How To Make Knowing A Good Thing: Thinning Handbooks

SI has developed a process to mitigate the negative outcomes of piping examination.

SI has developed a process to mitigate the negative outcomes of piping examination.  One part of that process is Thinning Handbooks, which have resulted in direct savings in excess of $10 Million for one nuclear plant.

Examination of Safety Related Service Water piping is driven by a number of factors, all of which tend to converge on the objective of finding localized thinning prior to the thinning becoming a problem.  In other words, examinations are performed to eliminate the risk of a leak and ensure that the wall thickness remains greater than tmin (the minimum required uniform wall thickness).  However, the rules, regulations, and economic realities mean that only bad things happen from an exam regardless of what is found.

Examinations that find thinning below tmin result in emergent repair or replacement activities.  If a repair is deferred, there are additional burdens taken on by the plant that may include submittal of a Relief Request to the NRC.  Emergent activities are significantly more costly, contain their own inherent risks, and erode regulatory margin.

Examinations that find little or no evidence of thinning have used resources that could have been better applied elsewhere.  This may impact the ability to obtain funding for additional examinations elsewhere in the system.

Even a “successful” examination that finds thinning before it becomes a problem is likely to generate the same question from the NRC, INPO, management, and others: Why didn’t you find this sooner?  It is reminiscent of the movie “Office Space” in which the main character laments that fear of losing his job “will only make someone work just hard enough to not get fired.”

Examinations help you understand the condition of your Service Water system.  Knowing the state of the Service Water system should not be a bad thing.  SI has developed a process to mitigate the negative outcomes of examinations.  Put simply, SI’s process helps to Make Knowing a Good Thing.

FIGURE 1. How to Make Knowing a Good Thing – The Overall Process

Typical examination campaigns start with selection of locations, then develop acceptance criteria, and end with the examination.  SI’s process is shown in Figure 1 and is a significant departure from this norm. A feedback loop is utilized to inform the next round of examinations.

SI has helped a number of utilities implement portions of this process.  This article focuses on the development of finite element model (FEM) based Thinning Handbooks.

FIGURE 2. Typical Analysis of Non-Uniform Thickness in FEM Thinning Handbook

The conservatively calculated tmin value is often used as acceptance criteria for examinations.  This approach does not account for the degradation mechanism most common in Service Water piping.  The tmin value assumes uniformly thinned piping, which would result from general uniform corrosion.  However, localized corrosion that produces uneven wall thickness is the dominant degradation mechanism.  SI’s acceptance criteria relies on FEM Thinning Handbooks that account for the non-uniform wall thickness observed in actual Service Water piping.

Thinning Handbooks are created before the examinations occur and are used to show that the non-uniform wall thickness meets the system Code of Construction stress limits.  They are applicable to both ASME Section III and B31.1 designed systems.  Typical handbook results show that thinning well below tmin meets the Code stress limits and is acceptable for continued operation.

FIGURE 3. Nuclear Plant Service Water Piping Showing Inspection Results Classifications

One nuclear plant has implemented SI’s FEM Thinning Handbooks extensively.  They applied Thinning Handbooks to historic NDE data to determine that many of the planned repeat examinations provided no value.  This allowed them to eliminate some repeat examinations while deferring others based on the additional margin obtained from the thinning handbook results.  This resulted in significant savings, especially for buried piping examinations.  The Thinning Handbooks also extended the end-of-life for one of the safety related system headers to beyond the date of the system retirement.  This allowed the plant to cancel replacement of the header, which would have resulted in a costly (and unnecessary) replacement.  To date, FEM Thinning Handbooks have resulted in direct savings in excess of $10 Million for this nuclear plant.

The plant also utilized FEM Thinning Handbooks to support their most recent round of examinations.  During this campaign, thinning below tmin was discovered in a line that presents considerable challenges to repair.  Rather than perform an emergent repair, the site utilized an FEM Thinning Handbook to show that the location meets the Code of Construction stress limits.  In fact, this location is predicted to meet the stress limits for an additional two years.  The plant is currently running with this thinned location while repair plans are being developed, which is exactly the point of the FEM Thinning Handbook.  An examination identified local thinning before it became a leak.  The FEM Thinning Handbook allowed the site to avoid an emergent repair, which would have significantly increased the cost and risk associated with this difficult location.  Instead, the site now has time to prepare for this challenging iteration.  The use of FEM Thinning Handbooks helped the site to Make Knowing a Good Thing.

To find out How to Make Knowing a Good Thing at your plant, contact Eric Houston (ehouston@structint.com) or Stephen Parker (sparker@structint.com).

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