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Science

Incidents and accidents in nuclear power plants - when are they really dangerous?

All incidents and accidents in civilian nuclear facilities must be reported to the International Atomic Energy Agency. There are eight defined levels - from harmless to catastrophe. DW takes a look.

All countries that use nuclear power (with the exception of North Korea) have committed themselves to report nuclear incidents and accidents to the International Atomic Energy Agency (IAEA). Those reports are principally public - the idea is that experts and media can freely inform themselves and openly discuss the gravity and significance of the events.

But there are doubts that this information flows as intended - even in western democracies. In the French power plant Fassene (in German: "Fessenheim"), there was an incident on April 9, 2014 in which water damaged part of the reactor protection system. The reactor was shut down and the case reported to the IAEA as a level 1 incident.

But the event should have been reported as an incident of the next higher level, according to a report issued Friday by investigative journalists from the German public broadcaster WDR and the daily Süddeutsche Zeitung.

Powerplant Fassene (Photo: Picture alliance/ dpa)

Only an "anomaly" or an "incident"? Engineers in Fassene were unable to use their control rods

This is because there were severe complications when trying to shut down the reactor. The engineers were unable to move the control rods, which are there to limit or stop the nuclear fission reaction. They are placed between the fuel rods and absorb neutrons - no more neutrons, no more fission.

Eventually, the engineers had to resort to a different method of stopping the chain reaction. They flooded the reactor with Boron. This element has the same qualities as the control rods; it absorbs neutrons and stems the fission process.

This incident was most definitely a "significant failure in safety provisions" and therefore should have been reported as a level 2 incident. This is how the different levels are defined, according to the International Nuclear Event Scale (INES).

'Deviations' from normality

To grasp incidents that have no immediate relevance for security, but which could eventually lead to more severe further incidents if neglected, there is a definition for a level 0 incident.

That could include minor leaks in cooling systems, a failure of the uninterruptible power supply or a failure in testing emissions. Also, a cable fire or one in a transformer could be qualified in that category.

Such events are rather common. For example, in the year 2014, there were 68 notifiable events of level 0 in all of Germany's nuclear facilities.

The lowest form of an incident is an "anomaly" (level 1). This can be the case if a faulty component such as a damaged power generator triggered a shut down of the reactor. Also, a severe mistake in the management of power plant rules and processes can be qualified as a level 1 incident. Throughout all of the 1990s, there were 75 such events in Germany.

A real "incident" (level 2) occurs when there is a significant failure in safety provisions. In such cases, another part of the safety infrastructure will step in, like in the case of the 2014 Fassene event. Other examples could include the failure of an important water pump or a dangerously low Boron concentration in a flooding container. Such level 2 incidents are rare. Throughout the 1990s, Germany's nuclear facilities reported only three of them.

A "serious incident" (level 3) involves the leaking of a small amount of radioactive material into the environment. This usually means an exposure of personnel to radioactivity, resulting in contamination or health problems. This always involves the failure of several parts of the safety provisions. Such events are extremely rare even on the global scale, happening around once a year around the world.

One example was the leaking of radioactive material from the British nuclear reprocessing plant in Sellafield in 2005. Large parts of the facility and the area around it were then contaminated.

An emtpy town near Fukushima (Photo: Picture alliance/ dpa)

Near Fukushima more than 120,000 People had to leave their homes

Accidents

An accident with local consequences (level 4), an accident with wider consequences (level 5) or a serious accident (level 6) mean that a significant amount of radioactive material has been released. The different levels are differentiated by the severity of radiation and the number of casualties. If the radioactivity of the spilled substances amounts to hundreds of Terabecquerels (1 Tbq = 1 Trillion radioactive fission processes per second), or if there is at least one fatality, the accident will be considered level 4.

If the radiation is at least 100 times higher, or are there several fatalities, the accident is level 5. Level 6 is reserved for accidents with tens of thousands of Tbq and widespread contamination.

Generally speaking, accidents around nuclear facilities are rare. Level 4 accidents include the partial meltdowns in the French power plant Saint-Laurent in 1969 and 1980. The Three-Mile-Island accident in Pennsylvania in 1979 was classified level 5, after a third of the reactor core melted then - but could be contained. The release of radioactivity led to a considerable increase of cancer rates downwind from the reactor by as much as 150 Percent.

An example of a level 6 accident was the massive release of radioactive substances from a military nuclear plant in Majak in Siberia in 1957. More than 10.000 people in the sparsly populated area had to be resettled. Today, Majak is considered the third most severe accident of the nuclear era.

The catastrophe

Since nuclear power was invented, there were two catastrophic accidents (level 7): Chernobyl in 1986, and 2011 in Fukushima. Both accidents saw a complete meltdown of one (Chernobyl) or more (Fukushima) core reactors.

In both cases, the outer walls of the reactor-building were destroyed by hydrogen explosions and nuclear substances could freely escape. In both cases, vast areas were contaminated. In Chernobyl, about 50 people died shortly after the accident from radiation sickness. Different estimates about the long term effects assume that there may have been tens of thousands of fatalities due to cancer.

Also for Fukushima, there are competing estimates ranging from 15 victims to thousands of possible victims, which the accident may yet claim, due to increasing cancer rates.

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