Structural Integrity Investigation of Class 1 Piping During a Seismic Event with an Assumed Failed Support

Abstract of the technical paper/presentation presented at:
ASME-sponsored International Conference on Nuclear Engineering (ICONE) 27
May 19-24, 2019

Prepared by:
Khalid Chaudhry
Canadian Nuclear Safety Commission

The objective of this research activity is to assess the structural integrity of the piping system that has an undetected degradation-related flaw and to quantify the remaining seismic design margin for a pipe support assumed to have failed completely during a seismic event.

The structural integrity of nuclear Class 1 piping is very important for the safe operation of nuclear power plants (nuclear fuel cooling and residual heat removal). A single piping support failure can challenge the structural integrity of the piping and may jeopardize the safety of the nuclear power plant. The scope of this investigation covers the structural integrity assessment of nuclear Class 1 carbon steel piping with a failed support under seismic loading that also has undetected aging-induced degradation in the form of a localized flaw though the pipe wall thickness.

A scenario is hypothesized in which a single piping support is assumed to fail during the strong motion period of the earthquake event. Although Canadian NPPs are designed to withstand earthquakes in the 0.05g to 0.20g PGA range, a 0.3g PGA-magnitude earthquake is considered a credible event in North America and is chosen for the investigation of the structural integrity of the piping.

The assumed support failure (in a typical 8″ NPS Schedule 100 piping section) is likely to produce increased free span, which can generate much higher loads on the piping system. The structural integrity of the degraded piping system containing an undetected through-pipe wall thickness flaw was assessed. Dynamic analysis simulations were used to calculate the piping’s load response-time history as a function of increased free span for the remaining duration of the strong motion. The response load time history was non-linear, but preferentially elastic load time history was calculated first. This time history was then be used to assess the integrity of the degraded piping and establish the remaining seismic design margins.

In summary, the full paper will provide the results obtained from dynamic simulations completed to assess the structural integrity of a degraded piping system with an undetected through-wall flaw.  The paper will quantify the margin to failure and describe whether this margin warrants any additional physical inspections and maintenance program for the degraded piping system with undetected through-wall flaws.

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