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  1. Release from the building

    Release from the building

    (Note) These numbers are to be used for the whole body dose evaluation together with those of noble gases.

    Figure 7.4.2(1) Process of iodine release during the main steam line break (major accident)

    1. Dose Evaluation

    Based on the released noble gases and iodine calculated by the above-mentioned methods, doses are calculated as follows.

    (1) Evaluation of individual doses

    In accordance with the "Meteorological Guide for Safety Analysis of Nuclear Power Reactor Facilities", individual doses are evaluated according to the following procedures. For this evaluation, the meteorological data for one year were collected before an application was made for approval of the NPP establishment

    1. For each sector of 16 directions, the maximum dose location is determined.

    For each of the 16 maximum dose locations, using all meteorological data for one year and changing the release initiating time (hour), the dose during duration of each release is calculated. The dose*2 corresponding to 97% frequency of appearance is determined by accumulating the obtained 8,760 dose data sets in ascending order (corresponding to the number of total hours for one year, i.e., the number of selected hours for initiating time (hour)).

  2. The maximum dose out of the 16 doses corresponding to 97% frequency of appearance

(*1 2) Equivalent to the dose under meteorological conditions rarely encountered during a postulated accident

NSRA, Japan

7“ 16

Chapter 7 Safety Evaluation of BWR Plants

Release from the building Release from the building

(Note) These numbers are to be used for the whole body dose evaluation together with those of noble gases.

Figure 7.4.2(2) Process of iodine release during the main steam line break (hypothetical accident)

obtained in the above is treated as the evaluated individual dose.

(2) Evaluation of the cumulative whole-body dose In evaluating the cumulative whole-body dose, a probabilistic evaluation method like the 97% frequency of appearance for individual doses is not used under the present guide, but the following evaluation method is followed under the uniquely determined meteorological conditions.

  1. The cumulative value of man-Sv in each sector of

16 directions is calculated under the following conditions ii) to iv).

  1. Wind speed is 1.5 m/s.

  1. T'he F-type atmospheric stability is used for the vertical diffusion (that is, there is little diffusion under stable weather conditions, which makes radioactive materials reach remote places).

  2. Thirty degrees is used for the horizontal diffusion (this corresponds to the diffusion angle 0 of 0.1 of the Type-C atmospheric stability, and larger diffusion than the F-type is assumed because of long-time diffusion).

  3. The maximum dose out of man-Sv cumulative values of each sector is treated as the evaluated cumulative whole-body dose.

Since the above-mentioned evaluation method is used for cumulative whole-body doses,

7- 17

NSRA, Japan

actual frequency of wind direction appearance is ignored, and the diffusion direction toward densely populated areas is naturally chosen as the direction to give the largest man-Sv value.

This method does not assess the adequacy of the

population distribution in the vicinity of a site and the separation, but tends just to show an indicator of "how far the site is from the densely populated areas (for example, Tokyo)." Some dose evaluation results are provided in Table 7.4.1.

Table 7.4.1 Dose evaluation results for major accidents and hypothetical accidents

Analytical result

Major accident

Loss of reactor coolant

Infant thyroid dose

3.1 mSv

Whole-body dose

5.6 x 102 mSv

Main steam line break

Infant thyroid dose

1.2 x 101 mSv

Whole-body dose

2.2 x 102 mSv

Hypothetical accident

Loss of the reactor coolant

Adult thyroid dose

7.6 x 101 mSv

Whole-body dose

2.8 mSv

Cumulative whole-body dose

2,800 person-Sv

Main steam line break

Adult thyroid dose

4.5 x 101 mSv

Whole-body dose

5.8 x 10'2 mSv

Cumulative whole-body dose

5.4 person-Sv