Horizontal Control Rods - Inner Room

The B Reactor’s nuclear chain reaction is controlled by long control rods coated with neutron-absorbing material that were inserted or withdrawn from the side of the pile to decrease or increase the pile reactivity, respectively.

The B Reactor employed nine horizontal control rods (HCRs) that entered the pile from its left side (as viewed from the front of the pile). The operator in the control room moved these rods in or out via remote controls to increase, decrease, or maintain the pile’s chain reaction.

The nine rods were laid out in three rows of three rods, about 5 ft apart both vertically and horizontally. Each rod was about 75 ft long and 3 in. across, and entered the pile through a hole in the shielding. The entire length of the rod’s rectangular hole into the pile was lined with an aluminum thimble that prevented the pile’s helium from escaping.

The HCRs were divided into two groups. The operator at the pile’s controls used seven shim rods to control the bulk of the pile’s reactivity and to adjust the neutron flux distribution within the pile. The other two regulating rods (the two top outside rods) handled the minute-to-minute adjustments. The shim rods were driven by hydraulic motors, while the regulating rods were electrically driven. All nine rods were otherwise the same.

The original design of the pile had included only three control rods, but nine were eventually included to give an added factor of safety and control. As it turned out, operating experience showed that the extra rods were not at all a luxury. Under some circumstances during startups, the pile’s energy level could accelerate much more rapidly than had been anticipated, and the extra control rods were needed to restrain the growth of the neutron flux.

Each HCR was divided into two sections. Only the inner section (the part nearest the pile) entered the pile. This 36 ft portion included three aluminum tubes that had been coated with boron-containing compound (for the first 29 ft of their lengths), which would absorb neutrons within the pile. The three tubes carried circulating cooling water, and were enclosed within an aluminum casing for rigidity.

The outer 39 ft section of the control rod, which never entered the pile, was mounted to a rack that engaged a pinion on the drive mechanism, and carried the cooling tubes to flexible hose connections at the end of the rod.

In normal operation, the section of the HCR that went into the pile became intensely radioactive, and workers would need protection when the rods were withdrawn from the pile. Instead of enclosing each control rod in a protective shield, the room that housed the rods was divided into two rooms. When the rods were withdrawn from the pile, the entire length of the inner sections of the rods were contained within the inner rod room. Workers would not enter this room if any of the rods were even partially withdrawn from the pile. The arrangement of these two rooms above the control room is shown in this cutaway view of the 105-B building.