RPV

Reactor Pressure Vessel - contains all of the reactor heat.

In BWR reactorBWR reactorA boiling water reactor uses Light Water as both coolant and neutron moderator. The second most used reactor, next to the PWR reactors, there are approximately 75 plants in current operation. The efficiency of these reactors is about 46%, with 33-34% in practice. Enriched uranium is used as nuclear fuel, as light water absorbs too many neutrons to use Uranium that is natural. Light water is not as good of a moderator compared to Heavy Water or graphite, but it is good as in the event of a LOCA os, the RPV contains the reactor core - basically the entirety of the main reactor assembly. The RPV is designed to withstand a very large amount of force considering that in a BWR it must withstand the pressure that both it operates at and at emergency designs -- this is due to the fact that in most designs, the RPV isn't considered to be at major risk: even during a major LOCA the RPV is considered to be at healthy conditons (this is a very huge simplification, as there are many, MANY systems in place in the event of a RPV rupture).

This is an diagram of such a RPV (in a BWR - that is important as a PWR uses a different system - a PWR does not have the same steam considerations) RPV

Interesting aside of the control rods

Interestingly, all of the control rods literally stick into the downcomer (waterfilled) region of the reactor, and that brings up the consideration -- how does the RPV seal these rods to prevent water leakage? This is extremely important, as otherwise highly radioactive water can leak out and cause major damage to the resulting plant (well, major is an overstatement, it will primarily leak out into probably the control rods mechanisms and there are safety systems to control this.)