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Thorium as nuclear fuel

Thorium fuel rig
The fuel testing whici is now under way at the IFE Halden Reactor will provide answers related to how thorium might be utilized as a nuclear fuel.
Photo: T. Tanberg
Thorium is not fissile – but fertile. In order to create a reactive thorium fuel capable of producing energy, some form of fresh or recycled fissile material is needed as a ‘driver component’. As the fuel operates, thorium transmutes to uranium-233 which is an excellent fissile material that then yields energy in the fuel. Reactor grade plutonium is a very good fissile driver since it is available from today’s spent nuclear fuel inventories.

Thorium will absorb neutrons in a thermal reactor and its reactivity will increase as its U-233 content grows. It is possible to achieve net ‘breeding’ of U-233 in thorium fuels in faster-spectrum variants of light water reactors (LWRs) and in heavy water reactors.

Ceramic thorium oxide (ThO2) has excellent material properties for serving as a nuclear fuel. ThO2 has a higher thermal conductivity and a higher melting point than uranium oxide and it is better able to retain fission products within its crystal lattice. Thorium oxide fuels can therefore operate with lower internal pellet temperatures and exhibit less fission gas release than uranium fuels (including MOX). It is therefore recognized that thorium oxide fuels can operate safely to high burn-ups.

It is possible to design viable thorium-plutonium fuels for LWRs balancing intersecting issues of fuel reactivity, safety margins and reactor operability. To this end, the design and licensing of uranium-plutonium (MOX) fuels has “paved the way” for thorium-plutonium fuels since it is only the added plutonium component that makes these different from simple enriched uranium fuels.

Thorium itself generates essentially no plutonium, and no minor actinides as it burns – unlike uranium fuel. Thus, a thorium-plutonium fuel will achieve much greater net plutonium consumption than a regular MOX fuel (which makes new plutonium as it burns). Thorium-plutonium fuel can be designed with a priority to maximise plutonium consumption – by maximizing the extent to which neutrons are moderated in the reactor.

Ceramic thorium oxide has excellent properties as a waste-form in its own right – even after irradiation. It is highly insoluble, it is non-oxidizable and it retains both fission products and actinides extremely well within its lattice. Thorium oxide would therefore serve as a good matrix for once-through fuel designed specifically to ‘burn’ plutonium.

Thorium-plutonium fuels have good long-term radio-toxicity credentials. Modelling shows that the decay profiles for dose rates from thorium-plutonium spent fuels are essentially the same as those for uranium-based fuels, but there are fewer chemical pathways for thorium fuels to be dissolved – especially in oxidising environments.

Thorium fuel cycles are highly resistant to nuclear proliferative actions. Both open and closed thorium fuelling options exhibit higher proliferation resistance than their corresponding uranium based cycles, as assessed by internationally recognised methodologies.

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