Episode:
52
Bohunice A1 HWGCR
Country:
Czechoslovakia
Years of Operation:
1972-1977
Category:
Prototype & Demonstration
Reactor Type:
Coolant:
CO2
Fuel Type:
Natural Uranium
Moderator:
Heavy Water
Thermal Power (MWth):
150
Electrical Power (MWe):
150
Status:
Prototype & Demonstration
timeline
First Criticality Year
1972
Commercial Op Year
1972
Shutdown Year
1977
Lessons Learned
Refueling is a safety system. Treat it with the same rigor as shutdown and decay heat removal.
Operational complexity compounds human error. Online refueling only works when failure modes are brutally simple.
Chemistry and materials matter. A relatively simple moisture control component became the initiating event.
Natural uranium is not “simpler.” It often shifts complexity elsewhere—usually into operations.
sources
ARTICLE
The Bohunice A1 nuclear power plant was one of the most unconventional commercial reactors ever connected to a grid—and one of the most sobering reminders that complexity often hides where you least expect it.
Construction of the Russian designed KS-150 reactor began in 1958 in what was then Czechoslovakia. Bohunice A-1 entered operation 14 years later in 1972, using a heavy-water-moderated, carbon-dioxide-cooled reactor fueled with natural uranium, similar to a CANDU NPP. On paper, it promised fuel independence and avoided enrichment.
In practice, it delivered operational fragility.
The reactor operated at 150 MWe, with online refueling—a feature intended to improve capacity factor but which dramatically increased procedural and mechanical risk. Unlike light-water reactors, this system relied on a complex refueling machine interacting directly with a pressurized CO₂ environment. Tolerances were tight. Margins were thin. Human factors mattered—a lot.
And that’s where things began to unravel.
Bohunice A-1 suffered multiple refueling-related incidents, including a serious fuel damage event in 1976. But the fatal blow came in February 1977.
During refueling, a silica-gel moisture absorber that coated new fuel elements (humidity control) was not sufficiently removed from a new fuel element before the fuel element was inserted into the core.
The silica-gel coating prevented proper fuel element CO2 cooling, which caused severe fuel overheating and fuel element failure. Radioactive material was released inside the reactor building. While there was no explosive criticality excursion, the damage was extensive, contamination was significant, and confidence in the plant evaporated overnight.
Bohunice A-1 never returned to service. It was permanently shut down after less than five years of commercial operation.
SLIDE DECK
