Molten Salt Reactors


March 2011

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The NB-36 made a number of flights in the 1950s carrying an operating nuclear reactor. The crew worked from a lead-shielded cockpit.

As America’s nuclear renaissance matures, proponents of molten-salt reactors (MSRs) increasingly expect the focus to shift their way, to a much simpler design. That design translates into lower capital costs, speedier construction, and fewer potential operating risks. Together they point to the likelihood of less-grueling regulatory processes for approvals and operating licenses, if and when MSRs get that far.

Not everyone is content to wait for the spotlight to shift. A small Canadian company, Ottawa Valley Research Associates Ltd., (OVRA) has filed for a broad MSR patent in the U.S. and internationally. The applications cover a redesign in reactor piping that addresses some long-standing technical problems.

The applications were filed by David LeBlanc, principal of OVRA and a physics researcher at Carleton University in Ottawa, Ont., Canada. The patent applications are to protect OVRA’s intellectual property or “IP,” arising from LeBlanc’s years of MSR research. “In almost any business case, it is the IP developed with the product itself that has the real value,” he noted. OVRA’s U.S. patent application was filed in May 2008. The international patents were filed for in November 2009. “There has been no opportunity like this since [John D.] Rockefeller gobbled up the entire oil industry” in the late 1800s.

LeBlanc is squarely in the mainstream of nuclear innovation as laid out by the Generation IV International Forum (GIF). MSRs are one of GIF’s the six chosen “Gen-4” technologies. Launched in 2001, GIF members include the U.S., the U.K., Canada, France, China, Japan, Russia and the European Atomic Energy Community (Euratom). Gen-4 goals are to improve nuclear safety, better resist proliferation, minimize waste and overuse of natural resources, and decrease costs overall. “MSRs have a great deal to offer for all of these goals,” LeBlanc pointed out.

Researchers designed a molten salt reactor for use on nuclear-powered bombers. Heat from the reactor would replace the combustion of fuel within the jet engines.

MSRs use molten salts for coolant rather than water. Getting rid of water allows MSRs to operate at 500 °C to 1,000 °C (roughly 900 to 1,800 °F) for greater efficiency. Water that cools current reactors is about 275 °C to 315 °C, or roughly 530 °F to 600 °F). Lower operating pressures mean no need for large pressure vessels or massive concrete and steel containment structures. Coolants in other low-pressure Gen-4 reactors include sodium and lead / bismuth.

Other MSR advantages:

  • Simpler fuel cycles, with the fuel dissolved in the fluid salt.
  • Lower capital costs, possibly 25 to 50 percent less than for today’s reactors.
  • “Burning” used fuel from conventional reactors, a solution to the problem of storing “spent” fuel rods.
  • Near zero long-lived radiotoxicity of MSR wastes—one-ten-thousandth that of current light-water reactors, which means no need for Yucca Mountain-type repositories.

 

In contrast to today’s ubiquitous light-water reactors, Gen-4s would be simpler to engineer, win speedier regulatory approval, get built sooner, and be simpler to operate. Many Gen-4s could also be built smaller, just a few hundred megawatts, making them feasible for large–scale, heat-driven industrial processes. Today’s power-generation units are well over 1,000 megawatts.

With Gen-4, “suddenly, the almost forgotten MSR technology was a hot research topic,” LeBlanc said. But it will not be a free lunch. Gen-4 is long-term internationally coordinated R&D with demonstrations in 2020-2030. Challenges include materials—high-temperature, corrosion-resistant metal alloys; ceramics; nuclear-grade graphites and composites; and nano-structured ferrites, according to the U.S. Department of Energy (DOE).

For the past 30 years, U.S. government backing and investment went to water-cooled reactors for generating electricity. Today U.S. utilities operate just over 100 reactors. They and the Navy’s aircraft carriers and submarines account for virtually all U.S. reactors.

A small but growing effort continues at the DOE’s Oak Ridge National Laboratory in Tennessee. That focuses mainly on the cooling capabilities of molten salts for solid fuels, and the requisite plumbing, rather than true MSRs.

In the past, said LeBlanc, that plumbing had been “a nightmare of interlaced fuel and blanket salts that caused Oak Ridge to abandon an otherwise promising two-fluid concept in 1968. It was an extreme engineering challenge.” He explained that his patent application covers “surprisingly simple solution to the plumbing, a tube within a tube: a large blanket salt tube enveloping a long and narrow core.”

Looming over everything nuclear in the U.S. is the protracted American regulatory process. “The robust, inherent and simple-to-understand safety of these reactors suggests that if given a rational regulatory overview,” LeBlanc said, “they may prove relatively simple to license.”

The other five Gen-4’s are very high temperature gas, sodium-cooled fast, supercritical water-cooled, gas-cooled fast and lead-cooled fast. Fast refers to a portion of the neutron spectrum. Improvements to existing reactors 2000 and later are considered third generation. Today’s operating units, 1970-2030, are second generation.  The first generation was 1950-1970 prototypes and demonstration units.

[Adapted from “Too Good to Leave on the Shelf” by David LeBlanc, for Mechanical Engineering, May 2010.]

For the past 30 years, U.S. government backing and investment went to water-cooled reactors for generating electricity.

 
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by Jack Thornton