Towers of concrete blocks as tall as skyscrapers, a ski lift-like contraption on the side of a mountain, underground caverns — the quest for more ways to back up the increasing amount of wind and solar energy on the grid has recently driven investment toward a number of unusual technologies.
Now, the ability to serve a cleaner but more variable grid has become part of the pitch for a technology that has been around for decades but is still seeking commercialization: molten salt nuclear reactors, which have been promoted for years by investors like Bill Gates, who say that a new version of nuclear power is necessary to provide on-demand, carbon-free energy.
Developers of so-called "advanced reactors" — those that use designs fundamentally different from the light-water reactors that make up the existing U.S. power reactor fleet — want to combine a number of revenue streams into a package for potential customers. These uses include load-following to help the grid deal with intermittent renewable energy, the ability to provide power when not connected to external transmission lines (blackstart capability) and heat for industrial processes.
Advanced reactor developers believe these additional capabilities can provide an escape from the current situation facing the nuclear industry, which is struggling with high capital costs that have made the construction of new conventional power reactors extremely challenging in North America and Europe for the foreseeable future.
“We are innovating to solve what we believe is the biggest limit on new nuclear builds in the west — conventional technology is unaffordable and non-cost-competitive,” Simon Irish, CEO of advanced reactor developer Terrestrial Energy, told Utility Dive in an interview.
The molten salt reactor is just one type of nuclear design but, due to the ability of molten salt to contain and store heat at extremely high temperatures, proponents say it is particularly well-suited to the grid of the future.
“Having the ability to sell ancillary services, the ability to sell heat on the side – that really helps the economics of the reactor,” TerraPower Director of External Services Jeff Navin also said in an interview.
U.K. and Canada-based developer Moltex Energy is working on a design it calls a “Stable Salt Reactor” that the company says could eventually store energy for around eight hours but up to 24, significantly longer than the four-hour duration period that lithium-ion batteries typically provide.
While various developers have been working to get non-light-water reactors off the ground for several years — TerraPower, for example, was founded by Gates in 2006 — the passage of the Nuclear Energy Innovation and Modernization Act (NEIMA) has given the nuclear industry a new boost in confidence that advanced reactors will make significant progress soon. Signed into law by President Trump in early 2019, NEIMA requires the Nuclear Regulatory Commission to create a new regulatory structure by the end of 2027 that will ease the licensing process for new reactor designs. The law “will improve the path to commercialization for advanced reactors that can play an important role in addressing climate change and global energy needs,” the Nuclear Innovation Alliance, a non-profit think tank that supports advanced reactors, said in a statement upon the bill’s passage.
More federal legislative support for advanced reactors is possible. On March 3, the House Committee on Energy & Commerce held a hearing about advanced reactors, and witnesses included TerraPower President and CEO Chris Levesque. TerraPower’s technology allows a molten salt reactor to act “like a giant thermal battery,” Levesque said. These kinds of reactors “will be essential for reducing carbon use in industrial and transportation sectors as we move toward a carbon-free economy,” he told the committee.
Despite the enthusiasm for the technology, commercialization could be many years away. Molten salt reactors still have several technical challenges that must be solved before they can become accepted in the power industry, according to Jacopo Buongiorno, TEPCO Professor of Nuclear Science and Engineering at the Massachusetts Institute of Technology and one of the co-chairs of the MIT Energy Initiative’s study, “The Future of Nuclear Energy in a Carbon-Constrained World.”
What’s more, while it provides a selling point, the ability to store energy doesn’t overcome these hurdles. “The questions that are open are with the reactor itself, not the storage medium,” Buongiorno said in an interview with Utility Dive.
The MIT Energy Initiative study found that while “more mature concepts” like the light-water small modular reactor design being developed by NuScale Power are ready for commercialization well within this decade, “less mature reactor concepts, including lead fast reactors, gas-cooled fast reactors, and molten salt systems, however, would not be expected to reach commercialization before 2050 if the traditional approach to nuclear development is followed.”
The 2050 projection is for an "nth-of-a-kind" reactor, meaning a commercial product that comes after a small-scale demonstration project and a full-scale prototype, referred to as "first-of-a-kind."
Some molten salt reactor developers are targeting the late 2020s and early 2030s for first commercial deployment of their technology — not necessarily "nth-of-a-kind," but a reactor that can be sold to a customer. But according to Buongiorno, even skipping the demonstration or first-of-a-kind step would likely not yield nth-of-a-kind production until 2040.
One of the biggest challenges that extends the timeline is determining the structural materials that would go into a molten salt reactor core, which may be very different than the materials used in conventional reactors because of the completely different chemical environment.
While the Oak Ridge National Laboratory operated an experimental molten salt reactor for several years in the 1960’s, that project was for research and not for producing power, so it only partially answers the many technical questions needed to determine how a molten salt reactor could operate as a power plant, according to Buongiorno. “We don’t know exactly yet how long the materials used for molten salt reactors will last before they corrode,” he said. “Another important issue to be resolved is access and maintenance of the reactor components during power operations and outages, since the molten salts can become highly radioactive.”
How nuclear energy storage could work
Conventional reactors use water as their primary coolant, but molten salt reactors use a liquid salt. That difference has a very significant impact on the temperature at which a power reactor can operate. Molten salt absorbs tremendous amounts of heat. Light water reactors can only safely reach about 300 Celsius (572 Fahrenheit), while molten salt reactors can operate at over 700 C (1,292 F). The absorbed heat can then be used as thermal energy for a number of different applications.
For example, Moltex has a deal with New Brunswick Energy Solutions Corp. to build a 300-MW demonstration reactor for the Canadian province. The developer’s concept for the Stable Salt Reactor involves a molten salt coolant for the reactor, accompanied by a set of additional molten salt tanks that store the reactor’s thermal energy. The heat trapped in the tanks can then be used to drive a turbine to generate electricity.
The goal is to be cost-competitive with existing, mature forms of energy storage, Moltex Energy Chief Executive for North America Rory O’Sullivan said in an interview with Utility Dive. “You don’t need to have expensive battery storage because you have our thermal heat,” he said.
The storage element is supposed to give the reactor-and-turbine project additional flexibility to ramp up or down, fitting with a cleaner grid dominated by variable wind and solar energy. Conventional reactors are typically run as close to around-the-clock as possible to maximize revenue to cover high capital costs and avoid operational inefficiencies that come with cycling up or down.
NuScale is also designing its light-water SMR to have the capability to load-follow renewable energy. But the stored thermal heat feature included in a design like Moltex’s gives another lever to pull to meet changing grid demands without having to ramp down the reactor. “It doesn’t make economic sense to ramp down. It means you are not getting the maximum MWh out of your plant,” O’Sullivan said. “You can’t do this with [light-water] nuclear power because the temperatures aren’t high enough.”
Ultimately, Moltex believes a fully commercial 1,000-MW stable salt reactor, combined with a 3,000-MW turbine driven by the thermal energy produced by the reactor, could have combined capital costs of under $1,000 per kW. Units 3 and 4 that are under construction at the Vogtle nuclear power plant in Georgia, the first brand-new power reactors built in the U.S. in decades, are currently estimated to cost $25 billion for a combined 2,200 MW, or more than $11,000 per kW.
Terrestrial Energy’s Integral Molten Salt Reactor (IMSR) design also uses a salt-cooled reactor core. Heat is captured by molten salt and circulated by a pipe loop to transport thermal energy to sites up to five miles away. The heat can drive a turbine to provide blackstart capability or renewable load-following, but can also serve energy-intensive industries like chemical production that might be co-located near a reactor. Irish said that the company does not have a specific estimate of the potential duration of energy storage from the IMSR.
Unlike the Moltex design, which uses high-level nuclear waste as its fuel, the IMSR uses the same uranium fuel used in current operating reactors. The fuel is part of a “strategy we believe creates the clearest path to market,” Terrestrial Energy CEO Irish said, which is to innovate in terms of the cost, operational flexibility and safety of the reactor, but to still have a recognizable product with which regulators and customers can be comfortable. Using conventional uranium means the same fuel supply chain is in place for any utility that adopts an IMSR.
The developer aims to make the IMSR commercially available by the end of the 2020s, and in the meantime has explored partnerships to demonstrate use cases for the combination of heat and nuclear power. Terrestrial Energy has worked with Southern Co. on a project to study the prospects of using heat produced by nuclear power to produce hydrogen more cleanly and efficiently.
TerraPower has also partnered with Southern Co. Beginning in 2019, the developer and utility started experimenting with a test molten salt loop to show how it could store energy. One of TerraPower’s designs, the Molten Chloride Fast Reactor, uses molten chloride salt as both fuel and coolant, and intends to create heat for industrial uses and thermal energy storage along with electricity.
TerraPower did not provide a specific estimate of the duration of energy storage the reactor could offer, but Navin pointed to existing examples of molten salt systems that are already being used to store thermal energy. For example, Cerro Dominador, a concentrated solar power and photovoltaic plant with molten salt storage that is currently under development in Chile, "is slated to operate off of stored energy for up to 17.5 hours at 110 MWe of generation, or just under 2 GWhe of storage," he said.
TerraPower sees its design as appealing to electric utilities that are trying to meet decarbonization goals and need a source of on-demand power that can also load-follow and meet peak electricity demand, Navin said.
Utilities have been embracing lithium-ion batteries to back up renewable energy in order to meet emissions reductions goals, aided by declining costs of batteries. While battery storage is available now, TerraPower does not expect commercialization of the MCFR until the late 2020s or early 2030s.
But with more and more utilities, cities and states adopting goals of net-zero carbon emissions by 2050, the more options to back up renewables, the better, Navin argues. “Batteries are going to continue to get better,” he said. “But if you look at what it takes to decarbonize the power sector, it’s a massive transformation. We need multiple shots on goal, all tools in the toolbelt.”