Addendum the second to The_Custodian's write-up:

Another huge advantage to using pressurized water concerns water's unique properties as a moderator. A moderator slows down and reflects neutrons back into the fuel, increasing the probability that they will be absorbed by the fuel, thus causing reactor power to increase. This increase in power, in turn, causes temperatures inside the reactor to rise. As temperatures rise, the density of the water decreases and thus reduces its ability to reflect neutrons back into the fuel. This causes reactor power to go down. This cycling makes a pressurized water reactor inherently more stable. This property is known as a negative coefficient of reactivity.

For an RBMK reactor, such as Chernobyl, water was used as a coolant, but graphite was used as the moderator. The process begins the same as in a pressurized water reactor, but when the temperature goes up, the ability of graphite to reflect neutrons does not change significantly, unlike water. This causes power and temperature to go up, which will eventually cause the coolant to boil off, removing the reactor's ability to cool itself. Since steam isn't a moderator, more neutrons reach the graphite, causing power and temperature to continue rising higher and higher in an unending cycle now that there is nothing cooling the reactor.

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