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:
The neutron flux causes a change in the composition of the cooling water by converting 16O to 17O and then further transforming 17O into carbon-14. This process is undesirable because carbon-14 can escape into the environment.