More than a quarter of the world's carbon dioxide emissions come from burning fossil fuels to produce heat and electricity. In the United States, the Department of Energy projects load growth of 22 percent by 2040.
Meanwhile, greenhouse restrictions are poised to go into effect under the Obama administration's Clean Power Plan.
In order to meet that growing demand while hitting a greenhouse gas reduction target of 26 to 28 percent below 2005 levels by 2025, a chorus of voices from industry and think tanks is calling for a nuclear energy renaissance in the United States and around the world.
But old fears and new economic realities stalk the nuclear industry, sending innovators back to the drawing board to come up with better designs.
The argument for nuclear energy, especially in the context of climate change, remains robust in many ways: Nuclear fuel is among the most energy-dense fuels known to humanity. One kilogram of coal can keep a 100-watt light bulb lit for four days, while the same amount of uranium nuclear fuel can keep it lit for 140 years without carbon dioxide emissions.
Because of its limited pollution and the gobs of electricity it produces, nuclear energy is also the safest power source on a per-kilowatt basis.
These promises fueled the freewheeling postwar era of nuclear ambitions, when the Air Force proposed nuclear aircraft and Detroit talked about nuclear cars. National laboratories like Argonne and Oak Ridge flirted with all sorts of reactor designs, but the massive research and development effort from the Navy led to the dominance of light water reactors.
Reactor boom hits roadblocks
The "light" in light water reactors refers to the hydrogen isotope in a water molecule. Light water, like the kind you drink, uses hydrogen that has a single proton in its nucleus. Heavy water has at least one hydrogen atom in the molecule with a neutron added on, increasing its weight.
During a typical nuclear fission reaction in a power plant, an isotope of uranium, U235, splits apart, resulting in two fragments and ejecting two or three neutrons. These neutrons go on to strike other atoms, splitting them and ejecting more neutrons. This chain reaction generates heat that can boil water to spin a turbine.
However, uranium's most common isotope, U238, absorbs neutrons without splitting and quenches nuclear reactions. To make uranium useful as a fuel, engineers must increase the ratio of U235 to U238 so enough material is present to propagate the reaction. This is a tedious and energy-intensive process known as enrichment.
Controlling the reaction requires a substance to act as a moderator, which slows down neutrons so that they propagate the reaction at a stable controlled rate. A heat transfer fluid helps move heat away from the reactor to generate electricity and to keep the process at a stable temperature. To stop the reaction, power plants typically insert graphite between the solid uranium fuel rods.
In a light water reactor, water acts as both a coolant and a moderator for high-energy neutrons, reducing cost and complexity. These designs use enriched uranium as fuel. Heavy water reactors, on the other hand, can use cheaper, lower-grade fuel, but have the added expense of producing heavy water.
The bulk of the world's nuclear power plants run on some version of the light water reactor, now in its second generation.
In the United States, there was a surge in nuclear construction starting in the 1950s, with more than 100 reactors constructed, providing 20 percent of the nation's electricity and 65 percent of carbon-free generation. But then nuclear development and deployment stalled as its downsides became impossible to ignore.
For one thing, the nuclear industry was notoriously unreliable with its cost and delivery schedules, as price tags ballooned and deadlines passed during the construction of these complicated machines.
Another problem is that nuclear power produces highly radioactive waste that remains dangerous for thousands of years. Disposal remains politically contentious, as evidenced by the back-and-forth in Congress over the Yucca Mountain nuclear waste repository in Nevada (E&ENews PM, May 21).
The accident at Pennsylvania's Three Mile Island nuclear plant poisoned nuclear energy's image in the eyes of the American public. Opposition to new nuclear plants surged, and new reactor construction stalled for more than 30 years.
Avoiding another Fukushima
Now fears have faded and a new generation of nuclear engineers and scientists is leading the charge, spurred by the need for low-carbon energy. And while reactor construction was on hold, enabling technologies like better materials and computer simulation tools have opened the door to new third-generation designs that improve safety and cut costs.
"The majority of Gen 3+ machines are still light water reactors," said Mark Peters, associate laboratory director for energy and global security at Argonne National Laboratory. "They have more advanced designs in terms of safety systems as such."
In particular, third-generation designs use passive systems to improve safety. The nuclear disasters at Three Mile Island, Chernobyl in Ukraine and Japan's Fukushima Daiichi plant all suffered when circulation pumps shut off and cooling water boiled off, allowing fuel and waste to overheat and melt down.
Third-generation reactors are engineered to avoid such a scenario.
One such design is the AP1000 from Westinghouse Electric Co. (the "AP" stands for advanced passive). In 2012, the U.S. Nuclear Regulatory Commission approved construction of the design, leading to the first new reactors in the United States since 1979.
Two AP1000 reactors are under construction at Vogtle Electric Generating Plant near Waynesboro, Ga., supported in part by $8.3 billion in federal loan guarantees with a projected total cost of $14 billion. They are expected to go online in 2018 and 2019 and will be operated by Southern Nuclear, a subsidiary of Southern Co.
"The AP1000 was selected because it builds on proven nuclear technology, while also incorporating improvements in that technology," John O'Brien, senior communications specialist at Georgia Power, a utility owned by Southern, said in an email. "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 containment from overheating."
What this means is that if a cooling pipe breaks or computer systems shut down, gravity and convection will kick in to keep the reactor core cool as the nuclear reaction coasts to a stop without any input from an operator.
O'Brien added that the AP1000 is constructed in such a way to curb construction costs and reduce maintenance needs.
Envisioning water-free reactors and new fuels
Another competing third-generation design is the EPR, which initially stood for European pressurized reactor. The design was a collaboration between French and German firms. Two units are under construction in China, one in Finland and one in France.
EPR also incorporates passive safety features and can run on fuel enriched to just 5 percent. It uses 17 percent less fuel per unit of energy than conventional second-generation designs. However, the current projects have all suffered cost overruns and delays.
Small modular reactors (SMRs) are yet another third-generation approach to nuclear power. The idea is to refine existing reactor designs and build them on smaller scales, thereby reducing the upfront costs of building a nuclear power plant and shrinking the time needed to build it.
"Instead of a huge 1,200-megawatt reactor, they are providing a smaller 300-MW reactor," explained Samuel Brinton, a clean energy fellow at Third Way, a think tank. The smaller size also makes it easier to match new construction to demand projections, allaying concerns for investors.
Third Way recently issued a report that found that the private sector is showing interest in nuclear power, with $1.3 billion in investments. Much of this is focused on approaches even further down the road, toward fourth-generation reactors.
"We're definitely looking at the current AP1000s, EPRs and SMRs as bridging technologies," Brinton said. "You are not just going to jump to a new type of reactor; you need training and support systems in place."
Fourth-generation reactor proposals look very different from current nuclear generators. For one thing, most eschew water altogether, instead relying on pressurized gases, molten salt or liquid metal to keep components cool. These coolants allow reactors to operate at higher temperatures and have low risks of boiling off.
The fuels are also different. More efficient designs mean fourth-generation reactors can use lower-grade fuel. Transatomic Power is one nuclear startup company developing a molten salt reactor that runs on nuclear waste from other nuclear power plants. TerraPower is engineering a reactor that would run on existing stockpiles of depleted uranium, converting it into fissile fuel inside the reactor.
Other companies are looking at entirely new fuel cycles like thorium, which eliminates the risk of nuclear weapons proliferation. However, a new fuel cycle means starting from scratch when it comes to infrastructure, engineering and know-how, so it will be years before thorium catches on.
Getting past the advanced design 'hurdle'
A big problem for the industry now is testing these approaches and getting regulations to catch up.
"The NRC is exceptionally good at regulating the light water reactors that we have," said Ashley Finan, advanced energy systems project director at the Clean Air Task Force. "They don't really have the opportunity to build up expertise in advanced designs. That's really a hurdle."
In May, officials from the advanced nuclear industry testified before the House Science, Space and Technology Committee and called for the Department of Energy to invest in reactor test beds at national laboratories (E&E Daily, May 14).
As many American nuclear plants approach retirement, upcoming greenhouse gas restrictions may make utilities weigh whether to extend lifelines to existing plants, invest in new reactors or make up the shortfall with other low-carbon sources. "At the highest level, it has to do with economics: What does it take to keep the machine within licensing phases and safe?" said Argonne's Peters.
Meanwhile, wind and solar power continue to advance in performance and drop in price. Energy storage is also gaining traction on the grid, and natural gas prices are scraping record lows. In this market, the nuclear industry can't afford another hiatus from research and development if it wants to remain competitive.
"By 2025, we can have operating commercial Gen 4 reactors," said Third Way's Brinton. "We're going to have more competition that will push them to lower their costs. They will have to keep innovating or they will be left behind."