Lead-Bismuth Eutectic or
LBE is a eutectic
alloyAn alloy is a partial or complete solid solution of one or more elements in a metallic matrix. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may be homogeneous in distribution depending on thermal history...
of
leadLead is a main-group element with symbol Pb and atomic number 82. Lead is a soft, malleable poor metal, also considered to be one of the heavy metals. Lead has a bluish-white color when freshly cut, but tarnishes to a dull grayish color when exposed to air...
(44.5%) and
bismuthBismuth is a chemical element that has the symbol Bi and atomic number 83. This trivalent poor metal chemically resembles arsenic and antimony. Bismuth is heavy and brittle; it has a silvery white color with a pink tinge owing to the surface oxide. Bismuth is the most naturally diamagnetic of all...
(55.5%) used as a
coolantA coolant is a fluid which flows through a device to prevent its overheating, transferring the heat produced by the device to other devices that use or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, and chemically inert, neither causing nor...
in some
nuclear reactorA nuclear reactor is a device in which nuclear chain reactions are initiated, controlled, and sustained at a steady rate.The most significant use of nuclear reactors is as an energy source for the generation of electrical power and for the power in some ships...
s, and is a proposed coolant for the
lead-cooled fast reactorThe lead-cooled fast reactor is a nuclear power Generation IV reactor that features a fast neutron spectrum, molten lead or lead-bismuth eutectic coolant, and a closed fuel cycle. Options include a range of plant ratings, including a number of 50 to 150 MWe units featuring long-life,...
, part of the
Generation IV reactorGeneration IV reactors are a set of theoretical nuclear reactor designs currently being researched. Most of these designs are generally not expected to be available for commercial construction before 2030, with the exception of a version of the Very High Temperature Reactor called the Next...
initiative.
It has a
melting pointThe melting point of a solid is the temperature range at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. When considered as the temperature of the reverse change from liquid to solid, it is referred to as the freezing point...
of 123.5°C (pure lead melts at 327°C) and a
boiling pointThe boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....
of 1670°C.
Lead-bismuth alloys with between 30% and 75% bismuth all have melting points below 200°C.
Alloys with between 48% and 63% bismuth have melting points below 150°C.
While lead expands slightly on melting and bismuth contracts slightly on melting, LBE has negligible change in volume on melting.
History
The Soviet
Alfa-class submarineThe Soviet Union/Russian Navy Project 705 was a submarine class of hunter/killer nuclear powered vessels . The class is also known by the NATO reporting name of Alfa...
s used LBE as a coolant for their nuclear reactors throughout the
Cold WarThe Cold War was the continuing state of political conflict, military tension, and economic competition existing after World War II , primarily between the USSR and its satellite states, and the powers of the Western world, including the United States...
.
Advantages
As compared to sodium-based liquid metal coolants such as liquid sodium or
NaKNaK is a sodium - potassium alloy usually liquid at room temperature. Various commercial grades are available. NaK is highly reactive with air or water and must be handled with special precautions...
, lead-based coolants have significantly higher boiling points, meaning a reactor can be operated without risk of coolant boiling at much higher temperatures. This improves thermal efficiency and could potentially allow
hydrogen productionHydrogen is commonly produced by extraction from hydrocarbon fossil fuels via a chemical path. Hydrogen may also be extracted from water via biological production in an algae bioreactor, or using electricity , chemicals or heat ; these methods are less developed for bulk generation in comparison...
through thermochemical processes.
Lead and LBE also do not react readily with water or air, in contrast to
sodiumSodium is a metallic element with a symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals within "group 1"...
and
NaKNaK is a sodium - potassium alloy usually liquid at room temperature. Various commercial grades are available. NaK is highly reactive with air or water and must be handled with special precautions...
which ignite spontaneously in air and react explosively with water. This means that lead- or LBE-cooled reactors, unlike sodium-cooled designs, would not need an intermediate coolant loop, which reduces the capital investment required for a plant.
Lead is also an excellent radiation shield, blocking gamma radiation while simultaneously being virtually transparent to
neutronThe neutron is a subatomic particle with no net electric charge and a mass slightly larger than that of a proton.Neutron are usually found in atomic nuclei. The nuclei of most atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of protons in a...
s. In contrast, sodium will form potent gamma emitters following intense
neutron radiationNeutron radiation is a kind of non-ionizing radiation which consists of free neutrons.-Sources:Neutrons may be emitted during either spontaneous or induced nuclear fission, nuclear fusion processes, very high energy reactions such as in the Spallation Neutron Source and in cosmic ray interactions,...
, requiring a large radiation shield for the primary cooling loop.
As heavy nuclei, lead and bismuth can be used as
spallationIn general, spallation is a process in which fragments of material are ejected from a body due to impact or stress. In nuclear physics, it is the process in which a heavy nucleus emits a large number of nucleons as a result of being hit by a high-energy particle, thus greatly reducing its atomic...
targets for non-fission neutron production, as in Accelerator Transmutation of Waste (see
energy amplifierIn nuclear physics, an energy amplifier is a novel type of nuclear power reactor, a subcritical reactor, in which an energetic particle beam is used to stimulate a reaction, which in turn releases enough energy to power the particle accelerator and leave an energy profit for power generation...
).
Both lead-based and sodium-based coolants have the advantage of relatively high boiling points as compared to water, meaning it is not necessary to pressurise the reactor even at high temperatures. This improves safety as it reduces the probability of a loss of coolant accident dramatically, and allows for passively safe designs.
Disadvantages
Lead and LBE coolant are more corrosive to
steelSteel is an alloy consisting mostly of iron, with a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most cost-effective alloying material for iron, but various other alloying elements are used such as manganese, chromium, vanadium, and tungsten...
than sodium, and this puts an upper limit on the velocity of coolant flow through the reactor due to safety considerations. Furthermore, the higher melting points of lead and LBE (327 °C and 123.5 °C respectively) may mean that solidification of the coolant may be a greater problem when the reactor is operated at lower temperatures. Finally, upon neutron radiation the bismuth in LBE coolant will undergo neutron capture and subsequent
beta decayIn nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted. In the case of electron emission, it is referred to as beta minus , while in the case of a positron emission as beta plus...
, forming
poloniumPolonium is a chemical element with the symbol Po and atomic number 84, discovered in 1898 by Marie and Pierre Curie. A rare and highly radioactive metalloid, polonium is chemically similar to bismuth and tellurium, and it occurs in uranium ores. Polonium has been studied for possible use in...
, a potent
alpha emitterAlpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle, and thereby transforms into an atom with a mass number 4 less and atomic number 2 less...
. The presence of radioactive polonium in the coolant would require special precautions during refueling of the reactor.