Lead-bismuth eutectic

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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 (pure lead melts at 327°C) and a boiling point of 1670°C.

History

The Soviet Alfa-class submarines used LBE as a coolant for their nuclear reactors throughout the Cold War.

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 investment 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.

As heavy nuclei, lead and bismuth can be used as spallation targets for non-fission neutron production, as in Accelerator Transmutation of Waste (see energy amplifier).

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.

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 refueling of the reactor.

Links

* [http://aaa.nevada.edu/pdffiles/LI.pdf ATW group, Los Alamos]