Previously we had a Pressurized Water Reactor (PWR) which was critical and adding heat to the coolant/moderator. After the reactor reaches the point where it is generating heat, and the moderator becomes less effective, it becomes neccessary to withdraw the control rods again to increase power output.
Since this is a PWR, we are not supposed to have large-scale boiling in the core. Steam doesn't transfer heat as readily as water does, and in a PWR, in-core boiling is a bad, bad thing. For this reason the pressure in the primary coolant loop must be kept well above the boiling point of water for the temperature found in the core. How do we do that? We have a pressurizer!
The pressurizer is a dual-purpose vessel that does two things: It acts as a surge volume for the primary coolant system, by having a compressible steam-filled void space. Secondly, it keeps the system pressure high, so that boiling cannot occur in the core. This is done by keeping a vessel half full of water and adding enough electrical heaters that the temperature in the pressurizer is maintained higher than that of the core. All boiling will occur in the pressurizer, not in the core.
Below is a simplified diagram of the primary and secondary systems of a PWR. The radioactive primary coolant (Red) is contained in a closed loop. The reactor core is the heat source, the control rod drive mechanism positions the control rods to increase/decrease reactor power, the primary pump circulates the primary coolant, and pressurizer serves as a static surge volume.
The steam generator segregates the radioactive primary coolant from the steam/water system, but allows heat to be transferred out of the primary coolant loop and generate steam for the turbine.
Pressure in the primary system is increased by energizing submerged electrical heaters in the pressurizer, and reduced by spraying a small slip-stream of primary coolant from the reactor coolant pump through a spray head at the top of the pressurizer. This cools and condenses some of the steam in the pressurizer, reducing system pressure.
Temperature is gradually increased by withdrawing the control rods. Remember in the previous post that the neutron population would increase with no control rod movement if the reactor were slightly supercritical? That is no longer the case when power level is high enough for the reactor to generate heat. After the core begins heating and the "negative temperature coefficient of reactivity" kicks in, withdrawing the control rods increases power, followed by a smaller drop in power as the primary coolant heats up. So control rods are withdrawn in a series bumps, and the coolant temperature is slowly increased.
The primary coolant pump circulates primary coolant (very pure water) between the reactor core and the steam generator. The reactor core adds heat created by fission. The steam generator removes heat by boiling secondary water in a heat exchanger. Thus, primary coolant is radioactive, but does not boil. Secondary water is non-radioactive, and does boil.
In a PWR, excess fuel is loaded, more than what is required to bring the reactor up to power. This is done so that the reactor can continue to run as the U-235 is expended, and fission products build up. Many of the atoms that are created by fission are strong neutron absorbers (poisons), and eventually result in the inability of the reactor to maintain full power. This is in spite of excess U-235 added at during refueling. Only about 3% of the U-235 is used up before the accumulation of poison halts reactor operation. This is why fuel recycling is so desirable from an industry standpoint.
Commercial PWRs are unique in that they add boric acid to the primary coolant. Boron is a neutron absorber. With the addition of boron to the coolant, the control rods may be entirely withdrawn from the core, using only boron to control neutron population. This is desirable because in the vicinity of control rods, neutron population, and fissions will be depressed. This forces other regions of the core to run hotter for a given power level. With control rods out of the core, neutron distribution is more consistent and hot spots due to uneven neutron flux are less likely to develop.
At a certain point, on the secondary side of the steam generator, steam is at an adequate pressure to begin warming up the steam turbine. This also has an interesting impact on reactor power. As we create steam on the secondary side of the steam generator, we are also removing heat from the primary coolant. This slightly cooler primary coolant returns to the reactor. Since it is now more dense, it is a better moderator, and more neutrons stay in the core to cause fissions. Reactor power now increases along with steam demand. This is a very desirable feedback loop, because now there is little need to adjust the control rods. We allow the steam turbine load to determine what power level the reactor will operate at.
The remainder of the steam plant operates in a very similar manner to a fossil-fueled plant. One difference: Since it is not possible to superheat the steam with this process, steam dryers are added between different stages on the steam turbine, to prevent mist from damaging the lower pressure sections.