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Nuclear reactor and primary coolant system

Primary Coolant System
Primary Coolant System
  1. Reactor Pressure Vessel
  2. Steam Generator
  3. Primary Coolant Pump
  4. Pressuriser

The fuel assemblies which form the reactor core, are loaded into a specially fabricated cylindrical steel pressure vessel (the reactor pressure vessel). The reactor pressure vessel is about 12 metres high and has a 20 cm thick steel wall with an inner diameter of about 4 metres. It weighs about 314 tonnes. The primary coolant system of a 900 MW class reactor consists of the reactor pressure vessel and the primary circuit with 3 identical loop (Top figure). Each loop has a primary coolant pump, a steam generator and the interconnected piping. A pressuriser is installed in one of the 3 loops. Each primary coolant pump will circulate the cooling water (ordinary water) around the loop through the reactor core at a high pressure of about 155 bar (1 bar = 100 kPa). In addition to its moderator function, the cooling water would also transfer the heat from the reactor core to the steam generator. The water temperature at the reactor pressure vessel outlet is about 330 degree C whereas the water temperature at the inlet of the vessel is about 290 degree C. The cooling water is in a sub-cooled condition at such high temperature and pressure to prevent it from boiling. The steam generator of about 20 metres in height is fitted with U tubes in the inside which serve as the heat exchanger to transfer the heat from the water in primary circuit to that in the secondary circuit. The heat will convert the feed water in the secondary circuit to steam for driving the turbine-generator (Right figure).

Primary Coolant Circuit
Primary Coolant Circuit


The pressuriser is mainly used to maintain the pressure in the primary coolant circuit and prevent overpressure. It is a cylindrical pressure vessel of about 2 m in diameter and about 13 m long, tapping off from one of the hot legs in the primary loops. The steam and water volumes occupy the top half and bottom half of the pressuriser respectively during normal operation. There are water spray nozzles at the top and a group of heaters at the bottom of the pressuriser. The water level inside the pressuriser and thus the pressure in the primary coolant circuit can be controlled by operation of the heaters and water spray. A sophisticated pressuriser level control system is used to regulate the water level inside the pressuriser so as to ensure a proper pressure control during reactor power change and transient plant operation. The heaters will be turned on to increase steam production if the pressure drops. If the pressure increases, the water spray will be turned on to condense the steam to reduce the pressure. In addition, the control system will provide a protection signal to shutdown the reactor automatically if the pressure inside the pressuriser is too high or too low.

At the first start up of a new reactor, primary source rods consisted of californium-252 are inserted into the reactor to produce sufficient neutrons to initiate the first fission. Secondary source rods consisted of antimony and beryllium are also inserted at the same time to provide a regenerative neutron source such that it will initiate nuclear fission in subsequent start up of the reactor throughout its service life. To ensure nuclear safety and allow control of the fission rate inside the reactor, some fuel assemblies are fitted with control rods. Each control rod assembly consists of a number of absorber rods attached to a spider assembly and coupled to the control rod drive mechanism. The absorber rods are made up of neutron absorbers such as silver, indium and cadmium. Hence, by adjusting the position of the control rods, the number of neutrons and thus the fission rate in the reactor can be controlled. The control rod assemblies are fitted with driving mechanism to move the control rods up and down in the reactor core for controlling the start up of the reactor, adjusting its power output, and enable the normal shutdown of the reactor and scram. In addition, the fission rate in the PWR can also be controlled by adjusting the boron (a neutron absorber) concentration in the primary coolant circuit. After start up of the reactor and attaining its desired power output, it would be maintained at criticality for stable operation at power. The reactor can be shutdown during emergency by cutting off the power supply to the control rod driving mechanism which then causes the control rods to drop down to the reactor core by gravity quickly and thereby stopping the nuclear fission immediately.

The above information is provided by EMSD

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