##### Residual decay power kinetics.

Up to now, the nuclear power density ft (W/cm³) was calculated from the fission neutron source sf (neutr/cm³/s) by (1) where κ/ν (skn) is tabulated along with the other neutronic properties.

Actually, sf represents the total "potential" fission neutron source which includes the prompt neutrons emitted at fission time, and the delayed neutron emitted by subsequent decay of radioactive fission product emitters cj generated at fission time.
We remind (2) the balance equation of precursor cj with decay constant λj.
For stationary condition, (4) applies, which implies (5).
A strict mathematical similitude may be established between the delayed neutron precursors and the power decay fission product (FP) emitters. The concentration cyj (the added "y" stands for power deca"Y") satisfies (7), where βyj is the number of FP emitters of type j with decay constant λyj.
The power density fyj deposited by emitters yj decay is proportional (factor μ) to the decay rate (9).
At steady-state, from (8) and (9) , we obtain (10) which shows that if we set (11), fyj could be interpreted as well as the steady-state ratio of cyj generated decay power fyj to the total potential power ft.
The actual fyj in transient condition is (12), and f (13) the actual instant power.
In (13), βyp is the prompt (major) fraction.
For yj emitters with time constant (1/ λyj) much larger than the application duration, the contribution of those emitters should more conveniently be approximated by a constant part βy0 ft0 of the initial power. At steady state, f=ft=ft0 and (8) holds.
Power decay model is activated under Lstcy and Lstcyj lists. (be=β, am=λ).
The prompt fraction beyp must not be entered: it is calculated from bey0 and the behj by means of (14).