MIT engineers work to improve nuclear power generation

With U.S. electricity demand projected to increase by nearly 50 percent over the next 25 years, the Bush administration and others see nuclear power as an increasingly attractive energy option.

Nuclear power has the potential to help make the United States less dependent on foreign fuel and to cut the carbon dioxide emissions that contribute to global warming.

Pilot models of next-generation nuclear plants are being built around the world, but such plants are not likely to produce consumer electricity in the United States for 20 years or more, said Pavel Hejzlar, a principal research scientist in MIT's Department of Nuclear Science and Engineering.

In a nuclear power plant, the fission of uranium atoms provides heat to produce steam for generating electricity. While nuclear plants are already energy-intensive – one pickup-truck full of uranium fuel can supply enough electricity to run a city for a year – Hejzlar and Mujid S. Kazimi, the TEPCO Professor of Nuclear Engineering, professor of mechanical engineering and director of the Center for Advanced Nuclear Energy Systems, wanted to make fuel go even further.

Uranium fuel typically is formed into cylindrical ceramic pellets about a half-inch in diameter. The pellets look like a smooth, black version of food pellets for small animals.

In a three-year project completed recently for the U.S. Department of Energy, Hejzlar and Kazimi teamed up with Westinghouse and other companies to look at how to make a fuel for one kind of reactor, the pressurized water reactor (PWR), 30 percent or more efficient while maintaining or improving safety margins.

They changed the shape of the fuel from solid cylinders to hollow tubes. This added surface area that allows water to flow inside and outside the pellets, increasing heat transfer. The new fuel turned out even better than Hejzlar dared hope. It proved to be easy to manufacture and capable of boosting the plant power output of PWRs by 50 percent.

The next step is to commercialize the fuel concept, which will include testing a limited number of rods filled with the new pellets in an operating reactor and examining the results to ensure the safety and performance of the new fuel.

Water is used in many nuclear reactors to help generate electricity and to ensure safe operation. Now Jacopo Buongiorno, assistant professor of nuclear science and engineering, has come up with a way to change water's thermal properties. This change may contribute to plants' safety while boosting their power density, or the amount of energy they can pump out.

In these reactors, energy released from fission of the fuel's atoms is harnessed as heat in water, which creates steam that drives turbines and produces electricity. In both PWRs and their close cousin, the boiling water reactor (BWR), that steam is turned back into water and reused. Water also is used as a coolant in the reaction process and in safety systems.

The efficiency of PWRs and BWRs is limited to around 33 percent because water can be heated to only a certain temperature and only a certain amount of heat can be taken out of water. If that limit were pushed higher, more heat could be extracted, and the plant would generate more energy at a lower cost.

This may soon be possible, thanks to Buongiorno.

His laboratory works on nanofluids – base fluids such as water interspersed with tiny particles of oxides and metals only billionths of a meter in diameter. Buongiorno's nano-spiked water, transparent but somewhat murky, can remove up to two times more heat than ordinary water, making it an ideal substance for nuclear plants.

The nanoparticles “change some key properties of the way water behaves when it boils,” Buongiorno said, improving its heat transfer capabilities.

The spiked water could provide an extra measure of protection in the event of a nuclear meltdown. In a meltdown, molten nuclear fuel sinks to the bottom of the big stainless steel pot containing it, which sits in a cavity of cooling water. If the excess heat is not removed, the molten fuel could breach the pot.

Nanoparticles in the water that cools the outer surface of the vessel raise the amount of heat that can be drawn away from the core, making the plant less susceptible to the negative repercussions of a possible meltdown.

The key issue to be resolved before nanofluids can be used in nuclear plants, Buongiorno said, is the stability of the nanoparticles, which could agglomerate and settle quickly if appropriate chemical and thermal conditions are not carefully maintained.

This work is funded by the Idaho National Laboratory, the nuclear energy vendor AREVA and the MIT Nuclear Reactor Laboratory.