This paper describes the engineering design and performance of a rapid-insertion control-absorber mechanism, for use within hollow, vertical, fuel elements of the DIDO class materials-testing reactors operating at a thermal flux of 1014 n/cm2 second. The initial downward acceleration of the absorber is that of gravity, and the full travel ofn24 in. takes 0.4 seconds. The magnet release time is 5 milliseconds. The absorber weighs 15 lb, has an effective area of 120 in2 of cadmium, and controls nearly 3% of reactivity. It can be positioned with an accuracy of 1.5 x 10-3 in. and withdrawn at speeds of 0.016 in./sec and 0.004 in./sec. The mechanism consists of a vernier stepping motor which drives a lead-screw and raises or lowers a close-fitting cylindrical shield plug carrying at its lower end an electromagnet which supports a tubular absorber.
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