Episode:
70
Saint-Laurent A1
Country:
France
Years of Operation:
1969-1990
Category:
Commercial & Power
Reactor Type:
Coolant:
CO2
Fuel Type:
Natural Uranium
Moderator:
Graphite
Thermal Power (MWth):
1690
Electrical Power (MWe):
1690
Status:
Commercial & Power
timeline
First Criticality Year
1969
Commercial Op Year
1969
Shutdown Year
1990
Lessons Learned
Which is a shame, because it taught three brutal lessons that still apply today:
1. Fuel handling is not logistics. It’s nuclear safety.Core damage didn’t start with physics. It started with inventory control and human error.
2. Channelized cores and gas cooling are unforgiving.Block one channel and you don’t get a warning. You get a local furnace.
3. “Foreign material exclusion” is not bureaucracy. It’s survival.Ten years later, the sister unit A2 melted fuel again—this time from corroded internal parts breaking loose and blocking cooling channels.
Different decade. Same crime. Same weapon: obstruction.
Complex machines don’t fail dramatically.
They fail because one small, stupid object shows up where it doesn’t belong… and nobody notices until the laws of thermodynamics file their report.
Case closed.
sources
#ForgottenReactors
#NuclearHistory
#NuclearEngineering
#LessonsLearned
#CO2Coolant
#ReactorSafety
ARTICLE
In the late 1960s, France was building reactors the way watchmakers build watches: graphite blocks, gas cooling, natural-uranium fuel, everything fitted together with mechanical elegance and a faint aroma of Gauloises.
One of those machines was Saint-Laurent A1, a UNGG reactor (uranium naturel, graphite moderator, CO₂ coolant) built on the Loire.
Construction began in 1963.First criticality: January 1969.Commercial operation: June 1969.Power: ~1650 MW thermal, ~390 MW electric.
A serious industrial power plant. Not a toy.
And then, nine months later, it quietly committed one of nuclear history’s most forgettable crimes.
During routine refueling operations, a graphite spacer block—a “rondin,” in French—was mistakenly inserted where a fuel element should have gone. Same shape. Same color. Same bad day at the office.
That single wrong object partially blocked a cooling channel.
CO₂ flow dropped.Fuel temperature climbed.Five fuel elements melted.
No explosion. No dramatic fireball. Just uranium quietly liquefying inside a concrete caisson like a crime scene no one wanted to photograph.
Roughly 30–50 kg of uranium ended up dispersed inside the reactor vessel.
The plant shut down, cleaned up, repaired… and kept operating until 1990, when France finally retired its graphite-gas fleet in favor of boring, reliable pressurized water reactors. The nuclear equivalent of trading a vintage Italian sports car for a Toyota Hilux.
Saint-Laurent A1 became a footnote.
SLIDE DECK
