MOLTEN-SALT REACTOR CHEMISTRY W. R. GRIMES Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 KEYWORDS: fused salt fuel, chemical reactions, reactors, beryllium fluorides, zirconium fluorides, Ii th i um fluorides, Received August 4, 1969 uranium hexafluoride, repro­ Revised October 7, 1969 cessing, protactinium, seporo­ tion pro c es s es, breeding, fission products, MSRE aggressive toward some otherwise suitable ma­ terial of construction and toward some suitable This document summarizes the large program of chemical research and development which led moderator material. The fuel must be stable to selection of fuel and coolant compositions for toward reactor radiation, must be able to survive the Molten-Salt Reactor Experiment (MSRE) and fission of the uranium (or other fissionable ma­ for subsequent reactors of this type. Chemical terial} and must tolerate fission product accumu­ lation without serious deterioration of its useful behavior of the LiF-BeFrZrFrUF4 fuel mixture and behavior of fission products during power op­ properties. We must also be assured of a gen­ uinely low fuel-cycle cost; this presupposes a eration of MSRE are presented. A discussion of low-cost fuel associated with inexpensive turn­ the chemical reactions which show promise for around of the unburned fissile material, and recovery of bred 233Pa and for removal of fission effective and economical schemes for recovery of product poisons from a molten-salt breeder reac­ tor is included. bred fissile material and for removal of fission­ product poisons from the fuel. A suitable secondary coolant must be provided to link the fuel circuit with the steam-generating equipment. The demands imposed upon this cool­ ant fluid differ in obvious ways from those im­ INTRODUCTION posed upon the fuel system. Radiation intensities will be markedly less in the coolant system, and A single-fluid molten-salt thermal breeder the consequences of uranium fission will be ab­ (MSBR) of the type described by Rosenthal et al.,1 sent. The coolant salt must, however, be com­ Bettis and Robertson,2 and Perry and Bauman3 patible with metals of construction which will makes very stringent demands upon its fluid handle the fuel and the steam; it must not undergo 4 6 violent reactions with fuel or steam should leaks fuel. - This fuel must consist of elements having low capture cross sections for neutrons typical of develop in either circuit. The coolant should be the energy spectrum of the chosen design. The inexpensive, possessed of good heat-transfer prop­ fuel must dissolve more than the critical concen­ erties, and it should melt at temperatures suit­ tration of fissionable material (235U 233U or able for steam cycle start-up. An ideal coolant 239 Pu), and high. concentrations. of fertile' material' would consist of compounds which would be easy 232 to separate from the valuable fuel mixture should ( Th) at temperatures safely below the tempera­ ture at which the salt leaves the MSBR heat they mix as a consequence of a leak. exchanger. The mixture must be thermally stable, This report presents, in brief, the basis for and its vapor pressure needs to be low over an choice of fuel and coolant systems which seem operating temperature range ( 1100 to 1400°F) optimum in light of these numerous-and to some sufficiently high to permit generation of high extent conflicting-requirements. quality steam for power production. The fuel mixture must possess heat transfer and hydro­ CHOICE OF FUEL COMPOSITION dynamic properties adequate for its service as a Compounds which are permissible major con­ heat-exchange fluid. It must be relatively non- stituents of fuels for single-fluid thermal breeders NUCLEAR APPLICATIONS & TECHNOLOGY VOL. 8 FEBRUARY 1970 137 Grimes MSBR CHEMISTRY are those which can be prepared from beryllium, The only good moderator material truly compati­ bismuth, boron-11, carbon, deuterium, fluorine, ble with molten-fluoride fuel mixtures is graph­ lithium-7, nitrogen-15, oxygen, and the fissionable ite. 4- 6 and fertile materials. As minor constituents one might tolerate compounds containing the other Phase Behavior Among Fluorides elements in Table I. Uranium tetrafluoride and uranium trifluoride Many chemical compounds can be prepared are the only fluorides (or oxyfluorides) of uranium from the several "major constituents" listed which appear useful as constituents of molten­ above, Most of these, however, can be eliminated fluoride fuels. Uranium tetrafluoride (UF 4) is after elementary consideration of the fuel re­ relatively stable, nonvolatile, and largely non­ quirements. 4-6 No hydrogen- (or deuterium-) hygroscopic. It melts at l035°C (1895°F), but this bearing compounds possess overall properties freezing point is markedly depressed by useful that are practical in such melts. Carbon, nitro­ diluent fluorides. Uranium trifluoride dispropor­ gen, and oxygen form high melting binary com­ tionates at temperatures above ~ l000°C by the pounds with the fissionable and fertile metals; reaction these compounds are quite unsuitable as constit­ ( 1) uents of liquid systems. The oxygenated anions 6 7 either lack_ the required thermal stability (i.e., It is unstable ' at lower temperatures in most N03- or N02-) or fail as solvents for high concen­ molten-fluoride solutions and is tolerable in reac­ trations of thorium compounds (i.e., C03=). It tor fuels only with a large excess of UF4 so that quickly develops, therefore, that fluorides are the the activity of u° is so low as to avoid appreciable only suitable salts indicated in this list of ele­ reaction with moderator graphite or container ments. metal. Fluoride ion is capable of appreciable neutron Thorium tetrafluoride (ThF 4) is the only known moderation, but this moderation is by itself in­ fluoride of thorium. It melts at llll°C (2032°F) sufficient for good neutron thermalization. An but fortunately its freezing point is markedly de­ additional moderator is, accordingly, required. pressed by fluoride diluents which are also useful with UF4. Consideration of nuclear properties alone leads TABLE I one to prefer as diluents the fluorides of Be, Bi, 7 Elements or Isotopes Which may be Tolerable Li, Mg, Pb, and Zr in that order. Equally simple 8 9 in High-Temperature Reactor Fuels consideration of the stability of these fluorides ' toward reduction by structural metals, however, Absorption Cross Section eliminates the bismuth fluorides from considera­ Material (barns at 2200 m/sec) tion. This leaves BeF2 and 7 LiF as the preferred diluent fluorides. Phase behavior of systems Nitrogen-15 0.000024 based upon LiF and BeF2 as the major constitu­ Oxygen 0.0002 10 Deuterium 0.00057 ents, has, accordingly, been examined in detail. Carbon 0.0033 Fortunately for the molten fluoride reactor con­ Fluorine 0.009 cept, the phase diagrams of LiF-BeF2-UF4 and Beryllium 0.010 LiF-BeF2-ThF4 are such as to make these ma­ Bismuth 0.032 terials useful as fuels. Lithium-7 0.033 The binary system LiF-BeF2 has melting points Boron-11 0.05 below 500°C over the concentration range from 33 10 11 Magnesium 0.063 to 80 mole% BeF 2. ' The phase diagram, pre­ Silicon 0.13 sented in Fig. 1, is characterized by a single Lead 0.17 eutectic (52 mole% BeF2, melting at 360°C) be­ Zirconium 0.18 tween BeF2 and 2LiF·BeF2. The compound Phosphorus 0.21 2LiF·BeF2 melts incongruently to LiF and liquid Aluminum 0.23 at 458°C. LiF·BeF2 is formed by the reaction of Hydrogen 0.33 solid BeF2 and solid 2LiF·BeF2 below 280°C. 0 11 Calcium 0.43 The phase behavior of the BeF2-UF/ • and 12 Sulfur 0.49 Be Fr ThF4 systems are very similar. Both sys­ Sodium 0.53 tems show simple single eutectics containing very Chlorine-37 0.56 small concentrations of the heavy metal fluoride. Tin 0.6 ThF4 and UF4 are isostructural; they form a con­ Cerium 0.7 tinuous series of solid solutions with neither Rubidium 0.7 maximum nor minimum. 138 NUCLEAR APPLICATIONS & TECHNOLOGY VOL. 8 FEBRUARY 1970 Grimes MSBR CHEMISTRY i :::~ 7001--~~-t--~~--1[~~~~-+~~~+-~~--t-~~--;~~~--r--~~--r~~~r-~~~ E600'--~-L-iF~+~L-IQ_U_ID~---+--~·--'~~~~~+-~~--+-~~----;~~~--+-~~--r~~~r-~~~ ~ ' ~~ ~ ~ _J.----f__ _, ~ 500f--~~-+-~~----j~~~~~-,-\-~--r~~~t-~~-t-~--:::....-ri....-::-:--'---~~-t--~~--j--~~--j 458 l ~ i-----+------i---+-.....,-._............ v BeF (HIGH QUARTZ TYPE) L 2 ~ +LIQUID -1 400 1----1---"'--i-h..l"~--+----+------i------36_0--fl I Li 2BeF.i + BeF2 (HIGH QUARTZ TYPE l 300~~·~-+-~~-+-+,_-.::.-.::.-.::.-:::.'.::..-.::.-.::.-.::.-.::.-.::.:'=--------J+----~J---~....---28_0_,I ~<;t mN Li2B•F4 "' LiBeF3 + BeF2 (HIGH QUARTZ TYPE) ..J + ~ Li8eF3 + BeF2 (LOW QUARTZ TYPE~ j 220 Li BeF3 ::::; J. " 200'----'-----''----_.____.__~---~--~--~---~-.-0.-~--~l Li F 10 20 30 40 50 60 70 80 90 BeF2 BeF2 (mole%) Fig. 1. The system LiF-BeF2 • 13 14 2 The binary diagrams LiF - UF4 and LiF-ThF4 3LiF·ThF4 can incorporate Be + ions in both are generally similar and much more complex interstitial and substitutional sites to form solid than the binary diagrams discussed immediately solutions whose compositional extremes are rep­ above. The LiF-UF4 system shows three com­ resented by the shaded triangular region near pounds (none are congruently melting) and a single that compound. Liquidus temperatures < 550°C eutectic, at 27 mole% UF4, melting at 490°C. The (1022°F) are available at ThF4 concentrations as LiF-ThF4 system contains four binary compounds, high as 22 mole%. The maximum ThF 4 concen­ one of which (3LiF·ThF4) melts congruently, with tration available at liquidus temperatures of 500°C two eutectics at 570°C and 22 mole% ThF4 and at (932°F) is seen to be just above 14 mole%. Inspec­ 560°C and 29 mole% ThF4. tion of the diagram reveals that a considerable 15 The ternary system LiF-ThF4- UF4, shown in range of compositions with > 10 mole% ThF4 will Fig. 2, shows no ternary compounds and a single be completely molten at or below 500°C.