An assessment of the Chernobyl disaster 38 years after it happened

Saturday, April 26, 1986, 01:23:45 AM local time. A terrifying accident was taking place in the northern Ukraine, in the former USSR. Few people realized it immediately, and even fewer were aware of the seriousness of the situation. Reactor No. 4 of the Chernobyl Nuclear Power Plant had exploded. This event was so important that, according to the then leader of the Soviet Union, Mikhail Gorbachev, it was the biggest cause of the collapse of the Soviet Union.

Chernobyl Power plant

The power plant was located very close to the border with Belarus, which was also greatly affected by the accident. The plant consisted of four graphite-moderated RBMK reactors (i.e. reactors that use carbon as a neutron moderator), with an electrical output of 1,000 megawatts each. At the time of the accident, the plant produced about 10% of Ukraine's electricity.

The plant was new (with the 4 reactors having started commercial operation between 2 and 8 years before the accident) and was built next to the also new (established in 1970) city of Pripyat.

Accident

The accident occurred during a test due to a combination of human error by operators and design flaws in the reactor itself. A series of unfortunate safety-defying decisions led to steam explosions and a meltdown of the reactor core. Due to the absence of a containment structure that could keep radiation inside the plant, airborne radioactive contamination spread over the next few days across the USSR and Europe.

No report was released by the Soviet state until May 28, when Sweden concluded that a nuclear accident had occurred in the Soviet Union, correlating the radiation levels with the direction of those days' winds and pressing for explanations.

Losses and damages

The immediate casualties in those days (due to the accident and direct exposure to high levels of radioactivity) were 30 people, although losses from increased cancer mortality due to the accident in the following decades are estimated at several thousand.

An exclusion zone of 10 km (6.2 mi) radius was established, which was later extended to 30 km (19 mi). 53,000 people were evacuated the next day, while by 2000 some 350,000 had been resettled.

Disaster mitigation efforts

To avoid contamination of the groundwater by the molten core, it was decided to excavate a tunnel under the reactor in order to install a cooling system (heat exchanger), consisting of water pipes covered with a layer of (thermally conductive) graphite. The tunnel was dug, but finding that in the meantime the melting of the fuel had stopped, it was instead filled with concrete to strengthen the foundations under the reactor.

This was followed by the removal of debris from the reactor roof, which was initially attempted using remote-controlled robots. But because of the difficult terrain and the fact that the high levels of radiation destroyed the robots' electronic circuits, 90% of the debris was removed by 3,828 soldiers, who worked for 40 to 90 seconds each to get a radioactive dose within permissible limits.

After the most urgent work was completed, 24 days after the accident, design began for the sarcophagus, which would enclose the reactor to prevent contamination from spreading through the air. The construction of this first sarcophagus lasted from June to the end of November 1986, in very difficult conditions and high levels of gamma radiation. Member of the 12-member advisory committee for the preservation of the sarcophagus, called “International Advisory Group” (IAG) and established in 1997 under the chairmanship of Dr. Carlo Mancini, was also the geotechnical giant Michele Jamiolkowski.

This first sarcophagus was built in haste and with a predicted lifespan of only 20 to 30 years. With signs of deterioration evident, and rain entering through holes in the roof eroding the supporting beams, the decision was made to build a new structure to enclose the reactor and the old sarcophagus. The construction of this structure, called the New Safe Confinement (NSC), began in September 2010 by a French consortium of Vinci and Bouygues and its installation was completed on November 29, 2016.

Although after Russia's invasion of Ukraine the region is again suffering from unrest, this dome, which was created to last 100 years, makes us all feel a little safer. Several reactors of the RBMK type are still operating today (all in Russia), however without serious incidents since most of their flaws were corrected after the Chernobyl accident.