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Re-Evaluating End of Life Projection Guidelines for Buried Assets at Nuclear Utilities
Posted on April 9th, 2018

In the past five years, the RSCS and CorrTech technical team have undertaken several long-term projects to evaluate the subsurface corrosive conditions at nuclear utilities in support of the NEI 09-14 Buried pipe and Tank Inspection Initiative.  Most nuclear plants have been in service for over 30 years and rely on original buried pipe systems to perform as designed.  In-scope piping and tanks are comprised of a dense complex of multiple systems in close proximity at depths sometimes exceeding 30-ft. These systems were manufactured using various metals that are bonded together to reduce electrical step and touch potential hazards, which impacts their susceptibility to corrosion.  Predictions of the service life and risk-rank of these buried assets at nuclear utilities using current industry guidelines do not adequately factor the subsurface soil galvanism present at nuclear sites. 

RSCS and CorrTech have been been implementing soil corrosivity, in-situ corrosion rate and electrical potential monitoring assessments that have involved the collection of soil samples from the subsurface for industry standard analytical corrosivity analysis followed by the installation of a permanent direct bury instrument, known as a Smart Stack, where the soil sample was collected and in the subsurface proximal to site structures of interest. 

Soil corrosivity assessments paired with co-located smart stack monitoring has been implemented at 5 geographically, and geologically unique sites between 2013 and 2017 totalling 45 assessment locations with sufficient data sets (more locations pending).  The data set includes more than 450 analytical results and more than 225 individual Smart Stack readings. 

The analysis and comparison of empirical corrosion rates with soil corrosivity results at nuclear power facilities do not agree with industry guidance provided by AWWA or EPRI.  These findings suggest that the dense system arrangements and subsurface environments at nuclear power facilities present a unique condition that does not apply to current industry guidance.  As a result, soil corrosivity assessments performed along buried pipelines at nuclear power facilities may under predict, or over predict outside diameter corrosion rates of buried pipes and tanks.  Incorrect system life projections could result in incorrect maintenance projections, license renewal delays and incorrect cost projections in plant extended periods of operation. A re-evaluation of the industry guidance is required to better estimate end of life for buried assets.

Findings to be presented by M.E. Darois, CGWP and H.G. Kleinfelder at the ANS 2018 Annual meeting in Philadelphia, PA, June 17-21.  Meeting details can be found here.

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