Description of VVER-1000 reactor.
1) Control Element Drive Mechanism
2) Reactor vessel head
3) Reactor pressure vessel
4) Coolant inlet – outlet nozzles
5) Downcomer for coolant
6) Neutron reflector
7) Fuel assemblies
Most common nuclear reactors are light water reactors (LWR), in which light water (ordinary water) is used as a moderator as well as the cooling medium. The use of ordinary water makes it necessary to use an enriched uranium fuel in order to maintain the criticality of the reactor along entire fuel cycle (e.g. for 18 month fuel cycle).
Light water reactors are thermal reactors which use thermal neutrons to sustain the chain reaction. In general, LWR’s are divided into two categories:
Pressurized water reactors use a reactor pressure vessel (RPV) to contain the nuclear fuel, moderator, control rods and coolant. They are cooled and moderated by high-pressure liquid water (e.g. at pressure of 16MPa). At this pressure water boils at approximately 350°C (662°F). Inlet temperature of the water is about 290°C (554°F). The water (coolant) is heated in the reactor core to approximately 325°C (617°F) as the water flows through the core. As it can be seen, the reactor has approximately 25°C subcooled coolant (distance from the saturation).
Boiling water reactors are cooled and moderated by water like a PWR, but at a lower pressure (of about 7MPa), which allows the water to boil inside the pressure vessel producing the steam that runs turbines. A BWR is like a PWR but with many differents. The BWRs don’t have any steam generator. Unlike a PWR, there is no primary and secondary loop. The thermal efficiency of these reactors can be higher, and they can be simpler, and even potentially more stable and safe. But the disadvantage of this concept is that any fuel leak can make the water radioactive and that radioactivity can reach the turbine and the rest of the loop.