moderator the temperature of the coolant the fuel temperature the void fraction in the coolant and certain fission products including xenon and samarium the effect of each of these factors can be determined by using known combinations of theoretical and empirical techniques the total reactivity present as a function of time can be determined by summing the reactivities associated with each factor relative to some initial condition usually an exactly critical reactor such a calculation is referred to as a reactivity balance f net reactivity the net reactivity is the total reactivity present this quantity is the end product of a reactivity balance calculation note relative to the controller disclosed herein it functions by restricting the net reactivity to that which can be controlled by altering the reactivity associated with the control mechanisms g one group decay constant there are several types of neutrons in a nuclear reactor including prompt delayed and source neutrons the generation time for the prompt ones is 100 microseconds there are six distinct groups of delayed neutrons each with a characteristic half life and decay constant there are also several groups of source neutrons the equations of reactor dynamics become mathematically tractable if they are written in terms of a single effective.