Reactor criticality. A – supercritical state; B – critical state; C – subcritical state

In preceding chapters, the multiplication factors have been analysed in order to understand a steady-state condition. Also the operational changes that influence the criticality of the reactor were described. It must be noted, nuclear reactors are not always critical. In order to start it up or to shut it down, the k_eff must be changed for a certain period, which in turn changes the neutron population in the reactor core. The study of behavior of the neutron population in a noncritical reactor is called reactor kinetics. From this point of view, the state of a reactor can be distinguished by following criteria.

The basic classification of states of a reactor is according to the multiplication factor as eigenvalue which is a measure of the change in the fission neutron population from one neutron generation to the subsequent generation.

• k_eff < 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.

• k_eff = 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.

• k_eff > 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.

The definitions described above are fully applicable to a reactor at zero power level, that is at such power level in which all thermal considerations are not important to the chain reaction (let say from zero power to 1% of rated power).

For power reactors at power conditions the reactor can behave differently as a result of the presence of reactivity feedbacks. Power reactors are initially started up from hot standby mode (subcritical state at 0% of rated power) to power operation mode (100% of rated power) by withdrawing control rods and by boron dilution from the primary coolant. During the reactor startup and up to about 1% of rated power, the reactor kinetics is exponential as in zero power reactor. This is due to the fact all temperature reactivity effects are minimal.

On the other hand, during further power increase from about 1% up to 100% of rated power, the temperature reactivity effects play very important role. As the neutron population increases, the fuel and the moderator increase its temperature, which results in decrease in reactivity of the reactor (almost all reactors are designed to have the temperature coefficients negative).

See also: Operational factors that affect the multiplication in PWRs

The negative reactivity coefficient acts against the initial positive reactivity insertion and this positive reactivity is offset by negative reactivity from temperature feedbacks. In order to keep the power to be increasing, positive reactivity must be continuously inserted (via control rods or chemical shim). After each reactivity insertion, the reactor power stabilize itself on the power level proportionately to the reactivity inserted. The total amount of feedback reactivity that must be offset by control rod withdrawal or boron dilution during the power increase is known as the power defect. The power defects for PWRs, graphite-moderated reactors, and sodium-cooled fast reactors are:

• about 2500pcm for PWRs,
• about 800pcm for graphite-moderated reactors
• about 500pcm for sodium-cooled fast reactors

The power defects slightly depend on the fuel burnup, because they are determined by the power coefficient which depends on the fuel burnup. The power coefficient combines the Doppler, moderator temperature, and void coefficients. It is expressed as a change in reactivity per change in percent power, Δρ/Δ% power. The value of the power coefficient is always negative in core life but is more negative at the end of the cycle primarily due to the decrease in the moderator temperature coefficient.

It is logical, as power defects act against power increase, they act also against power decrease. When reactor power is decreased quickly, as in the case of reactor trip, power defect causes a positive reactivity insertion, and the initial rod insertion must be sufficient to make the reactor safe subcritical. It is obvious, if the power defect for PWRs is about 2500pcm (about 6 βeff), the control rods must weigh more than 2500pcm to achieve the subcritical condition. To ensure the safe subcritical condition, the control rods must weigh more than 2500pcm plus value of SDM (SHUTDOWN MARGIN). The total weigh of control rods is design specific, but, for example, it may reach about 6000pcm. To ensure that the control rods can safe shut down the reactor, they must be maintained above a minimum rod height (rods insertion limits).

The basic classification of states of a reactor may be in some case insufficient and a finer classification is needed. The finer classification takes into account the two groups of neutrons that are produced in fission.

It is known the fission neutrons are of importance in any chain-reacting system. The fission of fissile nuclei produces 2, 3 or more free neutrons. But not all neutrons are released at the same time following fission. Even the nature of creation of these neutrons is different. From this point of view we usually divide the fission neutrons into two following groups:

• Prompt Neutrons. Prompt neutrons are emitted directly from fission and they are emitted within very short time of about 10^-14 second.

• Delayed Neutrons. 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 fact the neutron is produced via this type of decay and this happens orders of magnitude later compared to the emission of the prompt neutrons, plays an extremely important role in the control of the reactor.

The fact the prompt neutrons are emitted within 10^-14 and have significant shorter mean generation time (~ 10^-5s) than delayed neutrons (~0.1s) have crucial consequences. The period of ~10^-5s is very short and causes very very fast response of the reactor power in case of prompt criticality. The state of reactor, when the chain reaction is self-sustained only by prompt neutrons, is known as the prompt critical state. This state of the reactor is very unstable, because one neutron generation takes only  ~10^-5s.   Therefore nuclear reactors must operate in the prompt subcritical, delayed critical condition. All power reactors are designed to operate in a delayed critical conditions and are provided with safety systems to prevent them from ever achieving prompt criticality.

The prompt critical state is defined as:

• k_eff > 1; ρ ≥ β_eff, where the reactivity of a reactor is higher than the effective delayed neutron fraction. In this case, the production of prompt neutrons alone is enough to balance neutron losses and increase the neutron population. The number of neutrons is exponentially increasing in time (as rapidly as the prompt neutron generation lifetime ~10^-5s).

The prompt subcritical and delayed supercritical state is defined as:

• k_eff > 1; 0 < ρ < β_eff, where the reactivity of a reactor is higher than zero and lower than the effective delayed neutron fraction. In this case, the production of prompt neutrons alone is insufficient to balance neutron losses and the delayed neutrons are needed in order to sustain the chain reaction. The neutron population increases, but much more slowly (as the mean generation lifetime with delayed neutrons ~0.1 s).

The prompt subcritical and delayed critical state is defined as:

• k_eff = 1; ρ = 0, where the reactivity of a reactor is equal to zero. In this case, the production of prompt neutrons alone is insufficient to balance neutron losses and the delayed neutrons are needed in order to sustain the chain reaction. There is no change in neutron population in time and the chain reaction will be self-sustaining. This state is the same state as the critical state from basic classification.

The prompt subcritical and delayed subcritical state is defined as:

• k_eff < 1; ρ < 0, where the reactivity of a reactor is lower than zero. In this case, the production of all neutrons is insufficient to balance neutron losses and the chain reaction is not self-sustaining. If the reactor core contains external or internal neutron sources, the reactor is in the state that is usually referred to as the subcritical multiplication.