Nuclear chain reaction between being slightly subcritical and slightly delayed-supercritical, but must always remain below prompt-critical. It is impossible for a nuclear power plant to undergo a nuclear chain reaction that results in an explosion of power comparable with a nuclear weapon, but even low-powered explosions due to uncontrolled chain reactions, that would be considered "fizzles" in a bomb, may still cause considerable damage and meltdown in a reactor. For example, the Chernobyl disaster involved a runaway chain reaction but the result was a low-powered steam explosion from the relatively small release of heat, as compared with a bomb. However, the reactor complex
Nuclear reactor safety system control rods. In the case of a Loss of coolant accident (LOCA), the water-loss of the primary cooling system can be compensated with normal water pumped into the cooling circuit. On the other hand, the standby liquid control system (SLC) consists of a solution containing boric acid, which acts as a neutron poison and rapidly floods the core in case of problems with the stopping of the chain reaction. Pressurized water reactors also can SCRAM the reactor completely with the help of their control rods. PWRs also use boric acid to make fine adjustments to reactor power level, or reactivity,