that the exposure dose for 50 years after the intake (committed effective dose) is evaluated by modeling behavior of radioactive materials in the body, and the evaluated dose added to the external-exposure dose should be considered as the dose of the fiscal year concerned. Since the evaluation of internal exposure is so complicated, in all NPPs, strong efforts are made not to cause a significant internal exposure, as described in Section 6.3.2.

There are three kinds of methods for measuring and evaluating internal exposure, as shown in the following, but from the above-mentioned fundamental management concept, focus is on the actual control for determining if internal contamination exists or not.

1. A whole body counter (WBC) provided with a plastic detector, Naf detector, or Ge semiconductor detector is used to directly measure internal contamination.

2. The quantity of internal radioactivity is indirectly determined by measuring the quantity of radioactivity of excreta.

3. The concentrations, nuclides, and quantities of radioactive materials inhaled by workers are calculated using the measured values of particulates in the air of a workplace and then internal contamination is estimated. This method is used also for assessment of doses other than due to inhalation such as swallowing or absorption from the skin.

The measurement and evaluation of internal exposures of radiation workers are carried out at the time of each entry and exit and every one to three months as shown in Table 6.7.1.

3. Target of Individual Dose Control

First of all, the purpose of dose control is to comply with the dose limits provided in laws, but actually, each NPP establishes and applies voluntary dose limits lower than those of the laws as its control target for the fiscal year. Moreover, there are many power plants that provide dose monitoring targets for a day, a week, or a month for careful radiation control. On the other hand, by setting planned doses for individuals and for each work activity, they can be used as targets for dose reduction.

3. Records of Individual Dose

The exposure doses are recorded at each evaluation, and the reactor licensees and employers retain them in their radiation control ledgers. The dose data are registered and transferred to the Radiation Dose Registration Center by the reactor licensees. Moreover, individuals are notified about the data.

In addition to this, the employer enters the dose data in each individual worker’s Radiation Control Notebook as dose records and history confirmation. Moreover, the Notebook contains records on education and health examinations and covers all the information required for individual control.

3. Track Record of Individual Dose

Historical trends in Japan of the number of NPPs, radiation workers, and total doses are provided in Figure 6.7.1. As of 2007, there were 55 units in operation. . The total number of radiation workers who worked in NPPs in a fiscal year had continuously increased until around 1993 with the increase in the number of NPPs, but since 1994, it has been almost constant or declining slightly.

The total dose had a tendency to decrease yearly after peaking in 1978, and since the 1990s, it has been almost constant although there was some increase or decrease due to large-scale work projects.

The reason for the high total dose in 1975 and afterwards was due to work undertaken to implement countermeasure against damage of steam generator tubes in PWRs and against SCC and feedwater spargers in BWRs. The plants newly commissioned after those countermeasures were almost finished incorporated the measures, and from 1980, the total dose has been decreasing although the number of plants has increased.

From 1982 until around 1990, the number of radiation workers increased or stayed constant with an increase in the number of NPPs, but the total dose decreased. This is considered due to the results of work efficiency improvements by upgrading the working environment and the jigs and tools used, and of active application of rationalization technologies, such as automation, and widespread applications of exposure reduction measures, such as crud (corrosion product) reduction, installation

6- 19

NSRA, Japan

of shields, and implementation of decontamination procedures.

The periodical inspection works that account for 70 to 80 % of the annual total dose were those associated with the primary system, including the nuclear reactor and the coolant clean-up system, or the steam generators (for PWRs). The trend of annual dose per NPP (Figure 6.7.2) had a remarkable reduction for BWRs, and the dose was almost constant for both PWRs and BWRs since 1990s.

As shown in Figure 6.7.3, the annual average dose per radiation worker decreased year after year; in the 1990s, there were ups and downs depending on the scale and/or place of work projects and it has been slightly exceeding 1 mSv. Moreover, in recent years, the radiation workers who received one tenth or less of the annual dose limit (5 mSv/year or less) have accounted for 90 % or more of all radiation workers, that is, almost all radiation workers have received low doses, and no person received a dose exceeding 25 mSv/year.

The number of radiation workers who enter a NPP for any work activities (including periodic inspections) in one year is about 1200-1300 on average, and total number of persons required during normal plant operation is less than one half of that In addition, the number of radiation workers for PWRs is fewer than that for BWRs because workers for PWR turbines are not required to be registered for radiation control (Figure 6.7.4).

Figure 6.7.5 shows trends in major counties for the annual average total doses per NPP The annual average total dose per plant in Japan was lower than that in several of the countries until 1996. After that, the total dose in Japan remained roughly flat, but other countries’ total doses have continued to

decrease; since 1998, the total dose in Japan has been the highest. In the US, regulatory reform and introduction of high burnup fuel etc. shifted the operating cycle to operations of 18 to 40 months with refueling outages of 30 to 40 days, and the capacity factor has improved from about 70 % in 1997 to 90 % or more in recent years. The capacity factors in Finland, Switzerland, Spain, and Korea are also over 90 % nowadays. On the other hand, the Japanese capacity factor was the highest in 1998 at 84.2 %, but went down to 71.9 % in 2005. The plants in Japan operate for 14 months at the maximum with average periodical inspection outages of 80 days, which is considered as one of the reasons why the annual average total dose per NPP is higher. It would be possible to reduce the annual average total dose by reforming various systems, but, as the total doses are fundamentally represented by the dose rate of the working environment (Sv/h) times the number of working hours (h) times the number of radiation workers, it is necessary to make active efforts to reduce working hours by improving working efficiencies and decreasing the number of workers. Furthermore, in order to reduce the total dose, source reduction measures for reducing the dose rate of the working environment, etc. should be taken.

NSRA, Japan

6-20