8. Reactor Auxiliary Systems

Reactor auxiliary systems support the operation and ensure the safety of the reactor. They consist of the following systems.

• CVCS to clean up the reactor coolant, and to adjust the water chemistry and control the volume of the reactor coolant

• RHRS to remove the reactor decay heat, to transfer the reactor cavity water, and to function as the low pressure injection system of the ECCS.

• Component cooling water system to cool reactor auxiliary system components, such as the non- regenerative heat exchanger and residual heat exchangers.

•Sea water system (service water system) to supply sea water to cool component cooling water system heat exchangers.

• Spent fuel pit water cooling and clean-up system to remove decay heat from spent fuel and to clean up pit water.

• Fuel handling system used for the handling and storage operations of fuel assemblies, from the off-loading of new fuel assemblies to the shipment of spent fuel assemblies out of the plant (cf. Section 3.2.2 (4)).

1. Chemical and Volume Control System (cvcs)

1. System composition and functions

1. System composition

The CVCS (Figure 3.8.1) consists of various sub-systems, such as:

•a letdown/clean-up system to cool and clean up letdown flow from the reactor coolant system and to reduce its pressure, by employing a regenerative heat exchanger, letdown orifices, a non-generative heat exchanger, pressure control valves and demineralizers;

• a reactor makeup control system to supply makeup water to the reactor coolant system and to adjust the boron concentration of the reactor coolant; and

•a charging system, consisting of a volume control tank, charging pumps and filters, to return the letdown water to the reactor coolant system.

The CVCS components are aligned to establish a continuous reactor coolant letdown and charging loop before the plant start-up. During the plant start-up and shutdown periods, the CVCS maintains proper water inventories in the reactor coolant system, by bleeding off excess water from the system, or by making up water to add the system, in addition to normal letdown and charging systems consisting of letdown orifice lines and charging pumps and a flow control valve. During the plant normal operation, the letdown water is cleaned up by passing it through demineralizers and filters in the letdown line. Furthermore, a make-up water control system and a chemical mixing tank are connected to the system around the volume control tank and the two systems are used to adjust the boron concentration in the reactor coolant and to add necessary chemicals to the reactor coolant.

2. Functions

The CVCS performs has five functions.

1. The first function is to maintain the reactor coolant inventory. The charging andletdown systems maintain a proper water inventory in the reactor coolant system by regulating the pressurizer water level to meet a programmed profile in line with the plant power level, by changing the letdown and charging flow rates. The letdown flow rate is changed stepwise by inserting or taking out the letdown orifices, and the charging flow rate is controlled by a charging flow controller to automatically control the pressurizer water level.

2. The second function is to maintain the water chemistry of the reactor coolant

The chemical mixing tank is connected to the suction of the charging pumps, and it is used to add corrosion inhibitors and oxygen scavenging chemicals to the reactor coolant

In pH control, lithium hydroxide (LiOH) is used to control the pH of the reactor coolant, and the concentration of Ii in the reactor coolant is adjusted by making the letdown flow pass through a cation bed demineralizer. In an improved pH control system, the control ranges for pH and Li are as follows:

•B equal, or higher than 1100 ppm: pH=6.8 (at 285t}) +Ii 0.4ppm

3-95

NSRA, Japan