reactor coolant system, the pressurizer pressure is reduced to a specified value for reactor cold shutdown. When the pressure and temperature of the reactor coolant system become below 28kg/ cni and 178°C respectively, cooling down of the system is continued with the residual heat removal system. When the temperature of the reactor coolant is decreased below 60°C , shutdown operations are completed. If the reactor vessel or other reactor coolant system equipment needs to be opened, as in the case of plant periodical inspections, degassing and cleaning operations to remove radioactive gases and radioactive corrosion products contained in the system are performed, concurrently with the cooling down operations. These operations resultantly reduce dose received by workers. Figure

5. shows the standard set of shutdown curves of a PWR. Actual examples of shutdown curves were shown in Figure 5.1.3.

6. Chemistry Control

In a PWR plant, the primary system extracting heat from the reactor is completely separated from the SGs of the secondary system which generate steam to drive the turbine. Chemistries of both systems can be independently controlled and kept under the optimum conditions.

The primary system has a single-phase closed cycle with low temperature differentials in the cycle, though under high temperature and high pressure conditions. Hence, the water in the system has stable physicochemical characteristics, and the water chemistry of the primary system is finely controlled with respect to inhibiting the corrosion of system components and reducing radioactivity levels of the coolant and the system equipment

On the other hand, the secondary system, like the condensate and feedwater systems of conventional thermal power plants, has a two-phase (liquid and vapor) cycle. Since impurities are locally concentrated as the result of evaporation at the secondary sides of the SGs, the water quality of the secondary system must be controlled far more strictly than in conventional thermal power plants to keep the integrity of system components. However, one of the big advantages of a PWR plant compared to a BWR plant is that the secondary system equipment can be handled without any problem

caused by its radioactivity, thanks to the presence of SGs.

1. Outline of chemistry control of primary system

The primary system chemistry control is unique, as discussed in the following, by taking advantage of the features of the system, i.e. a single closed cycle with small temperature differential, though under high temperature and high pressure conditions.

i ) Since the physicochemical state of the reactor coolant is extremely stable, a so-called chemical shim control method is adopted: Neutron absorber (boron) is added to the reactor coolant to control the reactor reactivity.

2. Hydrogen gas is added to the reactor coolant to prevent dissolved oxygen concentration from increasing by radiolysis of water. A high concentration of oxygen might contribute to stress corrosion cracking (SCC) of system components.

3. Lithium hydroxide is added as a pH regulator to inhibit corrosion and subsequently to reduce radiation exposure of workers. Furthermore, concentrations of chloride ions, fluoride ions and other impurities hazardous to the integrity of system components are controlled and maintained at extremely low levels. Recently, primary system chemistry control techniques have been improved and implemented in actual plants, resulting in significant reduction in radiation exposures of workers.

2. Outline of chemistry control of secondary system

Purposes of chemically controlling the secondary system water are to prevent the heat transfer tubes of the SGs from failing due to corrosion and to maintain the integrity of the secondary system equipment including the steam turbine unit

Corrosion of the SG heat transfer tubes related to the secondary system water quality is considered to be caused by impurities dissolved in the water, which are concentrated at crevices between the heat transfer tubes and the tube support plate holes. Mechanisms of such failures are classified into several categories, such as thinning of tube walls (caused by phosphate), IGA/SCC (by free alkali) and denting (by seawater constituents), depending

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NSRA, Japan

Charging Pump Shutdown RC Pump Shutdown

Initiation of Pressurizer Filling

Condenser Vacuimi Break

Initiation of RHR System Operation Cooling by Turbine

Bypass Valve Stop

Maintain RCS Pressure

Letdown Line Pressure Set at 28.1 kg/arf

Safety Injection System Manual Blockage

Pressure Decrease by Manual Control of Preessurizer

Spray ( — 135 kg/arf)

Turbine Bypass Flow Increase

Initiation of Reactor Cooldown All Pressurizer

Heater Off Reactor

Boron Injection

Maintain Zero Power Pressure and Temperature by

Turbine Bypass Control

Subcriticality by Control Rod Adjustment

Tie-out. Turbine Shutdown

Generator Minimum Load

Turbine Bypass Control Change over to Pressure Control

Control Rod Control and Feed Water Control change over to Manual

Station Load Change over from Auxiliary Transformer to Startup Transformer Initiation of Load Decrease

Rated Power Operation:

Normal RCS Pressure and Temperature, Autonatic

Control Rod Control, Automatic Feed Water Control,

T_atc Control of Turbine Bypass Control System

Figure 5.1.5 PWR standard shutdown curve

NSRA, Japan

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