The shim rods alone could absorb a sufficient number of neutrons to shut down the pile, except when there was a complete loss of water from the pile. In that case, the pile’s neutron multiplication factor, k, would increase dramatically, say from 1.000 to 1.025. In the micro-world of fissioning nuclei, this would increase the rate of the pile’s chain reaction so quickly that it would soon become catastrophically explosive. To handle this possibility and to provide a backup mechanism for shutting down the pile, another set of rods hung above the pile, ready to be dropped at a moment’s notice.
Above the pile hung 29 vertical safety rods (VSRs), or drop safety rods. When their release mechanisms were triggered, they could drop into their respective aluminum thimbles in the top of the pile within 2.0 to 2.5 seconds. Their design and spacing meant they could reduce the pile’s value of k by about 0.038, normally more than enough to drop the value of k well below 1.000 and shut down the pile.
Each VSR was a 35 ft long, 2.25 in. diameter tube of steel with a 1.5 percent boron content to absorb neutrons. Unlike the HCRs, these rods normally remained withdrawn from the pile, and therefore needed no cooling. Each rod was suspended above the pile by steel cables to an electric winch some 40 ft above the top of the pile. The winch was locked by an electromagnetic clutch. In the event of a power failure or other safety-circuit fault, the clutch would be deenergized, freeing the cable and allowing the VSR to plunge into the pile.
Each VSR had a steel plug around its bottom and top, which effectively sealed the thimble and greatly reduced the radiation that would otherwise stream from those openings in the pile. The VSRs were normally inserted into their thimbles so that their lower ends aligned with the bottom of the biological shield blocks at the top of the pile, so that there was little radiation entering the thimble above the plug. Nonetheless, the 29 holes in the top of the pile meant that this area would always be a radiation hazard zone.