Fuel follower control rod failure

In August of 1969, after having accumulated 240 MWH and 52 transients, the regulating control rod fuel section failed during the performance of a $2.50 reactivity insertion. A continuous air particulate monitor (fixed filter GM detector type) provided the first indication of a fission product release. The leaking fuel follower control rod was finally identified as the source of fission products after (1) all experiments and experimental facilities were removed from the core, (2) the rotary specimen rack was completely sealed, and (3) the B- and C-rings of fuel were replaced with new fuel elements. It was concluded that the failure was due to imperfections within the stainless steel cladding which is normally 0.020 in. in thickness. Measurements of transverse bend and longitudinal elongation performed after the control rod failed did not reveal any anomalies. Approximately 2 mCi of noble gases and their particulate daughters were released in the initial rupture and during the investigations performed to locate the failed element. The incident was essentially unpredictable. Measurements of transverse bend and longitudinal elongation performed after the control rod was removed from the core did not reveal any anomalies. All of the safety systems designed to cope with this type of incident functioned properly. The incident did point out two weak points in the air particulate monitoring system: 1. The low alarm set point needs re-evaluation to provide more pertinent information to the operator, and 2. The low level alarm should include an audible as well as visual indication when alarmed. Both of these points have been corrected