This is a heat sensor. Place it next to a heat pipe and connect to it via red/green wires. Select it, enable Read temperature, and select the signal. Intended to be useful for nuclear control systems where you want to measure the cold end of a heat pipe, not just the reactor temp.

It will complain about no fuel due to workarounds I had to do to prevent the reactor prototype from outputting heat. The flashing icon is hidden. Feel free to ignore anything it says about fuel. It also says it produces 1W of heat, which it does not because it has no fuel.