Past exam of the MPhil in nuclear energy course of the University of Cambridge / 2021 / Module NE1 / 2 / e

After the short prompt adjustment, the prompt jump approximation takes the neutron amplitude to remain nearly in balance with the slowly changing delayed neutron precursor reservoir. For the source-free model,Substitute this quasi-steady relation into the precursor equation:For constant coefficients,The approximation is meaningful in the stated delayed-neutron regime with $\rho <\beta$ and a separated fast transient. The delayed neutron precursor reservoir remains continuous across an idealized instantaneous reactivity step; the approximation does not resolve the short prompt adjustment itself.