Chernobyl

Chernobyl (Ukrainian: Чорнобиль, Chornobyl) is a town in Central Ukraine, and known infamously for the accident in the nearby nuclear power plant on 26 April 1986. The scale of remediation efforts (officially the liquidation of the accident aftermath) and subsequent engineering challenges such as construction of the iconic reactor sarcophagus, have drawn the interest of many curious travelers over the last 32 years. Nowadays Chernobyl is visited by ca. 60,000 tourists annually. Radiation from the accident remains around the site, making access severely restricted, and leaving no doubt that the area is a dangerous place and decidedly not an amusement park. A visit to the area is a unique experience however, and offers an insight into the scientific, technological and humanitarian aspects of the disaster.

Chernobyl nuclear power plant’s reactor 4 encased in a sarcophagus. A replacement to the one shown in the picture was installed in 2016.

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The name Chernobyl refers to the area around the Chernobyl Nuclear Power Plant (often abbreviated to ChNPP) in the north of Ukraine bordering Belarus. Chernobyl is a town 15 km south of the power plant, and was the closest known settlement by western media in 1986 when a catastrophic accident occurred in one of the power plants 4 reactors. The city Pripyat, built to house power plant employees is actually right next to the power plant and thus much closer than Chernobyl, but was a closed city at the time and thus not known in the West. The accident thus became known as the Chernobyl disaster instead, and the name stuck.

The accident contaminated a large area around the power plant with radioactive fallout, and these areas were subsequently evacuated. This Zone of Alienation was expanded several times when the magnitude of the accident became clear, and eventually covered an area of 2,600 km², roughly the size of Luxembourg. Special procedures were put in place to minimize spreading of radioactivity, and access to the area was restricted. It became known as the Chernobyl Exclusion Zone. The exact borders of the Exclusion Zone have been adjusted several times to align them better with actual radioactive contamination levels, but the are remains roughly the same as in 1986. A lot of the fallout fell in Belarus rather than in Ukraine.

As radioactivity naturally decays away over time, radiation levels have been dropping over the past 32 years. In the town Chernobyl itself for example, radiation levels are about the same as in Kiev, and the once abandoned town is now inhabited again. Most of the Exclusion Zone remains deserted however, a largely forested area with lakes and rivers, dotted with abandoned settlements and industrial installations. Although not nearly as dangerous anymore as it once was, it remains a nuclear wasteland that draws curiosity from travellers from around the world. The Exclusion Zone has been featured in popular media, most notably mainstream computer games like S.T.A.L.K.E.R and Call of Duty, which spread the mysteries of the Exclusion Zone to the broader public. The Chernobyl Exclusion Zone is probably the closest any place in the world gets to the digital game worlds seen in the Fallout series, which explains its popularity as a tourist attraction.

Although access to the Exclusion Zone is still restricted, guided tours are organized, most including transport from Kiev to and from the Exclusion Zone. It was visited by 72,000 tourists in 2018.

The Chernobyl Nuclear Power Plant was constructed between 1972 and 1977 on the shore of the Pripyat river, about 100 km north of the Ukrainian capital Kiev. Its location was selected because of the safe distance from the capital, the thinly populated area, and the proximity of water for cooling purposes. The plant has 4 massive nuclear reactors of the RBMK type, with a total electricity output of 4 GW — enough to power roughly 4 million microwave ovens. The plant design was innovative for the time, featuring hydrogen cooled generators with integrated electrolysis cells to generate the necessary hydrogen on site, as well as advanced computer systems. The machine hall housing the turbines and generators is one of the longest buildings in Europe with a length of 600 m.

Aside from nuclear and electronic innovations, the plant also implemented automatic control and safety systems, which needed to be field tested in a live production environment, as was common with all Soviet technology at the time. Of particular concern was the safety system handling a so called station blackout, a situation in which external factors lead to a complete loss of electrical power to the power plant. The reactors, each with a thermal output of 3.2 GW, must be actively cooled in such a situation to avoid their cores from melting, and to do so powerful pumps are installed to pump cooling water to the reactor cores. Backup diesel generators were available to generate the required electricity to drive the water pumps, but due to their sheer size, they took over a minute to get up to speed — a minute during which the cores would remain uncooled. This was considered an unacceptable safety risk. Engineers came up with a clever solution, and proposed to use the residual momentum of the massive turbines and generators, acting as giant flywheels, to keep pumps running until the backup diesel generators produce enough power to take over the responsibility of cooling the reactors. The idea worked in theory but had never been tested, and Reactor 4 of the Chernobyl Nuclear Power Plant was selected to verify the theory with an experiment.

Engineers devised a test scenario in which the output power of Reactor 4 would be decreased to a much lower level, at which the steam pipes between the reactor and its turbines would be closed off to let the turbines flywheel down. Measurement equipment was installed to log the output power of the generators, and the crew in the reactor control room was briefed on the technical details of the test. Because closing steam valves were interpreted by the electronic safety systems as fatal incursions which caused an automatic reactor shutdown, it was decided to disable these safety systems and transfer manual control to the operators in the control room. Calculations showed safe operations at all time, the test was approved, and scheduled for the evening of 26 April 1986. As Ukrainians went to bed and power consumption dropped, the test began and the output power of Reactor 4 was throttled back according to plan.

An unexpected failure in another sub station elsewhere in Ukraine required the Chernobyl Nuclear Power Plant to take over power generation however, and electrical grid controllers demanded Reactor 4 to be brought back to full output power. Execution of the experiment needed to be postponed. By the time the issue was resolved and the experiment could resume, shifts in the control room staff had changed: the day shift had long gone home, and the evening shift was preparing to leave and hand over reactor control to the night shift. Because of the unexpected delay of the test, night shift operators had not been briefed, and instead of having to monitor decay heat in an otherwise shut down reactor, they were tasked to execute the test instead of their evening shift colleagues.

A series of human errors from the relatively inexperienced night shift operators resulted in the reactor being almost shut down completely, again causing the experiment to be postponed. It was decided to disable the last remaining automatic safety systems to get the reactor back online as quickly as possible, and all control rods were retracted manually. This left the reactor in an extremely unstable state that was not allowed by operational procedures. When the experiment finally started, shutting off the steam valves caused a positive feedback in the reactors output power, but alarms were ignored by the operators in the control room. With no automated safety systems counteracting reactor power fluctuations, the its output power soared exponentially to over 11 times its rated maximum power level.

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