Buried pipe degradation is an issue of increasing importance with respect to the nuclear industry due to an increased frequency of leaks over the past 10 years. The WCAP-17465-NP Handbook [1] was commissioned by the Pressurized Water Reactor Owners Group (PWROG) to address the need for a fitness-for-service evaluation methodology with the additional goal of providing a means of reducing the amount of time and work required to perform the evaluation.
The Handbook details the developed methodology, which is consistent with Revision 6 of draft Code Case N-806 for the evaluation of degradation in buried metallic pipe [2]. The developed methodology is applicable to ASME Class 2 and 3 buried metallic piping items (excluding cast iron piping items). The methodology consists of nine criteria that encompass stresses resulting from burial and surface surcharge loads, internal pressure, and external events (i.e., seismic, frost heave, etc. that generate shear, longitudinal, and bending stresses.) The methodology does not permit through-wall leakage.
The majority of the evaluation procedures come from existing ASME documents with the exception of the burial and surcharge loads. The definition of these loads is new to the ASME Boiler and Pressure Vessel Code, but not new to buried piping analysis for other fields of work, which have defined standards that are used to calculate these loads. If one of the requirements defined in the Handbook is not passed, one of several steps must be taken: a check may be performed to see if further refinement of soil, loads, and corrosion profile can be made to reduce conservatism, a more detailed analysis could be performed using a structural analysis tool such as finite element analysis, or the piping section containing the metal loss region would need to be repaired or replaced.
Additionally, the Handbook provides pre-calculated results for a range of piping materials, sizes, soil classifications, and surface loads, as well as a sample calculation. These results are included in the Handbook in chart and table format. The inclusion of the charts and tables changes a relatively complex, time consuming evaluation to one can be performed in a minimum amount of time by a seasoned evaluator. This attribute is very important because plant operability (or needed relief request) may be heavily dependent on appropriate but prompt fitness-for-service evaluations.
