Norwegian company Thor Energy began a five-year-long irradiation test of thorium fuel at the Institute for Energy Technology’s research reactor in Halden, Norway, marking the most recent investigation into the abundant radioactive element’s future role in the production of nuclear power.
The company loaded six rods (Figure 2) in the last week of April and plans to collect data from the test to support the licensing and eventual use of the mixed oxide (MOX) fuel variant, composed of thorium oxide and blended plutonium, in currently commercial light water reactors (LWRs). “The thorium-MOX fuel to be tested in this program is prototypical of commercial MOX fuel in its microstructure and its composition,” the company said.
Thorium is between three and five times more abundant in nature than uranium, but it occurs, similar to uranium-238, as a “fertile” isotope (Th-232). According to the International Atomic Energy Agency (IAEA), which is holding a technical meeting this September on thorium’s role in the future of nuclear power, the potential of Th-232 for breeding a human-made “fissile” isotope, U-233, in a thermal neutron reactor has been recognized for more than five decades. Several experimental and prototype power reactors were successfully operated from the mid-1950s to the mid-1970s using thorium and uranium fuels in high-temperature gas-cooled reactors, LWRs, and molten salt breeder reactors. But, as the organization points out, thorium fuels have lagged in commercial introduction because “estimated uranium [resources] turned out to be sufficient.”
In recent times, however, increased demand for carbon-free energy and the accelerated growth of nuclear power worldwide has underlined the need for longer fuel cycles, higher burn-up, improved waste form characteristics (thorium oxide [ThO2] is relatively inert and does not oxidize, unlike uranium oxide), reduction of plutonium inventories, and in-situ use of bred-in fissile material. Also increasingly highlighted is that “thorium based fuels, with their unique characteristics of difficult-to-dissolve and reprocess, and easy-to-detect hard gammas of [U-233] daughter nuclides (which also aids deterrence), offer ‘safeguards–friendly’ technological options of fuel cycle,” as the head of India’s Atomic Energy Commission, Dr. R.K. Sinha, pointed out at an IAEA panel discussion this June.
Thor Energy’s continued investment in thorium, meanwhile, is also pegged on forecasts that uranium prices are likely to surge after 2020.
As Sinha noted, the thorium option continues to be investigated in a number of countries besides Norway, notably in Germany, India, Canada, Japan, the Netherlands, Belgium, Russia, Brazil, the UK, and in the U.S., all of which are seeking to overcome a number of technical challenges afflicting thorium fuels and fuel cycles. One challenge is that the melting point of ThO2 is 3,350C—much higher than that of uranium oxide (2,800C)—which requires a much higher sintering temperature to produce high-density ThO2 and ThO2-based MOX fuels.
A unique thorium-fueled light water breeder reactor that operated from 1977 to 1982 at Shippingport, Pa., has already demonstrated U-233 can be bred from thorium. More recent developments include a 2009 agreement between Canada’s AECL, the Third Qinshan Nuclear Power Co., and related Chinese nuclear entities to jointly develop and demonstrate the use of thorium fuel and study the commercial and technical feasibility of its full-scale use in CANDU pressurized heavy water reactors (PHWRs).
India, meanwhile, has been advancing the design of an advanced heavy water reactor (AHWR) as part of the third and final phase of its national nuclear energy plan. That 300-MWe reactor, which could be operational within 30 years, will use thorium-plutonium or thorium-U-233 seed fuel in MOX form, sourced from India’s abundant thorium reserves—the largest in the world. A 500-MWe prototype fast breeder reactor under construction in Kalpakkam is designed to produce plutonium to enable AHWRs to breed U-233 from thorium.
The feasibility of using thorium fuels in pressurized water reactors (PWRs) was also studied in depth, collaboratively by Germany and Brazil, before that research program was terminated for nontechnical reasons. Yet recent interest has peaked in Thor Energy’s efforts to develop thorium-plutonium oxide (Th-MOX) fuels for LWRs because experts contend that option is the most readily achievable, as such fuel can be used in existing reactors with minimum modifications. The test fuel, which is in the form of pellets composed of a dense thorium oxide ceramic matrix containing about 10% plutonium oxide as the “fissile driver,” can also be made in existing uranium-MOX plants using existing technology and licensing experience.
Nevertheless, some international players disagree that thorium has a future as a nuclear fuel. The UK’s National Nuclear Laboratory in 2010, for example, concluded that in the short to medium term, a thorium fuel cycle is “likely to have only a limited role internationally for some years ahead. The technology is “innovative,” the entity said, but noted it is “technically immature and currently not of interest to the utilities, representing significant financial investment and risk without notable benefits. In many cases, the benefits of the thorium fuel cycle have been over-stated.”