contact controlled leakage seal to ensure long life and minimize leakages from the seal. The seal surfaces are composed of erosion-resistant ceramic seal plate which is bolted to a stainless steel retainer. Almost all of the leakage through the No.l seal is drained down to the chemical and volume control tank via the seal return line.

b. No. 2 seal

The No. 2 seal is a contact-type pressure- balanced mechanical seal which at normal operation conditions, seals the leakage from the No. 1 seal at a pressure of approximately 0.4 MPa [gage]. Hie seal assembly consists of a sintered hard alloy runner and a stationary carbon member. Pure purging water is injected to wash down boric acid precipitant, from a head tank into the pump gland behind the No. 2 seal. Hie purge water drain is sent to the containment coolant drain tank together with the leakage from the No. 2 seal. The No. 2 seal is designed to accommodate

No3. Seal

Bolling ring'

No2. Seal

IVol. Seal

Primary coolant discharge nozzl

Stud nuts

Cool ing water outlet

Spool piece

No3. Seal leak-off

Injection water inlet

Purge _ waler inlet

Cooling waler inlet

" Pump

driving shaft Radial bearing Thermal barrier heat exchanger

Impel ter

Casing

Primary coolant suction nozzle

Figure 3.4.9 Reactor coolant pump (Type 93A-1)

the full pressure of the system across it, as a back­up to the No. 1 seal under emergency conditions when the function of the No. 1 seal is lost.

3. No. 3 seal

The third seal, similar to the second one, is a contact-type pressure-balanced mechanical seal, consisting of a sintered hard alloy runner and a stationary carbon member. It seals the leakage from the No. 2 seal and the purge water which was injected into the pump gland between the second and third seals at a pressure of about 0.05 MPa [gage] during the normal operation. The leakage from the No. 3 seal is led to the containment coolant drain tank.

Figure 3.4.10 Shaft seal structure

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NSRA, Japan

Purge water outlet

Demineralized

Figure 3.4.11 Shaft seal system

4. Motor

A schematic drawing of the RCP motor is shown in Figure 3.4.12. It is a three-phase totally enclosed cooling squirrel cage type induction motor with a double-acting Kingsbury- type thrust bearing, and upper guide bearing, both located above the stator. The lubricating oil of these bearings is cooled by an oil cooler which is installed outside the motor. The lower guide bearing is located below the stator, and the lubricating oil is cooled by an oil cooler installed in the bottom of the oil sump. In order to prevent a rapid flow coast-down of reactor

coolant and a resultant rapid decrease in its heat removal capability for the reactor core in the event of electric power supply loss, a flywheel is installed on the top of the motor shaft to increase the rotating inertia of the motor. The motor is equipped with an anti-reverse rotation device and with an oil lift pump which sends high pressure oil to the thrust bearing during the start up of the pump.

6. Valves

The most important valves of the RCS are the pressurizer safety and relief valves, and the pressurizer spray valves. These valves prevent the RCS from over-pressurizing by regulating and limiting the pressurizer pressure. The valves are described next.

1. Pressurizer safety valves

The pressurizer safety valves are spring-loaded valves which discharge the pressurizer steam to the pressurizer relief tank when they are lifted. They are the back pressure compensation-type (equipped with bellows) to eliminate the effect of back pressure on their lifting pressure. A cross- sectional view of a typical pressurizer safety valve is shown in Figure 3.4.13.

Figure 3.4.12 Reactor coolant pump motor

NSRA, Japan

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Chapter 3 Systems of PWR Nuclear Power Plants

More than two such safety valves are installed on the top of the pressurizer; they are designed to provide the required steam discharge capacity to keep the RCS pressure below 1.1 times the maximum design pressure during a complete loss of load to prepare for the assumed opening of only one of the safety valves on the main steam line. Typical design specifications of pressurizer safety valves are given below. All parts of the valves in contact with the reactor coolant are made of corrosion-resistant materials such as stainless steel.

Typical design specifications of pressurizer safety valves

2. iii) Pressurizer spray valves

   Hie pressurizer spray valves are automatically controlled and they regulate the spray flow rate into the pressurizer, preventing the RCS over­pressurization.

   Similar to the relief valves, the spray valves are pneumatically operated. The valve opening is regulated by automatic control signals. The valve is equipped with a bellows seal to prevent leakage from the valve ground when the valve is at a midway position. All parts of the spray valves in contact with the reactor coolant are made of corrosion-resistant materials such as stainless steel.

   number of valves per reactor 3

   lifting pressurel 7.16 MPa [gage]

   discharge capacity of each valve approx. 157 ton/h

   Pressurizer relief valves

When the system pressure exceeds the normal operating pressure and reaches a set point, the relief valves are automatically opened to discharge the pressurizer steam to the pressurizer relief tank to prevent the system over-pressurization and to minimize the actuation of the safety valves. A cross-sectional view of a typical pressurizer relief valve is shown in Figure 3.4.14.

These valves, combined with the turbine bypass system, have the capability to control the RCS pressure within limits during the design­basis load reduction. Pressurizer relief valves are pneumatically operated and controlled by automatic control signals. If a relief valve fails to close, the valve inlet line is isolated by closing a motor-operated root valve installed upstream of the relief valve. Typical design specifications of pressurizer relief valves are given below. Similar to the pressurizer safety valves, all parts of the pressurizer relief valves in contact with the reactor coolant are made of corrosion-resistant materials such as stainless steel.

Typical design specifications of pressurizer relief valves number of the valves per reactor 3

set pressure approx. 16.1 MPa [gage)

discharge capacity of each valve approx. 95 ton/h

® Valve box	© Spring holder
© Valve seat	® Spring cover
® Valve body	® Adjusting boi t
@ Valve shaft	® Cap
® Spring seat	0 Bellows
® Spring

Figure 3.4.13 Pressurizer safety valve (example)

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I © Driving part	© Valve shaft
© Valve body	® Gland packing
® Bonnet	® Gasket for Valve seal
© Valve seat	© Gasket for bonnet
® Holding ring	® Stud
® Valve spindle	© Nut