Effective Precursor Decay Constant – Lambda-Effective
The effectively delayed neutron precursor decay constant (pronounced lambda effective) is a new term, which has to be introduced in the reactor period equation in the case of a single precursor group model. Creating a simple kinetic model conducive to understanding reactor behavior is useful to reduce the precursors to a single group further. But if we do this, the convention is to employ a constant precursor yield fraction and a variable precursor decay rate, as defined by lambda effective (λeff). In the single precursor group model, the lambda effect is not a constant but rather a dynamic property that depends on the mix of precursor atoms resulting from the reactivity.
The reason the constant decay constant cannot be used is as follows. There is a difference in the decay and the creation of short-lived and long-lived precursors during power transients.
During a power increase (positive reactivity), the short-lived precursors decaying at any given instant were born at a higher power level than the longer-lived precursors decaying at the same instant. The short-lived precursors become more significant. As the magnitude of the positive reactivity increases, the value of lambda effective increases closer to that of the short-lived precursors (let say 0.1 s-1 for +100pcm).
During a power decrease (negative reactivity), the long-lived precursors decaying at a given instant were born at a higher power level than the short-lived precursors decaying at that instant. The long-lived precursors become more significant. As the magnitude of the negative reactivity increases, the value of lambda effective decreases closer to that of the long-lived precursors (let say 0.05 s-1 for -100pcm).
If the reactor is operating at steady-state operation, all the precursor groups reach an equilibrium value and the λeff value is approximately 0.08 s-1.