SPECIAL REPORT | EMERGENCY DIESEL GENSETS rigour in the 2008 legacy design change process. The


process failed to address foreign material as a design consideration, which meant a selected and installed emergency diesel generator relay design was susceptible to foreign material intrusion. The licensee revised the Design Attributes Review Checklist, identified and implemented special modification considerations and controls concerning foreign material control susceptibility for the current relay design, and revised receipt and warehouse instructions for handling and opening packaging of the relays.


VC Summer At V C Summer, NRC wrote on 1 May 2024 on the root causes of the failure to identify and correct a failure mechanism that affected the fuel oil system piping of the ‘A’ EDG. They were first that SAP-0999, “Corrective Action Program,” Revision 13 and subsequent revisions, failed to drive effective evaluation and resolution of EDG fuel oil piping cracks. Second, the fuel oil piping design was less than adequate for maintenance adjustments and introduced challenges when realigning the piping to correct leaking connections. Earlier, in May 2023, NRC had assessed the root cause


of a failure to identify and correct oscillations on the ‘B’ EDG. It found that the organisation did not adequately challenge unanticipated oscillations observed during the EDG Surveillance Test Procedure, instead rationalizing the occurrences to grid fluctuations attributed to winter weather. In earlier letters the NRC questioned the operator’s analysis, saying its view that the ‘B’ EDG would “degrade and completely lose function at some point while in the isochronous mode of a required event… was not changed by the written response provided”. The NRC said the criteria used were not applicable for


generic steady-state operation. It disagreed with an analysis that assumed that the


open circuit conditions observed during a surveillance run, with the EDG synchronised with the grid, were bounding for all modes of operation and would not degrade further during operation, suggesting that the large observed EDG perturbations were minor and within the capability of the EDG to recover during steady operation. Instead, NRC said that the data set created by surveillance runs was limited. The engineering report did not address the randomness of


failures, develop any correlation between the two different modes of operation, and did not discuss the potential differences in expected engine vibrations. It said there was an upward trend in open circuit conditions and said “it is very likely that the wear and tear at the connection point would lead to a permanent open circuit resulting in complete failure of the electronic speed control system”.


Earlier issues Performance issues related to EDGs is not a new feature of NRC records. For example, two were recorded in the two years prior to the new decade. In a 27 June 2019 communication regarding Peach Bottom,


NRC found that the root cause of a failure of the an EDG in 2018 was that staff failed to establish measures to assure that conditions adverse to quality associated with scavenging air check valve were promptly identified and corrected. Because the EDG was inoperable for a period greater than the technical specification allowed, Peach Bottom 2 and 3 Technical Specifications on “Electrical Power Systems - AC Sources – Operating,” were violated. A 2018 finding with regard to the Perry plant involved


the failure to evaluate the effects of voltage suppression diode failure on the EDG control circuit. The introduction of new diodes into the control circuitry resulted in the eventual failure of the EDG control circuit, rendering the EDG inoperable. Also in 2018 the NRC had set out corrective actions for design weaknesses in all Catawba’s EDGs. It wanted Catawba to modify the EDG voltage regulator to address design weaknesses for all EDGs based on detailed simulation of the voltage regulator to prevent diode damage. It also called preventive maintenance in the form of diode and silicon-controlled rectifier replacement at a maximum interval of 18 months and asked the plant to revise its “Operating Experience Program,” for insight, crediting future engineering changes and reviewing historical performance trends to ensure preventive maintenance strategy changes are identified.


Lessons for the future industry This short tour around resolved issues relating to diesel generators does not show systematic issues across the industry. Instead it shows the variety of technologies and behaviours that may affect a plant system that is both familiar and vital for maintaining safety ‘defence in depth’. It should also be a reminder for the fast-growing industry developing and rolling out new small modular reactors (SMRs) and even for the parallel industry striving to commercialise fusion technology. Most developers take a far more ‘off the shelf’ approach to these designs than was the norm in earlier generations of nuclear technology. This approach to the technology should be more economic in construction, with factory-based fabrication, and allow for the fast incorporation of operating experience from the use of similar technology elsewhere. Some SMR developers are even aiming for designs that can be delivered to site at the start of their lifetime and recovered at the end, with waste management at specialist sites – allowing for as much ‘hands off’ operation as possible during its lifetime. However, as experience with emergency diesel generators


Above: Failing to select a speed switch within the safety-related emergency diesel generators led to an NRC root cause analysis at the Davis-Besse plant Source: Bechtel


26 | July 2025 | www.neimagazine.com


shows, familiar components remain an integral part of the nuclear unit and of its safety and operating regimes. ‘Off the shelf’ cannot mean ‘fit and forget’. ■


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