'Energy Frontiers' is the title of the second of this year's Industrial Physics Forums, which is being held in Nashville, Tennessee, in conjunction with the annual meeting of AVS: Science and Technology of Materials, Interfaces, and Processing.
The forum closed with a session entitled “Materials for a sustainable future.” Leading off the session, Todd Allen of the University of Wisconsin–Madison surveyed the ways that materials science can run more safely and efficiently, produce less harmful waste, and make nuclear reactors last longer.
Nuclear power has strong advantages. Thanks to the liberation of mass energy during nuclear fission, the energy density of nuclear fuel is a million times that of fossil fuels. Far less uranium ore needs to be mined to run a nuclear power station than coal to run a coal-fired station. Nuclear fuel is also cheap, it can be recycled for millennia, and it doesn’t emit climate-changing carbon dioxide.
Offsetting those advantages is a list of strong disadvantages. Even when a nuclear reactor is safely shut down, radioactive fuel rods continue to produce heat. If the heat isn’t dissipated, it can cause problems, as was the case at Fukushima Daiichi in March and Three Mile Island in 1979. Radioactive waste, though small in volume, is harmful and long-lasting. And whereas the cost of nuclear fuel is low, the price tag for a new nuclear power station is exorbitant.
Materials science, Allen told his audience, can—or has the potential to—mitigate the technical aspects of all of those disadvantages. But it can’t, in Allen’s view, do much to lower the risk of terrorists gaining access to bomb-making material, another disadvantage of nuclear power.
Managing heat
The explosion that blew the roof off Unit 1 at Fukushima Daiichi was caused by the buildup of hydrogen gas, which in turn was caused by a buildup of heat inside the unit. Allen explained how the hydrogen gas originated. The fuel rods used at Fukushima Daiichi are cooled by pressurized water, which absorbs the kinetic energy of the fission neutrons and ultimately produces the steam that drives the turbine.
Each rod consists of uranium oxide pellets encased in a zirconium tube. Zirconium is transparent to neutrons and is a good thermal conductor—hence its choice as a cladding material. In contact with water, zirconium forms an oxide plus hydrogen. At normal operating temperatures, the rate of hydrogen production does not pose a problem. But if the water gets too hot, the hydrogen-producing reaction accelerates dangerously.
Allen and others are investigating alternatives to zirconium in water-cooled reactors. Using a molten salt as a coolant could also mitigate the danger of heat buildup and would bring other advantages, such as more efficient neutron capture, higher and therefore more efficient operating temperatures, and lower and therefore safer operating pressures. But prototype reactors built by the French and Japanese show that molten salts are typically corrosive. Reaping the benefits of using molten salt coolants, Allen said, entails meeting a host of challenges, among them finding a suitable cladding material.
Reducing the toxicity of nuclear waste can, in principle, be achieved through materials science. Allen cited three aims: to produce waste in a form that (1) is less corrosive; (2) resists corrosion that leads to the release of radiation; (3) remains stable while being irradiated.
Extending lifetimes
The accident at Three Mile Island caused a freeze on building new reactors in the US. Since then, the operators of nuclear power plants have been upgrading and tweaking their existing reactors to extend their lifetime and improve their efficiency. In the 1990s the Nuclear Regulatory Commission agreed to extend the lifetime of most reactors from 25–30 years to 40 years.
Today, replacing the entire US fleet of nuclear power plants would cost $500 billion, which is roughly what the US paid last year to all 47.5 million Medicare beneficiaries. Rather than face that cost, the operators of nuclear power plants want to extend reactor lifetimes beyond the 40-year span of a current NRC operating license.
According to Allen, the NRC will likely grant 20-year extensions to 90% of the existing nuclear power plants in the US, but only if it can be convinced of the reactors’ material wellbeing. Because reactor vessels can’t easily be replaced or refurbished, their longevity largely determines a power plant’s longevity.
How long a reactor vessel lasts, said Allen, depends on the damage wrought by the fission neutrons. The 1- to 2-MeV energy of a typical fission neutron is more than enough to dislodge an atom inside the vessel’s wall. Once knocked off its lattice site, the dislodged atom leaves a vacancy and becomes mobile. Given that three of the ills that beset metals—corrosion, creep, and precipitation—are all caused by mobile atoms, neutron bombardment is a potentially serious problem.
Indeed, Allen’s research has shown that neutron bombardment creates defects on scales of around 100 nm. These defects, such as dislocation loops, precipitates, and voids, can cause a metal to become more brittle. The voids can even cause the material to swell.
To safeguard future vessels, Allen and others are investigating the possibility of introducing what are known as tailored interfaces—grain boundaries that frustrate the movement of dislodged atoms. Mitigating corrosion could especially benefit from this approach. Allen has found that the corrosion of two alloys, 690 and 800H, used to make reactor vessels depends strongly on the orientation of the grains and their boundaries.
Conducting R&D
Investigating the properties of materials in the extreme environments of a nuclear reactor is difficult and expensive. Third-generation synchrotrons, such as the Advanced Photon Source at Argonne National Laboratory, do not typically come with sources of 1-MeV neutrons. They are, however, beginning to accommodate researchers like Allen, who bring mildly radioactive samples on-site.
In April 2007 the Department of Energy opened its Advanced Test Reactor in Idaho for use by academia and industry. The 40-year-old facility had previously been used by the US Navy to provide research support for its nuclear-powered submarines and warships. Among the materials science equipment on-site at ATR are a dual-beam focused ion beam and an atomic force microscope.
The proponents of nuclear power have devised a range of new reactor concepts, dubbed Generation IV. Many of those concepts require high operating temperatures, which, Allen told his audience, could be a show-stopper from a materials science point of view.
Charles Day
