REACTOR OPERATION DOE-HDBK-1019/2-93 Reactor Theory (Reactor Operations) Reactor  Operation  Summary (Cont.) Three methods are used to shape or flatten the core power distribution. Use of reflectors Installation of neutron poisons Axial or radial variation of fuel enrichment Power tilt is a non-symmetrical variation of core power in one quadrant of the core relative to the other quadrants. Shutdown margin is the instantaneous amount of reactivity by which a reactor is  subcritical  or  would  be  subcritical  from  its  present  condition  assuming  all control rods are fully inserted except for the single rod with the highest integral worth, which is assumed to be fully withdrawn. The  stuck  rod  criterion  is  applied  to  the  shutdown  margin  to  ensure  that  the failure  of  a  single  control  rod  will  not  prevent  the  control  rod  system  from shutting down the reactor. Several  factors  may  change  during  and  after  the  shutdown  of  the  reactor  that affect the reactivity of the core. Control rod position Soluble neutron poison concentration Temperature of the fuel and coolant Xenon Samarium Decay heat is always present following reactor operation due to energy resulting from the decay of fission products. The amount of decay heat present in the reactor is dependent on three factors. The pre-shutdown power level How long the reactor operated The amount of time since reactor shutdown Decay  heat  immediately  after  shutdown  is  approximately  5-6%  of  the  pre- shutdown  power  level.   Decay  heat will  decrease  to  approximately  1%  of the pre-shutdown power level within one hour of reactor shutdown. NP-04 Rev. 0 Page 34