10. The boiling water nuclear reactor system according to claim 8, 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. 11. The boiling water nuclear reactor system according to claim 10, 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. 12. The boiling water nuclear reactor system according to claim 10, wherein the condensate return line further includes a split portion that returns condensate inside the downcomer region.