The results of the qualification of main steam isolation valves (MSIV) for water / steam-water medium have been analyzed taking into consideration the ―small series‖ WWER-1000 units. This analysis demonstrates that qualification of MSIV for water has not been substantiated sufficiently for the ―small series‖ of WWER-1000, so it could not been extended for serial WWER-1000. In addition, it is necessary to perform the qualification of MSIV for water for severe beyond design basis accidents taking into consideration possible human errors and multiple failures based on reasonably substantiated criteria of qualification.

Keywords: qualification, main steam isolation valve, steam-water medium.