Kirk Sorensen of Flibe Energy described the central role that fluorination plays in the handling of fission products in molten-salt reactors. New fluorination technology may resolve previous challenges, at the 4th MSR Workshop at Oak Ridge National Laboratory, on October 4, 2018.
U.S. Department of Energy is funding new research into liquid fluoride thorium reactor (LFTR) technology. LFTRs generate nuclear power with thorium carried in a solution of molten fluoride salts, a technology advocates say is safer and more efficient than conventional uranium reactors. Flibe Energy will receive $2.1 million from DOE and $525,500 from other sources to study the use of nitrogen trifluoride to remove uranium from the nuclear fuel solution.
In a conventional solid-fueled reactors, the consumption of fuel, and the degradation of cladding material are generally the reasons the reactor must be shut down for refueling rather than the buildup of fission products.
Long-term Operation of molten-salt reactors
In a fluid-fueled molten-salt reactor, the potential exists to refuel the reactor during operation by adding fissile material to the fuel salt. The cladding degradation issue does not apply. Molten-salt reactors that use fluoride salts as the chemical medium are impervious to radiation damage in the fuel itself, due to its ionically-bonded nature. This leaves fission product buildup as the only real threat to the long-term operation of the reactor.
Reductive extraction of fission products increasingly appears to be the most attractive suggested way to manage the long-term buildup of fission products in the fuel salt, especially if lithium metal is used as the reductant. Because lithium is one of the constituents of the FLiBe salt that makes up the solvent into which nuclear fuel is dissolved in the reactor, its addition over time will not be detrimental and more easily managed than a foreign species such as cerium. The metallic lithium can be alloyed with metallic bismuth to carefully manage lithium’s introduction into the fuel salt; bismuth is immiscible with the fluoride fuel salts that are generally favored for molten-salt reactors.
Flibe Energy proposes to evaluate is the use of a fluorinating/oxidizing agent to convert uranium, typically UF4 found in a liquid fluoride reactor to its gaseous state UF6. Depending on the fluorination/oxidizing agent and temperature, other actinides will also be fluorinated and/or oxidized from a trivalent or tetravalent state. Neptunium and plutonium do form volatile hexafluorides but plutonium hexafluoride is thermodynamically unstable. If fluorination could be undertaken prior to an attempt at reductive extraction, the uranium, neptunium, many of the transition metals, and non-metals present in the salt could be largely removed and reductive extraction could be employed much more productively to remove fission products.
The appeal of fluorination as a technique for the removal of uranium from fluoride fuel salt has been noted for many years and fluorination formed an integral part of most of the chemical processing flowsheets that were developed at Oak Ridge National Laboratory under the Molten-Salt Reactor Program from 1957 to 1976. Fluorinators were envisioned at a variety of locations in the chemical processing, universally under the assumption that they would remove uranium from the fuel salt. Despite the prevalence of fluorination as an envisioned chemical processing technique, the actual amount of development that was undertaken on continuous fluorination was surprisingly small.
Fluorination to remove uranium from molten salt fuel
Batch fluorination was utilized to remove uranium from the fuel salt of the Molten-Salt Reactor Experiment (MSRE) in 1968, but this was done in the drain tank of the reactor vessel and led to the introduction of a significant amount of corrosion products. Repeated fluorination of the MSRE fuel salt in this manner would have undoubtedly led to the structural failure of the drain tank due to corrosion.
But the aggressiveness of F2 led to many practical engineering challenges in the development of a continuous fluorination system. To protect the fluorinator from F2 attack, ORNL engineers envisioned using an extensive interior cooling system to freeze a layer of salt on the fluorination column’s inner surface. A fuel salt containing fresh fission products has considerable internal heat generation that can be opposed by a cooling system to form a frozen wall on the interior surface of a fluorination column. But a chemically-similar simulant salt, such as LiF-BeF2-UF4, where fission products are replaced with stable isotopes, has no such internal heat generation term. It was necessary to simultaneously heat the salt internally, to simulate the heating effect of fission product decay, while cooling the wall of the fluorinator to generate the frozen wall. Thus testing the frozen wall of the fluorinator under these conditions was very difficult. This was never satisfactorily resolved during the Molten-Salt Reactor Project.
In the years since the MSRE concluded in 1976, alternative fluorination agents have been advanced for consideration. Most notable among these is NF3. NF3 has been considered for rocket propulsion and is extensively used in the electronics industry to clean and etch microelectronic silica chips. It is minimally hazardous and not corrosive at temperatures below 70C and is likely less corrosive than other fluorinating agents. It is not known to react with moisture, is thermally stable at room temperature, and has been demonstrated by PNNL to be an effective, thermally tunable fluorination/oxidation agent for spent nuclear fuel constituents. By controlling the treatment temperature, NF3 will selectively fluorinate/oxidize spent nuclear fuel constituents. The different temperature sensitivities and NF3 concentration effects for the fluorination/oxidation of the different constituents potentially provides mechanisms to effect separations of the volatile fluorides.
The hazard level and chemical reactivity attributes potentially make NF3 a very attractive fluorinating/oxidizing agent for managing the composition of the fuel salt in a liquid-fluoride reactor where uranium is the dominant or even exclusive fissile material. Fluorination/oxidation of the fuel salt with NF3 would produce UF6 and remove uranium from the salt. Reductive extraction could then be employed to remove non-volatile fission and activation products from the salt. Hydrogen could be used to reduce UF6 back to UF4 and reconstitute the salt for return to the reactor.