Rev. B66, 054108, (2002).Google Scholar
13. Sickafus, K.E., Minervini, L., Grimes, R.W., Valdez, J.A., Ishimaru, M., Li, F., McClellan, K.J., Science, 289, 748 (2000).Google Scholar
14. Wuensch, B.J., Eberman, K.W., Heremans, C., Ku, E.M., Onnerud, P., Yeo, E.M.E., Haue, S.M., Stalick, J.K., Jorgensen, J.D., Solid St. Ionics, 129, 111 (2000).Google Scholar
15. Szymanski, J.T. and Scott, J.D., Can. Mineral. 20, 271 (1982).Google Scholar
16. Lumpkin, G.R., Smith, K.L., Blackford, M.G., J. Nucl. Mater. 289, 177 (2001).Google Scholar
17. Lian, J., Wang, L.M., Lumpkin, G.R., Ewing, R.C., Nucl. Instr. Meth. Phys. B191, 565 (2001).Google Scholar
18. Wang, S.X., Wang, L.M., Ewing, R.C., Phys. Rev. B63, 024105 (2001).Google Scholar
19. Nastasi, M., Mayer, J.W., Mater. Sci. Rep. 6, 1 (1991).Google Scholar