THORIUM AS AN ENERGY SOURCE - Opportunities for Norway ...

The Front End of the Thorium Fuel Cycle
with accumulated decay products also emits β-particles and penetrating γ-rays. However, as a
result of the very long half-life of thorium, these emissions do not present a real danger for direct
handling of limited amounts of thorium. Some shielding is required for large amounts. Thorium is
a very active metal and in finely divided form it can be pyrophoric and in dust form it may be
explosive. The presence of moisture and hydride as well as residual calcium may render thorium
metal pyrophoric. The pyrophoricity can be minimized by giving thorium metal powder a
protective treatment. This treatment usually consists in immersing the powder in an aqueous
solution of a salt of a metal that is less electro-positive than thorium.
4.2.2 Thorium Oxide Fuel
The use of thorium in reactors has been mainly envisaged in the form of thorium dioxide (thoria)
in view of the greater experience and proven performance of the oxide fuels. However, unlike the
sintering of UOx, where the sintering atmosphere and O/U ratio play an important role, ThO2
being the most stable oxide known, can be sintered in any atmosphere such as air, hydrogen or
vacuum.
Pelletizing ThO2-based fuel pellets out of calcinated kernels seems to present the most attractive
fuel fabrication process at present. It has been under development since the late seventies [27],
[28], [29], [30], [31], [32], [33].
For fabrication of thoria kernels the sol-gel process has been developed. On the basis of this
technology a procedure for production of pelletized fuel for water-cooled reactors has been
developed. External gelation of thorium sols is used in order to produce gel kernels which could be
calcinated and suitable for pelletization and sintering. The pelletizing of ex-gel-kernels avoids
dust generation, is easy to remotize, and produces high density pellets with the desired
microstructure at compaction pressures and sintering temperatures which may be even lower
than for the conventional UO2 manufacturing.
ThO2-pellets can also be manufactured by the classical pelletizing process extruded from
adequately pre-treated powder. The relevant experience is described in detail in the open
literature:
• All ThO2-based fuel in the US-programs from the early fifties until 1978 was produced by
pelletizing from different powders. Other sophisticated approaches, such as duplex pellet
manufacturing, were investigated on the laboratory scale. The necessary powders have been
produced by nearly all known and applicable processes.
• The experience from India in fabrication of thoria-pellets using ex-oxalate powder was
concentrated on powder granulation and on processing parameters as well as on sintering by
the use of Mg-dopant as a sintering aid. The objective of this work was to improve the
fabrication reliability for fast breeder blanket elements for the fast reactor at Kalpakkam,
India.
The extrusion processes were also investigated. However, it seems that this approach is not very
promising.
4.2.3 Mixed Oxide Fuel
Since thorium is a purely fertile material, a reactor containing thorium must also contain fissile
material. The “driver” or “seed” material can be separated from the fertile fuel elements, but it is
also frequently a mixed fuel: (Th,U)O2 or (Th,Pu)O2; where uranium could be U-235 or U-233. A
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