information:filtering, structural:reformatting
Protection

Hydrogen-rich ordinary water effects neutron absorption in nuclear fission reactors: Usually, neutrons are so strongly absorbed by normal water that fuel enrichment with a fissionable isotope is required. (The number of neutrons produced per fission depends primarily on the fission products. The average is roughly 2.5 to 3.0 and at least one, on average, must evade capture in order to sustain the nuclear chain reaction.) The deuterium in heavy water has a very much lower absorption affinity for neutrons than does protium (normal light hydrogen). Deuterium is, therefore, used in CANDU -type reactors, in order to slow (moderate) neutron velocity, to increase the probability of nuclear fission compared to neutron capture.