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Molinaroli College of Engineering and Computing

  • Molten salt reactor research

USC’s key contribution for developing more flexible and cost-effective nuclear technology

An evolving chemical database project led by Ted Besmann’s research team is helping energy companies design the next generation of nuclear reactors. 

The University of South Carolina’s SmartState General Atomics Center for Transformational Nuclear Technologies is playing a key role in the comeback of a clean energy technology first developed and then abandoned more than 50 years ago. 

From start-ups to industry giants, energy companies are in the process of developing their own brands of molten salt reactors (MSRs), a next generation technology that aims to provide more economical ways to generate nuclear electricity. The Department of Energy is currently funding research in the General Atomics Center, led by Mechanical Engineering Professor and SmartState Chair Ted Besmann, to provide critical information that companies’ and their suppliers need to bring their MSR designs to market. 

Today’s reactors, which currently supply over 50% of South Carolina’s power, use massive amounts of water to cool nuclear fuel rods containing uranium, the heat which produces steam to generate electricity. MSRs combine coolant and fuel into a single, stable mixture of liquid or molten salts, which eliminate thousands of fuel rods and allow plants to be smaller and less costly to operate. Among the U.S. companies interested in designing, building and selling the new models are Bill Gates’ Terrapower and Southern Company, which is partnering with the federal government on a new MSR demonstration.

Although MSR technology was first demonstrated in the 1950s, water-cooled reactors became the dominant model, in part because government-supported research followed a path laid down by nuclear powered submarines. The need to increase the flexibility and cost-effectiveness of reactors has led companies to consider MSRs as a next generation technology, but a current gap in understanding the chemistry of molten salts was a stumbling block. Materials such as chloride salts of lithium, sodium (table salt), potassium and uranium, the formula for one brand of MSR, form a potentially corrosive mixture that becomes more complicated as nuclear fission occurs and new elements are created. 

The solution to the problem of understanding salts is the goal of Besmann’s research team. Since 2018, they have created and expanded a chemical thermodynamic database that companies and government regulators can use to better understand salt behavior. Those enrolled to access the database include 25 universities, 11 U.S. companies, seven national laboratories and the Nuclear Regulatory Commission (NRC).

“We not only take measurements in our lab but also take advantage of others’ work from publications going back up to three quarters of a century to the latest developments in the broader community,” Besmann says. “With this information, we develop thermodynamic functions that allow computer codes to calculate the chemical state of the salt and predict phenomena such as corrosion. Currently in the database, we have around 290 chemical systems.”

Clara Dixon is a Ph.D. candidate in the nuclear engineering program and a member of Besmann’s research team. She says that one of the biggest challenges with the database is the work required to ensure that quality assessments are produced. 

“This is especially true as the database keeps growing and becomes more complicated, and we move into assessing more previously unstudied systems, which requires a lot of lab work,” Dixon says. “The challenge has been overcome by our research group working together effectively, checking each other's work and dividing up tasks, as the database development is a multidisciplinary team effort.”

Prior to arriving at USC in 2014, Besmann was a thermodynamics expert. During his nearly 40-year tenure at Oak Ridge National Laboratory in Tennessee, he led a research group  that studied the chemistry of nuclear fuels and related materials.

“Oak Ridge decided that it was important to have a database for the industry that was reliable and internally consistent. While the database started at Oak Ridge, the home and keeper of the knowledge has always been the USC College of Engineering and Computing,” Besmann says. 

According to Besmann, the database is valuable because the thermodynamic equations can be used with commercial software to calculate the state of a salt and its behavior, such as the volatility of certain components important for safety concerns. Users can take the database and software and combine it with larger computer programs for describing overall reactor properties and behavior, such as heat transfer and corrosion.

“Let’s say someone wants to know the melting point, the state of the salt with sodium, potassium, uranium and fluorides, with the element nickel found in the reactor piping. The software calculates the equilibrium state by taking the energetic functions from the database and finds the lowest energy state, revealing possible reactions” Besmann says. “In other words, it answers questions such as, ‘Will it form a solid or all stay a liquid; Will the piping rapidly corrode, or does it likely have a long lifetime?’”

Traditional water-cooled reactors use nuclear fuel in the form of ceramic uranium pellets stacked in metal tubes to create power. By contrast, MSRs use a mixture of salts such as sodium chloride combined with uranium chloride, which is heated above its melting point, forming a liquid that can be pumped around a loop. As the salt flows around the loop, one section is configured so that uranium can undergo a sustained nuclear chain reaction to generate heat. In another part of the loop, that heat is transferred to a second fluid which boils water and makes electricity. 

The two elements into which uranium splits can be any of up to 60 different elements whose formation is governed by statistical probability. Among the possible elements are cesium and iodine, which are considered radiological hazards due to their volatility and potential for its vapor to be emitted from the reactor in the unlikely event of an accident. 

“One of the goals of the database is to produce fundamental knowledge for understanding, simulating and predicting how molten salt reactors operate,” Besmann says. “This information is important for safety analysis by the NRC, which has ultimate responsibility for ensuring safe operation of plants. The NRC is using our database in their key software for simulating reactor behavior under accident conditions.” 

Besmann says that the database is an ever-evolving library of chemical thermodynamic values. While priority is given to the most abundant and influential elements and systems, the database is extensive because there are different salt combinations that companies are interested in using. 

“The elements we’re concerned with have a particular impact or are in relatively high concentrations. We want to include those because they will influence the behavior of the salt,” Besmann says. “The operator needs to know the chemical state and temperature maximums to control corrosion, to prevent solids from forming and blocking flow.” 

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