In order to get a good understanding of the Pressurized Water Reactor, which will hence forth be called the PWR, we are going to start with discussing the Boiling Water Reactor, or BWR, both of which will be refered to in the western design and not their eastern and soviet counterparts.
The reactor makes a reaction using a fuel such as U-235. This fuel is kept at an enrichment of about 3%, this is due to safety, without a neutron flux moderator the fuel is first and foremost going to act like nothing, it won't react and eventually the neutrons will get absorbed, however, with the moderator present the reactor can make a reaction. In the West we use water as a neutron flux moderator, this is because in the event of a LOCA, loss of coolant accident, the nuclear reactor will automatically scramble and stop the reaction. So how do we control this reaction? Control rods are the answer, cadmium is a great neutron absorber so it is used to decrease reactivity, when rods are put in the reaction slows down; when rods are pulled out the reaction increases. And at a certain point of rod position, depending on the current parameters, the reactor will be stable.
The key thing about a BWR is that the water in the reactor is what is getting boiled in order to drive the turbine. It is a binary looping system. The primary loop will encompass the reactor and the turbine, water is boiled and fed to the turbine. After the water passes over the turbine it enters the heat-exchanger known as a condensor. This condensor is the secondary loop of a BWR, it takes cold water from a lake, river, or by sending it into a cooling tower. The steam from the primary loop will condense via convection, evaporating the steam in the condensor.
The reaction happens in much the same way as a BWR, they even use the same control rods too. Even the moderator is the same in this type of reactor. So how does it stand out from the BWR then?
The PWR is a tertiary looping system. The primary loop is heated by the chain reaction, unlike the BWR, however, the water does not boil. This is due to the pressurizer keep the water at about 2200 Pounds per Square Inch. From there the heated water is passed to the steam generator, which is a heat exchanger, it then boils the secondary loop and makes steam which will drive the turbine. The steam is then sent to the condensor chamber which is a heat exchanger containing the tertiary loop. the secondary loop steam is condensed back into water with the condensor, which is uses water from another source and brought in with the tertiary loop. The secondary loop water, after it is condensed, moves back into the steam generator and cools off the primary loop via convection.
PWR does not have Positive or Negative Void Coefficients. The positive void coefficent happens when steam is generated, the generation of steam increases temperature making more steam in a positive feedback loop. The negative void coefficient happens because the steam is made. Water is the moderator of the reactor, when steam is made it can no longer moderate the speed of neutrons. When the neutrons speed up the reactivity slows down. The last coefficient, the PWR does have this one but I thought it was worth mentioning, the negative temperature coefficient is simply that once the temperature rises the reactivity decreases.