Chapter
2 Systems of BWR Nuclear Power Plants
(OG
: off gas)
good charcoal bed performance for noble gas absorption, the offgas
is typically passed through the bed after it has been sufficiently
dehumidified. The charcoal bed performance has been recently
maintained by controlling off-gas humidity through temperature
control of the room where the charcoal bed is installed.
The off-gas from the charcoal bed is then discharged to the
environment after HEPA filters remove fine particles, such as solid
daughter nuclides generated by radioactive decay.
The off-gas from the turbine gland seal can be directly discharged
to the environment from the stack because its activity is kept
negligible by providing the turbine shaft seal with steam generated
from the evaporator into which clean water in the condensate storage
tank has been fed.
The degree of vacuum in the condenser during startup and shutdown
operations is maintained by the startup and shutdown air bleeder
driven by steam generated by the house steam system and the off-gas
from the air bleeder is processed by the aforementioned noble gas
hold-up system. This keeps the vacuum pump off-gas activity so low
as to be negligible. Therefore the off-gas can be released to the
environment directly from the stack.
When the off-gas is discharged to the environment, its radioactivity
is measured at the stack. The exposure dose rate and inair
radioactivity concentrations outside of the environmental monitoring
area should not exceed the values, which are defined by METI in the
"Notification defining the dose limits according to the rules
for the installation, operation, etc. of commercial nuclear power
reactors." In addition,
target values for radioactive noble gas and iodine gas release
control, established according to the NSC Regulatory Guide L-RE-LO
*7 "Annual Dose Target for the Public in the
Vicinity of Light Water Power Reactor Facilities", should be
met
Example target values for typical plants are approximately 6.7 x
10lsBq/y
for radioactive noble gas and approximately 2.3 x 10nBq/y
for radioactive iodine (1-131).
(These numbers were obtained as the sum of seven units at one site;
Kashiwazaki-Kariwa site)
Main sources of liquid wastes generated in a BWR plant are leakages
from pumps and valves in the primary cooling systems, discharges
from sampling lines and back wash water for ion exchange resins
(collectively, they are called equipment drains or low conductivity
liquid wastes), miscellaneous house water used in the reactor
building (i.e. floor drains), liquid wastes generated by
regeneration operations of ion exchange resins in the demineralizer
(i.e. regeneration liquid wastes), and liquid wastes generated by
washing clothing used in controlled areas (i.e. laundry drains).
Floor drains and regeneration liquid wastes together are called high
conductivity liquid wastes.
The amount of liquid wastes generated during normal operations of a.
1,350 MWe class nuclear power plant can be estimated as follows:
equipment drains: approximately 55 m3/day;
floor drains: approximately 10 m3/day; and
laundry drains: approximately 5 m3/day.
(*7)
[Translator’s note] It
is downloadable
from http:/www.nsc.go.jp/NSCenglisli/.
2-
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NSRA,
Japan
Figure 2.9.1 a typical flow sheet of gaseous wastes treatment system (Example of a 1,100 mWe bwr plant)
Liquid Waste Treatment System
The regeneration liquid wastes are generated intermittently and
are approximately 10 m3/
regeneration. Since the amount of floor drains has decreased in
recent plants, floor drains and equipment drains are treated
together in the same system.
All the liquid wastes are collected and processed according to
their characteristics by the liquid waste treatment system to
remove chemical impurities and radioactive materials. The
processed liquids are reused in the plant as much as possible or
they are discharged to the sea after dilution with cooling water
from the condenser, if their radioactivity has been removed to a
level as low as practically achievable. Figure 2.9.2 shows an
outline of the liquid waste treatment system.
Equipment drains (Low
conductivity liquid wastes)
Equipment drains are leakages from pumps and valves in the
primary coolant system and back wash water for ion exchange
resins. They contain a
high concentration of radioactivity but they are relatively pure
chemically. They are
treated through filtration and ion exchange processes to remove
particles and dissolved impurities as ions. The processed liquid
is collected in the condensate storage tank as makeup water
after concentrations of radioactivity and chemicals are checked
to be under required limits.
As a filter component, previously pre-coat type filters with
filter aid were widely used, but mechanical
filters, such as membrane filters and electromagnetic
filters, have been used recently because they do not need the
filter aid which results in less secondary waste generation than
for precoat filters.
Floor drains and
regeneration liquid wastes
(High conductivity
wastes)
Floor drains are generated from miscellaneous liquids and they
generally contain low radioactivity but have high levels of
chemical impurities. Regeneration liquid wastes are generated
from ion exchange resign regeneration operations and contain a
relatively high concentration of radioactivity and high levels of
chemical impurities such as sodium sulfate. Both floor drains and
regeneration liquid wastes are processed by an evaporator. The
evaporated steam is condensed in the condenser and the condensate
is then collected in the condensate storage tank as makeup
water after demineralization, or the condensate is discharged to
the sea by diluting with condenser coolant after concentrations
of radioactivity and chemicals are checked to be under required
limits.
The concentrated liquid from the evaporator bottom is stored in
the temporary concentrate
storage tank to allow short half life nuclides to
decay and then it is sent for treatment in immobilization
processes.
Laundry drains
The laundry drains are
generated from washing clothing used in controlled areas and they
generally contain a very low concentration of radioactivity and
have high levels of chemical impurities. They are discharged to
the sea by diluting with the condenser coolant after
concentrations of radioactivity and chemicals are checked to be
under required limits. In order to further reduce radioactivity
discharged to the environment, some plants use a charcoal
adsorption process and an evaporator.
When the wastes are discharged to the environment from the above
treatment processes, the processed wastes are stored in the waste
sampling tank, where the concentration of
equipment
drain system
(tow
conductivity
liquid r
waste
system)
floor
drain system
(high conductivity
liquid
waste
system) regeneration
liquid waste
system(high
conductivity
Dquid waste system)
laundry drain
system
collection
tank —
filter |
|
concentrator |
|
|
|
collection tank |
|
concentrator |
|
|
|
collection tank |
|
filter |
|
-
receiving tank
demineralizer
condensate
storage tank
^outlet
to
canal
Figure
2.9.2 A typical flow sheet of liquid waste treatment system
NSRA,
Japan
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