ISO 4917-6:2024

Overview

ISO 4917-6:2024 - "Design of nuclear power plants against seismic events - Part 6: Post-seismic measures" provides guidance for actions to be taken before, during and after an earthquake at nuclear power plants with water‑cooled reactors. It is a decision‑support guideline for plant operators and safety engineers covering verification, classification and graded post‑seismic measures for continued operation, shutdown and restart. The standard explicitly does not apply to earthquakes stronger than the plant’s design basis earthquake (DBE).

Key topics

• Verification of an earthquake
  • Activation of seismic recorders, checking for faulty signals and using at least two instrument locations (trigger thresholds) to confirm an event.
• Classification using recorded response spectra
  • Evaluation against plant‑specific acceleration levels and reference spectra (e.g., design basis × factor). The document notes a recommended factor f = 1.5 (larger values require plant‑specific verification).
• Graded procedure for post‑seismic actions
  • Concept of graded measures tied to recorded ground motion: initial measures, in‑depth measures and resulting measures (continued operation, shutdown, restart).
• Initial measures
  • Control‑room checkups, immediate plant walk‑down inspections (visual checks per Annex B), and documentation as quality records.
• In‑depth and special inspections
  • Engineering analyses, special/seismic inspections (Annex C), and shutdown inspections where the inspection level is decisively exceeded.
• Decision criteria
  • Guidance on when specified normal operation can be maintained, when to initiate in‑depth measures, and when to shut down the plant (includes guidance for high‑frequency exceedances, e.g., > ~16 Hz).
• Documentation & quality records
  • Emphasis on documenting checks, faulty signals and inspection results for auditability.

Applications and users

Who should use this standard:

• Nuclear plant operators and control‑room staff for post‑quake decision making
• Plant safety and seismic engineers for inspection and analysis protocols
• Emergency planners and operations managers for pre/post‑earthquake action programmes
• Regulators and auditors assessing conformance with seismic post‑event procedures

Practical uses include drafting operator manuals, defining plant‑specific inspection levels and trigger thresholds, developing walk‑down checklists, and setting procedures for shutdown and restart decisions after seismic events.

Related standards

• ISO 4917‑1 - Principles for seismic design of nuclear power plants (normative reference)
• ISO 4917‑5 - Seismic instrumentation (normative reference)

Keywords: ISO 4917-6:2024, post-seismic measures, nuclear power plants, seismic events, plant walk-down, seismic instrumentation, design basis earthquake, operating basis earthquake, inspection level.