Pressurized Water Reactor (PWR)

Of all NPPs PWRs are the most frequently used type world-wide.

In PWRs water is used as coolant and moderator simultaneously. The core, where heat is produced by nuclear fission is located inside a pressure vessel. The coolant through the core is circulating through a separate coolant circuit, which in most PWRs is completely confined in the containment. Pressure in the primary coolant system is high enough to prevent coolant from boiling. Integrity of primary system is a crucial point for safety of a PWR. Neutron flux and stress to the components is high and leads to embrittlement and fatigue. The primary circuit is divided into several loops with the corresponding number of SGs and pumps. The number of loops is different and reaches from 2 e.g. some Westinghouse PWRs to 6 in WWER-440 reactors.

The steam which is necessary for driving the turbine is generated in a special heat exchanger, the Steam Generator (SG), where the heat from the closed primary coolant circuit is transferred to a second coolant circuit (secondary or feedwater circuit).

The containment is supposed to isolate the primary system in case of accidents. Because of several penetrations into the containment the isolating function can be degraded e.g. by valve failures. Hydrogen explosions and pressure build-up can cause destruction of the containment under accident conditions.
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  1. containment: prestressed concrete (2 m thick)
  2. secondary containment: steel
  3. accumulator tank
  4. concrete shield
  5. protection against missiles
  6. water-cooled fuel pool
  7. control rod drives
  8. steam generator
  9. reactor pressure vessel
  10. reactor core : consists of a large number of fuel elements composed of fuel rods