Chapter
3 Systems of PWR Nuclear Power Plants
Materials
The main structural materials of the RV are low- alloy
steels(manganese-molybdenum-nickel and nickel-molybdenum alloys).
These low alloy steels are used because of their well-established
material properties including the following.
(D Excellent high
temperature strength.
Satisfactory workability such as weldability.
Sufficient toughness to avoid brittle fracture.
Low and well known neutron irradiation embrittlement
characteristics.
The RV interior surfaces in direct contact with the reactor coolant
are clad with corrosionresistant stainless steel.
Irradiation embrittlement surveillance
The ductility of ferritic steel exposed to fast neutron dosage rates
greater than 10,8n/cm2
gradually decreases. Since PWR RV walls receive a fast
neutron dosage of lO'
Wcm2,
it is expected that the notch toughness of vessel materials will
decrease over their service life. To obtain information on the
extent of the irradiation embrittlement, capsules containing RV
material test specimens are placed in the vessel around the core and
removed from the vessel on a planned schedule basis after which
various metallurgical tests are performed on them. Data gained by
these tests are analyzed by linear fracture mechanics techniques to
determine the allowable operating pressure - temperature profiles as
seen
in Figure 3.4.4 to prevent the RV
from failing by a brittle fracture mechanism.
Stress analyses
The RV is subjected to transients including the plant start-up and
shutdown operations, load changes and accidents during the service
live. Stresses developed during these transients are analyzed in
detail, to confirm that the design criteria in the applicable
regulation and codes are satisfied.
Design specifications for a typical three-loop RV are given in Table
3.4.1. Also, Table 3.4.2
lists the laws, regulations, and codes, upon which the primary
system components are designed and manufactured.
Tests and inspections of the RV
In manufacturing the RV, composition analyses, mechanical tests and
non-destructive inspections are performed on the raw materials, and
various non-destructive inspections and pressure tests are carried
out during the manufacturing processes. After the commencement of
service, planned inservice inspections of the RV are performed
from its internal surface side, after the core internals are
removed.
Structure
The SG, as shown in Figure 3.4.5 is a vertical, recirculation type
heat exchanger. It consists of
Monitor
the ductility reduction by the encapsulated specimens
Control
the pressure and the temperature '
in
a way to satisfy the limiting condition
for prevention of
brittle fracture of
reactor vessel 7
Figure
3.4.4 Monitoring and prevention of embrittlement by irradiation
3-37
NSRA,
Japan
Steam generators
Table
3.4.1
Reactor vessel specifications (3-loop) |
Vertical cylindrical vessel with hemispherical lop and bottom heads |
Max. Design Pressure |
17.16 MPa (gage) |
Max. Design Temperature |
343’C |
Operating Pressure |
~15.4 MPa (gage) |
Coolant Inlet Temperature |
~284t’C (al rated power) |
Coolant Outlet Temperature |
~321‘C (at rated power) |
Major Dimensions |
cf. Fis.3.4.2 |
Material Basic |
Low-alloy steel and tow-alloy forged steel |
Clad |
Stainless steel |
Stud |
high tension steel |
a primary coolant channel head, a U-type heat transfer tube
bundle and a set of secondary side moisture separators.
The hemispherical primary coolant channel head is separated into
inlet and outlet sections by a partition, both of which are
connected to the reactor coolant piping. A manhole is provided in
each section to permit access for inspection and maintenance work
of the U-tubes.
The U-type tube bundle consists of over 3000 heat transfer tubes.
The U-tubes are seal welded to the tube sheet at both sides of
the tube to plate joints, after being fixed to the plate by a
pipe expanding process. The tubes are supported by horizontal
support plates at regular intervals and by antivibration
bars at their bent portions. An annular water path is provided
between the tube wrapper and the lower shell.
Moisture separating equipment is located above the top of the
tube wrapper. A blow-down piping is connected just over the tube
plate surface to drain
the secondary cooling water down to the blowdown system for
the water quality control. The secondary side moisture separation
equipment consists of screw type swirl vane steam separators and
a wave forming chevron type moisture separator unit The
main steam pipe is connected to the top of the upper shell, and
the main feed water pipe is connected to the cylindrical shell at
a level above the tube bundle.
Reactor coolant flows into the SG through the inlet section of
the channel head; it transfers heat to the secondary coolant
while flowing through the U-tubes, and flows from the SG through
the outlet channel to
the reactor coolant loop piping.
Feed water flows into the secondary side of the SG through the
inlet nozzle and the feed water ring located just above the top
of the heat transfer tube bundle. The water mixed with the
circulating water (separated from the steam by the moisture
separating equipment) flows down through the annular space
between the tube wrapper and the shell. As the water reaches a
point just above the tube plate, its flow direction changes
upward and it flows through the tube bundle, receiving heat from
the reactor coolant and generating steam voids. The
upward flow of the steam-water mixture enters the swirl vane
assembly, where the water droplets are separated from the steam
flow by centrifugal forces acting on the droplets. The separated
water flow is mixed with the feed water and it circulates through
the tube bundle. On the other hand, the steam flows out of the SG
through the outlet nozzle located on its top, after its moisture
contents are removed in the moisture separator unit, and then it
flows to the turbine via the main steam piping (see Figure
3.4.5).
Table
3.4.2 Laws, regulations and standards relating to primary coolant
system components |
Laws, regulations and standards |
Reactor vessel Steam generator Pressurizer Piping & valves composing the reactor coolant pressure boundary |
Ordinance of Establishing Technical Standards for Nuclear Power Generation Equipment Codes for Nuclear Power Generation Equipment (The Japan Society of Mechanical Engineers) Japan industrial standards (IIS) Electrical technology guidelines (Japan Electric Society) ASME (American Society of Mechanical Engineers) ANSI (American National Standard Institute) ASTM (American Society for Testing & Materials) |
NSRA,
Japan
3-38
Chapter
3 Systems of PWR Nuclear Power Plants
Steam
outlet nozzle
Transition
wrapper
Anti-vibration
bar
Moisture
separator
Steam
separator
Tube
wrapper
Heat
transfer tube
Stay
rod
Flow
distributor
Level
gage nozzle
Primary
side manhole
Drain
nozzle
Channel
head
Secondary
side manhole
Level
gage nozzle
Feed
water inlet nozzle
Level
gage nozzle
Inspection
hole
Inspection
hole
3.5m
Inspection
hole
Inspection
hole
Blow
down header nozzle
Blow
down header
Coolant
inlet & outlet nozzle
Figure
3.4.5 Steam generator
Feed
water distribution ring
e
Design of SGs
The primary sides of the SGs are
integral parts of the reactor coolant pressure boundary system , and
they play important safety roles in both preventing accidents from
occurring and limiting their consequences. Therefore, the SGs are
designed so that their integrity and proper functions are maintained
over their service life,
Materials
The materials used for the main structural
parts of SGs must have well-known properties that are qualified by
their rich service records. In particular, the following
requirements should be considered in the selection of materials for
nuclear use.
® Workability such as
weldability. ©Toughness to
avoid brittle fracture.
Corrosion-resistant characteristics (especially
for the heat transfer tubes).
The body of the SG is constructed of low alloy steel and low alloy
forged steel. The heat transfer tubes are made of nickel-based alloy
690 (Inconel ®-690) to
improve their corrosion resistance properties. The interior surfaces
of the channel heads in direct contact with the reactor coolant are
clad with stainless steel to minimize their corrosion, and the
primary coolant side of the tube sheets are clad with nickel-based
alloy (Inconel®), in consideration of the welding of the tubes to
the tube sheets.
Stress analyses
Similar to the RV, SGs are also subject to plant transients
including the startup and shutdown operations, load changes and
accidents, during their service lives. Stresses developed during
these transients are analyzed in detail to
NSRA,
Japan