Nuclear Power’s Future

Per Peterson’s Pebble bed advanced high Temperature reactor design

In Per Peterson’s Pebble Bed Advanced High Temperature Reactor design, the unpressurized molten salt coolant transfers heat at 704°C to helium gas that powers high-efficiency gas turbines. The thermodynamic efficiency of the design is 46 percent, considerably higher than the 33 percent figure for conventional pressurized water reactors.

A conversation with Per Peterson, the chair of Berkeley’s Department of Nuclear Engineering.

Some environmental organizations are proposing that the world’s future energy needs can be met without fossil fuels and without nuclear power. Per Peterson, the chair of UC Berkeley’s nuclear engineering department, ponders that for a moment and replies, “Do you really want to bet the climate on that possibility?”

Peterson suggests that putting all our eggs in the renewables basket, without designing and building better nuclear plants, will leave us with coal and other fossil fuels as our only backup. Although Peterson would prefer the option of nuclear power, he is tough-minded about its problems, which he sees as cost, safety, proliferation risk and waste disposal.

Says Peterson, “Nuclear power plants are expensive to build but cheap to run.” Most of the costs for nuclear power are capital costs. Buying the nuclear fuel and running the plant are relatively minor parts of the cost picture. “The cost of building a nuclear plant has run as high as $10,000 per kilowatt of electrical power,” says Peterson. “Those costs have to come down, and new modular construction techniques will make that happen.”

Imagine buying a car and having it arrive in a storage trailer, completely unassembled. To build the car, you would be required to hire a crew to paint the metal and bolt together thousands of pieces. “Not only would your car end up being very expensive,” says Peterson, “it would take a really long time to assemble it. Yet that is basically the way nuclear power plants have been built.”

The latest designs are different. Peterson cites as an example the new Westinghouse/ Toshiba AP1000. The reactor is built from a few hundred modular subassemblies that are manufactured in a factory under strict quality control and shipped to the site where the plant is assembled. “This is more like having your car arrive as 50 sub-assemblies that have to be bolted together,” say Peterson. “Still significant work, but a big step in the right direction.”

In Georgia, the Southern Company is building two AP1000s, which will be the first new reactors in the United States to come on line since 1996. The total capital costs will be approximately $6,000 per kilowatt. Over the 60-year planned lifetime of the plant, it may become competitive with electricity produced from natural gas, especially if carbon taxes are implemented.

China has four AP1000 reactors under construction, for which the costs are estimated to be less than $2,000 per kilowatt, and even less for future units. Says Peterson, “Labor costs are much lower in China, although they are expected to rise. But China also will benefit from economies of scale due to the number of units being built. And China is not only building the reactors, it is building the factories to make the modular components of the reactors.

“Even more important than the cost savings,” says Peterson, “are the improved safety features of newer reactor designs. The AP1000 is one of the first of the Generation III+ reactors to receive design certification from the U.S. Nuclear Regulatory Commission. Unlike existing Generation II and III plants built in the 1970s and ’80s, the AP1000 design is much less vulnerable to the prolonged loss of grid power, or ‘station blackout,’ that damaged the Fukushima reactors.”

According to the Westinghouse/Toshiba website, the AP1000 is designed to shut down “without any operator action and without the need for grid power or pumps. Instead of relying on active components such as diesel generators and pumps, the AP1000 relies on the natural forces of gravity, natural circulation and compressed gases to keep the core and containment from overheating.”

Says Peterson, “As impressive as these improvements are, all commercial nuclear reactors suffer from low thermodynamic efficiency. That’s because they use water to cool the reactor and steam to power the turbines, and to keep the water pressure reasonable, its temperature must stay relatively low. That limits the transfer of heat energy to the turbines, so much of the reactor’s power is lost as waste heat.”

Most existing commercial reactors are based on naval designs. This is no surprise—one of the most influential people in the early development of nuclear reactors was U.S. Admiral Hyman Rickover, known as the father of the nuclear navy. Commercial nuclear power plants are in many respects beached naval reactors, with turbines spinning generators instead of propellers.

These Generation II and III reactors rely on the pumps and diesel backup generators and pipes that are typical of a ship, and they need a constant supply of water—such as the ocean—to act as the heat sink for the vast quantities of waste heat they produce. Nuclear power plants convert only about one-third of their thermal energy to electricity.

Peterson is exploring a Generation IV design that has more in common with the aircraft nuclear reactor and the liquid fluoride thorium reactors developed at Oak Ridge, TN, in the 1960s. These designs are smaller, run much hotter, and transfer heat to a gas turbine that resembles a jet engine. The higher temperature improves the thermodynamic efficiency to 46 percent and reduces the waste heat to the point that the reactor can be air- cooled. These reactors don’t need to be located near oceans or rivers.

Along with colleagues at MIT and the University of Wisconsin-Madison, in October, 2011, Peterson was awarded a $7.5 million grant from the Department of Energy to pursue this design, the Pebble Bed Advanced High Temperature Reactor (see illustration). The reactor is designed to produce heat at 704°C using an unpressurized molten salt coolant. The fuel is in the form of pebbles slightly smaller than golf balls. The pebbles can be made of a variety of nuclear materials and can burn plutonium and other transuranics from spent fuel.

“Once we are familiar with running a pebble bed reactor with liquid salt cooling,” says Peterson, “the next step will be to put the nuclear fuel directly into the liquid salt, like the experimental reactors at Oak Ridge in the 1960s.

“Liquid fuel has several inherent safety features. If the reactor gets too hot, the fuel itself expands, reducing its density and power output. And these reactors can be built with a melt plug in their base, cooled by an electric fan. If the electricity supply fails, the fan stops, the plug melts and the liquid fuel flows out of the reactor and spreads out in a pan underneath, bringing fission to a rapid halt.

“Another possibility is to use thorium as the fuel. Thorium is much more abundant in nature than uranium, and unlike the uranium fuel cycle, the thorium fuel cycle is a much lower proliferation risk. Although it is in theory possible to build a nuclear weapon from isotopes purified from the thorium fuel cycle, in practice, it would be extremely difficult.”

Generation IV reactors might be operational by 2030. Until then, problems remain with the conventional commercial nuclear reactors, and Peterson works to help solve them as well. He is a member of the 15-person group, the Blue Ribbon Commission on America’s Nuclear Future, empanelled by Secretary of Energy Steven Chu at the request of President Obama. The panel has released a draft report on managing the country’s growing stockpile of spent fuel and high-level nuclear waste in the aftermath of the Yucca Mountain closure.

Along with Berkeley nuclear chemist Heino Nitsche, Peterson will lead a consortium of seven universities that has been awarded $25 million over five years from DOE’s Office of Proliferation Detection. The consortium will focus on education and hands-on training of undergraduate and graduate students in the core set of experimental disciplines that support the nation’s non-proliferation and nuclear security mission.

For Peterson, the future of nuclear power offers both good news and bad. The good news is that “there is no reason to expect that scarcity of fuel would ever in the future limit the ability to use fission as an energy source,” he says. “Current reactor technology uses a little less than one percent of the energy content of mined uranium. There are sufficient conventional uranium supplies so that no one expects shortages or high prices for decades. In the longer term, new reactor technologies could have the ability to use uranium and thorium more efficiently, and if this happens the supply becomes essentially infinite.

“But,” he warns, “two billion people on the planet don’t have reliable access to electricity but want it. And as we transition to electric vehicles, the demand for electricity will skyrocket. Right now nuclear power supplies about 17 percent of the world’s electricity. In the future, nuclear power may have to do more. In order to do so, we have to solve the proliferation and waste problems. If we don’t, the prospects for any effective future action to control carbon dioxide emissions look pretty grim.”