Liquid sodium metal is used in certain nuclear reactor designs as a cooling fluid, because it is very effective at transferring large amounts of heat from one place to another. Personally I'd rather have something a bit less violently explosive as the cooling fluid in my nuclear reactor, but so far as I know there haven't been any major accidents attributed to its use.
This object, which also uses liquid sodium metal for cooling, is not from a nuclear reactor. Is is a valve from a high-performance car engine, and it uses the sodium to transfer heat away from the valve head (which is in contact with the exploding fuel in the cylinder). This particular value has been broken in half to extract the sodium: You can see the hollow stem where the sodium was.
Look at the next sample down to see more clearly how this channel works.
Source: Nick Brandenburg
Contributor: Nick Brandenburg
Acquired: 10 August, 2005