As the moderator (water) increases in temperature, it becomes less dense and slows down fewer neutrons, which results in a negative change of reactivity. This negative temperature coefficient acts to stabilize atomic power reactor operations.
MODERATOR TEMPERATURE COEFFICIENT OF REACTIVITY
Meaning of MODERATOR TEMPERATURE COEFFICIENT OF REACTIVITY in English
NRC Nuclear Energy English Glossary. Английский глоссарий ядерной энергии NRC (Национальный научно исследовательский совет). 2012