the core. The amount of
rare gases released into the atmosphere is about 1.1 x
1014 Bq in the case of a major accident and about
5.3 x 1015 Bq in the case
of a hypothetical accident, compared with the quantity of about 3.0
x 1019 Bq accumulated in
the core.
(3) -1 Evaluation of individual dose
Obtain the maximum dose point for each of
16 sixteen wind directions
Obtain the doses at the maximum dose point for 16 directions during
the period of release while varying starting times of release using
meteorological data for one year
The obtained 8,760 (number
of hours in a year, i.e. the number of selected start times of
release) sets of doses are arranged in order of the smaller amount
of doses and the dose is determined as corresponding to a cumulative
occurrence frequency of 97%*1.
Obtain the maximum dose of 97% probability from the maximum points
for 16 directions and use this value as the individual dose
(3) -2 Evaluation of integral value of whole-body dose
For the evaluation of the integral value of whole-body dose, the
method employed for individual dose calculation with 97% probability
is not used; but the following procedure is used for deterministic
meteorological conditions.
From the following 4 conditions, the integrated value (man • Sv)
is taken for the 16 wind-direction sector with the largest dose:
-Wind speed of 1.5 m/s
-Atmospheric stability Type F for the vertical direction diffusion
(that is to say, assume stable meteorological conditions with little
diffusion at which the radioactive materials reach remote area)
30° for diffusion in the
horizontal direction
Use the maximum integrated dose from 16 direction sectors as the
integrated wholebody dose.
Because such a method is employed for the integral value of
whole-body dose, the direction of diffusion toward high population
areas is necessarily selected as the direction that gives the
maximum value, regardless of the actual frequency of wind direction
appearance.
The doses calculated by the above procedure (for a 3-loop plant) are
shown in Table 8.4.1.
On the assumption that one of the heat transfer tubes of a steam
generator ruptures and large amounts of radioactive materials are
released to the atmosphere through the secondary system, the effect
on the general public in the vicinity of a plant is evaluated in
order to ascertain the pertinence of reactor siting. The procedure
for the evaluation is as follows (Figure 8.4.2 (1)
and Figure 8.4.2 (2) are for a 3-loop
plant).
Assuming that 1% of the fuel rods were already failed before
occurrence of the accident and rare gases and iodine were at
equilibrium during the normal operation, additional release from the
fuel rods would take place when the reactor system was depressurized
at the time of the accident. The additional release is assumed as
follows:
(D For major accident: in
proportion to the reactor system depressurization
(2) For hypothetical accident: release of all the radioactive
materials immediately after occurrence of the accident
Reactor coolant containing radioactive materials leaks through the
hole of the steam generator tube to the secondary side. It is
assumed that 99% of the iodine leaked to the secondary side is
nonorganic iodine and the gas-liquid
partition factor is 100 at the secondary side, with some
part of it is transferred to the steam. The remaining 1% is organic
iodine (this is the same for both a major accident and a
hypothetical accident).
It is assumed that the rare gases and organic iodine, in the
radioactive materials transferred to the secondary system, are
released into the atmosphere
(*4)This
corresponds to the dose under very rare meteorological conditions at
the postulated accident.
NSRA,
Japan
8~
16
Steam Generator Tube Rupture
Release from fuel rods to the reactor system
Release from the primary system to the secondary system
Release into the atmosphere
Chapter
8 Safety Evaluation of PWR Plants
before the failed steam generator is isolated and some of the
inorganic iodine transferred to the steam side is released into
the atmosphere through the steam relief and safety valves or
leakage of the valves, etc.
The duration of the leakage is as follows:
Major accident: 1 day
Hypothetical accident 30 days
Dose evaluation
Evaluation is made by the same method as in 8.4.2 item (3)
above.
The evaluation results (for a 3-loop plant) are shown in Table
8.4.1.
Table
8.4.1 Result of safety evaluation for major/hypothetical
accident (examples for 3-loop plants ) |
Result of Evaluation |
Critical for Judgement |
|||
3 nJ l-l o 'H* |
Loss of Reactor Coolant |
Child Thyroid Dose :0.0053Sv Approx. Whole Body Dose Due External y -Ray : 0.0012Sv Approx. |
Thyroid (Child) <1.5Sv Whole Body <0.25Sv |
||
Steam Generator Heat Transfer Tube Reputer |
Child Thyroid Dose :0.0037Sv Approx. Whole Body Dose Due External y -Ray :0.0027Sv Approx. |
||||
Hypothetical Accidentt |
Loss of Reactor Coolant |
Adult Thyroid Dose :0.14Sv Approx. Whole Body Dose Due External y -Ray :0.057Sv Approx. Total Whole Body Dose Accumlated :490manSv Approx. |
Thyroid(Adult) <3Sv Whole Body <0.25Sv Total Whole Body Dose Accumlated <20,000manSv |
||
Steam Generator Heat Transfer Tube Reputer |
Adult Thyroid Dose :0.084Sv Approx. Whole Body Dose Due External y -Ray :0.011Sv Approx, Total Whole Body Dose Accumlated :290manSv Approx. |
||||
8-
77
NSRA,
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Unit
: Bq (1-131
Equivalent)
Unit
:
Bq (Converted to 0.5MeV y
Energy)
Total amount o(Iodine
Released : 7.0 x 1011 Bq Approx.
To
Atmosphere
Total
Amount of Rare
Gas
Released
: 7.7 x 10"
Bq Approx.
Organic
:
5.4 x 1011
Bq Approx. Inorganic : 1.5 x 1011
Bq Approx.
t
Total
Amount Relesed
*
before Steam Genereator Isolation
Release
of Inorganic Iodine
assumed through Steam
Generator
Safety
Valve,
etc after the
Isolation
of Steam Generator
Assume
Gas-Liquid Partition Factor 102
for
Inorganic
Iodine
<
Iodine >
Figure
8.4,2(1)
Steam genereator tube reputure (major accident)-Process
of release to atomosphere of iodine and rare gas
Time
for Release : One
Day
<
Rare
Gas >
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Japan
8~
18
Chapter
8 Safety Evaluation of PWR Plants
Unit
: Bq (1-131
Equivalent)
Unit
: Bq (Converted to 0.5MeV y
Energy)
Total amount of Iodine
Released : 3.2 x IO'2 Bq Approx.
To
Atmosphere
Total
Amount of Rare Gas Released :3.1xl016Bq
Approx.
Organic
: 1.7 x 10'-’
Bq Approx. Inorganic : 1.4 x IO'2
Bq Approx.
Total
Amount Relesed before Steam Genereator Isolation
Release
of Inorganic Iodine assumed through Steam Generator Safety
Valve,
etc after the
Isolation
of Steam Generator
Assume
Gas-Liquid
Partition
Factor 102
for
Inorganic Iodine
Time
for Release : 30
Days
<
Iodine > < Rare Gas >
Figure
8.4.2(2) Steam genereator tube reputure (hypothetical
accident)-Process of release to atomosphere of iodine and rare gas
8-
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