Smallest Size of Molten Salt Reactors: Design, Economics, and Applications

Molten salt reactors (MSRs) have garnered significant attention due to their potential to significantly enhance nuclear power generation. Unlike traditional reactors, MSRs can be designed to be relatively compact, making them suitable for various applications. This article delves into the smallest size an MSR can be, considering its design, economics, and applications.

Design Considerations for MSRs

The size of an MSR can vary based on several design factors. Factors such as fuel type, enrichment, heat transfer requirements, and safety systems sway the overall dimensions of an MSR. The fuel type, for instance, thorium or uranium, may necessitate different reactor sizes depending on the type of fuel and its level of enrichment. Efficient heat transfer within the reactor system is crucial and can also influence the reactor size.

Small Modular Reactors (SMRs)

Many MSR designs fall under the category of small modular reactors (SMRs), which typically have a thermal power output of less than 300 megawatts (MW). Some designs aim for even smaller outputs, ranging from 10 to 50 MW, which could be suitable for localized power generation or remote applications.

Experimental Designs

Some experimental MRS have been designed at even smaller scales, potentially in the range of a few megawatts or less, primarily for research purposes. While there is no strict lower limit, practical designs for operational MSRs usually range from about 10 MW to several hundred MW.

Real-World Examples: The IMSR Product Line

Terrestrial Energy, a company that specializes in MSRs, offers a product line with various sizes, each with distinct characteristics and economic implications.

Terrestrial IMSR Specifications

Terrestrial's IMSR product line includes the following specifications:

Big: 600 MW thermal and 300 MW electric power, reactor core cost of $0.65/W, fuel cost of 2 U235 Medium: 300 MW thermal and 150 MW electric power, reactor core cost of $2.00/W, fuel cost of 3 U235 Small: 80 MW thermal and 35 MW electric power, reactor core cost of $5.00/W, fuel cost of 4 U235

The economics degrade as the reactor size decreases, making the small IMSR roughly 8 times more expensive per watt and nearly twice the fuel cost. The small IMSR is also 1/40th the size of a large Pressurized Water Reactor (PWR) or Boiling Water Reactor (BWR).

Economic and Safety Implications

According to Dr. David LeBlanc, Terrestrial's CTO, the small IMSR is 35 MW electric, which can be problematic for the Canadian government. However, if the output is less than 35 MW, the options include running the reactor at lower power to extend its life or not pursuing nuclear power as a solution.

Nuclear power in general has a critical mass issue, where larger reactors use more fuel proportionally but offer better economic returns. MSRs, with their inherent better neutronic properties, can be viable even in smaller sizes. The large IMSR with 2 GW thermal and 1 GW electric power is cost-competitive with cheap US gas and is small enough to predict that 2/3 of orders will be for the larger model.

Before the flack comes in, it's worth noting that Terrestrial's large IMSR is a paper reactor that has undergone several reviews and certification. CNSC, Canada's NRC, is currently adopting a performance model for small reactors, even those considered large by CNSC's standards. Terrestrial needs to prove safety parameters but doesn't have to do so in a specific way, making their reactor order of magnitude more likely to succeed in 10 years than any US designs unless they seek CNSC certification and abandon US manufacturing and sales until the US NRC changes its regulations.

Applications for MSRs

MSRs, due to their compact size, are particularly suitable for niche markets. Terrestrial's IMSR is highly focused on the process heat market. With an outlet temperature of 600°C, their reactors can provide heat for various industrial processes, including shale oil extraction, oil refining, hydrogen/ammonia production, and other high-temperature applications. The smaller IMSR can be re-created to 800°C using a fraction of the gas to heat it to the required temperature.

Conclusion

The smallest size of an MSR is influenced by several factors, including design, economics, and safety. Terrestrial's IMSR product line showcases the viability of smaller MSRs, despite the economic challenges associated with their implementation. As MSRs continue to evolve, they are poised to play a significant role in the future of nuclear energy generation and industrial processes.