Generic Pressurized Water Reactor (PWR): Safety Systems Overview.

Generic Pressurized Water Reactor (PWR): Safety Systems Overview

Contents Pressurizer

The Chemical and Volume Control System (CVCS)

Containment structures and systems

Emergency Core Cooling System

Supporting and auxiliary systems

Pressurizer The pressurizer is a vertical, cylindrical vessel with the hemispherical top and bottom head constructed of carbon steel or stainless steel or Ni-alloys cladding on all surfaces exposed to the reactor coolant.

Electrical heaters are installed at the bottom head while spray nozzles, relief and safety valve connections are located at the top.

The pressurizer is used to:

-pressurize the RCS during plant start-up,

- to maintain normal RCS pressure during steady-state operation,

- to limit pressure changes during RCS transients, and

-To prevent RCS pressure from exceeding design values.

Pressurizer pressure, level, and temperature instrumentation is provided for indication, control, and protection.

Pressurizer

Pressurizer

Pressurizer

CHEMICAL AND VOLUME CONTROL SYSTEM (CVCS)The Chemical and Volume Control System (CVCS) main functions are :

1. Adjust the reactor coolant boric acid concentration

2. Maintain the proper water inventory in the RCS in conjunction with the pressurizer level control system

3. Provide seal water flow to the reactor coolant pump shaft seals

4. Add corrosion inhibiting chemicals to the reactor coolant

5. Purify the reactor coolant in order to maintain it within its design activity limits

6. Provide borated water for emergency core cooling

7. Process reactor coolant for the reuse of boric acid and reactor makeup water in the boron recovery system

Chemical and Volume Control System (CVCS)

Containment

The purposes of the containment are as follows:

1. Provides a barrier to prevent the escape of radioactivity during normal and accident conditions

2. Provides protection against internally and/or externally generated missiles

3. Provides biological shielding during normal and accident conditions

4. Provides Seismic Category I supports for the reactor coolant system (RCS) and its associated support systems.

Containment

Emergency Containment Cooling Systems- The containment spray system is installed to reduce the pressure inside the containment following a loss of coolant accident or a steam line break within the confines of the containment. The containment spray system satisfies the same design requirements as the emergency core cooling system.

- In addition to the containment spray system, other methods of controlling the containment pressure include containment fan coolers and hydrogen recombiners.

(NOTE: It can be seen from the above how the Defence in Depth principle have been applied : redundancy, diversity, single failure, active and passive systems).

Containment Spray System

Emergency Core Cooling Systems- Certain systems are installed to provide emergency cooling to the core in the event of a LOCA. These systems include the high and low pressure injection systems, and the cold leg accumulators .

- High pressure injection system

It consisting of two redundant trains, provides protection for small-break LOCAs (SBLOCAs). This system pumps water from the refuelling water storage tank (RWST) into the RCS cold legs during the injection phase. The high pressure system can also be used during the recirculation phase by connecting its suction to the residual heat removal system.

In addition, this system injects boric acid from the RWST into the RCS during steam break accidents to offset the positive reactivity added by the rapid cool down of the RCS. (Fundamental Safety Function : control of reactivity)

High pressure injection system

Emergency Core Cooling Systems- Low pressure injection system

The low pressure injection system (residual heat removal system) provides protection for a large loss of coolant accident. This system consists of two trains which can pump at a high rate of flow and at a low pressure from the RWST to the reactor coolant system. These pumps are also supplied with suction from the containment emergency core cooling recirculation sump.

This suction source will be used during the recirculation phase of the accident. In addition to supplying cooling water to the reactor vessel, these pumps can also supply the suctions of the intermediate and high pressure injection systems.

Characteristic: Design Pressure 4.5 Mpa Operating pressure 2.94 Mpa Inlet Temp < 180 C Min Flowrate 50l/hr

Low pressure injection system

Emergency Core Cooling Systems- Accumulators (passive injection system)

The cold leg accumulators consists of four tanks filled with borated water and pressurized with nitrogen.

When the RCS is pressurized and the reactor is at power, check valves, held closed by the higher reactor coolant system pressure, prevent the entry of water from the accumulators into the reactor coolant system.

However, if a loss of coolant accident results in a reactor coolant system pressure lower than the pressure in the accumulators, then borated water will flow from the accumulators to the reactor vessel, via the cold legs.

This system, unlike the high, intermediate, and low pressure injection systems, requires no actuation signal.

Accumulators (passive injection system)

Emergency Core Cooling SystemsIn addition :

The auxiliary feed water system is designed to provide feed water to the steam generators to remove decay heat in the event of a loss of main feed water. The auxiliary feed water system meets the same design criteria as the emergency core cooling system.

ECCS

Auxiliary Feed water (AFW) SystemThe purposes of the AFW system are as follows:

1. To provide feed water to the steam generators to maintain a heat sink for the following conditions.

a. Loss of main feed water (MFW),

b. Unit trip and loss of offsite power, and

c. Small-break loss-of-coolant accident (LOCA).

2. To provide a source of feed water to the steam generators during plant start-up and shutdown.

Auxiliary Feed water (AFW) System

The AFW system is activated automatically.

It has: Motor-Driven Auxiliary Feed water Pumps and Turbine-Driven Auxiliary Feed water Pump

The following conditions will automatically start AFW pumps:

1. Low-low water level in any single steam generator,

2. Loss of one main feed pump (MFP) if power is greater than 80 percent,

3. Loss of both MFPs at any power level,

4. Safety injection actuation signal, and

5. Loss of power to the Class IE power distribution system.

6. Low-low level in any single steam generator

Main Steam System

The purposes of the main steam system are:

1. To transfer steam from the steam generators to the turbine-generator and other secondary system components,

2. To provide overpressure protection for the steam generators, and

3. To provide diverse methods of decay heat removal.

Main Steam System

During normal operation the four steam generators deliver saturated steam through four separate steam lines to the main turbine.

These lines are cross-connected by a bypass header to ensure that the steam generators are loaded equally. The bypass header also supplies steam to a number of secondary plant components.

Each main steam line is provided with an isolation valve and check valve just outside containment. The main steam piping from the steam generators to these valves is Seismic Category I and contains several safety-related features.

The main steam isolation valves automatically close on either of the following signals: High steam line flow coincident with low steam pressure or low-low Tavg, or High-high containment pressure.

Main Steam System

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