Main steam line isolation valves (MSIVs) have important safety functions as well as operational functions in boiling water reactors. As being part of the reactor coolant pressure boundary (RCPB), they have to ensure its integrity, they are part of the containment function as well as they are contributing to avoid unallowable loss of reactor coolant and release of radioactive material. To fulfill all these tasks there are a couple of different requirements concerning closing time and closing characteristics, leak tightness, reliable function and others, on new MSIVs. The aim of this paper is to depict state of the art generic boundary conditions and requirements for MSIVs based on regulatory and system engineering requirements. This knowledge should contribute to avoid too much conservatism when specifying MSIVs both for new plants as well as modernizing projects. By showing examples and test results the main features of different MSIV types will be discussed and basics regarding leak tightness and closing characteristics will be given. Based on the above mentioned, an overview of state of the art requirements and boundary conditions for MSIVs will be derived as well as a new approaches. The results will be both applicable for MSIVs in new build plants as well as for modernizing projects in existing BWRs.

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