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.
sfrepresents 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
cjgenerated at fission time.
cjwith decay constant
cyj(the added "
y" stands for power deca"Y") satisfies (7), where
βyjis the number of FP emitters of type j with decay constant
fyjdeposited by emitters
yjdecay is proportional (factor
μ) to the decay rate (9).
fyjcould be interpreted as well as the steady-state ratio of
cyjgenerated decay power
fyjto the total potential power
fyjin transient condition is (12), and
f(13) the actual instant power.
βypis the prompt (major) fraction.
yjemitters 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 ft0of the initial power. At steady state,
f=ft=ft0and (8) holds.
For most of the transients at power, the effect of decay power is just reflected by a small time lagging and smoothing of power production behind the neutron flux and the activation of the effect is not worthwhile; the normal model is probably conservative.
For safety evaluation of the core immediately after shutdown however, the residual power depends of previous reactor operation history and the delayed neutron produced by remaining precursors may contribute to some fission power. Then the decay power model is necessary.
beypmust not be entered: it is calculated from
behjby means of (14).