In the course of research done it was concluded that circulation of pool water through the nuclear reactor core produces a bactericidal effect on microflora due to influence of radiation of various types. Contents of microbes returns to the initial level after 2-4 months after circulation was stopped. Microflora of pool water comprises big amount of coccus, G-positive rods and fungi and a lower content of G-negative rods if compared to water which had been used to fill reactor pool. There is an increased number of radioresistant forms with intensified production of catalase and nuclease. Supposedly, presence of these enzymes gives to the microbes certain advances to survive in high-radiation zones.