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.
Cooling is deployed in case the reactor starts to overheat, and that typically happens when water is not supplied fast enough to be converted into steam (or steam output gets stuck). Cooling is not mandatory if you are not using automatic power regulation. However, it is highly recommended to have it as it provides extra protection and helps to avoid any potential overheating.
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