Editing Control rod (section)

===Additional means of reactivity regulation===
Other means of controlling reactivity include (for PWR) a soluble neutron absorber ([[boric acid]]) added to the reactor coolant, allowing the complete extraction of the control rods during stationary power operation, ensuring an even power and flux distribution over the entire core. This [[chemical shim]], along with the use of burnable neutron poisons within the fuel pellets, is used to assist regulation of the core's long term reactivity,<ref name="EAGLE">{{cite web|title=Enriched boric acid for pressurized water reactors |work=EaglePicher Corporation |url=http://www.epcorp.com/NR/rdonlyres/71174EA7-B374-4934-8596-B51D105C4F30/0/w_c_01.pdf |access-date=September 25, 2008 |archive-url=https://web.archive.org/web/20071129121350/http://www.epcorp.com/NR/rdonlyres/71174EA7-B374-4934-8596-B51D105C4F30/0/w_c_01.pdf |archive-date=November 29, 2007}}</ref> while the control rods are used for rapid reactor power changes (e.g. shutdown and start up). Operators of BWRs use the coolant flow through the core to control reactivity by varying the speed of the reactor recirculation pumps (an increase in coolant flow through the core improves the removal of steam bubbles, thus increasing the density of the coolant/[[neutron moderator|moderator]], increasing power).