Neutron activation
Sodium and NaK (a eutectic sodium-potassium alloy) do not corrode steel to any significant degree and are compatible with many nuclear fuels, allowing for a wide choice of structural materials. They do, however, ignite spontaneously on contact with air and react violently with water, producing hydrogen gas. This was the case at the Monju Nuclear Power Plant in a 1995 accident and fire. Neutron activation of sodium also causes these liquids to become intensely radioactive during operation, though the half-life is short and therefore their radioactivity does not pose an additional disposal concern.