Reactors worth their salt

Why molten salt reactors could offer a revolution in clean energy.

The world has seen the greatest decline in global poverty in the past few decades. Since 2010, over 100 million people have been connected to the electricity grid every year.

This article was originally published in Power Engineering International, Issue 1-2021. Read the mobile-friendly Digimag or subscribe to receive a print version.

This growth was mainly achieved through the increasing reliance on fossil fuel burning. Various quality of life indicators in all countries suggest that an average total energy consumption of around 5 kW per person is required for an adequate standard of living.

Taking into account expected population growth, increasing the world population from poverty to modern comfort is likely to require global energy consumption that is three to five times our current consumption.

At the same time, we need to significantly reduce our production of greenhouse gases in order to avoid the catastrophic effects of climate change. Since the growth in energy use is taking place in developing countries that cannot afford to pay for too much energy, this rapid increase in energy production must fundamentally be cheaper than fossil fuels.

Here we have come to the dilemma of sustainability. In order to preserve modern civilization globally, we need an energy source that is emission-free, scalable, reliable and cheaper than coal – and we need it quickly. In the early days of the nuclear age, it was believed that light water reactors (LWR), which make up most of the current nuclear fleet, were not scalable because of
the perceived uranium shortage. Since then we have found that uranium is much more abundant, and instead of switching to various advanced (and more efficient) reactor concepts, we have stuck to the latest technology. Perhaps now is the time to rethink nuclear power.

Molten Salt Reactors (MSRs) offer a new approach to industrial fission performance. The nuclear fuel is dissolved in a high temperature alkali halide melt such as lithium fluoride or sodium chloride and circulates around the primary circuit between the fission core and the primary heat exchangers.

This has several fundamental advantages over existing LWR technology. First, the system provides high temperature heat in excess of 600 ° C, roughly twice the LWR outlet temperature. Second, the molten salts remain liquid at atmospheric pressure up to ~ 1400 ° C, allowing the system to operate at low pressure. Third, the pairing of alkali metals with halogens results in strongly bonded compounds that leave no chemical energy for rapid reactions.

Fourth, molten salt is an ionically bound liquid that, unlike covalently bound solid fuels, does not suffer radiation damage, so the fuel does not degrade over time.

Fifth, the molten salt is precisely the chemical medium that is suitable for distributing and extracting valuable elements, especially isotopes for medical and industrial applications. Sixth, the liquid fuel is continuously mixed and homogenized as it circulates around the loop, eliminating potential hot spots. Seventh, the molten salts dissolve uranium, thorium, and plutonium, providing fuel flexibility. Finally, in the event of a station power failure, the liquid fuel is much easier to cool, eliminating the risk of core meltdowns.

In short, MSRs provide a high temperature, low pressure system that can be fueled while in operation with relatively cheap thin-walled structural components, with no chemical or pressure drivers to propagate the radionuclides, with no restrictions on fuel life, and potentially completely passive for safety and the possibility of additional sources of income from valuable medical isotopes .

Additionally, it can consume problematic actinides from currently spent nuclear fuel or weapon grade plutonium supplies.

The technology

The basic technology of molten salt reactors was developed in the 1950s and 70s at Oak Ridge National Laboratory (ORNL), where three MSRs under the direction of Dr. Alvin Weinberg were constructed and operated. He coined the MSR promise as “burning stones” in “a pot, a pipe and a pump”.

The most notable system was the Molten Salt Reactor Experiment (MSRE), a graphite moderated fluoride salt reactor that operated in 1965-69 and served as a small 8 MW technology development platform. At the end of this operation, ORNL researchers were convinced of the merits of the technology, developed several concept designs for demonstrators on a commercial scale, and should continue construction in the 1970s.

Instead, due to changes in government funding priorities, the ORNL-MSR program died. In 2001 the MSRs were recognized by the Generation-4 International Forum as one of the next generation reactor concepts with improved safety, cost and efficiency.

Since then, interest in MSR technology has increased significantly internationally, especially in North America, Europe, Russia and China. The Chinese program is the most ambitious. Officially launched in February 2011 at the Shanghai Institute for Nuclear and Applied Physics (SINAP), it managed to replicate most of the previous ORNL technology. In 2017, SINAP researchers announced a target for their first protesters by 2020, which has apparently been delayed by the COVID-19 pandemic.

Big variety

Several private companies were formed in the US and Canada after 2010, many of which aim to develop demonstrators by the end of this decade. There is a wide variety of I&C concepts followed by these developers.

Flibe Energy is developing a two-liquid thorium breeder, the Liquid Fluoride Thorium Reactor (LFTR), which was the original goal of the ORNL program. Thorcon Power is aiming for a system most similar to the MSRE but incorporating shipyard construction and targeting Indonesia as the first market.

Terrestrial Energy is developing the Integral Molten Salt Reactor (IMSR), which integrates the reactor and heat chargers into a sealed, replaceable vessel. Terrapower, supported by Bill Gates, is working on a chloride salt powered reactor without a moderator, the Molten Chloride Fast Reactor (MCFR).

Kairos Power uses a fluoride salt to cool solid fuels in the form of pebbles, which were first invented for high-temperature gas-cooled reactors. This is not an exhaustive list, but rather shows the variety of technologies that are being pursued due to various technical tradeoffs.

Nuclear power is one of the most regulated areas of human activity for good reason. These regulations are tailored to the existing technology. Developing new regulations requires up-to-date experts. Melted until recently
Salt technology was neither taught nor known in the field of nuclear engineering. Nuclear reactor physics modeling tools did not consider flowing fuel.

We need a better understanding of the chemistry and thermophysical properties of hot molten salts with dissolved actinides and fission products.
The compatibility of molten salts with structural materials must be demonstrated and methods for accounting and control of fissile material must be in place
Developed for liquid fuels to meet international and national safety requirements.

Valves, flanges, pumps, exhaust systems, various sensors and detectors need to be designed and demonstrated. Universities, national laboratories and
The MSR developers develop the necessary expertise, tools and manpower with funds from the private and US Department of Energy. However, we already built and operated MSR systems in the 1950s and 60s. The real challenge is to be economically competitive with fossil fuels. Many believe that this is the best realistic option.

Sustaining our technological civilization will likely require a new source of energy that is vastly better than what is currently available. MSRs offer a novel approach to fission, high temperature heat provided by an inherently safe, low pressure system, the ability to deplete unwanted actinides in the spent fuel and plutonium supplies, and the production of desirable medical isotopes. Passive safety, high-temperature heat generation and the recycling of nuclear waste are important factors that contribute to both economic efficiency and public acceptance.

About the author

Dr. Ondrej Chvala is a research fellow in the Department of Nuclear Engineering at the University of Tennessee. He teaches Introduction to Energy Science and Technology, Nuclear Reactor Theory, Numerical Methods and Fortran and initiates and teaches a study abroad class in the Laboratory for Experimental Reactor Physics in Prague. Dr. Chvala’s current research focuses on the modeling of molten salt reactors, including depletion simulations, chemical control, nuclear protective measures and
System dynamic modeling.

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