Why is the double-circuit cooling system the most common layout of an NPP?

The main feature of the double-circuit cooling system is that the technology coolant circuit and the turbine fluid circuit are separated.

The primary circuit structure comprises a high-impact vessel containing water and the core of the reactor. Circulating pumps pump water (coolant) through the reactor core. High pressure is maintained inside the vessel, as well as in the whole primary circuit. This is why there is no steam component despite relatively high temperature at the output from the reactor core (324°С). Hence this type of reactor is called a pressurized water type reactor.

Heated water from the vessel passes to the steam generating unit – a special facility for transmitting heat to water of the secondary circuit, however, there is no direct contact or mixing between water of the primary and secondary circuits. The pressure in the secondary circuit is significantly lower which leads to the circuit steam being generated and passed on as working fluid to the turbine. The secondary circuit, similarly to the single circuit system, is looped through the condenser.

There are two points of utmost importance. First of all, neither water nor the steam produced in the secondary circuit are radioactive as they have no contact with the high neutron flux of the reactor core. Induced radiation of water from the primary circuit does not contain neutron components and does not activate them in the steam generator. Secondly, operational units of the radiation hazardous primary circuit (reactor core and steam generator) are designed as a compact unit with short pipelines, non-serviceable when the reactor is operating and equipped with an elaborate system of safety barriers. This prevents man-made radioactivity leaks into the environment. According to international classification this type of reactor has the abbreviation PWR (pressurized water reactor). Double-circuit cooling reactors produce around 60% of total world nuclear power.