The zinc injection has been applied to the Japanese PWR plants for reduction of dose rate of main equipment into the middle of the 2000s. As a result, the dose rate of main equipment was reduced about 40%. In addition to the zinc injection, new measure to reduce of dose rate is demanded. We note the primary water radiation changes significantly, even when water quality and a plant operation parameter are similar value. It is introduced that the approach for identifying the unexplained radiation factor.