Enhancing load-following and/or spectral shift capability in single-sparger natural circulation boiling water reactors

A method for obtaining load-following capability in a boiling water reactor (BWR) wherein housed within a reactor pressure vessel (RPV) is a nuclear core disposed within a shroud having a shroud head and which with said RPV defines an annulus region which is in flow communication with a core lower plenum region disposed beneath said nuclear core, an upper steam dome connected to a steam outlet in said RPV, a core upper plenum formed within said shroud head and disposed atop said nuclear core, a chimney mounted atop said shroud head and in fluid communication with said core upper plenum and with a steam separator having a skirt which is in fluid communication with said steam dome, the region outside of the chimney defining a downcomer region, there being a water level established therein under normal operation of said BWR, and said RPV containing a feedwater inlet, which comprises:

• disposing a single sparger connected to said feedwater inlet above the steam separator skirt bottom about the interior circumference of said RPV at an elevation at approximately the water level established during normal operation of said BWR;
• and
• adjusting the feedwater flow through said inlet and into said sparger to vary the water level to be above, at, or below the elevational location of said sparger in response to load-following need.