A pressurized water reactor (PWR) cannot run with natural uranium due to high neutron capture cross section of protium present in light water. If so, it seems that this protium after capture should be changed into deuterium. So, how much light water is converted into heavy water in PWR? Is it a significant amount? Should it be taken into account when calculating the reactivity? Could the light water from PWRs serve as a more efficient resource for heavy water production? Probably not, beacause the amount of water in PWR is low, but I would like to receive some more information about this.