The problem of installing a control rod in the V. P. I. subcritical reactor is complicated by the limiting two foot clearance between the top of the reactor and the ceiling. Within this space it is necessary to insert and operate an effective seven foot, solid, straight rod along the vertical axis of the reactor.

The problem is overcome by cutting the rod in six inch sections at an angle of 45 degrees and installing an internal pivoting arm which allows the sections to break 60 degrees with respect to one another. The rod is wrapped on a drum which takes the shape of a hexagonal helix when withdrawn from the pile. When inserted, an effective solid, straight rod is presented to the neutron flux. The control rod is motor driven and remotely operated. Provision is made for changing the absorber material in the control rod to permit the investigation of materials having various neutral absorbing properties.

Tests were performed using cadmium as the absorber material to determine the effect of the control rod on the neutron flux. Results indicate that the control rod, although flexible, is effectively a solid, straight rodo An equation is obtained for expressing the control rod effect as a function of pile level and control rod position.