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Chapter 3 Systems of PWR Nuclear Power Plants

  1. Functions

Functions of the annulus air clean-up system are summarized next

Following an unlikely LOCA, the first function of the annulus air clean-up system is to establish and maintain a negative pressure in the annulus, and to circulate the air to remove radioactive materials leaking from the containment into the annulus, in order to minimize radioactive materials released to the environment. In some plants, the annulus air clean-up system has a second function which is to establish and maintain a negative pressure in the fuel handling buildings and to clean up the exhaust air from the fuel handling buildings, reducing the quantities of radioactive materials released to the environment.

If a LOCA occurs, the annulus air clean-up fans will be automatically started by the ECCS actuation signal, and the air inside the annulus will be discharged to the atmosphere through the annulus air clean-up filter unit, an annulus full-flow exhaust valve and eventually the plant vent stack. A negative pressure in the annulus will be established within a specified time period after an accident. When the negative pressure is established, the full-flow exhaust valve is closed and a small-flow exhaust valve is opened, and the negative pressure is maintained at around a pre-set value by automatic control of the opening of an annulus air return valve.

If a fuel handling accident occurs during refueling operations, upon receiving a fuel assembly drop signal, the normal ventilating system of the fuel handling building is isolated and the annulus air clean-up fans will be automatically started. Air in the fuel handling building is discharged to the atmosphere through the annulus air clean-up filter unit, the full-flow exhaust valve and eventually the plant vent stack, maintaining a negative pressure in the fuel handling building.

  1. Components

The annulus air clean-up fans are a direct-coupled motor-driven type, and are designed to prevent the inside air from leaking during their operation. The electric motors are powered from the emergency buses connected to the emergency diesel generators, and they do not lose their power even if the plant off-site power is lost

The annulus air clean-up filter unit houses iodine and particle removal filters to reduce the concentrations of iodine and dust in the air discharged to the environment

Iodine and dust removal efficiencies of the annulus air clean-up filter unit are designed to be more than 95% and 99%, respectively.

  1. Safety Component Area Air Clean-up System

The safety component area air clean-up system consists of two independent safety component area air clean-up fans, a common safety component area air clean-up filter unit, and valves and ducts. The capacity of the safety component area air clean­up system is determined based on a requirement that the system must be capable of establishing a negative pressure in the safety component area within a specified time determined in the evaluation of radioactive materials released to the environment during plant accidents. The flow diagram of the safety component area air clean-up system is shown in Figure 3.7.8.

  1. Functions

Functions of the safety component area air clean­up system are summarized next.

If a LOCA occurs, the safety component area air clean-up system will establish and maintain a negative pressure in the safety component areas including the containment spray pump rooms and residual heat removal pump rooms. It will clean up exhaust air from these safety component areas, reducing the quantities of radioactive materials discharged to the environment.

In an unlikely event of a LOCA, upon receiving an ECCS actuation signal, the normal ventilating systems for the safety component areas are isolated, and the safety component area air clean-up fans are automatically started. Air inside the safety component areas is discharged to the environment through the safety component area air clean-up filter unit and eventually the plant vent stack, maintaining a negative pressure in the safety component areas.

  1. Components

The safety component area air clean-up fans are a directly coupled motor-driven type, and are

3-93

NSRA, Japan

designed to prevent air out-leakage during their operation. The electric motors are connected to separate emergency buses, and are powered from the emergency diesel generators even if the plant off-site power is lost

Similar to the annulus air clean-up filter unit, the safety component area air clean-up filter unit houses an iodine filter to remove iodine and a particle filter to remove dust particles, thereby reducing the quantities of both iodine and dust in the air discharged to the environment. Iodine and dust removal efficiencies of the safety component area air clean-up filter unit are designed to be more than 95% and 99%, respectively.

In some plants, the safety component area air clean-up system and the annulus air clean-up system are integrated into one system, and it is usually called the annulus air clean-up system.

Auxiliary building exhaust system

Legend

laid

Electric heating coil

Particle filter

Iodine filter

Automatic damper (pneumatic drive)

s

faiergency core cooling system actuation signal

Auxiliary building intake system

Safety component area

Safety component area air clean-up fan

■] Vent stack

Figure 3.7.8 Safety component area air clean*up system

3-94

NSRA, Japan