By dragging the orange slider to the right, you define the desired power level maintained in the reactor. This affects the amount of steam the reactor generates and how fast it burns through the fuel. All the heat generated needs to be used. Any extra heat can slowly build up, eventually leading to a reactor overheating. Basic reactors don’t have automated power regulation which makes their heat output fixed.
The current heat that has built up in the reactor. Heat is necessary to produce steam to run turbines. The pace of heat generation depends on the current power level of the reactor. When the heat reaches above the red marker, emergency cooling will try to reduce excess heat in the reactor. If the heat level goes beyond the heat bar maximum, the reactor will critically overheat, damaging the reactor and releasing radiation into the area.
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