The neutron flux slowly converts 16O in the cooling water to 17O by neutron capture, increasing its concentration. The neutron flux slowly converts 17O (with much greater cross section) in the cooling water to carbon-14, an undesirable product that can escape to the environment:
Neutron flux causes 16O in the cooling water to transform into 17O through capture. This process increases the concentration of 17O. However, the neutron flux also converts 17O into carbon-14, a byproduct that can escape into the environment.