Effective Delayed Neutron Precursor Decay Constant

TABLE 1 Delayed Neutron Fractions for Various Fuels

Figure 2 Reactor Power Response to Positive Reactivity Addition

Nuclear Physics and Reactor Theory Volume 2 of 2
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¯ eff ¯ I ¯ eff ¯ Reactor Theory (Reactor Operations) DOE-HDBK-1019/2-93 REACTOR KINETICS Rev. 0 NP-04 Page 13 where: = effective delayed neutron fraction = average delayed neutron fraction I = importance factor In a small reactor with highly enriched fuel, the increase in fast non-leakage probability will dominate the decrease in the fast fission factor, and the importance factor will be greater than one. In a large reactor with low enriched fuel, the decrease in the fast fission factor will dominate the increase in the fast non-leakage probability and the importance factor will be less than one (about 0.97 for a commercial PWR). Effective Delayed Neutron Precursor Decay Constant Another new term has been introduced in the reactor period ( ) equation. That term is _eff (pronounced lambda effective), the effective delayed neutron precursor decay constant. The decay rate for a given delayed neutron precursor can be expressed as the product of precursor concentration and the decay constant ( ) of that precursor. The decay constant of a precursor is simply the fraction of an initial number of the precursor atoms that decays in a given unit time. A decay constant of 0.1 sec , for example, implies that one-tenth, or ten percent, of a -1 sample of precursor atoms decays within one second. The value for the effective delayed neutron precursor decay constant, , varies depending upon the balance existing between the eff concentrations of the precursor groups and the nuclide(s) being used as the fuel. If the reactor is operating at a constant power, all the precursor groups reach an equilibrium value. During an up-power transient, however, the shorter-lived precursors decaying at any given instant were born at a higher power level (or flux level) than the longer-lived precursors decaying at the same instant. There is, therefore, proportionately more of the shorter-lived and fewer of the longer-lived precursors decaying at that given instant than there are at constant power. The value of is closer to that of the shorter-lived precursors. eff During a down-power transient the longer-lived precursors become more significant. The longer-lived precursors decaying at a given instant were born at a higher power level (or flux level) than the shorter-lived precursors decaying at that instant. Therefore, proportionately more of the longer-lived precursors are decaying at that instant, and the value of approaches eff the values of the longer-lived precursors. Approximate values for are 0.08 sec for steady-state operation, 0.1 sec for a power eff -1 -1 increase, and 0.05 sec for a power decrease. The exact values will depend upon the materials -1 used for fuel and the value of the reactivity of the reactor core.

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