Figure 3.4.1 Primary coolant system

1. : from Emergency core cooling system

2. : from Residual heal removal system

3. : to

4. ; to Chemical and vdlume control system

5. : from ”

The pressurizer heaters are divided into two different groups, namely, a proportional heater group used to compensate heat losses under normal operating conditions and a back-up heater group (on-off group). Under normal operating conditions, the proportional group heaters are used to suppress negative pressure deviations. However, when sharp negative pressure surges occur to the levels exceeding the proportional group control capability, the on-off group heaters are energized to recover the pressure to its set point

Also, during positive pressure surges, proportionally controlled spray flows are fed into the pressurizer by the spray valves to suppress the pressure increases. When steep positive pressure surges exceed the capacity of the spray valves, the relief valves will automatically actuate. If the pressure still continues to rise, the pressurizer

safety valves will open. These pressurizer safety valves serve as the last means to prevent the RCS from over-pressurizing.

The steam discharged from the pressurizer relief or safety valves, is piped to the pressurizer relief tank, where the steam is released from the nozzles on a sparger submerged in the tank water, and condensed by the tank water.

2. Main Components

1. Reactor vessel

1. Structure

The RV, as shown in Figure 3.4.2, consists of a body composed of a cylindrical shell and a hemispherical bottom head and a flanged hemispherical upper head.

The RV contains the fuel assemblies, core internal structures, control rod clusters and other appurtenances. The reactor coolant piping is

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Figure 3.4.2 Reactor vessel structure

O-ring

\Holding clip bolt

Holding clip

connected to the RV at an elevation between the head flange and the top of the core; therefore, even in a piping rupture event, the coolant level will not decrease to a level at which the reactor core is uncovered.

The upper head is bolted on the flange of the RV body, and it is removed during the periods of refueling operations and inspections. The pressure housings of the control rod drive mechanisms are connected to the upper head. Also, the air vent piping and valves are installed at the top of the upper head. A large number of in-core neutron flux instrument guide tubes penetrate the bottom head of the vessel.

As shown in Figure 3.4.3, two concentric circular grooves are cut into the flange surface of the upper head. Coolant leakage from the flange is prevented by two hollow, nickel-chrome- ferroalloy O-rings, placed in these grooves. Leakage detection lines are provided outside each of the two O-rings. If the coolant leaks past the metal O-rings, it will flow by a temperature sensor in these lines and a high temperature alarm will annunciate the coolant leakage.

Hie RV is supported by its support structures on a concrete pedestal. The inlet and outlet

Figure 3.4.3 O-ring seal for reactor vessel

nozzlesattached with support pads at their bottom are set on the support structures.

2. Design

The RV, as the principal part of the reactor coolant pressure boundary, serves as a barrier to the release of the radioactive materials in the reactor coolant. Since the ductility of the vessel wall gradually decreases due to the fast neutron dosages (this phenomenon is known as irradiation embrittlement*³), conservative approaches are taken in the material selection as well as design and manufacturing processes to maintain the vessel integrity over the plant service life.

It has been confirmed that even when the plant service life is extended, the integrity of the PWR RV exposed to an estimated irradiation dose (lxKFn/cm² at the interior surface of the vessel) is still maintained.

(*3) rpj_{ie ex}t_enf _of t|_ie irradiation embrittlement can be estimated by the embrittlement estimation equations introduced in “Recognition Test Methods of Fracture Toughness in Nuclear Power Plant Components" (JEAC 4206-1991), prepared by Nuclear Power Division, Electricity Technology Regulation Office, Japan Electricity Association (an aggregate corporation). Such estimation methods are established by statistical analyses based on the surveillance test data of domestic PWR and BWR pressure vessels, as well as those of US plants.

NSRA, Japan

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