For dealing with different types of loss of coolant accident (LOCA), pressurized water reactors (PWR) have emergency core cooling system (ECCS). ECCS are simply redundant pumps to inject water into core to cool fuel, thus preventing fuel melting in case of an accident (hypothetical) that involve break of primary coolant loop. Now these have further been divided into high head safety injection (HHSI), low head safety injection (LHSI) and accumulator (based on safety analysis study to fill core before LHSI start up). Some plant call HHSI as medium heads safety injection or simply safety injection (SI) because charging pumps flow is sufficient for up to some break size. Now my question after so may years in nuclear field is how all nuclear plant standardized on this arrangement of HHSI and LHSI. I have always wonder why redundant positive displacement pumps (like reciprocating pumps) have not been used to cater to all types of break (small, medium and large break). This way plant capital cost could have been saved. Am I right can some one prove me wrong. Can some one give me an example of other arrangement for ECCS.