Relay failures inside a pair of 1,280-megawatt Texas reactors hit eleven over three years, and a grinding subcontractor’s undersized rotor was hiding inside every single one of them
The chiller alarm came in quietly, the way the dangerous ones often do.
No explosion, no reactor trip, no emergency horn. Just a small relay inside a safety system at a sprawling Texas nuclear station, going dark when it should have held.
Then another relay failed. Then another.
By early June 2026, eleven identical relays had burned out across two reactor units over three years, each leaving an essential safety chiller unable to do its job.
By early June 2026, eleven identical relays had burned out across two reactor units over three years, each leaving an essential safety chiller unable to do its job.
The cause lived inside a part smaller than a human fist. How does a machining error survive qualification for a safety critical application?
Why the coil burned: a rotor ground a fraction too small
Every relay in the failed batch contained a rotor, a small pivoting metal component that sits inside a ring of copper wire called a stator. When the two fit correctly, the rotor’s mass induces eddy currents, tiny circulating electrical flows that limit how much current the coil draws and, by extension, how much heat the winding produces during normal operation.
The reduced rotor dimensions created a looser fit, shrinking those eddy currents and raising coil current consumption. That extra current translated directly into extra heat, and heat is the enemy of insulation. The increased coil wattage generated enough heat to short the relay windings at the finish lead, killing the relay outright.
So the relay did not fail because it was mishandled or improperly installed. It failed because the gap between rotor and stator was a fraction too wide, pushing a coil’s current draw just enough to cook the insulation from the inside over months of ordinary operation.
Where the flaw was born: a grinding shop and a missed specification
The relays at the center of this incident are MDR-styled devices, a compact and widely used industrial design made by a large connector and component manufacturer. The Part 21 evaluation identified a defect with MDR 131-1 and MDR 134-1 relays manufactured after mid 2020, bought as commercial off the shelf items and then formally reviewed for safety use at the plant.
The vendor informed the plant’s operating company that a subcontractor responsible for rotor grinding produced relay rotors outside of specification. The flaw was not in the design but in the machining of a single rotating part, one step removed from the company whose name appeared on the box.
Commercial grade components bought from a supply chain and then dedicated to safety use are only as reliable as the subcontractors those suppliers use, and this case shows exactly where that chain can slip without anyone upstream noticing.
Eleven failures, two units, three years of alarms
The South Texas Project station sits on a 12,200-acre site west of the Colorado River, about 90 miles southwest of Houston, running two large pressurized water reactors whose combined nameplate capacity reaches 2,560 MW. The plant’s capacity factors have historically ranked among the best in the country.
Since 2023, eleven of these relays across both units failed prematurely, leaving essential chillers inoperable and incapable of fulfilling their intended safety function. Essential chillers in a pressurized water reactor keep electronics and cables in key safety buildings from overheating during both normal operation and accident conditions. When a chiller drops offline, the plant must immediately account for the gap in its safety margin.
The NRC Resident Inspector was notified on June 10, 2026, at 1330 CDT. Written notification to the regulator was required within 30 days under Part 21 defect reporting rules governing components found substantially defective after installation.
The Part 21 clock and what it demands
Part 21 of federal nuclear regulations exists precisely for this kind of situation: a discovered defect that could affect the safety function of a component already installed and operating somewhere in the country’s reactor fleet. Once a facility confirms the defect, the reporting clock starts, and notification must reach other users of the same part, not just the regulator.
That obligation matters because the MDR relay family is a widely procured industrial component, and the manufacturing window in question covers several years of supply chain deliveries. Other plants that bought from the same production run would need to assess their own inventory. A single failed component can cascade through a plant’s operating margins faster than any single alarm suggests.
The lesson repeated here is that a component’s pedigree, not just its performance on the day it arrives, determines the risk it carries into service.
What the industry takes away from Bay City
No reactor at the South Texas Project scrammed because of these relay failures, and no member of the public or plant worker was harmed. The plant’s safety systems carry layers of redundancy precisely so that a single failed chiller does not become a crisis, and those layers held.
Eleven failures over three years, each traced to the same hidden dimensional error, represent exactly the kind of slow building defect that mature nuclear safety culture is supposed to catch earlier. Federal regulators were notified that the failures resulted from a subcontractor producing parts “outside of specification,” a finding that now travels to every facility that ordered from the same production run.
Commercial grade dedication relies on inspection and testing at procurement, and a flaw that only appears after months of thermal loading is far harder to catch at incoming inspection than one visible on a coordinate measuring machine. For a fleet pushing toward life extensions and new builds, including compact reactors now moving through testing, supply chain discipline at the subcontractor level is not a bureaucratic checkbox.
It is the physical boundary between a chiller alarm and something far harder to manage. Bay City’s eleven relays made that boundary visible in a way that will travel well beyond Texas.
Hugo is an engineer with strong technical expertise. Multilingual from an early age, his writing combines technical clarity with a strong interest in science and energy.
