You mean a reactor that is cooled with Sodium, a material that, according to Wikipedia, spontaneously explodes when it comes into contact with a rare substance called water?
Not spontaneously, only when contacting water. But you see, there is no water in such a reactor. And it can't get in, because if water can get in, other stuff can get out, and stuff getting out of a reactor is a disaster anyway, so the thing is a closed system. Even though, since sodium cooled reactors are not pressurized, stuff wouldn't necessarily come out of a small leak.
On the other hand, water at 300C and 200bar will explode spontaneously without coming into contact with anything. A steam explosion is no fun, according to Wikipedia.
