AbstractThe perturbation approach to control element evaluation is extended from the work presented in a previous paper. A two-group second-order perturbation expression for control element worth is obtained. This has, as its starting point, the unperturbed fast flux but considers the depression in the thermal flux caused by the control element and then, in turn, considers the perturbation on the fast flux caused by the perturbed thermal flux. Finally, the effect of the perturbed fast flux on the thermal flux is evaluated. It is shown that this process, if continued, converges to the correct answer. The perturbation results are compared to experiments for the case of a weak rod in the reflector region of the Bulk Shielding Reactor. The perturbation results are also compared to exact two-group calculations for a cylindrical rod on the axis of a bare cylindrical reactor. In both cases, excellent agreement is obtained.