Some other disadvantages are specific to some designs. Sodium is often used as a coolant in fast reactors, because it does not moderate neutron speeds much and has a high heat capacity. However, it burns and foams in air. It has caused difficulties in reactors, although some sodium-cooled fast reactors have operated safely for long periods (notably the and EBR-II for 30 years).
Another problem is related to neutron activation. Since liquid metals other than have low moderating ability, the primary interaction of neutrons with fast reactor coolant is the (n,gamma) reaction, which induces radioactivity in the coolant. Neutron irradiation activates a significant fraction of coolant in high-power fast reactors, up to around a of beta decays per kilogram of coolant in steady operation.
Some fast reactors also have positive boiling of the coolant in an accident would reduce coolant density and thus the absorption rate. This is dangerous and undesirable from a safety and accident standpoint. This can be avoided with a , since voids do not form in such a reactor during an accident; however, activation in the coolant remains a problem. A -cooled reactor would avoid this, since the elastic scattering and total cross sections are approximately equal, i.e. few (n,gamma) reactions are present in the coolant and the low density of helium at typical operating conditions means that neutrons have few interactions with coolant.