CLAIMS 2. The boiling water nuclear reactor system according to claim 1, wherein the condensate return line of the isolation condenser system includes a valve that opens flow through the condensate return line during a reactor shutdown. 3. The boiling water nuclear reactor system according to claim 1, wherein the condensate return line further includes a split portion that returns condensate inside the downcomer region. 4. The boiling water nuclear reactor system according to claim 1, further comprising: an isolation condenser system receives steam from the reactor pressure vessel and includes a condensate return line that selectively returns condensate inside the core shroud above the reactor core into one or both of an upper plenum or chimney region of the core shroud. a steam line connected to an upper end of the reactor pressure vessel; and a downcomer region disposed between an inner surface of the reactor pressure vessel and the core shroud; 1. A boiling water nuclear reactor system, comprising: a reactor pressure vessel; a reactor core disposed in the reactor pressure vessel; a core shroud surrounding the reactor core;