Nuclear power plants: Shaping new fuels for higher efficiency

MIT researchers have created innovative designs for nuclear fuels that will allow the cooling water inside a nuclear reactor to extract more heat from the uranium fuel. Their new fuels have channels that increase the exposed hot surface area and bumps that churn up the passing water, ensuring that fresh water is continuously brought to the hot surface, thus increasing the cooling effect. These new designs could boost the amount of energy recovered in the same volume of fuel by 30–50 percent, while reducing the cost of electricity by as much as 7 percent.

Nuclear power plants now provide about one-fifth of all the electricity used in the United States. Adding more nuclear plants—or getting more power out of the ones we have—could help us meet growing energy demand without adding to greenhouse gas emissions or oil imports.

At MIT, researchers are looking to improve both current and future plants by changing the design—or “geometry”—of the fuel inside the reactor. “We’ve had many years with the current geometry, so everyone is comfortable with it,” says Mujid S. Kazimi, the TEPCO Professor of Nuclear Engineering, professor of mechanical engineering, and director of MIT’s Center for Advanced Nuclear Energy Systems. “But we’re now trying to redesign it to benefit the efficiency, the economics, and the safety of nuclear power plants.”

The payoff will come from a design that enables the extraction of more power from a given volume of fuel. The benefits for new plants would be significant: for a defined power output, the entire plant could be downsized, leading to enormous cost savings.

But since only a few new plants are likely to come online in the near future, the researchers are also looking to use their more efficient fuel in today’s nuclear power plants—and soon. “A lot of our calculations were to confirm that the hydraulic behavior of our new fuel will be close to that of conventional fuel so that we can use them together as we gradually shift existing plants to the new fuel,” says Dr. Pavel Hejzlar, who worked with Kazimi on fuel design as a principal research scientist in the Department of Nuclear Science and Engineering until January 2009, when he became reactor design lead at TerraPower, LLC.

Doughnut-shaped fuel

Roughly two-thirds of the 104 nuclear power plants in the United States are pressurized water reactors (PWRs). In general, PWRs are fueled by metallic rods, or “pins,” that are 1 cm in diameter and 4 m long. They are packed with small, hard cylindrical pellets of uranium oxide fuel and are submerged in water inside the reactor. When the reactor runs, fission in the fuel releases huge amounts of heat, which transfers to the water and is subsequently used to make steam for the power-generating turbine.

Most attempts at increasing “power density”—how much energy can be extracted from a given volume of fuel—have involved making the fuel pins smaller to increase the surface-to-volume ratio so that the water can come into contact with more of the hot fuel. But the pins can be only so small before they lose their structural strength.

About five years ago Kazimi, Hejzlar, and their collaborators began work on a novel approach to increasing heat transfer. They designed a fuel that is annular in shape: instead of being a solid cylinder, each pellet is shaped like a hollow, thick-walled tube, about 1.5 cm tall (see the cross-sections at the right). To assemble a pin, the annular pellets are stacked up inside a long tube, and then a narrower tube is inserted down the middle. The final product is a fuel pin with an open channel down the center. Inside the reactor, water can flow along the inside wall as well as the outside one. The result: a 50 percent increase in exposed surface area.

Results of detailed simulations are encouraging. The new fuel should last about as long as conventional fuel does before it needs to be replaced by fresh fuel—but during that time, it will provide 50 percent more energy.

The new fuel also should improve reactor safety. During hypothetical adverse conditions such as “loss of coolant,” the annular fuel should get no hotter than conventional fuel would. Indeed, calculations suggest that the new fuel would operate at a much lower average fuel temperature than a typical solid fuel rod would at the same power output (700°C versus 1300°C) and also at a lower peak fuel temperature (800°C versus 2300°C).

To examine the feasibility and cost of manufacturing the annular fuel pellets, research collaborators at Westinghouse performed test runs in their commercial plant in Columbia, South Carolina. Using commercial fabrication techniques, they were able to manufacture the annular pellets with sufficient dimensional accuracy, little wasted material, and at reasonable cost. They also successfully loaded the pellets into sample 1.2-meter-long pins.

Incorporating the new fuel into today’s operating plants should be straightforward. The new pins are larger in diameter than conventional ones are (1.5 cm instead of 1.0 cm). But they can be grouped to form “assemblies” that have the same outside dimensions as assemblies of conventional pins. When a spent assembly in an operating plant needs to be removed, an annular one could be inserted in its place.

However, getting the full benefit of the new fuel in an existing plant would require the installation of larger pumps, different steam generators, and other changes in power-related equipment. Such changes would be costly, so one question is how much extra power could be achieved without large component changes.

The South Koreans are now finding out. They are independently preparing to test MIT’s annular design in their existing reactors and are making just one operational change: starting with cooler water at the entrance to the nuclear core, so that a greater temperature increase can occur before the water exits. Their hope is to get a 20 percent increase in power.

According to the MIT analyses, the effects of using the new fuel—the generation of more energy, the slightly increased manufacturing cost, and the higher cost of the larger pumps, steam generators, and turbines needed to remove 50 percent more power—should have the net impact of reducing the cost of electricity by 7 percent.

Twisted fuel

While continuing to develop their annular fuel, the MIT team has also been working on better fuel for the other prevalent type of nuclear plant, the boiling water reactor (BWR). In that design, the water coolant boils inside the reactor core, becomes steam, drives the power-generating turbine, and then condenses and returns to the reactor vessel to go through the cycle again.

To increase heat extraction in a BWR, the researchers designed a novel fuel rod that has a cross-section with four lobes, somewhat like a four-leaf clover. The four-lobed tube is first gently twisted and then filled with matching cross-shaped fuel pellets.

This new design offers several benefits. The cross shape increases the surface-to-volume ratio, and the twisted surface causes the water flowing by to become more turbulent, ensuring that fresh, cool liquid is constantly brought to the hot surface. Also, when gathered into an assembly, the twisted rods touch at intervals, so they support one another without grid spacers—horizontal plates that conventional rods pass through every few feet to keep them stable and separated.

To test their design, the researchers built three sets of 16 rods, each set with a different twist angle, plus a reference set of untwisted rods (see photo at right). They then developed a facility in which they could test water flows through the assemblies under conditions similar to those inside a BWR.

As expected, they found that their assemblies had significantly less resistance to flow than conventional assemblies do, largely due to the absence of the grid spacers. As a result, less power is required to pump the water through. In addition, the open channel ensures that the water and steam can circulate freely around the rods—a critical feature in BWR operation. The increased turbulence also prevents the formation of hot spots on the fuel rods.

According to computer simulations, the combined effects of the new fuel should allow BWRs to operate at 20–25 percent higher power density. Moreover, during normal operation, the highest centerline temperature of the new fuel should be about 960°C—some 225°C lower than that of a standard cylindrical fuel rod—adding to the ability of the materials to tolerate the effects of sudden changes in operating conditions.

The researchers are continuing to work on both their PWR and BWR fuels. Results to date suggest that the new fuels may provide a means of significantly expanding power generation from today’s well-proved nuclear plant technologies while next-generation reactor designs are being developed and demonstrated.

This research was supported by the U.S. Department of Energy and Tokyo Electric Power Company.

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