Occupational exposure during BWR refueling and annual maintenance periods is determined by radioactive corrosion products, such as 6°C o and 58 Co deposited on major components and piping of the primary cooling system.The authors have investigated behavior of radioactive corrosion products, e.g. generation, activation and deposition processes, in the primary cooling system, and then expressed the findings as mathematical equations. These provide corrosion product simulation models to predict the amounts of radioactive corrosion products depositing on components and piping.The effects of corrosion product reduction procedures on shutdown dose rate were evaluated using the model The procedures were incorporated into the Japanese Improvement and Standardization Program. Improvements of operational procedures to control water chemistry, such as NifFe ratio, as well as application of hardware improvements resulted in an extremely low occupational exposure of less than 90 man·remfyr for currently constructed BWR plants.

In order to cause fewer environmental impacts, nuclear power plant systems and operational procedures with fewer radwaste sources, lower occupational exposures and smaller operational areas contaminated by radioactivity which will minimize internal exposure, have been established. Collaborative efforts have demonstrated that the combined application of major improvements in the systems and better operational procedures (e.g., (1) prevention of fuel defects, ( 2 ) application of low cobalt containing and corrosion resistant materials for the primary cooling system, (3) improvement of condensate water cleanup system and the resins applied to it, and (4) careful water chemistry control) is resulting in BWRs involved in the Japanese Improvement and Standardization Program (JISP BWRs) having occupational exposure of less than 0.5 man.Sv/yr, minimized contaminated areas in turbine buildings to only around the main steam turbines, and radwaste sources producing fewer than 500 drums/yr. Additionally, the JISP BWRs have high reliability with more than a 75% duty factor and unscheduled plant shudown occurrences of fewer than once in 10 years. K E Y W O R D S : B W R type reactors, nuclear power plants, cobalt 60, occupational exposure,contamination, radwaste, water chemistry, radioactivity, reliability, environmental exposure

In order to cause fewer environmental impacts, nuclear power plant systems and operational procedures with fewer radwaste sources, lower occupational exposures and smaller operational areas contaminated by radioactivity which will minimize internal exposure, have been established. Collaborative efforts have demonstrated that the combined application of major improvements in the systems and better operational procedures (e.g., (1) prevention of fuel defects, ( 2 ) application of low cobalt containing and corrosion resistant materials for the primary cooling system, (3) improvement of condensate water cleanup system and the resins applied to it, and (4) careful water chemistry control) is resulting in BWRs involved in the Japanese Improvement and Standardization Program (JISP BWRs) having occupational exposure of less than 0.5 man.Sv/yr, minimized contaminated areas in turbine buildings to only around the main steam turbines, and radwaste sources producing fewer than 500 drums/yr. Additionally, the JISP BWRs have high reliability with more than a 75% duty factor and unscheduled plant shudown occurrences of fewer than once in 10 years. K E Y W O R D S

No abstract

The No. 2 Unit of Fukushima-Daini Nuclear Power Plant (2F-2; I ,1 00 MWe) was commercially operated for 10,320 effective full power hours (EFPH) as its first fuel cycle. The basic design concept of the 2F-2 incorporated the following two features : (1) Application of procedures for reducing shutdown dose rate based on the Japanese Improvement and Standardization Program (2) Low crud generation to minimize radioactive waste by careful material selection for the primary system. Thus, it was possible to keep the average Fe concentration in the condensate water at less than 6 ppb during the first fuel cycle. As a result of this low value, the average life of powdered resin precoated prefilters was extended to about a month, and the average chemical regeneration period of the deep bed demineralizers was extended to more than one year. The water chemistry of the 2F-2 was characterized by low 6°C o and high 58 Co radioactivities in the reactor water, which resulted in a low shutdown dose rate determined mainly by 58 Co depositing on the primary piping. For example, average dose rate around the primary piping just after reactor shutdown was about 70 mR/h, about 7 5% of which was from 58 Co depositing on the pipe inner surfaces. The contribution of 6°C o was about 25%.