The lead-cooled fast reactor is a nuclear reactor design that uses molten lead or lead-bismuth eutectic as its coolant. These materials can be used as the primary coolant because they have low neutron absorption and relatively low melting points. Neutrons are slowed less by interaction with these heavy nuclei (thus not being neutron moderators) so these reactors operate with fast neutrons. The concept is generally similar to sodium-cooled fast reactors, and most liquid-metal fast reactors have used sodium instead of lead. Few lead-cooled reactors have been constructed, except for the Soviet submarine K-27 and the seven Soviet Alfa-class submarines (though these were beryllium-moderated intermediate energy reactors rather than fast reactors). Some proposed new nuclear reactor designs are lead-cooled. Fuel designs being explored for this reactor scheme include fertile uranium as a metal, metal oxide or metal nitride. The lead-cooled reactor design has been proposed as a generation IV reactor. Plans for future implementation of this type of reactor include modular arrangements rated at 300 to 400 MWe, and a large monolithic plant rated at 1,200 MWe.
Operation
Lead-cooled fast reactors operate with fast neutrons and molten lead or lead-bismuth eutectic coolant. Molten lead or lead-bismuth eutectic can be used as the primary coolant because especially lead, and to a lesser degree bismuth, have low neutron absorption and relatively low melting points. Neutrons are slowed less by interaction with these heavy nuclei (thus not being neutron moderators), and therefore help make this type of reactor a fast-neutron reactor. If a neutron hits a particle with a similar mass (such as hydrogen in a pressurized water reactor (PWR), it tends to lose kinetic energy. If it hits a much heavier atom such as lead, the neutron will "bounce off" without losing this energy. The coolant serves as a neutron reflector, returning some escaping neutrons to the core. Smaller capacity lead-cooled fast reactors (such as SSTAR) can be cooled by natural convection, while larger designs (such as ELSY) use forced circulation in normal power operation, but will employ natural circulation emergency cooling. No operator intervention is required, nor pumping of any kind to cool the residual heat of the reactor after shutdown. 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 theoretically high enough to support thermochemical production of hydrogen through the sulfur-iodine cycle, although this has not been demonstrated. The concept is generally very similar to sodium-cooled fast reactors, and most liquid-metal fast reactors have used sodium instead of lead. Few lead-cooled reactors have been constructed, except for some Soviet nuclear submarine reactors in the 1970s, but some proposed new nuclear reactor designs are lead-cooled, with one under construction.
Fuel Fuel designs being explored for this reactor scheme include fertile uranium as a metal, metal oxide or metal nitride.
Small modular reactors
Reactors that use lead or lead-bismuth eutectic can be designed in a large range of power ratings. The Soviet Union was able to operate the Alfa-class submarines with a lead-bismuth cooled intermediate-spectrum reactor moderated with beryllium from the 1960s to 1998, which had approximately 30 MW of mechanical output for 155 MW thermal power (see below). Other options include units featuring long-life, pre-manufactured cores, that do not require refueling for many years. The lead-cooled fast reactor 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 production and desalinisation process for the production of potable water).
Advantages of lead in fast reactors The use of lead as a coolant has several advantages when compared to other methods for reactor cooling:
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