Thorium dioxide (ThO2) is undoubtedly the best-characterized ceramic com­pound of thorium. Although this partly stems from its study for nuclear pur­poses, the majority of information exists because of the non-nuclear

usefulness of the material. Since thoria has the highest melting point (~3300 °C) and is the most stable to reduction of all the refractory oxides, it is a superior crucible material for the melting of reactive metals. Thoria is gener­ally prepared in powder form by the thermal decomposition of a purified salt, generally the oxalate. This powder can be consolidated by usual ceramic fabri­cation techniques, such as slipcasting, pressing, and sintering, or hot pressing. The fabricability and ceramic properties can often be related to conditions of preparation of the starting salt and firing.

Thorium dioxide exists up to its melting point as a single cubic phase with the fluorite (CaF2 type) crystal structure and is isomorphous and completely miscible with UO2 to a measurable extent. Therefore, it is stable to high temperatures in oxidizing environments. In vacuum, it darkens with loss of oxygen, even though the loss is insufficient to be reflected in chemical analysis or lattice constant mea­surement. Unlike UO2, thoria does not dissolve oxygen even on prolonged heating to 1800-1900 °C. By reheating in air to 1200 or 1300 °C, the white color can be brought back.

When uranium dioxide is incorporated in thoria, the lattice can take up extra oxy­gen in proportion to the uranium content. Table 7.6 summarizes some important physical and mechanical properties of thoria, along with analogous properties of uranium dioxide taken from the compilation by Belle.

Thorium carbide and thorium mononitride fuels may have potential for use as nuclear fuels, but have not been thoroughly studied.

Weld Fuel Rod

‘ Fuel Rod ‘ Characterization Acceptable? .

Yes

Figure 7.25 Process flowchart for fabrication of metallic fuels for fast reactors. Burkes et al., Ref. [19].

7.4

Summary

The topic of nuclear fuels is vast and is not easy to cover in a single chapter. None­theless, here a succinct review of both metallic and ceramic nuclear fuels is made and their various properties are discussed. Among metallic fuels, uranium, pluto­nium, and thorium are discussed. Among ceramic fuels, uranium dioxide, ura­nium nitride, and uranium carbide as well as plutonium-based oxide fuels and thorium oxide are covered. The metallic and ceramic fuels are found to have both advantages and disadvantages of their own.

Problems

7.1 What are the advantages and disadvantages of metallic nuclear fuels?

7.2 Describe various requirements imposed on a nuclear reactor fuel.

7.3 How many allotropic forms does uranium have? Discuss the effect of allo — tropic transformation on the properties of uranium.

7.4 Describe the plastic deformation mechanisms of alpha-uranium.

7.5 Discuss the beneficial effects of alloying on the properties of uranium as a nuclear fuel.

7.6 Distinguish between thermal cycling growth and radiation growth of alpha — uranium.

7.7 Martensitic transformation may occur in uranium system. Describe one example and its advantages and disadvantages.

7.8 How is plutonium produced?

7.9 Why is plutonium said to have fickle nature?

7.10 Discuss the origin of self-irradiation behavior of plutonium and its effects.

7.11 Discuss the origin of superplasticity in plutonium.

7.12 Can plutonium be alloyed with other metals? If so, compare it with uranium — based systems.

7.13 What are the main mineral sources ofthorium?

7.14 What are the main impediments to realizing thorium cycle on a wider scale?

7.15 State the advantages and disadvantages of ceramic nuclear fuels.

7.16 Describe the effects ofhypostoichiometry, stoichiometry, and hyperstoichiom­etry on the properties ofuranium dioxide.

7.17 Compare and contrast between UO2, UN, and UC fuels.

Bibliography

instability. Metallurgical and Materials Transactions A, 35 (8), 2207-2222.

9 Merz, M. D. and Nelson, R. D. (1970) Proceedings of the 4th International Conference on Plutonium and Other Actinides 1970 (ed. W. N. Miner), The Metallurgical Society of AIME, New York, p. 387.

10 Gschneider, K. A., Jr., Elliott, R. O., and Waber, J. T. (1963) Acta Metallurgica, 11 (8), 947-955.

11 Ray, H. S., Sridhar, R., and Abraham, K. P. (1985) Extraction ofNonferrousMetals, East-West Press, New Delhi, India.

12 IAEA (1997) Thermophysical Properties ofMaterials for Water Cooled Reactors/IAEA-TECDOC — 949, IAEA, Vienna, Austria.

13 Peterson, S., Adams, R. E., and Douglas, D. A., Jr. (1965)

Properties ofThorium, Its Alloys and Its Compounds, ORNL Report ORNL-TM — 1144.

14 World Nuclear Association, http://www. world-nuclear. org.

15 Webb, J. A. and Charit, I. (2012) Analytical determination of thermal conductivity of W-UO2 and W-UN cermet nuclear fuels, Journal of Nuclear Materials, 427, 87-94.

16 Webb, J. A. (2012) Ph. D. Analysis and Fabrication of Tungsten CERMET materials for Ultra-High Temperature Reactor Applications via Pulsed Electric Current Sintering, University ofIdaho.

17 Rondinella, V. and Wiss, T. (2010) The high burn-up structure in nuclear fuel. Materials Today, 13, 24-32.

18 Hayes, S. and Peddicord, T. (1990) Material properties correlations for uranium mononitride IV. Journal of Nuclear Materials, 171, 300-318.

19 Burkes, D. E., Fielding, R. S., Porter, D. L., Crawford, D. C., and Meyer, M. K. (2009) A US perspective on fast reactor fuel fabrication technology and experience. Part I: metal fuels and assembly design. Journal of Nuclear Materials, 389, 458^89.

Additional Reading

Allen, T., Busby, J., Meyer, M., and Petti, D. (2010) Materials challengesfor nuclear systems. Materials Today, 13 (12), 14—23.

Buckley, S. N. (1961) Irradiation growth, Atomic Energy Research Establishment, Harwel ARE-R 3674, UK.

Appendix A