Terrestrial Energy – Molten Salt Reactor Designed to Be Commercial Success

There is a growing roster of innovative organizations populated by people who recognize that nuclear technology is still in its infancy. Terrestrial Energy is one of the most promising of those organization because of its combination of problem solving technology, visionary leadership, and strong focus on meeting commercial needs.

Nearly all of the commercial nuclear power plants operating and under construction today use the basic design components of solid fuel arranged into a critical mass that can produce massive quantities of heat in a compact volume. The heat produced in that solid fuel is almost always moved by water, with the heat energy that has been transported from the fuel transformed into steam that turns a turbine generator.

During a relatively brief period of rapid manufacturing, a capable component production infrastructure was established with tooling, processes, quality assurance procedures, and skilled personnel. Regulators learned how to review water cooled reactor designs.

Nuclear engineering departments focused on solid-fueled, water-cooled reactor technology because they recognized there was an established market for graduates who were primed with the applicable knowledge. Even though that period of construction came to a virtual halt, operators have continued to invest in developing their skills in extracting the greatest possible value from the plants that were completed.

All that infrastructure results in a substantial inertia that has encouraged most developers of new nuclear technology to stay with the basic model of prior generations — with some evolutionary improvements based on many decades worth of documented lessons learned.

However, one of the lessons learned from using the conventional technology is that there are certain unavoidable cost and schedule limitations associated with the technological choice. Water does not want to remain in liquid form at the temperatures that are desirable in a steam power plant; the solution is to use high pressures to control the physical state of the water. High pressures mean thick-walled containers and piping; thick walls add to welding challenges, make it difficult to control forming processes and lead to lengthy production cycles due to the need to control rates of heating and cooling to add process inspection stages to ensure all quality standards are met.

There are organizations that have developed procedures, built the necessary tooling, and demonstrated that they can perform the difficult tasks well, but the best in the business have reached a stage where there are only marginal improvements available. When done by the best at a sufficient scale, water-cooled, solid-fueled nuclear power plants can compete against most fossil fuels, especially when they get credit for producing clean power that replaces fossil fuel power that inherently must discharge at least some of its waste materials into the environment.

Unfortunately, the marginal advantage is often not sufficient to overcome the risk that the nuclear plant will not be built by the very best and will suffer schedule interruptions that drive prices into the uncompetitive range.

Innovators like Terrestrial Energy believe there are fundamental choices that can alter the competitive balance. TEI’s choice has been to design a reactor that is more akin to a chemical reactor, with fuel that is a dissolved reactant in a solution (in this case, a salt solution) where the solution provides the transport mechanism for the heat produced in a strongly exothermic reaction. Of course, the reaction in this case is not a chemical reaction; it is a fission chain reaction.

The hot reactor fluid is circulated through multiple redundant heat exchangers sealed into the same container as the reactor. Fluoride salt without any actinides circulates on the other side of the primary heat exchangers to transport the reactor heat to a second set of heat exchangers where water receives the heat and boils into high temperature, high pressure steam.

The salt circuits operate at high temperature but low pressure. Low pressure enables containers that are simpler, cheaper and quicker to produce compared to the containers performing similar functions in a water-cooled reactor.

The heat and pressure conditions in the steam generator are more similar to those in a fossil fuel boiler than those in a pressurized water steam generator.

Terrestrial Energy has chose to operate its molten reactor on low enriched uranium — which it describes as a dry tinder — vice thorium, which is the frequently targeted molten salt reactor fuel. According to TEI’s web page explaining that choice, thorium is analogous to “wet wood” and needs a “torch” like plutonium-239 or highly enriched uranium (either 235 or 233) in order to be lit and sustained.

TEI knows there is plenty of available natural uranium at an affordable cost, and that there is plenty of capability to produce the correct enrichment with the ability to expand capacity as needed. Uranium fuel has a well-established supply chain; using it will simplify licensing. TEI is aggressive about commercialization; it is aiming to simplify both designs and related processes in order to drive down schedule-related costs.

TEI understands that graphite is a well-proven and understood moderator and structural material for high temperature, liquid-fueled reactors, but TEI also understands graphite’s characteristics of storing energy and changing dimensions under a sustained neutron flux. Replacing graphite components would be complicated; designing them to last the lifetime of the reactor would require research and development with uncertain results.

TEI has a solution for that issue in the form of producing sealed reactor/primary heat exchanger units with installed redundancy that will last for roughly seven years before needing to be replaced. Each unit will have a shielded space for two reactor modules, one will be in use and one will be cooling off. The design philosophy is similar to that used in staged rockets; the difference is that TEI will not throw away used reactors; they will contain useful materials that can be recycled when conditions are right for that activity to begin.

TEI has developed conceptual designs for three different power outputs aimed at various niches in the power market, ranging from 29 MWe to 290 MWe. Any of the basic power modules can be combined at a power station to provide a large total output power level.

One of the more important commercial decisions that TEI has made is to put its headquarters in Canada and to plan to use the Canadian performance-based licensing process. That process should take several years less than the one that would be required for a US license. Once there are operating units in Canada, presumably it will be easier to show US regulators how their system works.

Terrestrial Energy has a lot of work to do to achieve their goal of producing power by the early part of the 2020s, but the principals have established a plan that has strong potential for success.

Additional Reading

April 12, 2013 A simple and “SMAHTR” way to build a molten salt reactor, from Canada (Note: TEI’s current design reflects several refinements since this 16-month-old article.)

Corrected copy (9/5/2014) – Based on feedback from TEI’s Chief Technology Officer, this version corrects the secondary fluid from “solar salt” to fluoride salt.

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