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. A smaller capacity LFR such as SSTAR can be cooled by natural convection, larger proposals such as ELSY use forced circulation in normal power operation, but with natural circulation emergency cooling. The reactor outlet coolant temperature is typically in the range of 500 to 600 °C, possibly ranging over 800 °C with advanced materials for later designs. Temperatures higher than 800 °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).

Advantages

 * 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.
 * Liquid lead-bismuth systems can't cause an explosion and quickly solidify in case of a leak, further improving safety.
 * Lead is very dense, and therefore a good shield, against gamma rays
 * Lead's nuclear properties allow it to prevent a positive void coefficient, which is difficult to prevent in large sodium fast reactor cores.
 * The operating pressure is very low and lead has an extremely high boiling point of 1750 degrees Celsius, which is over 1100 degrees Celsius higher than the peak coolant operating temperaturer. This makes significant reactor pressurization by overheating virtually impossible.
 * Lead does not react significantly with water or air, unlike sodium which burns readily in air and can explode in contact with water. This allows easier, cheaper and safer containment and heat exchanger/steam generator design.

Disadvantages

 * Lead and lead-bismuth are very dense, increasing the weight of the system therefore requiring more structural support and seismic protection which increases building cost.
 * Lead is cheap and abundant, though bismuth is expensive and quite rare. A lead-bismuth reactor will require hundreds to thousands of tonnes of bismuth depending on reactor size.
 * Solidification of the lead-bismuth solution renders the reactor inoperable. However, lead-bismuth eutectic has a comparatively low melting temperature of 123.5 C, making desolidification a relatively easily accomplished task. Lead has a higher melting point of 327.5 °С, but is often used as a pool type reactor where the large bulk of lead does not easily freeze.
 * By leaking and solidifying, the coolant may damage the equipment (see Soviet submarine K-64).
 * Lead-bismuth produces considerable amount of polonium, a highly radioactive and quite mobile element. This can complicate maintenance and pose a plant contamination problem. Lead produces orders of magnitudes less polonium, and so has an advantage over lead-bismuth in this regard.

Russia/USSR
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 to 495 C 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

USA
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 Lawrence Livermore National Laboratory developed SSTAR is a lead-cooled design.

Belgium
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 in the nuclear reactor. 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).