Criticality and a stable operating point
In a pressurized water reactor, water is used both as a moderator and a coolant. The criticality (or criticity) peaks above 1 for the repartition (A) between fuel and water. Conversely, the reactor does not function without fuel or moderating water (points 1 and 0 on the % axis). Between these extremes, there are two distributions B and C for which the criticality is equal to 1. The distribution (B) is chosen by engineers because it provides stable operation: If the reactor enters into the zone of divergence of the chain reaction, the temperature increases and the amount of water (moderator) decreases. More neutrons are lost, which naturally brings the criticality back to 1.