•B lower than 1100 ppm: Li 1.8 to 2.2ppm + pH=7.3±0.1(at285V)

In oxygen concentration control, hydrazine is added to the reactor coolant to remove oxygen during the plant start-up operation. During the plant normal operation, oxygen concentration is controlled by supplying hydrogen gas to the reactor coolant This is done by maintaining a hydrogen gas atmosphere in the volume control tank.

3. The third function is to control the boron concentration. For this, a reactor make-up water system is connected to the discharge side of the volume control tank. This make-up system consists of a boric acid blender to mix pure water from a make-up water storage tank and boric acid solution from the boric acid tank, and connecting piping and control valves.

The reactor make-up water control system is used to maintain a constant water level in the volume control tank and to adjust the boron concentration in the reactor coolant A make­up water control mode is selected out of five different modes, namely, an automatic mode, a manual mode, a concentration mode, a dilution mode and a fast dilution mode. Under the plant normal operating conditions, the automatic make-up mode is selected, and when the water level in the volume control tank is decreased to the pre-set minimum value, make-up water of the same boron concentration as the reactor coolant is added to the volume control tank,

4. The fourth function is to compensate leakage of the reactor coolant. The CVCS is designed to provide make-up water to compensate either small leakages from the reactor coolant pressure boundary, or coolant releases following ruptures of small size piping or breaks of small components directly connected to the reactor coolant pressure boundary. In this regard, flow restriction nozzles are inserted into the connections of the 3/4 inch branch piping to the reactor coolant piping, to ensure that a coolant leakage from a small break in a branch line, with an equivalent diameter up to 3/4 inch, is compensated by the addition of make-up water using the charging pumps.

5. The fifth function is to remove hydrogen and

noble gases in the reactor coolant. The CVCS removes hydrogen and noble gases from the reactor coolant prior to the reactor shutdown for refueling or maintenance operations. This is done by replacing the cover gas in the volume control tank with nitrogen. The gas effluent from the volume control tank is sent to a waste gas treatment system.

Finally, in the APWR plant design, the CVCS has an additional special function to add negative reactivity to the reactor which suppresses the reactor reactivity increases posed by overcooling events, including main steam line break accidents. The control rod clusters, together with the ECCS supported by the CVCS performing this safety function, give sufficient negative reactivity to the reactor core to establish and maintain its sub-criticality.

2. Components

1. Regenerative heat exchanger

The letdown flow entering the CVCS flows through the shell side of a regenerative heat exchanger, where its temperature is reduced to below the flashing temperature at the letdown orifices. Tins temperature drop transfers the heat of the letdown flow in the shell to the charging flow in the tubes which reheats the charging flow to a temperature close to the reactor coolant temperature.

2. Non-generative heat exchanger

After the regenerative heat exchanger has cooled the letdown flow, a non-generative heat exchanger further cools it to approximately 4613 which is the operating temperature of the demineralizers downstream from the non- generative heat exchanger.

3. Mixed bed demineralizers

Two mixed bed demineralizers remove ionic radioactive materials in the reactor coolant. One of the two is continuously used during the plant operation, with the other one serves as a stand-by.

4. Cation bed demineralizer

A cation bed demineralizer, intermittently used during the plant normal operation, reduces concentrations of cations, such as Li and Cs, in the reactor coolant

5. Boron removal demineralizer

3-97

NSRA, Japan