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On August 12, 2012, the licensee shutdown … On August 12, 2012, the licensee shutdown the plant to investigate the source of elevated PCS unidentified leakage. During a containment walk down post shutdown it was discovered the source of the leakage was from the housing of CRDM-24. The leak was classified as pressure boundary leakage. The licensee performed NDE as well as destructive testing on the housing of CRDM-24 at a laboratory contracted by the licensee. The preliminary results from this laboratory testing concluded that cracking in the housing of CRDM-24 was attributed to TGSCC. This form of cracking is prevalent when susceptible material such as the housing of CRDM-24 (austenitic stainless steel) is subjected to a corrosive environment (some level of oxygen and a small amount of chlorides) under applied stresses. The corrosive environment in this case is the primary coolant and the applied stresses include thermal and structural stress. The preliminary results from this laboratory testing concluded that cracking in the housing of CRDM-24 was attributed to TGSCC. This form of cracking is prevalent when susceptible material such as the housing of CRDM-24 (austenitic stainless steel) is subjected to a corrosive environment (some level of oxygen and a small amount of chlorides) under applied stresses. The corrosive environment in this case is the primary coolant and the applied stresses include thermal and structural stress. The licensee also examined the fracture surface of the housing of CRDM-24 at the location of the through-wall crack and identified six concentric rings (beach marks) propagating in a radial direction from the inside diameter out towards the outside diameter of the housing. The licensee considered the width of each of these beach marks as indicative of consistent periods/intervals corresponding to crack growth during operation (corrosive environment and stresses present). The relatively narrow band between successive concentric rings was interpreted by the licensee to represent periods of crack arrest during refueling outages (housing not in contact with corrosive environment and stresses are relaxed). However, more oxygenated water is introduced when the coolant system is refilled after refueling, starting the crack propagation process again when stress is applied. Based on the above interpretations of the fracture surface of the housing of CRDM-24, the licensee extrapolated the crack growth rate for the through-wall crack that caused leakage in the housing. The licensee applied this calculated crack growth rate to consider TGSCC in the remainder of the 44 CRDM housings based on the maximum size crack that could avoid detection. This licensee assessment revealed a time frame of over four years for the crack to propagate through-wall. The results of this crack growth rate as applied to the remainder of the 44 CRDM housings was documented in the licensees technical justification for startup. The inspectors have outstanding questions and concerns with regards to the methodology used by the licensee to extrapolate a crack growth rate due to TGSCC from the fracture surface of the housing of CRDM-24. The NRC Office of Nuclear Reactor Regulation (NRR) has been involved with detailed technical discussions with the licensee regarding these questions and concerns over the calculated crack growth rates. It is also possible that the concentric rings represent different periods where the temperature and pressure stresses were removed during heatup and cooldown cycles, irrespective of reintroducing oxygen into the system. In this case, the timeframe would be around 2 years for crack propagation. The NRC concluded that it was still acceptable for startup, but future detailed inspections may be needed sooner than the licensees assessment. The resolution of this URI is pending further discussions and potential independent examination of the fractured housing of CRDM-24 by NRC staff. (URI 05000255/2012012-02, Potential Inadequate Degradation Evaluation of CRDM Housings)e Degradation Evaluation of CRDM Housings)
23:59:59, 30 September 2012 +
23:59:59, 30 September 2012 +
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00:47:54, 21 February 2018 +
23:59:59, 30 September 2012 +