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Q&A:Thorium Think thorium Our expert panel answers your questions on thorium- fuelled fission reactors. Stephen Harris reports T horium is increasingly being promoted as an alternative fuel to uranium in civil nuclear fission reactors. Proponents argue it’s safer, more abundant and much harder to weaponise. But the most significant development of thorium-fuelled reactors at Oak Ridge National Laboratory in the US was cancelled in the 1970s and the technology would need large amounts of investment to continue readying it for commercial use. For our latest reader Q&A, we put your questions to a panel of thorium experts: n Julian Kelly, chief technology officer for Thor Energy, a Norwegian firm developing and testing thorium-plutonium (Th-MOX) fuels for use in commercial light water reactors (LWRs); n David Martin, deputy director of research at the Weinberg Foundation, a not-for-profit thorium lobby group; n Fiona Rayment, director of fuel cycle solutions at the UK’s National Nuclear Laboratory (NNL); n Kirk Sorensen, founder of US firm Flibe Energy, which aims to build a demonstration liquid-thorium-fuelled reactor (LFTR). Old idea: Thorium research dates back to the 1960s. removal and high thermal conversion efficiency achieved at low pressures. That final attribute is unique among reactors in this class. The disadvantages are a small and aged base of scientists and engineers who have actual experience in the technology, regulatory challenges that will be faced to adapt solid-fuel reactor regulations around liquid-fuel technology, the production of tritium in the salt and its containment, and the challenge of developing a materials database to support 30–60 year licensing of this class of reactors. n What are the advantages and disadvantages of thorium-fuelled Fiona Rayment: The main advantage is that reactors over other proposed next-generation the use of thorium is a potential strategic nuclear designs? The use of thorium alternative to using uranium/plutonium, and so David Martin: First, it is essential to an secure distinguish between thorium fuel and is a potential strategic could be energy additional contributor to fuel cycle future reserves. The thorium advanced reactors. In practice, most current also generates much lower quantities of highly and future reactors could be fuelled by alternative to using radioactive ‘transuranic’ materials, which are thorium-based fuel, but only certain reactors uranium/plutonium generally viewed as waste. Under some are able to fully exploit thorium’s potential. The circumstances, it is predicted that the thorium two most promising thorium-fuelled reactors fuel cycle could give an economic benefit, although this remains to are high-temperature reactors (HTRs) and molten salt be proven in practice. The main disadvantages are that thorium (in reactors (MSRs). the form of Th-232) is only fertile, not fissile and converting the Thorium’s high melting point makes it ideally suited to HTRs [but] thorium to fissile uranium-233 involves a neutron capture that is MSRs offer a step-change in fuel efficiency and inherent safety. time-consuming and expensive to implement. Another disadvantage Liquid fluoride thorium reactors [a type of MSR] would breed the is that utilising thorium will involve the development and fissile isotope U233 and would automatically ‘burn up’ their own deployment of new fuel-cycle technology, which has to compete with wastes, producing minimal long-lived radioactive waste. the established uranium fuel cycle. While it is true that using thorium produces only trace amounts of plutonium, it does produce very Kirk Sorensen: The advantages of LFTRs are the potential for much high-quality fissile material that is a significant proliferation issue, greater use fraction of fuel, the elimination of the need to fabricate as is the case with the conventional uranium/plutonium fuel cycle. solid-fuel elements, a completely passive approach to decay heat “ 32 | theEnGineeR | DECEMBER 2013