From analyses of the isotopic contribution of radioactive contamination on major components around the main boiling water reactor turbine system and the isotopic distribution along the system, the following conclusions are drawn:1. Contamination, especially on components that are installed downstream of the low-pressure turbine, is caused by the drain of surplus reactor water into the main condenser.2. Contamination is from radioactive fission products and radioactive corrosion products that are carried with the main steam flow.To reduce radioactive contamination, it is most important to drain the hot water directly into the radwaste system. To reduce contamination from carryover radioactivity, it is essential to suppress the radioactivity in the reactor water. Preventing fuel defects and suppressing the radioactivity of crud in the reactor water can decrease contamination of the main turbine system and minimize the contaminated area, which lessens the possibility of internal exposure.