collectively have sufficient capacity to perform the required function. Each accumulator is isolated from the reactor coolant system by two check valves which are closed during the plant normal operation. A motor-operated valve, which is open during the plant normal operation, is installed in the accumulator injection line, upstream from the check values, to isolate the line when the reactor coolant system pressure is lower than the accumulator pressure. The accumulators are filled with borated water and pressurized with nitrogen gas. Both the boron concentration and the water level of the accumulator water can be manually adjusted during the plant normal operation. Test lines connected to the piping between the motor-operated isolation valve and the upstream check valve, and to the piping between the two check valves, are used to test leakage from the valve seats of the check valves during the plant normal operation.

ii) Low pressure injection system

The residual heat removal system (RHRS) serves as the ECCS low pressure injection system for LOCAs. The low pressure injection system consists of the residual heat removal pumps, the residual heat exchangers, the piping and valves and the related instrumentation. The residual heat removal pumps are designed to satisfy the requirements shown in the “Performance Evaluation Guidelines of the Emergency Core Cooling System of Light Water Power Reactors" together with the high head injection pumps and the accumulators, providing a sufficient volume of cooling water to re-flood the core following the blow-down phase during a LOCA

Two full capacity motor-driven residual heat removal pumps, each of which is powered from one of the separate emergency electrical buses, automatically start upon receiving an emergency core cooling actuation signal, and send the borated water in the refueling water storage tank to the reactor core via the cold legs of the reactor coolant piping. In the recirculation mode of the emergency core cooling operation, these pumps take their suctions from the containment recirculation sump and inject the water into the core through the cold leg piping, after it is cooled by the residual heat exchangers using the component

cooling water. Some PWR plants are designed to supply the cooled sump water discharged from the residual heat exchangers to the suction of the safety injection pumps or the charging / high head injection pumps when needed. The motor-driven residual heat removal pumps are the centrifugal type. Each pump has a minimum flow bypass line branched off from the residual heat exchanger outlet and connected to the pump suction, to prevent the pump from being operated without flow, when the reactor coolant system pressure is higher than the discharge head of the residual heat removal pump. The minimum flow bypass lines are also used for the periodic operational tests of the pumps. Although the RHRS performs both the residual heat removal and the low pressure injection function, the two functions are not required simultaneously. The residual heat removal function of the system is described in Section 3.8.2.

3. Reactor Containment Facility

1. Functions and configuration

The reactor containment facility ensures the safety of the plant employees and the public in the vicinity of the plant, by preventing radioactive materials from being released to the environment during accidents including LOCAs.

Two types of containment vessels are employed in PWR plants: the pre-stressed concrete containment vessel(PCCV) and the steel containment vessel(SCV). A PCCV, as shown in Figure 3.7.3, consists of a containment vessel, an annulus space and their associated systems. A SCV, as shown in Figure 3.7.4, consists of a containment vessel, an outer shield, an annulus space and their associated systems. The annulus space, an airtight enclosed space, is formed between the lower section of the containment vessel cylindrical shell and the external shielding to serve as a secondary barrier for leakages of radioactive materials from the containment vessel to the environment, composing a double containment system. Air in the annulus is cleaned up by the annulus clean-up system which removes radioactive materials that may leak into the annulus from the containment vessel through potential leakage paths such as containment

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concrete foundation

Figure 3.7.3 Reactor containment (PCCV)

about 77n

Figure 3.7.4 Reactor containment (SCV)

2. Functions and structure of the different containment vessels

i) Functions

In general, the reactor containment vessel prevents hazardous materials, which include radioactive materials released from the reactor coolant system to the containment vessel in the event of an accident, from being released to the environment. The containment vessel acts as a barrier to the pressure caused by accidents including LOCAs, and serves as the final barrier against the release of radioactive materials to the environment. To perform these functions, the containment vessel and the piping penetrations and other parts composing the containment boundary are designed based on the following design policies: For the PCCV, the vessel wall, by itself, performs the external shielding function as well.

1. The containment barrier components enclosing the reactor and the reactor coolant system should be able to withstand the maximum pressure and temperature that might be developed following design basis LOCAs.

2. Ferritic materials used for the containment boundary components should have sufficient ductility at their working temperature ranges, so that no fast propagating ruptures will occur in these structural components.

3. The containment vessel should be designed to keep its air leakage rate less than 0.1 wt% of the containment vessel air per day at 90% of the maximum working pressure of the containment vessel.

4. All lines penetrating the containment vessel, that must be closed following accidents to ensure the reliable performance of the containment system following accidents, should have isolation valves or shut-off flanges to isolate the containment atmosphere from the outside atmosphere.

5. Necessary engineered safeguard systems should be incorporated into the plant design to ensure that the containment vessel can carry out its design functions.

6. The reactor containment vessel should be

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designed to allow its leakage tests to be carried out. Also, leakage rate tests should be able to be conducted for all piping penetrations with expansion joints, electric cable penetrations and air-locks, separately or on a small group basis.

ii) Structures

the PCCVs (Figure 3.7.3), are used in the latest four-loop plants, and the cylindrical SCVs (Figure 3.7.4), are used in two or three-loop plants. The design and manufacturing processes of the SCVs are relatively simple, compared to those of PCCVs. However, the SCV for a four-loop plant would be huge with a height of more than 100 m. Because of difficulties in the seismic design of such tall containment vessels, the PCCV design which provides a maximum working pressure higher than that of the SCV design and better seismic characteristics thanks to its smaller vessel size, is employed in the four-loop plants.

A containment vessel has three access hatches, i.e. a normal airlock, an emergency airlock and an equipment hatch. In addition, the containment vessel is equipped with a large number of penetrations for piping, electrical cables, ducts, etc. passing through its walls.

1. Steel containment vessel (SCV)

The cylindrical SCV (Figure 3.7.4) consists of a hemispherical upper section, a bottom dished plate and a cylindrical body. The steel structure is supported by a concrete foundation and fixed to the foundation by studs attached to its bottom dished plate and embedded in the foundation concrete. To ensure that the containment structure will not fail due to a negative pressure in an unlikely event of the containment spray system malfunctioning, which causes a rapid decrease of the containment pressure, the containment vessel is provided with a vacuum relief system to allow the outside air to flow into it through check valves and to balance pressure differences.

2. Pre-stressed concrete containment vessel

(PCCV)

The PCCV (Figure 3.7.3) consists of a reinforced concrete bottom section that is in itself the base slab of the reactor building directly based on hard bedrock, and a pre-stressed concrete portion consisting of a hemispherical dome and a cylindrical body. The PCCV inner surfaces are

lined with steel liner plates of several millimeters thickness.

As shown in Figure 3.7.5, the concrete shell is pre-stressed by a post-tensioning system consisting of circumferential horizontal tendons anchored at buttresses spaced around the containment body shell, and by longitudinal vertical tendons anchored at a tendon gallery located in the base slab. The post-tensioning system imposes compressive forces on the concrete shell, larger than tensile forces imposed by the maximum working pressure in the containment vessel.

The structural integrity of the containment vessel is ensured by the containment shell and the base slab concrete structures, and the containment air tightness is ensured by the liner plate backed-up with the annulus. The liner plate is anchored to the inside surface of the containment vessel concrete by liner anchors embedded in the concrete, so that the liner plate follows expansions and shrinkages of the containment concrete structures.

After tendons are inserted into the sheaths and are applied with tensioning forces, a rust inhibitor is injected into the sheaths. An unbonding method

Figure 3.7.5 Internal structure of PCCV

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