Figure
2,4.4
Irradiation embrittlement monitoring and brittle fracture prevention
Control rod drive housing
The control rod drive
housings are welded to the bottom head of the RPV through
the control rod drive penetrations. The control rod drive housings
transmit the gravitational loads of the control rods, control rod
drives, control rod guide tubes, fuel support pieces and fuel
assemblies to the RPV bottom head. A failure of the control rod
drive housing may lower the control rods and damage the fuel
elements due to neutron population changes. To prevent such
accidents, support pieces are provided to the housing.
In-core monitor housing
The in-core monitor
housings are welded to the inner surface of the RPV
bottom head, penetrating the RPV through the in-core penetrations.
At its top, an in-core monitor guide tube is welded, and at its
bottom end, a sealing flange is fastened with bolts.
RPV support skirt
The RPV support skirts are fixed, via a flange, onto the concrete
and steel support pedestal, with anchor bolts.
RPV stabilizer
The RPV stabilizers are
located around the top of the RPV, surrounding the vessel. Their two
ends are fixed to the top of the shielding wall and the stabilizers
carry the vessel bracket loads to the shielding wall. They also bear
the RPV lateral support
ABWR RPV
In an ABWR, the shape of the RPV bottom head is changed from the
conventional hemispherical shape to a dished head, in conformity
with the RIPs. In the conventional BWR RPV, the support skirts are
fixed to the RPV bottom head, whereas in an ABWR, the conical
support skirts are fixed to the RPV cylindrical body wall to avoid
interference with the RIPs. The shape of the main steam nozzles of
the ABWR is significantly changed from that of the BWR nozzle shape
to provide flow limiter functions to their safe ends.
The recirculation pumps and its motors are designed based on the
following philosophies.
The recirculation pumps are operated at variable speeds between
20-100% of the rated speed, being powered by the variable-frequency
electric power sources.
At the rated reactor water temperatures (about 27813),
the recirculation pumps provide a sufficient coolant flow to the
reactor at the rated speed.
The coast down features of the recirculation pump and the motor
ensure the core integrity under reactor transient conditions.
A shaft seal system of high reliability is used to dispense with
daily maintenance of the recirculation pumps, since they are
installed
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Recirculation pumps
Chapter
2 Systems of BWR Nuclear Power Plants
inside the PCV. In addition, the recirculation pumps are equipped
with monitoring devices for accurate detection of any anomalies.
Figure 2.4.5 illustrates a recirculation pump structure.
The recirculation pumps are stainless steel single- stage
double-volute vertical centrifuge pumps.
Figure
2.4.5 Reactor recirculation pump
Shaft seal
The shaft seal of a recirculation pump is a cartridge seal
consisting of two serially-installed mechanical seals. Each
mechanical seal has the capability to seal the entire reactor
pressure of about 7 MPa [gauge]. The cartridge seal has a built-in
pressure regulator that distributes the system pressure equally to
the two mechanical seals.
Figure 2.4.6 shows one example of three different types of internal
structures of cartridge
Figure
2.4.6 Recirculation pump mechanical seal
seals. Part of the seal water is discharged from the seal, in order
to divide the seal pressure equally between the two mechanical seals
(controlled bleed-off). The cartridge seal structure is designed so
that, if either of the mechanical seals fails, all of the system
pressure will be automatically borne by the other seal.
Seal conditions are monitored by measuring the temperatures and
pressures both in the primary (lower) and secondary (upper) seal
rooms. The leak rate and the controlled bleed-off flow from the
secondary seal are also measured. During normal reactor operations,
the pressures in the primary and the secondary seal rooms are about
7 MPa [gauge] and 3.5 MPa [gauge], respectively. Low- temperature
clean water is injected into the primary seal room from the
recirculation pump seal purge system for preventing the seal surface
damage due to the unplanned introduction of foreign items. The
injected purge water also prevents seal contamination by preventing
the reactor water inflow into the seal assemblies. Ulis
also prevents the reactor water leakage to the outside via the
controlled bleed-off.
Heat exchanger and thermal barrier
The heat of seal assemblies is rejected by an internal heat
exchanger built in the casing cover of the pump. The rejected heat
is transferred to the heat exchanger cooling water supplied from the
highly reliable reactor building closed cooling water (RCW) system.
Even when this cooling water supply is lost, the seal continues
being cooled by the purge water injected from the recirculation pump
seal purge system. In the mixing zone of the cool purge water and
the hot reactor water, a heater-mounted thermal barrier is installed
to prevent the surrounding parts from thermal fatigue cracking due
to temperature fluctuations in the mixing zone.
Pump bearing
The pump bearing is a non-contact hydrostatic submerged bearing with
automatic alignment capabilities, located above the impeller. The
pump discharge pressure is used for automatic aligning of the
bearing; the auto-aligning force is provided by flowing part of the
impeller discharge water through the small gap between the bearing
and the journal. To this end, a few pockets are formed
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on the internal surface of the bearing. The axial thrust of the
pump, generated from the reactor pressure, is borne by the thrust
beating mounted on the top of the motor.
Decoupler
A decoupler is placed between the pump shaft and the motor shaft.
This allows the replacement
of a mechanical seal cartridge without hoisting the motor. This
space for the coupling spacer between the motor and the pump is
secured by the motor frame, on which the motor is mounted. The
pump shaft is engaged with the coupling spacer through a special
gear that simplifies the shaft centering for connection.
Pump motor
The recirculation pump motor is a squirrel-cage induction-type motor
with a built-in thrust bearing and a radial bearing on its top and a
built-in radial bearing on its bottom.
The thrust bearing consists of pivoted segmental shoes, allowing the
shaft to rotate in either normal or reverse directions. As such, the
thrust bearing maintains its integrity even if the pump temporarily
rotates in the reverse direction when one of the two operating
recirculation pumps suddenly trips. In designing the radial bearing,
sufficient care is given to vibration, since the recirculation pump,
its motor and connected piping are all suspended. Every bearing is
the self-lubricating type. They are lubricated by simply being
immersed in an oil tank. No forced- lubrication oiling devices are
needed. Metal temperatures and winding temperatures of each bearing
are measured as part of operations monitoring, together with
vibrations of the motor shaft and the top of the motor.
The reliability, pumping capacity and performance of the
recirculation pumps and motors are checked by running tests,
performed during the manufacturing process. The running tests are
done under the same operating temperature and pressure conditions as
those of the reactor. They are regularly disassembled and checked
during their service.
Main steam isolation valve
(MSIV) Following are the design philosophies of the
MSIVs.
The MSIVs stop the discharge of reactor coolant (steam) within a
required time (5 s), upon a main steam line rupture.
The valves close at the timing (minimum 3 s) when simultaneous
closure of all MSIVs does not cause the reactor system pressure to
exceed the critical value.
The valves are designed to be fail-safe.
The valves have a slow-closure
function that allows for on-line performance tests during normal
operation of the plant
The structure of an MSIV is shown in Figure 2.4.7. The carbon steel
Y-pattern glove valve is
driven by the pneumatic (air or nitrogen) and spring mechanisms. The
MSIVs are welded to the main steam line.
Figure
2.4.7 Main steam isolation valve (MSIV)
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