The process inherent ultimate safety (PIUS) reactor is a 600-MW(electric) pressurized water reactor based on a concept developed and verified at ABB Atom during the last 10 years. It is designed to eliminate any possibility of a core degradation accident. Its basic design features a core that is openly connected, in a natural-circulation circuit, to a large pool of heavily borated water. This pool is kept in place by a prestressed concrete pressure vessel provided with redundant leakage barriers. The coolant pumps are operated so that there is hydraulic balance in the openings between the primary coolant loop and the pool. Therefore, the hot, low-boron primary loop water is kept separated from the pool water in spite of the always open natural-circulation path. In severe transients, such as loss of feedwater, this balance is affected, and pool water ingress occurs. Reactor shutdown and long-term residual heat removal are ensured without monitoring and intervention. Thus, safety is independent of potentially failure-prone devices and cannot be jeopardized by mistakes or malicious human acts. During normal operation, boron ingress to the primary loop due to turbulent diffusion can be kept at an adequately low level, as shown by a series of investigations and experiments. In transients the thermally stratified layer that separates hot primary coolant from cold high-boron pool water moves vertically. Large movements result in boron ingress and operational disturbances. These movements can be reliably calculated by computer simulations, as confirmed by test loop operation. Simulations and tests have shown that PIUS can survive even a grid voltage disturbance, implying loss-of-coolant pump power supply for >0.5 s, without boron ingress to the primary loop. (This severe operational transient is a Scandinavian requirement.) Hence, the risk that PIUS will experience spurious shutdowns due to the special arrangement for prevention of core degradation accidents is very small. The arrangement instead provides for plant simplification because of fewer safety systems and improved operator comprehension. PIUS should become a user friendly plant that can be run with high availability and for a long plant life.