Complex sentence: When a nuclear chain reaction in a mass of fissile material is self-sustaining, the mass is said to be in a "critical" state in which there is no increase or decrease in power, temperature or neutron population.
Split into simpler sentences: When a nuclear chain reaction in a mass of fissile material is self-sustaining, the mass is said to be in a "critical" state. In this state, there is no increase or decrease in power, temperature, or neutron population.