Key Characteristics of Delayed Neutrons
- The presence of delayed neutrons is perhaps most important aspect of the fission process from the viewpoint of reactor control.
- Delayed neutrons are emitted by neutron rich fission fragments that are called the delayed neutron precursors.
- These precursors usually undergo beta decay but a small fraction of them are excited enough to undergo neutron emission.
- The emission of neutron happens orders of magnitude later compared to the emission of the prompt neutrons.
- About 240 n-emitters are known between 8He and 210Tl, about 75 of them are in the non-fission region.
- In order to simplify reactor kinetic calculations it is suggested to group together the precursors based on their half-lives.
- Therefore delayed neutrons are traditionally represented by six delayed neutron groups.
- Neutrons can be produced also in (γ, n) reactions (especially in reactors with heavy water moderator) and therefore they are usually referred to as photoneutrons. Photoneutrons are usually treated no differently than regular delayed neutrons in the kinetic calculations.
- The total yield of delayed neutrons per fission, vd, depends on:
- Isotope, that is fissioned.
- Energy of a neutron that induces fission.
- Variation among individual group yields is much greater than variation among group periods.
- In reactor kinetic calculations it is convenient to use relative units usually referred to as delayed neutron fraction (DNF).
- At the steady state condition of criticality, with keff = 1, the delayed neutron fraction is equal to the precursor yield fraction β.
- In LWRs the β decreases with fuel burnup. This is due to isotopic changes in the fuel.
- Delayed neutrons have initial energy between 0.3 and 0.9 MeV with an average energy of 0.4 MeV.
- Depending on the type of the reactor, and their spectrum, the delayed neutrons may be more (in thermal reactors) or less effective than prompt neutrons (in fast reactors). In order to include this effect into the reactor kinetic calculations the effective delayed neutron fraction – βeff must be defined.
- The effective delayed neutron fraction is the product of the average delayed neutron fraction and the importance factor βeff = β . I.
- The weighted delayed generation time is given by τ = ∑iτi . βi / β = 13.05 s, therefore the weighted decay constant λ = 1 / τ ≈ 0.08 s-1.
- The mean generation time with delayed neutrons is about ~0.1 s, rather than ~10-5 as in section Prompt Neutron Lifetime, where the delayed neutrons were omitted.
- Their presence completely changes the dynamic time response of a reactor to some reactivity change, making it controllable by control systems such as the control rods.
Table of main kinetic parameters.
Example – Infinite Multiplying System Without Source and Delayed Neutrons
An simplest equation governing the neutron kinetics of the system with delayed neutrons is the point kinetics equation. This equation states that the time change of the neutron population is equal to the excess of neutron production (by fission) minus neutron loss by absorption in one mean generation time with delayed neutrons (ld). The role of ld is evident. Longer lifetimes give simply slower responses of multiplying systems.
If there are neutrons in the system at t=0, that is, if n(0) > 0, the solution of this equation gives the simplest point kinetics equation with delayed neutrons (similarly to the case without delayed neutrons):Let us consider that the mean generation time with delayed neutrons is ~0.085 and k (k∞ – neutron multiplication factor) will be step increased by only 0.01% (i.e. 10pcm or ~1.5 cents), that is k∞=1.0000 will increase to k∞=1.0001.
It must be noted such reactivity insertion (10pcm) is very small in case of LWRs. The reactivity insertions of the order of one pcm are for LWRs practically unrealizable. In this case the reactor period will be:
T = ld / (k∞-1) = 0.085 / (1.0001-1) = 850s
This is a very long period. In ~14 minutes the neutron flux (and power) in the reactor would increase by a factor of e = 2.718. This is completely different dimension of the response on reactivity insertion in comparison with the case without presence of delayed neutrons, where the reactor period was 1 second.
Reactors with such a kinetics would be quite easy to control. From this point of view it may seem that reactor control will be a quite boring affair. It will not! The presence of delayed neutrons entails many many specific phenomena, that will be described in later chapters.
Interactive chart – Infinite Multiplying System Without Source and with Delayed Neutrons
Press the “clear and run” button and try to increase the power of the reactor.
Compare the response of the reactor with the case of Infinite Multiplying System Without Source and without Delayed Neutrons (or set the β = 0).