BALANCE OF PLANT | VALVE SAFETY
Learning from valve safety experience
The US Nuclear Regulatory Commission has published ‘lessons learned’ from its inspection programme of power operated valves in US nuclear plants
THE US NUCLEAR REGULATORY COMMISSION (NRC) has issued a supplement to its May 2021 Information Notice 2021-01, “Lessons Learned from US Nuclear Regulatory Commission Inspections of Design-Basis Capability of Power-Operated Valves at Nuclear Power Plants”. The US regulator wanted to alert operators to lessons
learned from NRC inspections of the design-basis capability of power-operated valves (POVs) at nuclear power plants. While issuing the Information Notice, NRC was also working on Inspection Programme 71111.21N.02 to assess the reliability, functional capability and design-basis capability of risk-important POVs to determine whether licensees are maintaining POVs’ capability to perform as intended under design-basis conditions. The inspection programme was completed at the end
of 2022 and has been discussed at industry meetings, but participants requested that the NRC provide a complete list of the lessons as soon as possible. The NRC staff suggests that licensees review this information for applicability to their facilities and consider actions, as appropriate, to identify and address similar issues. The NRC found that:
● Licensees did not always ensure that valves were properly included and categorised within the scope of the in-service testing (IST) programme in Title 10 of the Code of Federal Regulations. The NRC inspectors found limitations, for example, in POVs with leakage limitation safety functions, or with either remotely or manually- operated safety functions.
● Licensees did not always properly determine the operating requirements and actuator capability for
POVs to perform their safety functions. Appropriate parameters to be addressed when calculating valve operating requirements or actuator capability may include valve friction coefficients or valve factors, maximum differential pressure conditions, motor torque temperature derating factors, stem friction coefficients, and butterfly valve bearing friction coefficients. Licensees did not always justify the use of POV parameters from outside sources.
● Licensees did not always ensure that valve-specific valve factors were used, if determined to be higher than generic valve factors. For example, for globe valves, there is a potential for increased thrust and torque requirements to operate globe valves under high-flow dynamic conditions. The unwedging load required for valves is part of the evaluation of the capability of POVs to open to perform their safety functions.
● Licensees did not always ensure that all normal operating loads that act simultaneously with seismic loads were addressed.
● Licensees did not always ensure that sufficient information and test data were developed to validate the assumptions for rate-of-loading and load-sensitive behavior for plant-specific motor operated valve (MOV) applications.
Most licensees agreed in 1996 to implement a ‘Joint Owners Group (JOG) Program on motor operated valve Periodic Verification in response to an NRC letter (Periodic Verification of Design-Basis Capability of Safety-Related Motor-Operated Valves).
Right: Operators did not always ensure that valve-specific factors were used to calculate appropriate thrust and torque
26 | March 2024 |
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