Effective Multiplication Factor
In this section, the effective multiplication factor, which describes all the possible events in the life of a neutron and effectively describes the state of a finite multiplying system, will be defined.
The required condition for a stable, self-sustained fission chain reaction in a multiplying system (in a nuclear reactor) is that exactly every fission initiate another fission. The minimum condition is for each nucleus undergoing fission to produce, on the average, at least one neutron that causes fission of another nucleus. Also the number of fissions occurring per unit time (the reaction rate) within the system must be constant.
This condition can be expressed conveniently in terms of the multiplication factor. The effective multiplication factor is the ratio of the neutrons produced by fission in one neutron generation to the number of neutrons lost through absorption in the preceding neutron generation. This can be expressed mathematically as shown below.
It is obvious the effective multiplication factor in a multiplying system is a measure of the change in the fission neutron population from one neutron generation to the subsequent generation.
- keff < 1. If the multiplication factor for a multiplying system is less than 1.0, then the number of neutrons is decreasing in time (with the mean generation time) and the chain reaction will never be self-sustaining. This condition is known as the subcritical state.
- keff = 1. If the multiplication factor for a multiplying system is equal to 1.0, then there is no change in neutron population in time and the chain reaction will be self-sustaining. This condition is known as the critical state.
- keff > 1. If the multiplication factor for a multiplying system is greater than 1.0, then the multiplying system produces more neutrons than are needed to be self-sustaining. The number of neutrons is exponentially increasing in time (with the mean generation time). This condition is known as the supercritical state.
Six Factor Formula
But the effective multiplication factor can be defined also in terms of the most important neutron-physical processes that occur in the nuclear reactor.
There are six factors that describe the inherent multiplication ability of the system. Four of them are completely independent of the size and shape of the reactor and these are:
k∞ = η.ε.p.f
The effective multiplication factor (keff) may be expressed mathematically in terms of the infinite multiplication factor (k∞) and two additional factors which account for neutron leakage during neutron thermalisation (fast non-leakage probability) and neutron leakage during neutron diffusion (thermal non-leakage probability) by following equation, usually known as the six factor formula:
keff = k∞ . Pf . Pt
Operational factors that affect the multiplication in PWRs.
Detailed knowledge of all possible operational factors that may affect the multiplication factor of the system are of importance in the reactor control. It was stated the keff is during reactor operation kept as near to the value of 1.0 as possible. The criticality of the reactor is influenced by many factors. For illustration, in an extreme case also the presence of human (due to the water, carbon, which are good neutron moderators) near fresh uranium fuel assembly influences the multiplication properties of the assembly.
If any operational factor changes one of the contributing factors to keff (keff = η.ε.p.f.Pf.Pt), the ratio of 1.0 is not maintained and this change in keff makes the reactor either subcritical or supercritical. Some examples of these operational changes, that may take place in PWRs, are below and are described in a separate article in detail.
- change in the control rods position
- change in the boron concentration
- change in the moderator temperature
- change in the fuel temperature
- change in the pressure
- change in the flow rate
- presence of boiling of the coolant
- presence of burnable absorbers
- fuel burnup