Koeberg PS Operating Model
Koeberg operates on 3 separate water systems. The water is also known as the coolant. In other types of nuclear reactors, gas is used as the coolant. The fact that the 3 systems are seperate is important because it means that the water in the reactor, which is radioactive but is in a closed system, does not come into contact with the other two systems and therefore does not contaminate the water in these systems.
The primary system takes heat away from the fuel in the (1) reactor to the tubes in the (2) steam generators The water is then returned to the reactor by means of a (3) pump. In this primary system, Koeberg uses a three-loop system which is kept under pressure by a (4) pressuriser hence the name Pressurised Water Reactor or PWR. As we have said, this system is closed and water from it does not come into contact with the secondary or tertiary system.
The secondary system is also closed. Water is pumped into the (2) steam generator. This water is allowed to boil and form steam which drives one (5) high pressure turbine, three (6) low pressure turbines and a (7) generator. The generator produces 921MW of electricity. Once the steam has driven the turbines it flows to the (8) condensers where it is cooled back to water and circulated back to the (2) steam generator.
The tertiary system is used in the condensers. The cooling water system for the condensers uses sea water at the rate of 80 tons/sec to cool the steam in the (8) condensers. Once it has cooled the steam down it is returned to the sea.
Pump The reactor coolant pumps circulate reactor coolant (water) through the reactor vessel and the steam generators tubes. There is one pump for each coolant loop, located on the cold leg of the steam generator. The reactor coolant pumps ensure an adequate core cooling flow rate and hence sufficient heat transfer. The pump is a vertical suction, horizontal discharge single-stage centrifugal unit sealed with a combination of three water-cooled mechanical seals. The motor is a constant-speed, air-cooled, vertical. squirrel-cage induction motor |
Pressuriser
The pressuriser and associated components establish and maintain the reactor coolant system pressure within prescribed limits and provide a surge chamber and a water reserve to accommodate reactor coolant density changes during operation. Relief valves connected to the pressuriser protect all reactor coolant system components from exceeding the design pressure. The pressuriser has sufficient steam and water volumes to prevent uncovering of the heaters or discharging water through the safety valves during the most severe reactor coolant pressure changes expected. |
When the pressure has to be decreased, cold water is sprayed through a spray nozzle located in the top of the pressuriser which condenses a part of the steam and consequently decreases the pressure. |
The surge line is sized to limit the pressure drop between the reactor coolant system and the safety valves with maximum allowable discharge flow from the safety valves.
The surge line connects the pressuriser to one reactor hot leg. The pressuriser is a vertical, cylindrical vessel with hemispherical top and bottoms heads, made of carbon steel with austenitic stainless steel cladding on all surfaces exposed to the reactor coolant.