Basically it means that you lose the ability to control reactor temperature. Sodium cooled reactors general have a positive thermal coefficient for reactivity, meaning when temperature increases the reactor power goes up.

When you lose cooling your temperature rises, which results in increased reactor power which results in increased temperature which results in power which ... in a self reinforcing loop until a melt down occurs and everyone gets Chernobyled.

Even if the reactor isn't susceptible to this cycle, liquid sodium is a solid at relatively low temperatures. So in some situations the liquid sodium will stop circulating and then start to cool and solidify. This effectively blocks the cooling pipes and it is very difficult to get the sodium heated up enough to liquefy and restore cooling circulation. This can again lead to getting Chernobyled.

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