FLiBe is a
molten salt
Molten salt is salt which is solid at standard temperature and pressure but enters the liquid phase due to elevated temperature. Regular table salt has a melting point of 801 °C (1474°F) and a heat of fusion of 520 J/g.Proc. Roy. Soc. Bibli ...
made from a mixture of
lithium fluoride
Lithium fluoride is an inorganic compound with the chemical formula LiF. It is a colorless solid, that transitions to white with decreasing crystal size. Although odorless, lithium fluoride has a bitter-saline taste. Its structure is analogous to ...
(LiF) and
beryllium fluoride (BeF
2). It is both a
nuclear reactor coolant and solvent for fertile or fissile material. It served both purposes in the
Molten-Salt Reactor Experiment (MSRE) at the
Oak Ridge National Laboratory
Oak Ridge National Laboratory (ORNL) is a U.S. multiprogram science and technology national laboratory sponsored by the U.S. Department of Energy (DOE) and administered, managed, and operated by UT–Battelle as a federally funded research an ...
.
The 2:1 mixture forms a
stoichiometric
Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions.
Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equ ...
compound, Li
2BeF
4, which has a melting point of 459 °C, a boiling point of 1430 °C, and a density of 1.94 g/cm
3.
Its volumetric
heat capacity
Heat capacity or thermal capacity is a physical property of matter, defined as the amount of heat to be supplied to an object to produce a unit change in its temperature. The SI unit of heat capacity is joule per kelvin (J/K).
Heat capacity ...
is similar to that of water (4540 kJ/m
3K = 1085 kcal/m
3K): 8.5% more than the standard value considered for water at room temperature), more than four times that of sodium, and more than 200 times that of helium at typical reactor conditions.
Its specific
heat capacity
Heat capacity or thermal capacity is a physical property of matter, defined as the amount of heat to be supplied to an object to produce a unit change in its temperature. The SI unit of heat capacity is joule per kelvin (J/K).
Heat capacity ...
is 2414.17 kJ/kg K, or about 60% that of water.
Its appearance is white to transparent, with crystalline grains in a solid state, morphing into a completely clear liquid upon melting. However, soluble fluorides such as UF
4 and NiF
2, can dramatically change the salt's color in both solid and liquid state. This made
spectrophotometry
Spectrophotometry is a branch of electromagnetic spectroscopy concerned with the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength. Spectrophotometry uses photometers, known as sp ...
a viable analysis tool, and it was employed extensively during the MSRE operations.
The
eutectic mixture is slightly greater than 50% BeF
2 and has a melting point of 360 °C. This mixture was never used in practice due to the overwhelming increase in viscosity caused by the BeF
2 addition in the eutectic mixture. BeF
2, which behaves as a glass, is only fluid in salt mixtures containing enough molar percent of
Lewis base. Lewis bases, such as the alkali fluorides, will donate fluoride ions to the beryllium, breaking the glassy bonds which increase viscosity. In FLiBe, beryllium fluoride is able to sequester two fluoride ions from two lithium fluorides in a liquid state, converting it into the tetrafluorberyllate ion BeF
4−2.
Chemistry
The chemistry of FLiBe, and other fluoride salts, is unique due to the high temperatures at which the reactions occur, the ionic nature of the salt, and the reversibility of many of the reactions. At the most basic level, FLiBe melts and complexes itself through
:
2LiF(s) + BeF2(s) -> 2Li+(l) + BeF4^(l).
This reaction occurs upon initial melting. However, if the components are exposed to air they will absorb moisture. This moisture plays a negative role at high temperature by converting BeF
2, and to a lesser extent LiF, into an oxide or hydroxide through the reactions
:
BeF2(l) + 2H2O(g) <=> Be(OH)2(d) + 2HF(d).
and
:
BeF2(l) + H2O(g) <=> Be(O)(d) + 2HF(d).
While BeF
2 is a very stable chemical compound, the formation of oxides, hydroxides, and hydrogen fluoride reduce the stability and inertness of the salt. This leads to corrosion. Its important to understand that all dissolved species in these two reactions cause the corrosion—not just the hydrogen fluoride. This is because all dissolved components alter the
reduction potential
Redox potential (also known as oxidation / reduction potential, ''ORP'', ''pe'', ''E_'', or E_) is a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respe ...
or redox potential. The redox potential is an innate and measurable voltage in the salt which is the prime indicator of the corrosion potential in salt. Usually, the reaction
:
HF(g) + e- -> F- + 1/2 H2(g).
is set at zero volts. This reaction proves convenient in a laboratory setting and can be used to set the salt to zero through bubbling a 1:1 mixture of hydrogen fluoride and hydrogen through the salt. Occasionally the reaction:
:
NiF2(d) + 2e- -> Ni(c) + 2F-.
is used as a reference. Regardless of where the zero is set, all other reactions which occur in the salt will occur at predictable, known voltages relative to the zero. Therefore, if the redox potential of the salt is close to a specific reaction's voltage, that reaction can be expected to be the predominant reaction. Therefore, it is important to keep a salt's redox potential far away from reactions which are undesirable. For example, in a container alloy of nickel, iron, and chromium, the reactions of concern would be the fluorination of container and subsequent dissolution of these metal fluorides. The dissolution of the metal fluorides then alters the redox potential. This process continues until an equilibrium between metals and salt is reached. It is essential that a salt's redox potential be kept as far away from fluorination reactions as possible, and that metals in contact with salt be as far away from the salt's redox potential as possible in order to prevent excessive corrosion.
The easiest method to prevent undesirable reactions is to pick materials whose reaction voltages are far from the redox potential of the salt in the salt's worst case. Some of these materials are tungsten, carbon, molybdenum, platinum, iridium, and nickel. Of all these materials, only two are affordable and weldable: nickel and molybdenum. These two elements were chosen as the main portion of
Hastelloy-N, the material of the MSRE.
Altering the redox potential of FLiBe can be done in two ways. First, the salt can be forced by physically applying a voltage to the salt with an inert electrode. The second, more common way, is to perform a chemical reaction in the salt which occurs at the desired voltage. For example, redox potential can be altered by
sparging
Sparging may refer to:
*Sparging (chemistry), a process in which a gas is bubbled through a liquid to remove other gases or volatile compounds
*Air sparging, a remediation process in which air is pushed through contaminated water or soil to remove ...
hydrogen and hydrogen fluoride into the salt or by dipping a metal into the salt.
Coolant
As a
molten salt
Molten salt is salt which is solid at standard temperature and pressure but enters the liquid phase due to elevated temperature. Regular table salt has a melting point of 801 °C (1474°F) and a heat of fusion of 520 J/g.Proc. Roy. Soc. Bibli ...
it can serve as a
coolant
A coolant is a substance, typically liquid, that is used to reduce or regulate the temperature of a system. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, chemically inert and neither causes nor promotes corros ...
which can be used at high temperatures without reaching a high
vapor pressure
Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phase ...
.
Notably, its optical transparency allows easy visual inspection of anything immersed in the coolant as well as any impurities dissolved in it. Unlike
sodium
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
or
potassium
Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosp ...
metals, which can also be used as high-temperature coolants, it does not violently react with air or water. FLiBe salt has low
hygroscopy
Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance ...
and
solubility
In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.
The extent of the solub ...
in water.
Nuclear properties
The low
atomic weight
Relative atomic mass (symbol: ''A''; sometimes abbreviated RAM or r.a.m.), also known by the deprecated synonym atomic weight, is a dimensionless physical quantity defined as the ratio of the average mass of atoms of a chemical element in a give ...
of
lithium
Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense solid ...
,
beryllium
Beryllium is a chemical element with the symbol Be and atomic number 4. It is a steel-gray, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with other elements to form m ...
and to a lesser extent
fluorine make FLiBe an effective
neutron moderator. As natural lithium contains ~7.5%
lithium-6, which tends to
absorb neutrons producing
alpha particle
Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be pro ...
s and
tritium
Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with half-life about 12 years. The nucleus of tritium (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus ...
, nearly pure
lithium-7 is used to give the FLiBe a small
neutron absorption cross section; e.g. the MSRE secondary coolant was 99.993% lithium-7 FLiBe. When Li-7 does absorb a neutron, it nigh-instantaneously decays via successive
beta- and then alpha decay into a beta particle and two alpha particles.
Beryllium will occasionally disintegrate into two alpha particles and two neutrons when hit by a
fast neutron. Fluorine has a non-negligible cross section for (α,n) reactions, which needs to be taken into account when calculating
neutronics.
Applications
In the
liquid fluoride thorium reactor (LFTR) it serves as
solvent
A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
for the
fissile
In nuclear engineering, fissile material is material capable of sustaining a nuclear fission chain reaction. By definition, fissile material can sustain a chain reaction with neutrons of thermal energy. The predominant neutron energy may be typ ...
and
fertile material
Fertile material is a material that, although not itself fissionable by thermal neutrons, can be converted into a fissile material by neutron absorption and subsequent nuclei conversions.
Naturally occurring fertile materials
Naturally occurrin ...
fluoride salts, as well as moderator and coolant.
Some other designs (sometimes called molten-salt cooled reactors) use it as coolant, but have conventional solid
nuclear fuel
Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission.
Most nuclear fuels contain heavy fissile actinide elements that are capable of undergoi ...
instead of dissolving it in the molten salt.
The liquid FLiBe salt was also proposed as a liquid blanket for tritium production and cooling in the
ARC fusion reactor, a compact
tokamak
A tokamak (; russian: токамáк; otk, 𐱃𐰸𐰢𐰴, Toḳamaḳ) is a device which uses a powerful magnetic field to confine plasma in the shape of a torus. The tokamak is one of several types of magnetic confinement devices being ...
design by MIT.
See also
*
FLiNaK
FLiNaK is the name of the ternary eutectic alkaline metal fluoride salt mixture LiF- NaF- KF (46.5-11.5-42 mol %). It has a melting point of 454 °C and a boiling point of 1570 °C. It is used as electrolyte for the electroplatin ...
*
Molten salt reactor
A molten salt reactor (MSR) is a class of nuclear fission reactor in which the primary nuclear reactor coolant and/or the fuel is a molten salt mixture. Only two MSRs have ever operated, both research reactors in the United States. The 1950's ...
*
Liquid fluoride thorium reactor
References
{{lithium compounds
Nuclear reactor coolants
Fluorides
Metal halides
Beryllium compounds
Molten salt reactors
Lithium compounds
Neutron moderators
Alkali metal fluorides