Editing Boiling water reactor (section)

=== Control rod issues ===
* Control rods are inserted from below for current BWR designs. There are two available hydraulic power sources that can drive the control rods into the core for a BWR under emergency conditions. There is a dedicated high-pressure hydraulic accumulator and also the pressure inside of the reactor pressure vessel available to each control rod. Either the dedicated accumulator (one per rod) or reactor pressure is capable of fully inserting each rod. Most other reactor types use top-entry control rods that are held up in the withdrawn position by electromagnets, causing them to fall into the reactor by gravity if power is lost. This advantage is partially offset by the fact that hydraulic forces provide much greater rod insertion forces than gravity, and as a consequence, BWR control rods are much less likely to jam in a partially inserted position due to damage to the control rod channels in a core damage event.  Bottom-entry control rods also permit refueling without removal of the control rods and drives, as well as testing of the control rod systems with an open pressure vessel during refueling.