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Three Mile Island (USA)

This accident occurred on 28 March 1979, at Three Mile Island in Pennsylvania.

Course of the accident
  • The accident started with an equipment failure which caused a loss of reactor coolant through a partly-opened safety valve.
  • The operators misinterpreted the accident and switched off the properly-working emergency core cooling system.
  • The loss of reactor coolant caused core melting.
  • The undamaged reactor vessel contained the damaged core.
  • A small amount of radioactive material escaped into the containment building.
  • The accident had no radiological effects on the environment or the population.



Consequences of the TMI accident

were incomparably milder than at Chernobyl or Fukushima,

because the containment fulfilled its function.



In all similar plants in the world,

appropriate measures were taken to prevent a similar accident from happening again.

General view of the plant

General view of the plant

Melted core in the TMI reactor

Melted core in the TMI reactor
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Chernobyl (The Soviet Union / Ukraine)

On 26 April 1986, the worst nuclear accident so far occurred at the Chernobyl 4 NPP in the Ukraine.

Main causes of the accident:
  • design of the reactor
  • poor safety culture


The direct cause of the accident

was a  violation of operating procedures.

The operators brought the reactor into an unstable state,

the power drastically increased,

this caused an explosion and fire in the graphite moderator,

releasing large quantities of radioactive material over a wide area.

The Chernobyl NPP had no containment.

The design of PWRs

does not allow the reactor

to become similarly unstable

as was the case in Chernobyl.

The reactor at the Krško Nuclear Power Plant

has negative feedback,

which makes it self-controlling:

an increase in temperature

causes a decrease in power.


Consequences of the accident

  • 47 people died.
  • About 4000 cases of thyroid cancer among children were diagnosed, 9 died, others were cured.
  • Among around 600,000 emergency workers and residents of the most contaminated areas about 4000 deaths from radiation-induced cancer and leukemia are expected.

(This is about 3% of the spontaneous cancers not caused by Chernobyl radiation.)

  • Except in the 30-kilometre exclusion area, the radiation levels are again mostly normal.

Chernobyl - Course of the accident

Chernobyl – Course of the accident

A “Sarcophagus” built over the ruined reactor

A “Sarcophagus” built over the ruined reactor

A new protection building will be built over the degraded Sarcophagus.

A new protection building will be built over the degraded Sarcophagus.
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Fukushima (Japan)

On 11 March 2011, an exceptional tsunami (8 m higher than considered in the design) following a magnitude 9 earthquake flooded the Fukushima Daiichi Nuclear Power Plant (6 BWR units) and disabled the safety systems.

Scheme of the Fukushima power plant

Three reactors lost cooling, fuel overheated and produced hydrogen which exploded in the reactor building. The fuel was damaged and released radioactive materials.



Consequences of the accident
  • No victims due to radiation
  • 70,000 evacuated from a 20 km radius before radioactivity release
  • Part of the evacuated areas is already safe for normal life


Lessons learned

  • Resilience against extreme natural events was analysed in NPPs of the world (“stress tests”) and additional safety features were introduced.

Daiichi NPP - Floods

Daiichi NPP – Floods

Daiichi NPP – map of Japan
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