Reactor Period - h1019v2_103

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P Po  et / ¯ eff eff ¯ eff Reactor Theory (Reactor Operations) DOE-HDBK-1019/2-93 REACTOR KINETICS Rev. 0 NP-04 Page 11 Reactor Period (  ) The reactor period is defined as the time required for reactor power to change by a factor of "e," where "e" is the base of the natural logarithm and is equal to about 2.718.  The reactor period is usually expressed in units of seconds.  From the definition of reactor period, it is possible to develop the relationship between reactor power and reactor period that is expressed by Equation (4-6). (4-6) where: P =  transient reactor power P =  initial reactor power o =  reactor period (seconds) t =  time during the reactor transient (seconds) The smaller the value of   , the more rapid the change in reactor power.  If the reactor period is positive,  reactor  power  is  increasing.    If  the  reactor  period  is  negative,  reactor  power  is decreasing. There are numerous equations used to express reactor period, but Equation (4-7) shown below, or portions of it, will be useful in most situations.  The first term in Equation (4-7) is the prompt term and the second term is the delayed term. (4-7) where: * =  prompt generation lifetime =  effective delayed neutron fraction =  reactivity =  effective delayed neutron precursor decay constant eff =  rate of change of reactivity Effective Delayed Neutron Fraction Recall that    , the delayed neutron fraction, is the fraction of all fission neutrons that are born as delayed neutrons.  The value of     depends upon the actual nuclear fuel used.  As discussed in Module 1, the delayed neutron precursors for a given type of fuel are grouped on the basis of half-life.  The following table lists the fractional neutron yields for each delayed neutron group of three common types of fuel.


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