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).