The European Pressurized Water Reactor's (EPR's) safety injection system (SIS) comprises four trains, each of them consisting of a medium head safety injection, an accumulator, and a low head safety injection (LHSI). Injection mode is into the cold legs of the main coolant line for the short term. This emergency core cooling (ECC) mode is quite different from the typical German concept with combined injection, i.e., safety injection into both the cold and the hot legs of the main coolant line at least for accumulator and LHSI. Therefore, the German Safety Authority requested justification for giving up the ECC-mode used in German pressurized water reactors, the so-called "combined injection." Furthermore, the Reaktor-sicherheitskommission requested a comparison between cold-leg injection and combined injection in terms of ECC efficiency over all relevant accident sequences.

The evolution from combined injection to cold-leg injection is described and results of comparative analysis are summarized. It is demonstrated that EPR's SIS is a well-balanced system, which ensures high ECC efficiency and limits loads to containment over the whole accident spectrum. For the entire loss-of-coolant accident (LOCA) spectrum, ECC efficiency of EPR's SIS is practically equivalent to ECC efficiency of a SIS of the KONVOI type with combined injection. The smaller the break, the more insignificant are differences. The ECC mode has a negligible impact on containment pressure and temperature evolution during a LOCA. Neither with combined injection nor with cold-leg injection is a containment spray system needed.