A STRUCTURAL IMPACT ASSESSMENT OF FLAWS DETECTED DURING ULTRASONIC EXAMINATION OF TANK 15
Ultrasonic (UT) inspection of Tank 15 was conducted between April and July 2007 in accordance with the Tank 15 UT inspection plan. This was a planned re-inspection of this tank, the previous one was performed in 2002. Ten cracks were characterized in the previous examination. The re-inspection was performed to verify the present models and understanding for stress corrosion cracking. During this re-examination, one indication that was initially reported as a 'possible perpendicular crack <25% through wall' in 2002, was clearly shown not to be a crack. Additionally, examination of a new area immediately adjacent to other cracks along a vertical weld revealed three new cracks. It is not known when these new cracks formed as they could very well have been present in 2002 as well. Therefore, a total of twelve cracks were evaluated during the re-examination. A critical review of the information describing stress corrosion crack behavior for the SRS waste tanks, as well as a summary review of the service history of Tank 15, was performed. Each crack was then evaluated for service exposure history, consistency of the crack behavior with the current understanding of stress corrosion cracking, and present and future impact to the structural integrity of the tank. Crack instability calculations were performed on each crack for a bounding waste removal loading condition in Tank 15. In all cases, the crack behavior was determined to be consistent with the previous understanding of stress corrosion cracking in the SRS waste tank environment. The length of the cracks was limited due to the short-range nature of the residual stresses near seam, repair and attachment welds. Of the twelve cracks, nine were located in the vapor space above the sludge layer, including the three new cracks. Comparison of the crack lengths measured in 2002 and 2007 revealed that crack growth had occurred in four of the six previously measured vapor space cracks. However, the growth remained within the residual stress zone. None of the three cracks beneath the sludge showed evidence of growth. The impact of the cracks that grew on the future service of Tank 15 was also assessed. Tank 15 is expected to undergo closure activities including sludge waste removal. A bounding loading condition for waste removal of the sludge at the bottom of Tank 15 was considered for this analysis. The analysis showed that the combination of hydrostatic, seismic, pump and weld residual stresses are not expected to drive any of the cracks identified during the Tank 15 UT inspection to instability. Wall thickness mapping for general thinning and pitting was also performed. No significant wall thinning was observed. The average wall thickness values were well above nominal. Two isolated pit-like indications were observed. Both were approximately 30 mils deep. However, the remaining wall thickness was still greater than nominal specified for the original construction plate material. It was recommended that a third examination of selected cracks in Tank 15 be performed in 2014. This examination would provide information to determine whether any additional detectable degradation is occurring in Tank 15 and to supplement the basis for characterization of conditions that are non-aggressive to tank corrosion damage. The in-service inspection program is re-evaluated on a three year periodicity. The Type I and II tanks are not active receipt tanks at present, and are therefore not a part of the In-Service Inspection Program for the Type III Tanks [1]. Changes to the mission for Tank 15 and other Type I and II tanks may be considered by the In-Service Inspection Review Committee (ISIRC) and the program adjusted accordingly.
- Research Organization:
- Savannah River Site (SRS), Aiken, SC (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- DE-AC09-08SR22470
- OSTI ID:
- 936602
- Report Number(s):
- SRNS-STI-2008-00028; TRN: US0805696
- Country of Publication:
- United States
- Language:
- English
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