by Richard Martin (Program Manager) & Dr. Hui Lu (Lab Manager)
The pressurizer spray valve in a pressurized water reactor (PWR) nuclear power plant failed to position as required, ultimately resulting in a plant shutdown. The spray valve was an air operated valve with an actuator that was air to open, spring to close. A pneumatic positioner was used to control operation of the spray valve actuator.
During power testing, the spray valve failed to shut from full open upon sending a control signal to the positioner to close the valve. Isolating control air (0 psi signal) to the positioner had no effect on the position of the spray valve. Trouble shooting indicated that the positioner had failed and continued to pass 85 psi air to the valve actuator, causing the spray valve to remain open.
The positioner and the associated control air transducer were shipped to Imperia’s Materials Engineering Laboratory for failure analysis. The components were received by Imperia utilizing Imperia’s radioactive material license. No decontamination of the components was required prior to shipping.
Imperia’s failure analysis was performed on a priority basis with an expedited schedule to support the plant’s root cause determination team. The client also had a concern for applicability of the failure to other units. Another unit’s pressurizer spray system utilized the same configuration.
Results of the failure analysis revealed debris in the 85 psi air supply to the positioner accumulated in the positioner and specifically within a close-tolerance spool valve that directed air flows to cause the spray valve to open or close. Most of the debris particles, which were predominately organic material, were larger than the spool valve tolerance. The debris became wedged between the spool and valve body causing misalignment and resistance to movement of the spool, preventing spool repositioning which allows the spray valve to close.
Imperia’s failure analysis of the positioner allowed the plant to complete a root cause determination and identify a solution to prevent recurrence.
by Raymond M. Pace, P.E. – Director of Nuclear Engineering
The VPs of Engineering and Operations at a nuclear power plant reached out to Imperia Engineering Partners, Subject Matter Experts (SMEs) (i.e., Mark I Program Structural and Coatings) for support because of discovery with respect to the Primary Containment. The plant was recovering from an extended outage hold due to COVID-19. They were in the process of completing planned internal examinations after recoating of the torus shell. The examination of the Vent System identified degradation of the internal coating due to standing water during the operational cycle. The coating degradation resulted in localized wall loss due to corrosion that exceeded the previously calculated minimum thickness requirement. Imperia was contracted to act as a 3rd party reviewer for the Finite Element Model (FEM) structural analysis, coating selection/application and corrosion product review for the Vent System. We were authorized to be in direct contact with both the structural and corrosion consultants previously selected.
The structural consultant prepared a symmetric finite element model of a Vent System bay to perform an ASME Code analysis of the localized thinning areas. Imperia reviewed the preliminary Finite Element Analysis (FEA) and recognized that a boundary condition required adjustment. In addition, Imperia reviewed the Mark I Program Plant Unique Analysis Report for all hydrodynamic loading conditions and validated the analytical approach proposed by the structural consultant. Imperia also identified that the preliminary results for two of the vent bays appeared inconsistent. Follow up by the structural consultant resulted in additional changes to the finite element model.
Imperia recommended that the utility could benefit by performing an ASME SC XI Code reconciliation from the 1977 Edition of ASME III to a later edition with lowered safety factor to gain 14% margin on material allowable stress values. The utility agreed and Imperia reviewed the Code reconciliation and the updated uniform wall thickness requirements for the Vent System. Imperia also reviewed FEA results to the reconciled allowable stress values and confirmed locations requiring an ASME SC XI repair. The number of repair locations was reduced because of the Code reconciliation.
Recoating the Vent System areas with localized degradation was deemed impractical due to the need for a comprehensive plan including as a minimum, confined space and enhanced ventilation requirements. The qualified coatings are also difficult to apply due to Volatile Organic Compounds (VOCs) and application requirements. Therefore, the corrosion consultant prepared a calculation to determine additional accumulated corrosion products if left uncoated for the cycle. The calculation was required to qualify the Emergency Core Cooling System (ECCS) suction strainers for additional debris loading.
Imperia performed a 3rd party review for the following products to support the plant’s successful startup to full power operation:
- Structural consultant’s Technical Report containing the ASME Code structural evaluation for localized thinning in the Vent System.
- Corrosion consultant’s calculation of factors refining corrosion rate of carbon steel in water with a nitrogen blanket and the corrosion consultant’s report presenting an estimation of the rust mass caused by corrosion of exposed steel in the vent system over an 18-month cycle.
- Structural consultant’s calculation of corrosion product generation in the vent system over an 18-month cycle and the structural consultant’s report on the impact of corrosion products on the plant’s ECCS performance.
- Site’s operability evaluation prepared to support full power operation for one cycle of operation demonstrating acceptability for the degraded/nonconforming condition (e.g., degraded vent system wall thickness and coating with the potential for additional debris generation).