A new type of boiling water reactor (BWR)—the process inherent ultimate safety (PIUS) BWR—has been conceived. A PIUS BWR is an advanced BWR that differs from the typical BWR in that a prestressed concrete reactor vessel (PCRV) with special internals replaces the conventional pressure vessel, emergency core cooling system, containment shell, spent fuel storage ponds, and most other components on the nuclear island. The reactor core and balance of plant are similar to current BWR designs. This approach effectively eliminates the possibility of reactor core meltdown and simplifies plant design. The PIUS BWR does not require operation of any mechanical or electrical components in any emergency for reactor shutdown or afterheat cooling, nor does it depend on any equipment outside the PCRV. The PCRV contains a natural circulation BWR; cool, borated water for emergency core cooling; a mechanism “X” that allows the cool, borated water to enter the reactor core if there is insufficient water in the core; and a mechanism “Y” that limits core power levels to available cooling capabilities.