By Jens Conzen, Manager, Structural Services & Vibration, Fauske & Associates, LLC
Recent and past earthquakes have shown that power plants and other engineering process facilities can suffer damage from seismic excitation. If damage occurs, it is often due to insufficient seismic design and anchorage. For new plant components, conservative seismic design analyses are commonly performed such as finite element modeling. For older plants, this was often not required, thus, most seismic vulnerabilities are typically identified in the existing fleet of nuclear power plants.
Fauske & Associates, LLC (FAI) recently developed a seismic evaluation method for piping systems that was specifically developed for a nuclear power utility. It should be noted that not every component in a nuclear power plant needs to be seismically qualified. Only those items that are designated essential for safe shut down and subsequent decay heat removal need seismic qualification.
Since the 2011 earthquake in Japan, engineers and power utilities pay closer attention to seismic design. In particular, components required for long term coping for loss of AC power scenarios are reviewed. It is also a requirement for the US and European power utilities to review their seismic plant analysis in response to the NTTF 2.3 Seismic letter and the European Union stress test, respectively. Consequently, seismic evaluation tools that have been developed several years ago are now being brought back to evaluate seismic capacity versus demand or to assess seismic margin. These tools can be used to perform an in-situ seismic evaluation of equipment. However, a time efficient assessment is often difficult due to the formality of these approaches.
Earthquake experience has shown that piping systems may undergo damage (or fail to deliver flow) if they are not designed for seismic loads. Further, the Fukushima accident has shown the importance of redundancy in piping systems. Intact piping can assure the delivery of service- or fire water that can be used for spent fuel cooling, for example. Hence, it would be beneficial if redundant options could be identified by a rather simple approach. FAI found out by conducting research and by evaluating seismic experience data that piping systems that are built according to general industrial standards may be rugged enough to cope with a design basis earthquake.
As a result, FAI has developed the FAI Screening Methodology for the Seismic Robustness of Installed Piping Systems that can be used to establish confidence in the seismic capacity of installed piping systems. The methodology has been applied at a nuclear power plant site to investigate the possibility to refill the on-site fire water storage tanks following a seismic event. The methodology encompasses supported pipe, buried pipe, valves, pumps and tanks. It is designed in a simple way so that an experienced engineer can use the method to perform conservative walkdown screening assessments of the seismic adequacy of the piping system. The walkdown screening also provides the benefit of a comprehensive as-built documentation of the piping system.
A case example was recently presented at the Nordic Nuclear Symposium, receiving attention industry wide:
- Motivation - Investigation of the possibility to use existing piping to refill the fire water storage tanks from the on-site fresh water reservoir following a seismic event at a nuclear utility site.
- Objective - Perform a conservative assessment of the seismic adequacy of the piping system by using the FAI seismic evaluation method.
- Basis for Screening Criteria
- Supported Pipe - ASME B31.1-2001, ASME Code - Power Piping
- - Piping/Seismic Handbooks (screening tables)
- Valves, Pumps and Tanks - GIP for Seismic Verification of Nuclear Plant Equipment by SQUG
- Buried Pipe - ASCE Guideline for Water Transmission Facilities
- - EPRI Guideline for Buried Pipe
- - NEI Guideline for Underground Piping and Tank Integrity
- - NUREG-1801 Aging Lessons Learned
- Tanks - NEI Guideline for Underground Piping and Tank Integrity
- - NUREG-1801 Aging Lessons Learned
- - ACI Guideline for Concrete Structures
- All - General Research Publications
- Methodology - The screens shall establish confidence that the system can deliver flow and remains leak tight. Features of a section that fail to pass a screening item are considered outliers. Outliers require resolution to assure seismic adequacy of the system.
- Outline of Supported Pipe Screens
- Construction Quality
- Pipe Degradation – External/Internal
- Span Between Vertical and Horizontal Supports
- Pipe Support Capacity
- Anchor Motion (differential displacement between buildings, for example)
- Joint Capacity
- Valve Capacity
- Seismic Interaction Concerns (proximity effects e.g. falling of nearby structures)
- Outline of Buried Pipe Screens
- Pipe Diameter
- Pipe Material
- Pipe Degradation and Corrosion Protection
- Joint Capacity
- Soil Category
- Soil Uniformity and Backfill Adequacy
- Buried Depth
- Buried Age
A conservative screening methodology has been developed by FAIand was successfully applied at a nuclear power utility. The results provided confidence in the seismic adequacy of the examined piping system. In addition, areas for improvement were identified and the evaluation report can serve as a basis for structural modifications.