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Science & Technology

September 13, 2010
Volume 88, Number 37
pp. 29 - 31
DOI: 10.1021/CEN090710142031

Nuclear Efficiency

With new fuel formulations, reactors could extract more energy, reduce hazardous waste

Jyllian N. Kemsley

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EFFICIENT ENERGY
EFFICIENT ENERGY An artist’s rendering shows the core of GE Hitachi Nuclear Energy’s sodium fast reactor.

When it comes to nuclear energy, the world is not exactly an early adopter of new technology: The vast majority of nuclear reactors running today falls into the so-called Generation II category and uses technology from the 1970s. Generation III reactors—the ones being built now or in the near future—are fundamentally based on the same water-cooled design, with improvements in safety, reliability, and efficiency.

It is in the development of Generation IV reactors—the ones that will start up around 2030—that nuclear energy will see a significant change in technology. The six models put forth by the Generation IV International Forum, chartered in 2001 to carry out nuclear energy research and development, aspire not only to be even safer and more reliable than previous generations, but also to get a greater return from the energy source—by extracting up to 90% of the available energy in their fuels instead of the 5% more typical of today’s reactors. In some cases, the reactors will use reprocessed or recycled waste fuel from other reactors. The fuels may also incorporate some of the longest lasting radioisotopes from waste fuel, including americium, curium, and neptunium, thereby turning these radiotoxic isotopes into less hazardous materials while providing a little extra energy in the process. To reach those goals, and especially to reach them safely, nuclear scientists are working to develop and evaluate new fuel formulations and materials.

Current reactors use either uranium dioxide or a mix of uranium dioxide and plutonium dioxide. The fuel powder is pressed into pellets that are about 1 cm in diameter. The pellets are then inserted into thin tubes to form rods. The tube material, known as cladding, is considered an integral part of the fuel. In traditional reactors, the cladding is a zirconium alloy. After the rods are sealed, they are assembled into bundles of dozens to hundreds of rods; several hundred of the bundles make up the core of a reactor.

When a neutron collides with atoms of 235U or 239Pu in the fuel rods, it can set off a fission reaction that splits the atoms into smaller atoms while producing more neutrons and heat. The initial neutrons come from a 252Cf neutron source in the reactor, but the neutrons produced from fission set off additional fission reactions. Control rods composed of neutron-absorbing materials help mediate the reactions. The heat gets transferred first to the coolant water and then to other parts of the power plant.

The four classes of Generation IV reactors work on the same basic principles as previous-generation reactors, but they operate at higher temperatures and may differ in terms of coolant, fuel assemblies, and neutron energy.

KERNEL Idaho National Laboratory
KERNEL A false-color photograph shows the UO2 core of a 1-mm diameter TRISO fuel pellet surrounded by layers of pyrolitic carbon (gray) and silicon or zirconium carbide (brown).

The supercritical-water-cooled reactor is akin to current reactors but uses supercritical water as its coolant and has an outlet temperature of 600 °C rather than the 300 °C of current reactors.

The “very high temperature” reactor is cooled by helium and has an outlet temperature of 1,000 °C.

Molten salt reactors also operate at very high temperatures but disperse the fuel into the salt coolant.

And the “fast” reactors use “fast” neutrons that are about 103 to 106 electron volts more energetic than the “thermal” neutrons used for fission in conventional reactors. (Both types of neutrons are produced in conventional reactors, but the water coolant saps energy from fast neutrons, converting them to thermal neutrons). Fast reactors are cooled with gas, either helium or carbon dioxide, or a liquid metal, either sodium, lead, or a lead-bismuth combination.

All of these next-generation reactors aim to achieve higher fuel burnup, which means more uranium or plutonium atoms undergoing fission. It also means an increase in the detrimental effects of fission and radiation that cause fuel materials to degrade and that reduce heat flow. Fission products include xenon and krypton gases that, along with helium produced by α decay of actinides, increase pressure in fuel rods. Radiation also knocks atoms off their lattice sites, creating vacancies and interstitial atoms and causing distortion and swelling of the fuel. Additionally, neutrons not consumed by fission can produce heavier transuranic elements through neutron capture and transmutation.

Another issue for the new reactors involves extreme thermal gradients in fuel pellets. The poor conductivity of UO2 and PuO2 means that fuel pellets can reach 1,500 °C or higher at the center, whereas the outside will be a much cooler 300 °C—a temperature difference that occurs over a mere half a centimeter in distance. “This causes all kinds of problems because the way the fuel behaves at 1,500 °C is totally different from 300 °C,” says Gary Was, a professor of materials science and engineering at the University of 
Michigan.

Fission products and radiation can also react with and corrode fuel cladding. If the cladding fails, the coolant becomes contaminated with radioactive fuel waste products.

None of these effects are new to nuclear researchers, but they are magnified when fuels run hotter and for longer, as planned for the new reactors. “If we want to reach higher burn rates and to have fuel that goes longer in a reactor,” researchers will have to find ways to handle or limit these various effects, says Martine Tourasse, deputy head of the fuel studies department in the nuclear energy division of the French Atomic Energy & Alternative Energies Commission (CEA).

One class of fuels being considered for fast reactors is ceramics—for example, uranium carbide or uranium nitride. “Carbides should be able to withstand the temperatures better” than UO2 in very high temperature reactors, says Marjorie Bertolus, a CEA research scientist, although there’s not a lot of data yet to back up that prediction. A better understanding of ceramic fuels is one of the goals of a European collaboration known as F-BRIDGE (Basic Research for Innovative Fuel Design for Gen IV systems), for which Bertolus is the deputy coordinator.

Another option is metal fuels, such as the uranium-plutonium-zirconium alloy being considered for sodium-cooled fast reactors. Because metals are conductive, they would eliminate the thermal-gradient issues. Metal fuels, however, tend to have bigger problems with swelling and distortion in response to radiation than their counterparts, Was says.

Alternatively, Idaho National Laboratory is investigating sticking with uranium dioxide but adding something such as carbon fibers to improve the thermal conductivity of the material, says Kemal Pasamehmetoglu, a scientist at INL and technical director for nuclear fuel programs for the Department of Energy. Yet another option is to create a fuel rod with a hole through the middle through which coolant can flow.

For new cladding materials, researchers are investigating ferritic-martensitic stainless steel; oxide dispersion-strengthened steels, which incorporate nanoparticles of titanium and tungsten oxides; nickel-iron-chromium alloys; and ceramics. But the behavior of those materials in reactors is not understood as well as the traditional zirconium alloy, and “that’s a big worry,” Was says. At the fuel-cladding interface, fission products could react with the cladding and create brittle phases or corrode the cladding. One solution researchers are looking at is to insert a thin ceramic barrier layer between the fuel and the cladding to isolate the two.

Was adds that scientists investigating cladding material for next-generation reactors have been surprised at the number and amount of radiation-induced precipitates that form in the new cladding materials during intense irradiation. Researchers are working to understand the effects and whether they’re benign or detrimental.

“You can only get more energy out of a nuclear fuel if you control the degradation processes so it works better for longer.”

At the extreme temperatures of the very high temperature reactors, metal cladding won’t work at all. The so-called tristructural-isotropic (TRISO) fuel being studied for such systems consists of a small fuel kernel with four layers of coating that act as the cladding: a porous buffer of carbon, then pyrolitic carbon, then silicon or zirconium carbide, and finally pyrolitic carbon again. The coated, 1-mm diameter kernels can be compacted with graphite into tennis ball-sized “pebbles” or formed into rods and inserted into a graphite block. Coolant can flow around the pebbles or through channels in the graphite.

Was has been studying the effects of radiation on silicon carbide layers, to understand how they swell, strain, and distort from radiation. “When these TRISO particles are being irradiated and there’s stress on the layers, we don’t know how they’re going to respond,” Was says. In particular, he’s using particle accelerators to irradiate the layer materials to try to understand how the materials distort. “How they respond is going to be important in whether they do their job and maintain their structural integrity,” Was says.

In molten salt reactors, the fuel is suspended into the circulating coolant. No cladding is necessary, but the fission products would be released directly into the coolant. “It’s a very simple system to analyze and track compared to other fuel structures,” Was says. But “the molten salt can be pretty corrosive and that also creates issues,” he adds. “The challenge will be to keep the coolant from leaking out.” Sophisticated chemical engineering will also be necessary to separate fission products from the salt.

Another route to improving the efficiency of nuclear reactors involves finding a way to deal with the “minor actinides”—americium, curium, and neptunium—produced in fuels by neutron capture reactions. Those elements are the primary sources of radiotoxicity in spent fuels. “If you look at the radiotoxicity of nuclear waste as a function of time, the contribution up to 250 to 300 years is mainly due to shorter-lived fission products,” says Joseph Somers, head of the nuclear fuels unit at the Institute for Transuranium Elements (ITU), one of seven research institutes of the European Commission’s Joint Research Centre. It is plutonium and the minor actinides in spent fuel that push radiotoxicity timelines out to hundreds of thousands of years.

Instead of considering the minor actinides a problem, some fuel developers see them as an opportunity. These researchers are separating the minor actinides from spent fuel—typically through electroplating techniques—and incorporating them into fuels for fast reactors at amounts ranging from 2 to 15%. The higher-energy neutrons in fast reactors can then induce fission in the minor actinides, yielding the dual benefit of rendering them less hazardous and extracting some energy from the process, notes Eric Loewen, chief consulting engineer for advanced plants technology for GE Hitachi Nuclear Energy, which is developing a sodium-cooled fast reactor known as PRISM, for Power Reactor Innovative Small Module.

Future Generation Idaho National LaboratoryView Enlarged Image
FUTURE GENERATION In a sodium-cooled fast reactor, molten sodium absorbs the heat from nuclear fission reactions to produce steam that, in turn, is used to generate electricity.

Understanding and managing what happens in a fuel pellet is particularly challenging when the minor actinides are added, INL’s Pasamehmetoglu says. “You’re not just dealing with one species but multiple species that can go through chemical reactions, as well as thermal diffusion and relocation within fuels.” The reactivity of the minor actinides may also change cladding requirements.

And the materials are not easy to handle. Whereas researchers can do hands-on fabrication of UO2 and PuO2 pellets in a glove box, extra lead shielding is required with the minor actinides to limit exposure to γ emissions. The materials have to be located in a “hot cell” and manipulated remotely. Also, conventional powder metallurgy methods yield a lot of radioactive dust that must be controlled. “It’s difficult to manage in a lab and very, very difficult on an industrial scale,” Somers says. Somers’ group is developing a method they call gel-supported precipitation as a dust-free means of converting droplets of solution into particles that are 20 to 250 μm in diameter that can then be formed into fuel pellets.

Researchers from INL, ITU, and Japan’s Central Research Institute of Electric Power Industry (CRIEPI) have experimented with adding small amounts of the minor actinides to a metal uranium-plutonium-zirconium fuel intended for sodium fast reactors. Of particular interest were the effects on the fuel performance of fuel heterogeneity—the trivalent actinides are not highly soluble in the fuel and tend to distribute around the grain boundaries of the matrix—and of fission gas production. Early results indicate that those factors don’t significantly change the fuel behavior under irradiation, say Pasamehmetoglu and Tadashi Inoue, executive research scientist at CRIEPI. Nor did the fuel appear to swell any more than did metal fuels without the additional actinides.

Historically, nuclear fuel research has been very empirical, Pasamehmetoglu says. “People tended to observe what happened and stay within those experimental limits” to ensure that they could get the license required to operate a reactor. With the effort to develop Generation IV reactors, however, that attitude has changed. Researchers across the board are now increasingly focused on trying to come up with a more fundamental understanding of fuel behavior and to be more predictive about it.

Accordingly, nuclear energy scientists are bringing new tools to the table that will enable them to understand fuel behavior down to the microstructure and grain level. Laser-based thermal conductivity measurements, electron microscopy, nuclear magnetic resonance spectroscopy, and X-ray diffraction and absorption are all techniques being adapted for work on radioactive fuels to help scientists pin down how fuel properties are affected by different parameters, such as fuel microstructure or grain size and reactor power or temperature.

“You can only get more energy out of a nuclear fuel if you control the degradation processes so it works better for longer,” says Robin Grimes, a materials physics professor at Imperial College London. “And it’s not just about formulating fuels differently but running the reactor differently—there are myriad things an operator can do. Sometimes, for example, a slightly higher temperature releases stresses or strains.”

There is also significant interplay between experiment and computational calculations. By the time a nuclear fuel is prepared, irradiated, cooled, and analyzed, a single experiment can take up to seven years to complete. “Experiments are so expensive and so time-consuming, if we can use computer simulation to point the way toward the critical experiments to do, that can save the experimentalists tremendous amounts of time and effort,” Grimes says.

On the analysis end, he adds, “The computer simulation allows you to rapidly discover potential mechanisms that might be operating.” Recent work from his lab has focused on modeling the effects of fission gases on UO2. “Simulations allow you to think how the fuel might have worked and compare that to the experimental data,” Grimes says.

Even as nuclear scientists try to make the harnessing of nuclear energy ever more efficient, a renaissance of nuclear energy still has many political, regulatory, and financial hurdles to overcome, not to mention the need for a waste repository in the U.S. (C&EN, March 8, page 31). World energy needs are not decreasing, however, so researchers should see the fruits of their labors come to life over the next few decades in new reactors that rely on highly productive and reliable fuels.

Chemical & Engineering News
ISSN 0009-2347
Copyright © 2011 American Chemical Society
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