Diesel generators have been workhorses since Rudolf Diesel invented the engine that bears his name in the closing years of the nineteenth century. But familiarity cannot mean inattention, especially for those diesel generators that provide emergency back-up power to nuclear stations. While they might be among the most easily-recognised pieces of equipment on site to visitors from outside the industry, their task is uniquely testing for both equipment and the personnel who maintain them, as they have to be ready to start up immediately, even if they are never used in an emergency over the many decades of the plant lifetime.
Emergency diesel generator (EDG) issues have been cited several times in performance issue notices published by the US Nuclear Regulatory Commission (NRC) since the start of the century, most of which detail resolution of an issue that allowed plants to be returned to a ‘green’ state from a slightly elevated ‘white’ state. The performance issues were all resolved to NRC’s satisfaction, but they illustrate how important it is that a positive safety culture is pervasive within a nuclear environment and that it extends to the conventional balance of plant.
Plant by plant
At Calvert Cliffs, a communication dated 24 April 2023 deals with an event when an EDG experienced an engine cylinder failure and tripped during routine testing, due to foreign material in the engine.
Constellation performed and documented a root cause evaluation and found the foreign material was most likely introduced during maintenance on the 1A EDG on 22 August 2020, and remained there until the failure. It said that larger pieces of foreign material had been present in the system, which had to degrade to a size that would allow introduction into the fuel injector before the performance issue would be detectable. It said there were no opportunities to identify the foreign material present because no maintenance occurred on the 1A EDG diesel fuel oil system after August 2020 that would have allowed identification of any foreign material present.
In October 2023 the NRC discussed failure by Calvert Cliffs to adequately establish and implement maintenance instructions and practices that reasonably ensured the reliability, availability and operability of the 1A EDG since the engine was originally commissioned. It said: “Adequate instructions, guidance, and part replacement frequencies relevant to the 1A EDG and the associated lube oil and fuel oil systems, in part, would have ensured appropriate preventive maintenance actions.” The failure to perform these maintenance tasks consistent with station and vendor guidance caused or contributed to an imbalanced fuel condition in the engine, ultimately resulting in its failure.
Root cause analysis by the company and NRC identified two root causes:
- Site leadership did not consistently engage workers to reinforce foreign material exclusion programme requirements and behaviours necessary to achieve sustained event-free performance prior to the August 2020 1A emergency diesel generator system outage window.
- Excellence in maintenance foreign material exclusion programme behaviours were not implemented and applied during the August 2020, 1A emergency diesel generator system outage window.
A contributing cause was that the site did not effectively use performance improvement tools to drive foreign material exclusion programme improvements prior to the 1A emergency diesel generator system outage window in August 2020.
Finally in April 2024 the NRC concluded that:
- Engineering personnel did not successfully identify fuel injector preventive and predictive maintenance strategies for the Société Alsacienne De Constructions Mecaniques De Mulhouse (SACM) diesel generators, which includes the 1A emergency diesel generator.
- System engineers and engineering managers did not exercise adequate technical human performance behaviours and technical conscience principles when developing the preventive maintenance strategy for the SACM diesel generators.
Catawba
At the Catawba plant, in a communication on 24 April 2025, NRC identified the root cause of a failure to implement measures to maintain functionality of the ventilation system in one of the diesel buildings. It said the licensee failed to recognise that the alternate method to operate the system was not described in the Updated Final Safety Analysis Report (UFSAR) and inadequate actions were taken to ensure UFSAR-defined testing was incorporated into surveillance test procedures.
Davis-Besse
At the Davis-Besse plant, in January 2022 the NRC confirmed a root-cause analysis regarding the unavailability and inoperability of its Division 2 EDG. The issue was failing to select a speed switch which was suitable for operation within the safety-related EDGs when it was being relied upon for plant safety. Rather than an issue with the EDG itself, NRC cited less than adequate incorporation, internalisation and anchoring of operating and in-house experiences around the unique design vulnerabilities in the direct current (DC) distribution system.
Joseph M Farley
At the Joseph M Farley plant, the NRC put to rest a performance issue in a 24 January 2024 letter. The issue involved a failure to provide adequate qualitative or quantitative acceptance criteria in work instructions during maintenance activities on the ‘B’ EDG.
Inadequate qualitative or quantitative acceptance criteria in the work instructions on reassembly of the valve cap for a lubricating oil check valve led to the valve cap being damaged on 11 October 2023, which ultimately resulted in a 2.3 gallons per minute lubricating oil leak.
Plant staff evaluation identified two root causes. The first was that the emergency EDG circulating lube oil pump discharge piping restraint was inadequate to prevent coupling separation. The second was that the circulating lube oil system failure mode was not identified and corrected in November 2022 because of “deficiencies and implementation weaknesses” in the troubleshooting process.
Maintenance leadership did not enforce compliance with the company’s ‘Management Model’ processes during the planning of the work order to repair a leak on the ‘EDG lube oil gallery supply check valve. As a result, the risk and potential consequences associated with installation of a gasket was not recognised, and work order instructions did not contain sufficient detail to install the gasket successfully. Additionally, the work order was set to ‘Ready’ status before the engineering work was completed. Engineering leadership did not reinforce the use of technical rigour or enforce compliance with the Management Model processes. As a result, the addition of a gasket was not identified as a configuration change, the risk and potential consequences associated with the repair strategy were not recognized, applicable design considerations and impacts were not evaluated, and no torque value was specified.
North Anna 2
At North Anna 2 the NRC confirmed in a communication dated 24 January 2025 that the root cause of a failure to have documented instructions appropriate to the circumstances for foreign material control was inadequate 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 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’.
