Evaluation of seismic margins for nuclear power plant piping systems using ratcheting criterion

• Published in: The International journal of pressure vessels and piping Vol. 202; p. 104894
• Main Authors: Ravi Kiran, A., Agrawal, M.K.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.04.2023
• Subjects: Floor response spectrum; Iterative response spectrum analysis; Ratcheting; Seismic margin; Strain based performance levels; Seismic margin; Iterative response spectrum analysis; Ratcheting; Strain based performance levels; Floor response spectrum
• ISSN: 0308-0161; 1879-3541
• DOI: 10.1016/j.ijpvp.2023.104894

In the wake of recent beyond design basis earthquakes, it is very essential to evaluate seismic margins of nuclear power plant piping systems. Better seismic margins will ensure the safety in the event of less probable bigger seismic event beyond consideration of design level. At present, permissible codal stress limits are used to evaluate these margins and it has been experimentally demonstrated that ratcheting is the significant failure mode for piping systems subjected to reversing dynamic load. Hence, it is more prudent to use actual failure criterion for evaluation of seismic margins. In this paper, a detailed framework is provided for evaluation of seismic margins of piping systems using ratcheting criterion. As ratcheting is the potential mode of failure for piping systems under earthquake load, two strain-based performance levels based on code practice and experience gained with experiments carried out by Bhabha Atomic Research Centre, are used for margin assessment. It is usual practice to carry out stress-based response spectrum analysis for design and qualification of piping systems under earthquake load. In order to offer a practical, simpler and feasible solution for ratcheting evaluation, the same response spectrum analysis in iterative way, can be used to evaluate ratcheting in the piping systems. Experimentally validated numerical procedure is used for ratcheting evaluation. This procedure is demonstrated for a piping system of a typical nuclear power plant. Seismic margins for the piping system are evaluated using codal provision and two ratcheting based performance levels for deterministic and probabilistic methods. •A framework for estimation of seismic margins of NPP piping systems using ratcheting criterion is provided.•The conventional method of response spectrum analysis when used iteratively is found to be suitable for ratcheting based seismic margin evaluation.•The seismic margins using ratcheting criterion are observed to be higher than conventional seismic margins.•It is proposed to use ratcheting criterion for the evaluation of realistic seismic margins.