Lead-cooled fast reactor
Molten lead or lead-bismuth eutectic can be used as the primary coolant in a nuclear reactor, as lead and bismuth have low neutron absorption and relatively low melting points. Neutrons are less slowed by interaction with heavy nuclei, so lead and bismuth are not neutron moderators, making this type of reactor a fast-neutron reactor. The coolant does serve as a neutron reflector returning some escaping neutrons to the core.
Few have been constructed, including some Soviet nuclear submarine reactors in the 1970s, but a number of proposed new nuclear reactor designs are lead-cooled. Some designs are claimed to be able to circulate the primary coolant via convection without requiring pumps, at least in emergency shutdown conditions.
Generation IV reactor design
The Gen IV lead-cooled fast reactor is a nuclear reactor that features a fast neutron spectrum, molten lead or lead-bismuth eutectic coolant. Options include a range of plant ratings, including a number of 50 to 150 MWe (megawatts electric) units featuring long-life, pre-manufactured cores. Plans include modular arrangements rated at 300 to 400 MWe, and a large monolithic plant rated at 1,200 MWe. The fuel is metal or nitride-based containing fertile uranium and transuranics. The LFR is cooled by natural convection with a reactor outlet coolant temperature of 550 °C, possibly ranging over 800 °C with advanced materials. Temperatures higher than 830 °C are high enough to support thermochemical production of hydrogen.
Modular nuclear reactors
The LFR battery is a small turnkey-type power plant using cassette cores running on a closed fuel cycle with 15 to 20 years' refuelling interval, or entirely replaceable reactor modules. It is designed for generation of electricity on small grids (and other resources, including hydrogen and potable water).
- Instead of refueling, the whole core can be replaced after many years of operation. Such a reactor is suitable for countries that do not plan to build their own nuclear infrastructure. Lifetime without refueling can be increased more easily, in part due to higher efficiency.
- As no electricity is required for the cooling after shutdown, this design has the potential to be safer than a water-cooled reactor (see Fukushima I nuclear accidents).
- Liquid lead-bismuth systems can't cause an explosion and quickly solidify in case of a leak, further improving safety.
- These reactors are lighter and smaller than water-cooled reactors
- Lead is a strong absorber, and therefore a good shield, against gamma rays
- Solidification of the lead-bismuth solution renders the reactor inoperable. However, lead-bismuth eutectic has a comparatively low melting temperature of 123.5 °C (254.3 °F), making desolidification a relatively easily accomplished task.
- By leaking and solidifying, the coolant may damage the equipment (see Soviet submarine K-64).
- Due to (n,gamma) reactions and minimal scattering in the coolant, lead-cooled fast reactors can have a positive Void coefficient. This is considered unsafe in water-cooled reactors due to the possibility of water starting to boil or surpassing the critical point at 374 degree celsius, creating voids in the process. Such reactors typically operate at 330 degree. The boiling temperature of lead,however, is much higher at 1749 degrees, over 1000 degrees above operating temperatures. Thermal expansion provides a large negative reactivity feedback, stopping the chain reaction before that point with a large margin of safety, which is part of the passive safety concept.
Two types of LFR reactor were used in Soviet Alfa class submarines of the 1970s. The OK-550 and BM-40A designs were both capable of producing 155MWt. They were significantly lighter than typical water-cooled reactors and had an advantage of being capable to quickly switch between maximum power and minimum noise operation modes.
A joint venture called AKME Engineering was announced on 25 December 2009 between Rosatom and En+ Group, to develop a commercial lead-bismuth reactor. The SVBR-100 ('Svintsovo-Vismutovyi Bystryi Reaktor' - lead-bismuth fast reactor) is based on the Alfa designs and will produce 100MWe electricity from gross thermal power of 280MWt, about twice that of the submarine reactors. They can also be used in groups of up to 16 if more power is required. The coolant increases from 345 °C (653 °F) to 495 °C (923 °F) as it goes through the core. Uranium oxide enriched to 16.5% U-235 could be used as fuel, and refuelling would be required every 7–8 years. A prototype is planned for 2017.
Another two lead cooled reactors are developed by Russians: BREST-300 and BREST-1200 
According to Nuclear Engineering International, the initial design of the Hyperion Power Module will be of this type, using uranium nitride fuel encased in HT-9 tubes, using a quartz reflector, and lead-bismuth eutectic as coolant.
The MYRRHA reactor can work in either of the two configurations, critical or subcritical, and is cooled by lead-bismuth eutectic. In the subcritical configuration the external proton beam produced by the linear proton accelerator is hitting a spallation target, thus providing an additional neutron flux needed to reach criticality of the nuclear reaction. Such approach means that the reactor can be stopped fast and completely, if so needed. It is also capable of decreasing the toxicity of the waste by a factor 1000, and decrease the volume of the waste by a factor 100 (compared to the nuclear power plants that are currently in operation). The design of the nuclear reactor will be completed by 2014. The project was started by SCK•CEN (Studiecentrum voor Kernenergie, Centre d'Etude de l'Energie Nucleaire).
- "Initiative for small fast reactors". World Nuclear News. 2010-01-04. Retrieved 2010-02-05.
- "Heavy metal power reactor slated for 2017". World Nuclear News. 2010-23-03. Retrieved 2012-09-26.
- "Design features of BREST reactors and experimental work to advance the concept of BREST reactors". US DoE, Small Modular Reactor Program. Retrieved 2013-05-16.
- "Hyperion launches U2N3-fuelled, Pb-Bi-cooled fast reactor". Nuclear Engineering International (Global Trade Media). 2009-11-20. Retrieved 3 December 2009.
- Science connection magazine, April/May/June 2011
- Advanced reactor, fuel cycle, and energy products workshop for universities
- Generation IV International Forum LFR website
- Idaho National Laboratory Lead-Cooled Fast Reactor (LFR) Fact Sheet
- Heavy-Metal Aerosol Transport in a Lead-Bismuth Cooled Fast Reactor with In-Vessel Direct-Contact Steam Generation
- Comparison of sodium and lead-cooled fast reactors regarding reactor physics aspects, severe safety and economical issues
- RBEC-M Lead-Bismuth Cooled Fast Reactor Benchmarking Calculations
- New York Times