Associated with the ever increasing tendency to build nuclear reactors with long-life cores is the development of more stable nuclear fuels. Higher fuel loadings accompanied by a substantial depletion of fissioning atoms imply large changes in core reactivity. Under these conditions adequate continuous control becomes extremely difficult unless additional neutron absorbers are used. However, unlike the control rod poison material, these additional neutron absorbers must be consumed at a rate which can be matched to the depletion of the fuel. The element boron possesses qualities which make it ideal for this application.