STRUCTURAL INTEGRITY INVESTIGATION OF CLASS 1 PIPING DURING A SEISMIC EVENT WITH AN ASSUMED FAILED SUPPORT

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Abstract

The objective of this research activity is to assess the structural integrity of a piping system during a seismic event and to quantify the remaining seismic design margins, for a scenario in which, the piping has an undetected degradation-related flaw and a pipe support is also assumed to have failed completely during the 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 impairment of a support under seismic loading that also has an undetected aging-induced degradation in the form of a localized part through wall embedded flaw. 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 structural integrity investigation. 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 is assessed. Dynamic analysis simulations (response spectrum analysis) are used to calculate the piping’s load response as a function of increased free span for the remaining duration of the strong motion. The piping structural response to higher earthquake loading can be non-linear, but preferentially elastic design methods are used first. The calculated stresses are then used to assess the integrity of the degraded piping by code comparisons and establish the remaining design margins from flaw evaluation for ‘fitness-for-service’.

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