Molten-Salt Technology and Fission Product Handling Kirk Sorensen Flibe Energy, Inc. ORNL MSR Workshop October 4, 2018 2018-10-16 Hello, my name is Kirk Sorensen and I’d like to talk with you today about fission products and their handling in molten-salt reactors. One of the things that initially attracted me to molten-salt reactor technology was the array of options that it gave for the intelligent handling of fission products. It represented such a contrast to solid-fueled systems, which mixed fission products in with unburned nuclear fuel in a form that was difficult to separate, one from another. While my focus will be on our work on molten-salt reactor fission product handling, many of the principles are general to molten-salt reactors as a whole. Fundamental Nuclear Reactor Concept In its simplest form, a nuclear reactor generates thermal energy that is carried away by a coolant. That coolant heats the working fluid of a power conversion system, which generates electricity from part of the thermal energy and rejects the remainder to the environment. coolant working fluid fresh fuel electricity Power Nuclear Heat Conversion Reactor Exchanger System spent fuel heated water or air coolant working fluid The primary coolant chosen for a nuclear reactor determines, in large part, its size and manufacturability. The temperature of the coolant determines the efficiency of electrical generation. Fundamental Nuclear Reactor Concept In its simplest form, a nuclear reactor generates thermal energy that is carried away by a coolant. That coolant heats the working fluid of a power conversion system, which generates electricity from part of the thermal energy and rejects the remainder to the environment. coolant working fluid fresh fuel electricity Power Nuclear Heat Conversion Reactor Exchanger System Fundamental Nuclear Reactor Concept spent fuel heated water or air coolant working fluid The primary coolant chosen for a nuclear reactor determines, in large 2018-10-16 part, its size and manufacturability. The temperature of the coolant determines the efficiency of electrical generation. All fission reactors turn fissile material like uranium or plutonium into an array of fission and activation products (such as plutonium or nep- tunium). These fission and activation products have a wide variety of physical and chemical properties. 2018-10-16 Some are noble gases, some are metals, some are volatile at the op- eration temperature, and others remain part of or dissolved in the fuel matrix. At the moment of their formation, they are furiously radioac- tive at the moment of their creation, throwing off beta particles as they rapidly transmute to a stable nuclear form within a month of their birth from fission. Regardless of their final form or properties, fission and activation products will accumulate in the fuel of a nuclear reactor. In conventional solid-fueled reactors, the consumption of fuel, and the degradation of cladding material are generally the reasons the reac- tor must be shut down for refueling rather than the buildup of fission products. 2018-10-16 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, on the contrary, 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. Uranium Absorption UF4 UF6 and Reduction UF6 Fluoride Fluoride Vacuum Volatility Volatility Distillation Fertile Fuel Salt Fission Salt Product Waste Recycle Recycle Fertile Salt Reactor Fuel Salt core Reactor blanket Uranium Absorption UF4 UF6 and Reduction UF6 Fluoride Fluoride Vacuum Volatility Volatility Distillation Fertile Fuel Salt Fission Salt Product Waste Recycle Recycle Fertile Salt Reactor Fuel Salt core 2018-10-16 Reactor blanket A fluid-fueled reactor also has attractive options for the long-term man- agement of fission products. They can be chemically isolated and sep- arated from the fuel salt in a manner analogous to the way that the kid- ney processes and removes waste products from the bloodstream. A variety of different approaches to the removal of fission/activation prod- ucts have been proposed for liquid-fluoride nuclear reactors. These include distillation of the fuel salt itself leaving the fission/activation products in the heel. Another proposed method is to selectively pre- cipitate certain neutron-absorbing fission products by overwhelming the salt with another material transparent to neutrons, such as cerium. 2018-10-16 Reductive extraction is yet another process, here the fuel salt is con- tacted with a metallic chemical reductant that will preferentially reduce fission products from fluoride salts to metals. ORNL two-fluid reactor chemical processing lne atmnsPaF minus blanket salt Bi(Th) Extractor Blanket Th0 Reductant 0 Li Addition 4 -UF 4 Bi(Th,Pa,U) m H2-HF _ = ea Fluorinator Decay Hydrofluorinator F Reduction UF6 Decay HF-H Tank 2 F2 H2 Bi ulFluorinator Fuel Freeze valve carrier salt Drain makeup Tank Distillation F2 to final fluorination and disposal ORNL two-fluid reactor chemical processing lne atmnsPaF minus blanket salt Bi(Th) Extractor Blanket Th0 Reductant 0 Li Addition 4 -UF 4 Bi(Th,Pa,U) m H2-HF _ = ea Fluorinator Decay Hydrofluorinator F Reduction UF6 Decay HF-H Tank 2 F2 H2 ORNL two-fluid reactor chemical processing Bi ulFluorinator Fuel Freeze valve carrier salt Drain makeup Tank Distillation F2 2018-10-16 to final fluorination and disposal The MSRE considered distillation as a viable approach since it ap- pears simple in concept. The high temperatures required for the dis- tillation of a carrier salt of LiF-BeF2 (FLiBe) from a fuel salt are chal- lenging. There is also the inefficiency issue of repeatedly attempting to boil a large inventory of carrier salt away in an attempt to concentrate a small inventory of fission products. Inevitably some of the valuable carrier salt will follow the fission products into the waste stream. 2018-10-16 Precipitation with cerium is also a challenging method for fission prod- uct management because it introduces another chemical species into the carrier salt at ever increasing concentrations rather than actually removing the fission products and leaving no residual behind. 2018-10-16 Reductive extraction of fission products increasingly appears to be the most attractive suggested way to manage the long-term buildup of fis- sion 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 in- troduction into the fuel salt; bismuth is immiscible with the fluoride fuel salts that are generally favored for molten-salt reactors. 2018-10-16 Reductive metal extraction is a technique that can either be employed in a "chemical" manner, contacting bismuth containing lithium with the fuel salt, or in an "electrolytic" manner, where an electrical potential is applied to more carefully control the addition of lithium reductant and the removal of metallic fission products. 2018-10-16 Ideally, a technique for the removal of fission products from the fuel salt of a liquid-fluoride reactor would be employed that could operate directly on the fuel salt without any pretreatment. But the nature of reductive extraction is that it will tend to remove the most "noble" con- stituents of the fuel salt first, and the actinides tend to be substantially more noble than the fission products. This figure provides the overview of how extractable various species are from the LiF-BeF2-ThF4 sys- tem. Zr, U, Pu, Pa, and other actinides tend to reduce from a salt in preference to the lanthanides. Thorium is a notable exception. 2018-10-16 Zirconium, uranium, plutonium, and several other minor actinides not depicted in this graph are all more susceptible to be removed from the fluoride salt mixture than the neodymium and europium, which rep- resent the larger class of lanthanide fission products. Thorium which would be present in the breeder salt blanket has an even lower propen- sity to be removed from the fuel salt than the lanthanide fission prod- ucts. But if one can imagine a fuel salt that does not contain thorium and where uranium is the dominant actinide species, a solution to the challenge may present itself. 2018-10-16 The approach which we propose to evaluate is the use of a fluorinat- ing/oxidizing agent to convert uranium, typically UF4 found in a liquid- fluoride reactor to its gaseous state UF6. Depending on the fluorina- tion/oxidizing agent and temperature, other actinides will also be fluo- rinated and/or oxidized from a trivalent or tetravalent state. Neptunium and plutonium do form volatile hexafluorides but plutonium hexafluo- ride 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. ORNL one-fluid reactor chemical processing H2 to purge columns H2 (10 scfm) (95%) (5%) Al O Sorber Charcoal Sorber