How do the pressurizers in a PWR nuclear reactor (Pressurized water reactor) work? I did a bit of googling and wikipedia informed me that the pressurizer is just a vertical tank of sitting coolant water that directly connects to the reactor. Then electrical heaters in this vessel raise the pressure in the pressurizer by raising the temperature (since the two are directly related). What I don't get is how is this the best way to go about it? Your $coolant$ is being heated. I read that the temperature difference between input and output of the reactor vessel is $30^o C$. Intuitively, that seems kind of poor...
But that confuses me less than the following: if the whole point of a PWR is that the coolant/moderator is pressurized so that it cannot boil, wouldn't heating it up to hundreds of degrees C in the pressurizer make it boil!?
How is it that in a PWR, heating of the water in the pressurizer increases the pressure but doesn't cause boiling, but in something like a boiling water reactor, the coolant is heated directly by the reactor which increases the pressure AND the coolant boils? What differentiates these two? Both are just cylindrical vessels with a heat source in them, yet one causes pressure increase and no boiling, while the other increases pressure and causes boiling.
I'm not too knowledgeable about this because I'm not in this industry or field, I'm just a guy who likes to read about this stuff and this part didn't make sense to me. Thanks!
 A: The pressurizer is at a higher temperature than the reactor core by design and it does indeed contain both water and steam.  Note that the Wikipedia article on PWRs says it is “partially filled with water”.  The rest of its volume is occupied by steam.  Boiling in the reactor core is undesirable in a PWR, so by having the pressurizer operate at a higher temperature than the core, the pressure in the whole primary coolant loop is maintained at the value where it boils at the temperature in the pressurizer.  Since the pressurizer is now the hottest part of the primary coolant system, boiling in the core (and the rest of the primary coolant loop) is then avoided.
The proportion of water/steam content of the pressurizer can then adjust to coolant volume changes (e.g. caused by temperature changes in the other parts of the primary coolant system) by boiling or condensing within the limits of the pressurizer’s volume.  If the pressure of the primary coolant system drops, part of the water inside the pressurizer flows out of it, equalizing the pressure and part of it evaporates, slightly lowering the temperature inside the pressurizer.  The other way around applies for pressure increases.
This design is used because it achieves a very soft response of the pressure in reaction to changes in water volume and it does this to a certain extent passively and without moving mechanical parts.  Consider the following cases of what will happen if a small extra volume of water enters quickly:


*

*If the gas volume is filled with an ideal gas and is thermally isolated, pressure and temperature rise (adiabatic compression).

*If the temperature can equalize, the pressure increase will be less (isothermal compression).

*If the gas is water vapor instead of an ideal gas, the isothermally compressed steam will be supersaturated, so a part of it will condense to water.  This has higher density than steam, so the pressure increase will be even lower.

*Without gas in the system, any volume change would have to be absorbed by the compressibility of the water and the elasticity of the piping, both of which are very stiff.


The isothermal case is conceptually similar to an expansion tank.  With condensation/evaporation, the response is softer and there is no need for a flexible membrane or movable piston to separate the gas from the liquid.  With active temperature control, the pressure changes can be reduced further, but the soft response of the passive behaviour means that a longer reaction time of the active control can be tolerated and the pressure will still be within limits.
A: Regarding the amazonprime comment of May 29, 2017.  Yes the primary system pressurized, hot, subcooled water is essentially incompressible.  However its density changes with temperature and without the pressurizer to control pressure an increase in temperature as the primary is heated from startup to operating conditions would result in excessive pressure.  Note also that the pressurizer contains steam, not a non-condensable gas, so in addition to the steam being compressible, heating the water in the pressurizer (using heaters) and condensing the water in the pressurizer (using water sprays) increases the ability to control pressure.
