therefore, to have sufficient strengths so that its integrity is not lost under the elevated conditions of pressure and temperature in a LOCA. Leak tests are conducted when the construction is completed and periodically thereafter. Hie design leak rate is 0.5% / day of the PCV air volume at room temperature and at the pressure of 0.9 time of the service peak pressure.

A biological shielding wall of about 2m thick is built around the PCV and shields radiation. It also works to protect the reactor from external incidents.

Hie PCV consists of a drywell that accommodates the RPV, the recirculation loops and other primary system components, and a suppression chamber that has a suppression pool therein. The drywell and the suppression chamber are connected by steel vent piping.

The steam-water mixtures released to the drywell in a LOCA are transported to the pool in the suppression chamber through the vent piping, where the steam is cooled and condensed. Thus, the pressure increase of the PCV can be effectively controlled. The isolation valves installed on the PCV piping penetrations hold the released fission products in the PCV. Hie PCV is designed, therefore, to have sufficient strengths so that its integrity is not lost under the elevated conditions of pressure and temperature in a LOCA Leak tests are conducted when the construction is completed and periodically thereafter. The design leak rate is 0.5% / day of the PCV air volume at room temperature and at the pressure of 0.9 time of the service peak pressure.

A biological shielding wall of about 2m thick is built around the PCV and shields radiation. It also works to protect the reactor from external incidents.

3. Containment spray system (css)

Figure 2.7.11 shows the schematic configuration of the CSS. The CSS consists of two independent loops. Either of the two can remove the released coolant energy and decay heat in a primary system pipe break, and can prevent the PCV pressure and temperature from exceeding the design values. This system also removes airborne radioactive iodine in the PCV atmosphere. This system functions in one operation mode of the RHR system. In a LOCA the RHR system is automatically actuated in the

drywell

Figure 2.7.11 RHR system configuration in CSS mode

LPCI system mode, and the operator maneuvers it remotely and manually to the CSS mode, by switching a solenoid valve, once the core is reflooded.

This system uses the suppression pool as its water source, and sprays the water into the drywell and the suppression chamber after removing heat in the heat exchangers. The sprayed water returns to the suppression pool via the vent piping.

4. Flammable gas control system (fcs)

Hie FCS prevents a rapid combustion of hydrogen and oxygen gases generated in the PCV in a LOCA

The FCS has two independent loops, each of which has a full capacity (100 %).

As shown in Figure 2.7.12, each loop has a blower, a heater, a recombiner, a cooler, piping, valves, and the I&C systems.

The FCS is actuated manually from the main control room after initiation of an accident, and draws the gases from the drywell by the blower, recombines the hydrogen and oxygen gases in the recombiner, and cools and condenses the gases at the cooler before returning them to the suppression chamber.

2-89

NSRA, Japan