Accident at the Lucens nuclear power plant
Construction of a heavy water reactor at the Lucens site in Switzerland began in 1962 (Figure 2.3). It was constructed completely underground, with the exception of a few storage and operations buildings. At the beginning of 1969, after a period of revision, the Lucens power plant reactor was returned to service. On January 21, 1969, when the power was increasing, the pressure in the primary cooling system dropped sharply. The instruments also reported a significant increase in radioactivity in the facility enclosure and a significant loss of heavy water. This meant that the moderator tank was damaged. An emergency shutdown of the reactor was carried out, and the caverns were isolated from the outside by closing the ventilation ducts. A few hours after the accident, radioactivity in the access tunnel decreased and the reactor continued to be cooled. Measurements taken on the night of the accident itself, and subsequently, showed that the level of radioactivity in the vicinity had hardly changed from a background level. After this incident, the facility was completely dismantled and the caverns decontaminated. The dismantling work, the decontamination, the analyses of the causes of the accident, the various expert reports and the publication of the final report took more than 10 years [CAN 11].
Figure 2.3. Diagram of the Lucens experimental nuclear power plant (adapted from [CAN 11]). For a color version of the figure, see www.iste.co.uk/amiard/industrial.zip
2.2.3.3. Nuclear accidents in Saint-Laurent-des-Eaux in 1969 and 1980
During a loading operation of the no. 1 graphite-gas reactor (SLA1) in Saint-Laurent-des-Eaux, France on October 17, 1969, the fuel loading and unloading apparatus in the operating reactor was controlled by a programmable displacement system. By mistake, a flow control valve was introduced over already-loaded fuel elements into one of the reactor core channels. This resulted in a drastic reduction in the cooling circuit of the fuel elements and a subsequent rise in temperature at the level of the magnesium and zirconium alloy sheaths of five fuel elements and their degradation. The reactor was automatically shut down due to the rise of radioactivity in the reactor vessel. These five fuel elements corresponded to about 50 kg of uranium dioxide that melted in the reactor core [IRS 15a].
The contamination would have been limited to the site. But because of its seriousness, it should have been classified at level 4 on the INES. However, EDF described it as an incident because it would not have caused any damage to persons, property or the environment outside of the site.
Clean-up operations began 1 year after the accident, while the nuclear fuel cooled and a full-scale model was built. These cleaning operations were mainly carried out using remote-controlled means. To finish this operation, several hundred (300–400) “cleaners” were mobilized, each one being able to intervene for only about 10 minutes, even 2 minutes for some. At the end of the clean-up operations, only 47 kg of uranium was recovered and the reactor was restarted on October 16, 1970.
The second graphite-gas reactor in Saint-Laurent-des-Eaux (SLA2) was automatically shut down on March 13, 1980 following a sudden increase in radioactivity in the reactor vessel. The next day, EDF estimated that a significant amount of spent uranium had melted. From March 22 to 26, after checking the proper operation of the iodine traps, decompression of the reactor vessel to the atmosphere was carried out in order to return to atmospheric pressure. This same reactor suffered several incidents during the same quarter of 1980 [GUI 16].
Examinations undertaken on March 27, 1980 showed that the accident originated in a total or partial plugging of six channels by a metal sheet detached from the fairing device due to its corrosion. Two fuel elements melted (about 20 kg of uranium) and two others showed significant traces of fusion. The cleaning was very difficult and took more than three and a half years [IRS 15a].
2.2.3.4. Accident at the Bohunice nuclear power plant
The Bohunice nuclear power plant is located 2.5 km from the village of Jaslovské Bohunice in the Trnava district of western Slovakia. On February 22, 1977, the A-1 reactor suffered a major INES 4 accident during reloading. A desiccant bag accompanying the package was inadvertently placed in the core with the nuclear fuel. This limited the circulation of the coolant, causing local overheating and serious damage. As in Simi Valley, this reactor was finally closed in 1978 and is still in the dismantling phase.
2.2.3.5. Accident at the Greifswald or Lubmin nuclear power plant
The Greifswald nuclear power plant, also known as the Lubmin nuclear power plant, was located in former East Germany. On December 7, 1975, a short circuit on the transformer of reactor no. 1 caused a small fire that destroyed the supply of five main cooling pumps out of a total of six. If the last pump had not cooled the reactor, core meltdown would have occurred. The rapid intervention of the fire brigade allowed the fire to be extinguished and the power supply to all the pumps to be temporarily restored. The incident was only revealed to the public in 1989 after the fall of the Berlin Wall. The IAEA has classified the accident as a level 4 accident on the INES.
2.2.3.6. Three Mile Island accident
On March 28, 1979, one of the two generating station’s reactors at Three Mile Island (TMI-2) in Pennsylvania (United States) suffered serious damage. These were 900 MW-pressurized water reactors. They were located on a small island (3.3 km2) in the Susquehanna River near Harrisburg. The cause of the accident was a result of the leakage of the primary water circuit enclosure (second protective barrier) because of a pressurizer relief valve that remained blocked in the open position. As a result, the core was no longer cooled, resulting in the melting of a substantial part of the fuel. The containment, the third barrier, played its role with the exception of a slight radioactive release that is difficult to quantify [IRS 12c]. This accident was classified as a level 5 accident on the INES.
Further analysis [NSA 80] showed that serious damage to the reactor fuel did not begin until about 1 hour and 40 minutes after the accident. During the accident, water containing radioactivity was removed from the reactor building at various times. Some of the radioactive gases dissolved in this water spread into the atmosphere of the auxiliary building and dispersed in the vicinity of the plant through the auxiliary building’s ventilation system.
Six years after the accident, it was possible to enter the enclosure, and a camera introduced into the tank showed that a significant portion of the fuel had melted (45%) and that it had partially flowed (20%) to the bottom of the tank but had not passed through it. The corium (the lava-like mixture of material created during the nuclear meltdown) had stratified at the bottom of the tank without causing an explosion.
Currently [NRC 11], the damaged core has been completely removed from the tank, including the parts melted during the accident; the containment has also been cleaned, and the plant is awaiting a decision on its future that could potentially be a complete dismantling, making the current right-of-way usable.
2.2.3.7. Serious incidents at various nuclear power plants
Serious incidents of level 3 on the INES occurring at nuclear power plants are relatively numerous. Among these incidents were the discovery in 2002 at the Davis–Besse power plant reactor (United States) of a cavity in the tank cover as a result of corrosion of the metal by boric acid; the loss of power supply in 1993 to Unit 1 of the Narora nuclear power plant (India) caused by a fire in the turbine hall; the emergency shutdown of reactor 1 at the Kola nuclear power plant (Russia) resulting from network disruptions following a tornado; and, in 1991, the exceeding of operating limits during
