[Source]
Atomic Energy Society of Japan, Special Committee for Research of
High Temperature Water Chemistry, "Water Chemistry Control and
Fundamental
Technology of Nuclear Power Plants"
3.0
2.5
f'2.0
£
iD
KJ
^1.5
C.O
o
.g1
■ &I.0
u.
Ph
0.5
0.0
0 5 1 0 1 5 20 25
EFPY
NSRA,
Japan
6-6
Figure 6.3.1 (1) Trends in dose equivalent rates of the bwr plr system piping
Chapter
6 Radiation Control
EFPY SE
Japanese
plants
[Source]
Atomic Energy Society of Japan, Special Committee for Research
of High Temperature Water Chemistry, "Water Chemistry
Control and Fundamental Technology of Nuclear Power Plants"
Figure
6.3.1 (2) Trend in dose equivalent rates of the PWR steam
generator water box
(man-Sv)
0
10
8
6
4
2
75 76 77 78 79 80 81 |
PWR A |
H-l Hamaoka -1 |
TK - 2 Tfikahama - 2 |
IF-5 Fukushima daiichi - 5 |
M-3 Mthama - 3 |
TO-2 Tokai daini |
1-1 Jkata - 1 |
1F-4 Fukushima daiichi - 4 |
O-l Ooi - 1 |
H-2 Hamaoka-2 |
-2 Ooi-2 |
IF"6 Fukushima daiichi - 6 |
G-l Gcnkai - 1 |
2F-1 Fukushima daini - 1 |
G-2 Genkai - 2 |
2F-2 Fukushima daini - 2 |
1-2 Ikato - 2 |
ON-1 Onagawa - 1 |
S’i Scndai-1 |
2F-3 Fukushima daini - 3 |
S-2 Sendai - 2 |
K-l Kfidiiwaziikj-kariwa -1 |
TK-3 Tfdmhama - 3 |
2F-4 Fukushima daini - 4 |
'IK-4 Tnkahama • 4 |
H-3 Hamaoka- 3 SH-2 Shimanc - 2 |
TS-2 Tsumga - 2 |
TO-2
w
o-l
IM.
H-2
2F-2
cvILjl2F-4.lU~3
i xx—
82 83 84 85 86*
1 87 88 89 90
Fiscal year
[Source]
Periodical inspection reports of power plants
Figure
6.3.2 Trends in dose equivalent at the 1st plant periodic inspection
6-7
NSRA,
Japan
Table
6.3.4 Example measures for dose reduction |
BWR Plants |
PWR Plants |
||
Measures for reduction at preceding plants |
1. Reduction of crud amount
■ Installation of condensate filters
|
1. Reduction of crud amount
|
||
2. Reduction of cobalt
|
2. Reduction of cobalt ■ Application of low-cobalt materials to fuel assembly grids • Zinc injection into the primary system |
|||
3. Decontamination
|
3. Decontamination
■ RCP internals
|
|||
4. Installation of shields ■ Piping with high dose equivalent rate in the reactor containment and the reactor building |
4. Installation of shields
|
|||
5. Automation / rationalization
|
5. Automation / rationalization ■ Working robots in the S/G water box • Seal plates etc. |
|||
6. Component improvement
|
||||
|
BWR Plants |
PWR Plants |
||
Additional measures for new plants |
1. Crud reduction • Application of low alloy steel to feedwater heater shells |
1. Crud reduction ■ Improvement of S/G tube surface treatment method |
||
2. Reduction of cobalt • Application of low-cobalt materials to feedwater heater tubes • Control of feedwater crud concentration |
2. Reduction of cobalt • Application of low-cobalt materials to materials of the primary system |
|||
3. Decontamination
|
3. Decontamination • RCP internals |
|||
4. Component improvement ■ Application of advanced containment
|
4. Installation of shields • Increase of permanent shields |
|||
5. Component improvement
|
||||
[Notes]
Preceding
BWR
plants: BWR plants commissioned in the 1970s Preceding PWR plants:
PWR plants commissioned by 1985
NSRA,
Japan
6-8
Chapter
6 Radiation Control
Behavior of
Radioactive Materials at NPPs and Protective Measures
Radiation exposure can be divided into two types:
External exposure from
pipes, pumps, valves, etc. of systems with fluids containing
radioactive materials; and
Internal exposure due to the working environment being
contaminated with gaseous or liquid radioactive materials
released from pumps and valves, etc. when they are disassembled
for maintenance etc.
Reduction of workers' radiation exposure requires measures
according to the type, i.e. external
or internal
exposure. Some of the measures should be considered
in advance at the facility design stage (permanent shields,
remote operation, automation, permanent ventilation facilities,
etc.), and other protective measures should be taken at the
given working environment after commissioning (lead wool
shields, temporary tools and devices, local ventilation devices,
etc.).
In planning radiation protection for an area where both external
exposure and internal exposure exist, the latter is currently
controlled so that it does not cause any significant intake
(zero as dose) even if both exposures mean the same thing in
terms of effective dose. Wearing a fully equipped mask to avoid
inconsiderable internal contamination may increase working time
due to lowered working efficiency and as a result, may increase
external exposure resulting in an increase in total dose. It is
necessary to select respiratory protective equipment which does
not increase a burden.
When monitoring data of the regular work environment or
measurement results of individual doses show a necessity to
improve the work environment, or when temporary measures are
taken for improving repeatedly performed works, it is desirable
to take permanent measures for dose and contamination reduction
(i.e. adding a permanent facility).
Lead wool is used in many cases as a typical temporary shield,
but one sheet of lead wool (equivalent to a lead plate of 3 mm
thickness) cuts only 15% of
ft’Co
gamma rays, and 5 laminated sheets are needed to cut it by half,
and 15 sheets to cut it to one tenth (Figure 6.3.3). The
exposure dose accompanying the work of installing and removing
the lead wool could be non-negligible
compared with the dose reduction effect during work activities,
so it is important to consider the net gain.
(one
lead wool is
equivalent to 'S
mm
thick lead plate)
[Source]
"2000 Practical Manual for Shielding Calculation of Radiation
Facilities", Table 6.3.2, Nuclear Safety
Technology
Center
Figure
6.3.3 Shielding effect of lead wool
Design Base for
Radiation Protection Facilities
Design Base Dose
The design base for
radiation protection facilities of NPPs has been established for
safe operation of activities at nuclear reactor facilities,
including operation, shutdown, inspection, maintenance and
repair, and periodic inspection, and for reduction of dose
equivalents of radiation workers and the general public to the
as low as achievable. The design base doses can be summarized as
shown in Table 6.4.1.
Shielding Design Base
The shielding design of a NPP is done to satisfy the design base
which makes the dose equivalent received at each area less than
the legislative limits taking into consideration the standard
occupancy time in the area for work. The standard occupancy
Table
6.4.1 Design base doses |
50 mSv / year |
General public |
Contribution of all sources 1 mSv / year |
Contribution of discharged radioactive materials 50 jiSv / year |
6-9
NSRA,
Japan
time is roughly classified taking into account frequencies of
operation and maintenance, and the shielding design base dose
rates are established as shown in Table 6.4.2. In addition, in
consideration of practical control, some power plants further
lower values for the high dose equivalent rate area.
When the radiation level during operation becomes higher than
that of the design area classification of concern, the occupancy
time may be restricted and/or a temporary shield may be
provided.
Table
6.4.2 Examples of shielding design classification and shielding
design base dose rates |
Plant-1 |
Plant-2 |
Plant-3 |
Plant-4 |
||||
1 |
Design base dose rate due to external radiation |
I |
Design base dose rate due to external radiation |
Classification |
Design base dose rate due to external radiation |
Classification |
Design base dose rate due to external radiation |
|
Outside of controlled area |
A |
1.3 mSv / 3 months or less |
A |
1.3 mSv / 3 months or less |
I |
1.3 mSv / 3 months or less |
I |
1.3 mSv / 3 months or less |
Within controlled area |
B |
Less than 0.01 mSv/h |
A |
0.06 mSv or less |
II |
0.01 mSv/h or less |
II |
0.01 mSv/h or less |
C |
Less than 0.05 mSv/h |
B |
0.01 mSv or less |
III |
0.02 mSv/h or less |
III |
0.15 mSv/h or less |
|
D |
Less than 0.25 mSv/h |
C |
0.06 mSv/h or less |
IV |
0,15 mSv/h or less |
IV |
More than 0.15 mSv/ h |
|
E |
Less than 1 mSv/h |
D |
0.12 mSv/h or less |
V |
1 mSv/h or less |
|
|
|
F |
1 mSv/h or more |
E |
No entry during normal operation |
VI |
More than 1 mSv/h |
|
|
|