Skip Ribbon Commands
Skip to main content
Operating method

Koeberg experience

Operating method

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.

Reactor

The reactor vessel, which contains the nuclear fuel, is the central component of the reactor coolant system.

Its purpose is:

  • to contain the nuclear core, the internal structures and the control rod drive mechanisms:

  • to ensure a complete leak tightness and a sufficient resistance to internal pressure:

  • to contribute to radiation protection.

The reactor vessel is a weld-fabricated structure composed of the vessel body and the closure head. It is 13m high and 25cm thick. It is made of low-carbon steel with less than 0,2% cobalt. All internal surfaces of the vessel are clad with stainless steel to avoid corrosion.

Waste re-racking

 

 

 

 

 

 

 

Diagram of the reactor

Steam generators

The steam generator is a tubular heat exchanger of the natural circulation type, with mechanical drying of the produced steam. The primary reactor coolant flows through the tubes and gives up heat to the secondary feedwater on the shell side, producing steam at approximately 1 800t/h at rated pressure of 5,8MPa.

The steam generator is designed to withstand the thermal stresses associated with thermal cycling from cold to hot operating conditions. Particular care has been given to corrosion conditions (compatibility of Inconel tubing with primary and secondary coolant) and to possible flow-induced vibration problems.

The steam generator is a vertical shell containing the U-tube heat exchanger with integral moisture separating equipment.

The reactor water flows through the inverted U-tubes, entering and leaving through the nozzles located in the hemispherical cast bottom head. The head is divided into inlet and outlet chambers by a vertical Inconel partition plate. The interior surface of the bottom head and nozzles are clad with austenitic stainless steel.

Feedwater is pumped into a preheater section where it is heated almost to saturation temperature before entering the boiler section. Subsequently, the water-steam mixture flows upward through the tube bundle and into the steam drum section. A set of centrifugal moisture separators, located above the tube bundle, removes most of the entrained water from the steam. Steam driers are used to increase the steam quality to a minimum of 99,75% dry steam.

Manholes and handholes are provided in the bottom head and in the steam drums for maintenance and inspection.

One of Koeberg's six steam generators

             Diagram of a steam generator

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. 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.

When the pressure has to be increased the electrical heaters are automatically switched on and a certain amount of water is vaporised, increasing the steam volume with a consequent increase in pressure. There are 60 of these heaters distributed on three concentric circles around the surge line nozzle.

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.

Turbines & generator

The steam generated by the steam generators drives a set of turbines. The steam drives one high pressure steam turbine and three low pressure turbinesat the rate of 1800 rpm. The turbines are connected to a generator. Each generator can produce a maximum of 920MW of electricity. The combined output of Koeberg's two units (1840MW) is enough to supply the whole of the Western Cape with electricity in summer

                                                                Koeberg's turbines and generators whilst in operation