Electricity generated by nuclear power is now common in Japan. The present commercialized status of nuclear power generation technology has been established due to efforts of many engineers and also as a result of information accumulated from other advanced countries. Presently 23 percent of total electricity generated in Japan depends on nuclear power, and in the long run, this proportion is expected to increase further. Although accumulation of various important technological knowledge is required in nuclear power generation, the conversion of heat generated in nuclear reactors into electrical energy by means of a working medium is one of the most essential problems in the relevant technology. This paper deals with rotary shaft seals used in the circulation pump for the primary coolant, as an important functioning component in such a system. Once this sealing system causes trouble such as excessive leakage, it results in quite serious social problems; i.e., not only power failure but also possibly radioactive contamination.