A thought for thorium3 November 2009
Could thorium fuel be used in future nuclear power plants? Recent indications from Areva and AECL show they are not ruling it out yet. By Caroline Peachey
In July, both Atomic Energy of Canada Limited and Areva signed agreements to investigate the use of thorium fuel in their reactors.
On 14 July AECL signed an agreement with three Chinese organisations – China’s Third Qinshan Nuclear Power Company, China North Nuclear Fuel Corporation and Nuclear Power Institute of China – to jointly develop and demonstrate the use of thorium fuel in CANDU reactors in China. AECL has previously built twin 728MWe CANDU 6 units at Qinshan III, located southwest of Shanghai and is already investigating the use of different types of fuel at the site.
The new agreement initiates the second phase of a cooperation programme that began in November 2008. The first phase focused on demonstrating the use of recovered uranium in the Qinshan III CANDU reactors. The second phase will look at both the commercial and technical feasibility of full-scale use of thorium fuel. The first task, to be completed by end of October 2009, is a joint study to examine the economic feasibility of the thorium proposal.
A collaborative agreement between Areva and Thorium Power, signed on 23 July, is to assess the use of thorium fuel in Areva’s light water reactors, specifically the EPR. The first phase of the project will involve the general study of evolutionary thorium fuel concepts for PWRs.
In September 2008, Thorium Power entered into an agreement to begin post-irradiation examination work programme at the Kurchatov Institute in Russia, which will provide key data for licensing its fuel. The company aims to start full-scale product demonstrations in an operating VVER reactor in 2012-2013. The first demonstrations in a PWR are scheduled for 1-2 years later and commercial use could happen as early as 2019.
That timeline might be too tight. Commercial use of thorium would likely take at least a decade, says Albert Machiels, senior technical executive at the USA’s Electric Power Research Institute. “Approval would follow a path similar to that of mixed-oxide fuel, including the insertion of 1-4 test assemblies in a power reactor. The performance of these assemblies would need to be evaluated over a few fuel cycles, at least four years. Test data would then be used to gain approval from the Nuclear Regulatory Commission for further testing of a larger batch of fuel assemblies.”
Developing a new type of fuel is also expensive. So far, Thorium Power has spent $10 million in research and development on its VVER fuel development programme and estimates another $17 million will be spent during the final three phases: detailed design, full scale testing and validation and commercial integration. The firm has budgeted $25 million to develop the PWR fuel; just over a million has been spent so far.
Other companies, including Norway’s Thor Energy, are also trying to develop thorium based fuels for light water reactors. India has long-since committed to a thorium fuel cycle and currently has an ambitious programme of R&D in this area.
But people have been talking about thorium fuels for years, so is it going to happen this time?
The question of thorium fuel comes up every so often, says Machiels. “I really cannot claim that there is a great interest in thorium fuel – it is more a matter of curiosity. Based on historical and expected nuclear industry experience, if there is anything that will be the subject of debate by US utilities in the next decade, it will be the introduction of MOX fuel.”
Thorium, which occurs as 100% Th-232, is a fertile material similar to uranium-238. Before it can be used as a source of neutrons for nuclear fission it has to be converted into fissile uranium-233 in a reactor, via a neutron capture process. A fissile material such as U-235 or Pu-239 is therefore needed to start the reaction. The reaction is: 232Th + n? 233 Th (22 min) ?233Pa (27 d) ? 233U (150,000 y). Th-232 captures a slow neutron forming Th-233, which decays via a two-stage process, through Pa-233 (t1/2=27 days) to U-233.
U-233 is a good fissile material because it uses neutrons efficiently: a high percentage of nuclei undergo fission when bombarded with neutrons, rather than a neutron capture process.
Other advantages of thorium are its high melting point compared with uranium; ThO2 melts at 3300°C while UO2 melts at 2800°C. Thorium also has a higher thermal conductivity than uranium. Both these properties allow for higher margins for the design and for the operation of reactor cores.
Thorium Power’s fuel assemblies are the same size as existing LWR fuel, which means they can be used in existing and future reactors with little modifications required. They are ‘swap-in’ options, the firm says, somewhat analogous to unleaded fuel.
The fuel design is based on the seed-blanket concept. Assemblies are made up of uranium-zirconium metal or metal oxide ‘seed’ rods and thorium-uranium dioxide ‘blanket’ rods. The purpose of the seed rods (which could be comprised of plutonium, or uranium enriched to less than 20%) is to supply neutrons to the blanket. The thorium-232 in the blanket captures the neutron and decays to uranium-233 which burns in situ. The inner seed rods last for three years (if the reactor is on 12-month cycle) or 4.5 years (for an 18-month cycle) while the outer blanket lasts for nine years.
Machiels of EPRI suggested that this could potentially be an issue during refuelling outages. “A main operational issue with this is that you will need to replace, in principle, the inside rods first because they have a lifetime of three years, and then the outside blanket rods much later because of their much longer lifetime. This will require design and engineering forethought to effectively execute refuelling outages,” he says.
But Seth Grae, CEO of Thorium Power said that only ‘minor tweaks’ would be needed to refuelling machines and that there would be “no additional time or complexity to refuelling outages.”
Thorium fuel is not new. It has been used in several reactors in the past including heavy water reactors, PWRs, HTRs and molten salt reactors. It was used to power the first commercial nuclear power plant in the USA, Shippingport in Pennsylvania. But at the time, governments decided to pursue uranium fuel cycle instead – perhaps for military reasons.
Dominique Greneche, deputy director for research and development at Areva NC, told NEI the time has come to take a ‘fresh look’ at thorium.
Thorium is three to four times more abundant that uranium, according to a number of estimates including the IAEA and the NEA’s Red Book. It is widely distributed in nature with an average concentration of 10ppm in earth’s crust and occurs in many phosphates, silicates, carbonates and oxide minerals. Thorium is most commonly found in monazite, which sometimes contains as much as 12% ThO2. The countries with the highest percentage of thorium deposits are Australia, USA and India, although there has been little exploration due to the lack of market for thorium.
To process the ore, the monazite is finely ground and in most countries dissolved in 50-70% sodium hydroxide at ~1400°C. Then it is subjected to a series of chemical operations, including solvent extraction and ion exchange processes to obtain pure thorium nitrate, which is precipitated in the form of thorium oxalate and subjected to controlled calcinations to obtain ThO2 powder. The ceramic fuel pellets of ThO2 or Th.UO2 are fabricated via the same powder-pellet process that is used for conventional UO2 pellets for light water reactors.
Thorium Power claims that its fuel will generate much less waste than conventional uranium fuels used today. There will be approximately a 50% reduction in spent fuel volume and 70% reduction in spent fuel weight, the firm says.
Experts disagree about whether thorium fuel is more proliferation-resistant than uranium.
Many in the industry remain sceptical with regard to thorium. Now that uranium infrastructure is in place, developing a thorium fuel cycle is ‘a big risk,’ ‘unnecessary’ and a ‘distraction,’ according to some in the industry.
I put the question to Thorium Power; if thorium fuel is so good why aren’t we using it? Their response:
“Essentially the answer is because the nuclear industry started using UO2 on a large scale first and they’ve had 50 years to improve it and become comfortable with it. Due to a highly conservative nature of nuclear utilities (‘why change something that works just fine’), there has been little incentive for a commercial utility to switch from UO2 fuels even though ThO2-based fuels have many advantages.”
For this reason, if thorium fuel is going to take off it will need to be introduced in light water reactors first, notwithstanding the interesting reactor concepts currently being developed that use thorium. In accelerator-driven systems, or ADS, a particle accelerator knocks neutrons off a heavy element such as mercury, and those neutrons cause thorium to breed fissile uranium- 233. In molten salt reactors, thorium dissolved in a 650°C fluoride salt coolant breeds uranium-233, which undergoes fission.
“ADS and breeder reactors, such as molten-salt reactors, are so far in the future that if thorium has to wait for one of those developments it’s not going to happen. The point of entry must be the existing infrastructure, at least for the United States,” Machiels says.
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