Liquid metal reactors [37]

The most documented type of liquid metal reactor is the sodium cooled reactor. Here the coolant is liquid sodium at a temperature of 500 °C, while the boiling temperature of sodium at atmospheric pressure is 883 °C. The pressure within the vessel is slightly above 1 atm of argon gas, in order to prevent air intrusion. The most modern sodium cooled reactors like Phenix and Superphenix [63] are of the swimming pool type. These characteristics practically prevent loss of coolant accidents. The negative temperature reactivity coefficients have been shown to lead to a break-off of the chain reaction and to a moderate temperature increase within minutes, for the small EBR2 (20 MWe) reactor. For Phenix (250 MWe) natural convection was demonstrated to evacuate the residual heat. In general it appears that sodium cooled reactors are much safer than water cooled reactors with respect to core melting. The risk for an individual living in the vicinity of the reactor to die from a cancer induced by accidental radioactivity release is estimated to be around 10~10 for these reactors.

The main safety concern with sodium cooled reactors is, precisely, with the use of sodium which burns spontaneously when in contact with air and dissociates water, with the consequent risk of a hydrogen explosion. The risk associated with violent sodium-water reactions explains the choice made by the former USSR to use a molten bismuth-lead eutectic as coolant for their most modern nuclear submarine reactors. It also prompted the recent proposal of the CERN group [76]. In addition lead has a boiling temperature of 1749 °C, which makes coolant boiling completely impossible, due to radiation cooling. A swimming pool type lead cooled reactor would have a very high safety level.

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