REACTOR OPERATIONDOE-HDBK-1019/2-93Reactor Theory (Reactor Operations)FlowAt low reactor power levels, changing the flow rate of the coolant through the reactor does notresult in a measurable reactivity change because fuel and moderator temperatures and the fractionof steam voids occurring in the core are not changed appreciably.When the flow rate is varied, however, the change in temperature that occurs across the core(outlet versus inlet temperature) will vary inversely with the flow rate. At higher power levels,on liquid cooled systems, increasing flow will lower fuel and coolant temperatures slightly,resulting in a small positive reactivity insertion. A positive reactivity addition also occurs whenflow is increased in a two-phase (steam-water) cooled system. Increasing the flow rate decreasesthe fraction of steam voids in the coolant and results in a positive reactivity addition. Thisproperty of the moderator in a two-phase system is used extensively in commercial BWRs.Normal power variations required to follow load changes on BWRs are achieved by varying thecoolant/moderator flow rate.CoreBurnupAs a reactor is operated, atoms of fuel are constantly consumed, resulting in the slow depletionof the fuel frequently referred to as core burnup. There are several major effects of this fueldepletion. The first, and most obvious, effect of the fuel burnup is that the control rods mustbe withdrawn or chemical shim concentration reduced to compensate for the negative reactivityeffect of this burnup.Some reactor designs incorporate the use of supplemental burnable poisons in addition to thecontrol rods to compensate for the reactivity associated with excess fuel in a new core. Thesefixed burnable poisons burn out at a rate that approximates the burnout of the fuel and theyreduce the amount of control rod movement necessary to compensate for fuel depletion early incore life.As control rods are withdrawn to compensate for fuel depletion, the effective size of the reactoris increased. By increasing the effective size of the reactor, the probability that a neutron slowsdown and is absorbed while it is still in the reactor is also increased. Therefore, neutron leakagedecreases as the effective reactor size is increased. The magnitude of the moderator negativetemperature coefficient is determined in part by the change in neutron leakage that occurs as theresult of a change in moderator temperature. Since the fraction of neutrons leaking out is lesswith the larger core, a given temperature change will have less of an effect on the leakage.Therefore, the magnitude of the moderator negative temperature coefficient decreases with fuelburnup.NP-04Rev. 0Page 30
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