Companion piece · 2026-10-18
At 5:20 p.m. on January 21, 1969, operators at an experimental nuclear power station in western Switzerland received an automatic reactor shutdown alarm.
The control room was protected by concrete and the surrounding rock. The reactor itself was located in an underground cavern.
Inside that cavern, a uranium fuel element had overheated, melted, and caught fire. Its pressure tube ruptured, followed by a second explosion approximately one second later.
Radioactive gas and heavy water escaped into the reactor enclosure.
There were no personnel inside the cavern. No deaths or injuries were reported.
The reactor, however, was beyond repair.
The incident occurred at the Lucens Experimental Nuclear Power Plant, a facility intended to demonstrate Switzerland's ability to develop its own nuclear power technology.
It would never operate again.
Switzerland's Nuclear Ambitions
During the 1950s, Switzerland began investing heavily in nuclear research.
The country's industrial sector saw opportunities in electricity generation, reactor manufacturing, and international exports. Developing a domestic reactor design also promised greater independence from foreign technology suppliers.
In 1961, the National Society for the Promotion of Industrial Atomic Technology was established to coordinate the development of a Swiss nuclear power system.
Construction began at Lucens, in the canton of Vaud, in 1962.
The installation was built inside excavated sandstone caverns, partly to provide additional protection against radioactive releases.
The programme also developed alongside Switzerland's Cold War discussions about acquiring nuclear weapons.
Swiss military authorities seriously considered an independent nuclear deterrent. The Lucens reactor's technology could theoretically have contributed to plutonium production, although no conclusive evidence establishes that the facility was ever used for military purposes.
The reactor was officially designed as an experimental civilian power station.
Jean-Paul Buclin, the facility's technical director, later stated that, to his knowledge, personnel involved in its construction and operation were unaware of military intentions.
An Unusual Reactor Design
Lucens was a heavy-water-moderated, carbon-dioxide-cooled reactor.
Its rated thermal output was approximately 30 megawatts, with an intended electrical capacity of roughly 8 megawatts.
The reactor contained 73 vertical fuel assemblies. Each housed seven metallic uranium fuel rods enclosed in magnesium-zirconium alloy cladding.
Pressurized carbon dioxide circulated through the assemblies to remove heat.
Heavy water served as the neutron moderator, slowing neutrons to sustain controlled nuclear fission.
The design combined several established reactor technologies, but its particular arrangement introduced maintenance and engineering challenges.
The reactor first achieved criticality in December 1966. Subsequent testing revealed technical problems, including difficulties with coolant circulation and fuel overheating.
By 1968, the facility had operated at substantial power levels.
A lengthy shutdown beginning in October was intended to resolve outstanding mechanical problems before regular operation.
October 1968–January 1969: The Hidden Damage
The maintenance programme focused partly on defective seals associated with the main carbon-dioxide circulation blowers.
During this work, water from the blower sealing system apparently entered the primary coolant circuit.
The intrusion was not recognized as a serious threat.
During a blower test on December 11, 1968, water was carried into several fuel assemblies, including assembly number 59.
The magnesium alloy surrounding the uranium fuel was vulnerable to corrosion when exposed to water.
Over the following weeks, corrosion damaged the cladding. In some locations, it penetrated completely.
Corrosion products accumulated near the bottom of the assembly, obstructing the passages through which carbon dioxide was supposed to circulate.
The reactor's instrumentation did not detect the condition.
Consequently, when operators prepared to restart the facility, one fuel assembly was already severely compromised.
January 21, 1969: The Final Startup
At 4:23 a.m., the reactor achieved criticality.
Operators gradually increased its output throughout the day. Instrument readings initially appeared normal.
At 5:14 p.m., reactor power reached approximately 12 megawatts, about 40 percent of its rated thermal output.
Inside assembly 59, however, cooling gas was no longer circulating adequately.
The fuel temperature increased.
At approximately 600 degrees Celsius, the magnesium cladding melted. The uranium fuel subsequently melted and began reacting with the carbon-dioxide coolant.
The resulting fire damaged neighboring rods and released radioactive fission products into the coolant circuit.
The reactor automatically shut down at approximately 5:20 p.m.
The nuclear chain reaction stopped.
The fuel fire did not.
Continued heating caused the surrounding graphite structure to deform and contact the pressure tube.
The tube overheated and ruptured under an internal pressure of approximately 50 bar.
The pressure surge damaged the heavy-water moderator system, releasing approximately 1,100 kilograms of heavy water and contaminated material into the reactor cavern.
A second explosion followed, associated with a chemical reaction between the released water and molten metal.
These were mechanical and chemical explosions, not nuclear detonations.
Emergency Response and Containment
The reactor's ventilation systems responded automatically, but radioactive gases escaped through small leaks into adjoining spaces.
Radiation levels inside the reactor cavern reached approximately one sievert per hour.
The operating team switched ventilation to an emergency system equipped with iodine filters and subsequently stopped ventilation to limit further releases.
Specialist monitoring teams assessed radiation levels outside the facility.
They detected only small increases, well below the applicable public exposure limits.
The underground construction played an important role in limiting the consequences.
Although the reactor enclosure was heavily contaminated, most radioactive material remained within the underground complex.
No fatalities or radiation injuries were reported.
Investigating the Failure
A formal investigation examined the accident over several years.
In 1979, a commission led by Andreas Fritzsche, technical director of the Federal Institute for Reactor Research, issued its final report.
Investigators reconstructed the sequence using damaged reactor components and operating records.
They concluded with very high probability that water entering through the coolant blower seals had caused the corrosion and subsequent cooling blockage.
They also identified shortcomings in instrumentation. Not every fuel assembly had temperature monitoring capable of detecting localized overheating.
The failure therefore developed without providing operators with an adequate warning.
Dismantling and the Aftermath
Lucens was permanently retired.
Between 1969 and 1973, specialists recovered fuel, removed contaminated equipment, and dismantled much of the reactor system.
In 1988, authorities decided to return the site to non-nuclear use.
During the early 1990s, the reactor and associated underground spaces were filled with concrete, and a drainage system was installed.
Most of the site was released from nuclear regulatory control in 1995.
The remaining radioactive waste containers were removed in 2003. The final portion of the site was released from nuclear regulatory oversight in December 2004, although environmental radiation monitoring continued.
Part of the former complex was subsequently repurposed for archival and cultural storage.
The accident also marked the definitive end of Switzerland's experimental indigenous power-reactor programme.
However, the project had already lost commercial support before the accident. Swiss utilities were increasingly purchasing established reactor designs from foreign manufacturers.
Lucens confirmed a decision that the industry had largely begun making independently.
The facility had been intended to demonstrate the viability of a Swiss-designed nuclear power station.
Instead, it became a case study in coolant chemistry, corrosion, inadequate instrumentation, and the consequences of undetected damage.
The reactor's safety systems successfully terminated the nuclear chain reaction.
They could not reverse the physical damage already underway.
No one died in the Lucens accident.
The reactor was its only permanent casualty.
This has been Nuclear Shadows. I'm Morgan Miller.
#NuclearShadows #TrueHorror #NuclearHistory