A number of inherent reactivity variations occur during the operation of a power reactor and must be considered in designing reactor control systems. Some of these can be used to assist in reactor control, others require attention in the control-system design.

A key effect to be considered is the temperature coefficient of reactivity. Temperature variations change neutron cross sections and dimensions of reactor materials and thus change the reactivity. A desirable condition is for the reactivity to decrease as the reactor temperature increases. Such a negative temperature coefficient has a stabilizing effect since, as the power increases and raises the reactor temperature, the negative temperature coefficient reduces the reactivity and tends to limit or level out the rise in power. Most of today’s reactors have a negative

temperature coefficient of reactivity, usually————- 1CT4/0F in

5k/k at operating temperatures

In boiling-water reactors (Vol. 2, Chap 16), the reac­tivity changes as the void volume in the core changes. Since the water (steam) acts both as neutron moderator and absorber, the void coefficient of reactivity can be either positive or negative

A generalization often used m preliminary calculations is to lump all the reactivity effects into a single power coefficient of reactivity, the change in reactivity resulting from a unit change in reactor power. This coefficient is typically ^10“4 decrease in 5k/k per megawatt (thermal) change in power level

A power reactor is inherently stable The degree of stability is temperature dependent since the reactivity coefficients are temperature dependent. Thus a reactor may be less stable when cold than at operating temperatures. When inherently stable, the negative temperature effects dominate the positive, and the requirements placed on the control system are less stringent. In fact, with a net negative coefficient of reactivity, some reactors may be controlled over a limited range without resorting to control-rod movement.

Long-time inherent changes in reactivity are those attributable to the increase of fission-product poisoning and fuel depletion. These can be controlled by chemical shimming or by incorporating burnable poisons in the fuel

7- 1.3 Methods of Reactivity Control

Most power reactors are controlled by rods that are inserted into or withdrawn from the reactor core The rods contain neutron-absorbing or fissionable material or a combination of the two Some power reactors have been controlled by the rotation of control drums on the core periphery, the control drums being made of combinations of neutron-reflecting and neutron-absorbing materials.

For power reactors where changes in neutron level may be accomplished over relatively long time periods, but where constant power levels are wanted once full power is achieved, burnable poisons and chemicals dissolved in the coolant (so-called “chemical shimming”) have proved ef­fective Burnable poisons can be added to the fuel elements to decrease fuel-element absorption of neutrons in propor­tion to the decrease in the fissionable material content of the fuel elements. Either type of control, chemical shimming or burnable poisons in the fuel, reduces the total reactivity that must be offset by the control-rod system