Next-Generation Nuclear Energy Reactors: A Primer

The next generation of nuclear power reactors promises to be safer, more fuel efficient and less water intensive — but the world must wait at least 20 years to see them in action.

Known as Generation IV reactors, the models are “revolutionary” in design, said David Lochbaum, director of the Nuclear Safety Project at the Union of Concerned Scientists. Ironically, though, the reactors’ leading-edge features could end up being the greatest impediment to their initial adoption, he said.

Utilities are likely to be leery to shell out billions and billions without proof of operational success, Lochbaum told SolveClimate News, and that could “slow down the market” for the new designs.

The goal is to make the Gen IV fleet “competitive” on price with today’s plants, said Robert Hill, a senior nuclear engineer at the Department of Energy’s Argonne National Laboratory, though it’s too early to tell if that’s possible. A large nuclear reactor today costs between $4 and $10 billion.

For the moment, at least, the point is immaterial. Gen IV reactors still need a great deal of research and development, and the U.S. Department of Energy estimates they won’t be commercially viable until 2030 at the earliest.

Development of the reactors is led by the Generation IV International Forum, a collaboration of 13 member nations, including the United States, Canada, Japan, China and the European Atomic Energy Community (Euratom). Each country contributes its own share of funding. In total, the forum’s research and development work costs $400 million dollars a year.

Since its establishment in 2001, members have selected six basic designs of Gen IV technology.

Safety Improvements

Most of the world’s existing reactors are Generation II plants designed in the 1970s. Over the past ten years, several countries have built Generation III reactors that are safer and simpler in design, supposedly reducing upfront capital costs.

Many Gen III reactors include “passive safety” mechanisms that get triggered by gravity or other natural forces in the event of trouble, said Hill. A reactor vessel, for instance, might have circulation systems that kick in even when electricity goes out to cool the facility through convection, making the plant less vulnerable to meltdown.

There’s also a subset of reactors called Gen III-plus that can be hard to distinguish from Gen III designs, though generally they have safety enhancements.

Ted Quinn, former president of the American Nuclear Society, an industry group, said one of the Gen III-plus designs includes a “swimming pool,” a reservoir of water positioned high up in the reactor. Since the water can be released through gravity there’s no need to find power for water pumps during emergency situations, he said.

If Japan’s Fukushima Daiichi reactor, a Gen II design, had such safety features, “we could have prevented some of the core damage,” Quinn added.

The Gen III-plus plants also have backup batteries with a 72-hour lifespan, which gives emergency responders three days to restore power. It’s an improvement over older reactors like Fukushima, whose batteries only last four to eight hours, he said.

Four Gen III-plus units are being licensed in the United States. Two are proposed for the Vogtle plant in Burke County, Georgia, and the other two for the V.C. Summer station near Jenkinsville, South Carolina.

Water as Coolant

Despite variations in age and design, most of the pre-Gen IV plants have one thing in common: They all use water as a coolant.

A coolant is the fluid that brings heat from the reactor core to other parts of the plant. The heat boils water into steam, which spins turbines to generate electricity. Then the steam is either condensed by pumping in cold water, or cooled through cooling towers.

Increasingly, reactors’ dependence on water is making them vulnerable to climate change impacts. Nuclear plants have been forced to decrease capacity due to heat waves, while drought and water scarcity are adding new constraints.

The Gen IV reactors could help mitigate that problem. Five of the six designs use hot gas, molten salt or liquid metal as a coolant.

Tim Leahy, senior adviser at Idaho National Laboratory, which is part of the U.S. Energy Department, said vulnerability to drought wasn’t a big concern when developing the Gen IV reactors. As it turns out, most of them operate at very high temperatures, making it impractical to use water as a coolant.

This design change has the advantage of increasing efficiency, said Leahy. Today’s reactors are about 33 percent efficient, meaning that for every three units of thermal energy produced by the reactor core, two units are rejected as waste heat and only one unit gets converted into electricity.

With a new type of coolant and reactor design, the Gen IV plants can reach nearly 50 percent efficiency.

The U.S. has an active role in developing two out of the six Gen IV designs — the Very High Temperature Reactor (VHTR) and the Sodium-Cooled Fast Reactor.

Gas Reactors

David Petti is director of the Very High Temperature Reactor Technology Development Office at the Idaho National Laboratory. He has spent years developing the VHTR, a gas reactor that can use helium to turn the reactor turbines.

These reactors still need water to cool the gas, but they have smaller condensers that suck up “much less water” than current plants, Petti said. 

The design also boasts passive safety features in the form of graphite. In the case of emergency, heat from the nuclear fuel will slowly move into surrounding layers of graphite.

According to Petti, the graphite can hold “a lot” of heat, and over time the heat will transfer onto the reactor vessel that holds the reactor core. From there, it moves into another heat removal system.

“In Fukushima, it didn’t take long before you had high temperatures,” Petti said. “In a gas reactor, the heat moves slowly because of the graphite.”

Petti called the gas reactor a potential “game changer” for energy efficiency. It can achieve 46-50 percent efficiency and runs at very high temperatures, which makes it a good energy source for the chemical industry.

It takes high temperatures to produce plastics, cosmetics and fertilizer. As of 2006, the chemical industry was responsible for 24 percent of total energy consumption in the U.S. manufacturing sector.

Manufacturers currently depend on coal and natural gas plants, but a nuclear gas reactor could provide “carbon-free” heat from a more energy-efficient source, said Petti. “It [would] help us deal with a big piece of the carbon footprint we don’t talk about.”

Some prototype gas reactors are already in use. Japan and China each have a test reactor, and the U.S. operated a commercial gas reactor in Fort St. Vrain, Colorado, during the 1970s and 1980s.

Petti called these reactors “cousins” of the Gen IV design — similar in basic concept but lacking some of the advanced safety features of the Very High Temperature Reactor.

Fast Reactors

Another Gen IV design under development in the U.S. is the Sodium-Cooled Fast Reactor.

The advantage of a fast reactor, said Argonne’s Hill, is the ability to recycle actinides, uranium and other elements that can be reprocessed for fuel.

Current reactors depend on enriched uranium. For every five to seven tons of processed uranium, only one ton ends up as useable fuel. The rest is stored as depleted uranium.

In a fast reactor, even the depleted uranium can be used as fuel, but it requires the construction of reprocessing facilities to recycle the actinides back into the reactor.

The older reprocessing technology, which is currently used on a commercial scale in Japan and France, produces a stream of pure plutonium, and some fear it will encourage nuclear proliferation.

Hill emphasized that the reprocessing facilities built for Gen IV plants would use new technology that doesn’t produce pure plutonium. This emerging technology has been demonstrated at a laboratory scale but hasn’t been commercialized.

In a recent study, researchers at the Massachusetts Institute of Technology showed there is enough uranium to fuel nuclear reactors for the next 100 years.

Both Hill and the MIT report authors believe that reprocessing could be useful in the long run, but there’s no immediate rush to invest in the technology.

As with the Very High Temperature Reactor, the U.S. has run several prototypes of the Sodium-Cooled Fast Reactor. The test reactors are not commercially available. Meanwhile, the Gen IV designs include even greater safety features such as passive decay heat removal systems that remove excess heat from the reactor vessel walls.

Will There Be a Market?

Petti says development of the gas reactor could be finished in 12 to 15 years. Similarly, Hill’s estimate for completion of the Sodium-Cooled Fast Reactor is between 2025 and 2030.

Building Gen IV reactors by 2030 is “achievable” if the only considerations are safety and other technologies, said Gary Vine, an independent consultant who formerly worked at Electric Power Research Institute, a power-sector research organization.

“The big unknown is whether there’s sufficient market for these to be built in large numbers.”

One important factor is the price of natural gas, which has been dropping. If natural gas keeps “ruling” for the next decade, said Vine, it could prove a serious barrier for Gen IV construction.

Lochbaum of UCS believes utilities will be “less enchanted with Gen IV designs because they’re so experimental.” This is where the government could step in and sponsor pilot Gen IV plants, he said, subsidizing extensive testing to help ease the reactors into the market.