Lead-bismuth eutectic

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Lead Bismuth Eutectic or LBE is an eutectic alloy of lead and bismuth 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.

[edit] History

The Soviet Alfa class submarine used LBE as a coolant for their nuclear reactors throughout the cold war.

[edit] Advantages

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 investments required for a plant. Lead is also an excellent radiation shield, blocking gamma radiation while simultaneously being virtually transparent to neutrons. In contrast, sodium will form potent gamma emitters 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.

[edit] Disadvantages

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 the bismuth in LBE coolant will undergo neutron capture and subsequent beta-decay, forming polonium, a potent alpha-emitter. The presence of radioactive polonium in the coolant would require special precautions during refuelling of the reactor.