Lead-Bismuth Eutectic or LBE is a eutectic alloy of lead (44.5%) and bismuth (55.5%) used as a coolant in some nuclear reactors, and is a proposed coolant for the lead-cooled fast reactor, part of the Generation IV reactor initiative. It has a melting point of 123.5 °C/255.3 °F (pure lead melts at 327 °C/621 °F, pure bismuth at 271 °C/520 °F) and a boiling point of 1,670 °C/3,038 °F.
Lead-bismuth alloys with between 30% and 75% bismuth all have melting points below 200 °C/392 °F. Alloys with between 48% and 63% bismuth have melting points below 150 °C/302 °F.  While lead expands slightly on melting and bismuth contracts slightly on melting, LBE has negligible change in volume on melting.
The Russians are the acknowledged experts in lead-bismuth cooled reactors, with EDO "Gidropress" (the Russian developers of the VVER-type LWRs) having special expertise in their development. The SVBR-75/100, a modern design of this type, is one example of the extensive Russian expertise with this technology.
Gen4 Energy Inc (formerly Hyperion Power Generation), a United States firm connected with Los Alamos National Laboratory, announced plans in 2008 to design and deploy a uranium nitride fueled small modular reactor cooled by lead-bismuth eutectic for commercial power generation, district heating, and desalinization. The proposed reactor, called the Gen4 Module, will be a 70 MWth reactor of the sealed modular type, factory assembled and transported to site for installation, and transported back to factory for refueling.
As compared to sodium-based liquid metal coolants such as liquid sodium or NaK, 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 production through thermochemical processes.
Lead and LBE also do not react readily with water or air, in contrast to sodium and NaK 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.
Both lead and bismuth are also an excellent radiation shield, blocking gamma radiation while simultaneously being virtually transparent to neutrons. In contrast, sodium will form the potent gamma emitter sodium-24 (half-life 15 hours) following intense neutron radiation, requiring a large radiation shield for the primary cooling loop.
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.
Lead and LBE coolant are more corrosive to steel 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 bismuth-209, the main stable isotopes of bismuth present in LBE coolant, undergo neutron capture and subsequent beta decay, forming polonium-210, a potent alpha emitter. The presence of radioactive polonium in the coolant would require special precautions to control alpha contamination during refueling of the reactor and handling components in contact with LBE.
- Subcritical reactor (accelerator-driven system)
- http://www.nea.fr/html/science/reports/2007/pdf/chapter2.pdf Handbook on Lead-bismuth Eutectic Alloy and Lead Properties
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- "The Gen4 Module, Safety & Security". Retrieved 25 Jun 2012.
- Long-lived radionuclides of sodium, lead-bismuth, and lead coolants in fast-neutron reactors.