Beam-Port Perturbations in Water and Graphite

The influence of the port void on the thermal-neutron scalar flux at the source plane and on the current at the exit plane of a beam port is examined experimentally and analytically. Activation experiments on 1 1/2, 3-, and 5-in. diam ports inserted into the water and graphite reflectors of a swimming-pool reactor are compared with an elementary P₁ analysis. The exit current is found to be well-predicted by the theory, which depends only upon a knowledge of the scalar-flux distribution when the beam port is absent.