CBS Evening News for
Tuesday, Apr 29, 1986

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Headline: USSR / Nuclear Accident

The "Chernobyl disaster", reactor accident at the Chernobyl nuclear power plant, or simply "Chernobyl", was the worst nuclear power plant accident in history and the only instance so far of level 7 on the International Nuclear Event Scale, resulting in a severe nuclear meltdown.

On 26 April 1986 at 01:23:40 a.m. (UTC+3) reactor number four at the Chernobyl Nuclear Power Plant located in the Soviet Union near Pripyat in Ukraine exploded. Further explosions and the resulting fire sent a plume of highly radioactive fallout into the atmosphere and over an extensive geographical area. Nearly thirty to forty times more fallout was released than Hiroshima. The plume drifted over parts of the western Soviet Union, Eastern Europe, Western Europe, Northern Europe, and eastern North America. Large areas in Ukraine, Belarus, and Russia were badly contaminated, resulting in the evacuation and resettlement of over 336,000 people. According to official post-Soviet data, about 60% of the radioactive fallout landed in Belarus.

The accident raised concerns about the safety of the Soviet nuclear power industry, slowing its expansion for a number of years, while forcing the Soviet government to become less secretive. The now-independent countries of Russia, Ukraine, and Belarus have been burdened with the continuing and substantial decontamination and health care costs of the Chernobyl accident. It is difficult to accurately tell the number of deaths caused by the events at Chernobyl, as the Soviet-era cover-up made it difficult to track down victims. Lists were incomplete, and Soviet authorities later forbade doctors to cite "radiation" on death certificates.

The 2005 report prepared by the Chernobyl Forum, led by the International Atomic Energy Agency (IAEA) and World Health Organization (WHO), attributed 56 direct deaths (47 accident workers, and nine children with thyroid cancer), and estimated that there may be 4,000 extra deaths due to cancer among the approximately 600,000 most highly exposed and 5,000 among the 6 million living nearby. Although the Chernobyl Exclusion Zone and certain limited areas will remain off limits, the majority of affected areas are now considered safe for settlement and economic activity.

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The Chernobyl nuclear power plant

 

The Chernobyl Nuclear Power Plant is located near the city of Pripyat in north central Ukraine.

The Chernobyl station ( 51°23′14″N, 30°06′41″E) is located near the town of Pripyat, Ukraine, 18 km northwest of the city of Chernobyl, 16 km (10 mi) from the border of Ukraine and Belarus, and about 110 km (68 mi) north of Kiev. The station consisted of four reactors of type RBMK-1000, each capable of producing 1 gigawatt (GW) of electric power, and the four together produced about 10% of Ukraine's electricity at the time of the accident. Construction of the plant began in the 1970s, with reactor no. 1 commissioned in 1977, followed by no. 2 (1978), no. 3 (1981), and no. 4 (1983). Two more reactors, no. 5 and 6, capable of producing 1 GW each, were under construction at the time of the accident.

The accident

On April 26, 1986 at 1:23:40 a.m., reactor 4 suffered a catastrophic steam explosion resulting in a nuclear meltdown, a series of additional explosions and a fire; the radiation was not contained and radioactive particles were carried by wind across international borders.

Test planning

During the daytime of April 25, 1986, reactor 4 at 7 51°23′22.39″N, 30°05′56.93″E was scheduled to be shut down for maintenance. A decision was made to test the ability of the reactor's turbine generator to generate sufficient electricity to power the reactor's safety systems (specifically the water pumps), in the event of a loss of external electric power. A RBMK-1000 reactor requires water to be continuously circulated through the core for as long as the nuclear fuel is present.

Chernobyl's reactors had a pair of backup diesel generators, but because there was a 40-second delay before they could attain full speed, the reactor was going to be used to spin up the reactor's turbine generator. Once at full speed, the turbine would be disconnected from the reactor and allowed to spin under its own rotational momentum. The aim of the test was to determine whether the turbines in the rundown phase could power the pumps while the generators were starting up. The test was previously successfully carried out on another unit (with all safety provisions active) with negative results — the turbines did not generate sufficient power, but because additional improvements were made to the reactor's four turbines, there was a need for another test.

Conditions prior to the accident

As conditions to run this test were prepared during the daytime of April 25, and the reactor electricity output had been gradually reduced to 50%, a regional power station unexpectedly went offline. The Kiev grid controller requested that the further reduction of output be postponed, as electricity was needed to satisfy the evening peak demand. The plant director agreed and postponed the test to comply. The ill-fated safety test was then left to be run by the night shift of the plant, a skeleton crew who would be working Reactor 4 that night and the early part of the next morning. This reactor crew had little or no experience in nuclear power plants, as many had been drafted in from coal powered plants, and Anatoly Dyatlov, deputy chief engineer of the plant and the effective crew chief during the experiment, had some experience installing nuclear reactors in submarines.

At 11:04 p.m., April 25, the grid controller allowed the reactor shut-down to continue. The power output of reactor 4 was to be reduced from its nominal 3.2 GW thermal to 0.7–1.0 GW thermal in order to conduct the test at the prescribed lower level of power. However, the new crew was unaware of the prior postponement of the reactor slowdown, and followed the original test protocol, decreasing power too rapidly.

A major product of nuclear fission is the isotope iodine-135. I-135 decays with a half life of 6.7 hours into xenon-135. Xe-135 is a potent reactor poison, i.e., it is extremely effective at absorbing neutrons and slowing the chain reaction. Once an atom of Xe-135 absorbs a neutron, however, it becomes the stable Xe-136 that doesn't absorb neutrons. In normal high power operation, an equilibrium is reached where the Xe-135 is "burned" by the reactor's high neutron flux as fast as it is produced by I-135 decay. But because reactor 4's power and neutron flux were rapidly decreased, the decay of large amounts of I-135 from previous high power operation produced Xe-135 faster than it could be eliminated, so it built up and dampened the nuclear reaction further.

When the operators commanded a small power reduction, the reactor power dropped to 30 MW thermal, approximately 5% of what was expected. The operators, unaware of the poisoning phenomenon, believed that the rapid fall in output was due to a malfunction in one of the automatic power regulators. To increase power, automatic control rods were pulled out of the reactor beyond the correct position for the desired power output in normal operating conditions, and also beyond what is allowed under safety regulations.

The reactor's power still only increased to 200MW, less than a third of the minimum required for the experiment. Yet the crew's management continued the experiment. As part of the experiment, at 1:05 a.m. on April 26 the water pumps that were to be driven by the turbine generator were turned on, increasing the water flow beyond what is specified by safety regulations. The water flow increased at 1:19 a.m., and since water also absorbs neutrons, this decreased reactor power further and prompted the removal of the manual control rods. This produced an extremely hazardous condition; with nearly all of the control rods removed, the only thing keeping the reactor at such a low power level was the build-up of Xe-135.

Fatal experiment

At 1:23:04 the experiment began. The unstable state of the reactor was not reflected in any way on the control panel, and it did not appear that anyone in the reactor crew was aware of any danger. The steam to the turbines was shut off and, as the momentum of the turbine generator drove the water pumps, the water flow rate decreased, decreasing the absorption of neutrons by the coolant. The turbine was disconnected from the reactor, increasing the level of steam in the reactor core. As the coolant heated, pockets of steam formed voids in the coolant lines. Due to the RBMK reactor-type's large positive void coefficient, the steam bubbles increased the power of the reactor. As soon as the reactor power increased, the positive feedback that had acted to drive reactor power down now acted to increase it further. As power increased, the Xe-135 poison began to be burned faster than it was being produced by I-135 decay, which increased power, resulting in a faster Xe-135 burn, and so on. With the manual and automatic control rods removed, nothing prevented a runaway reaction.

At 1:23:40 the operators pressed the AZ-5 ("Rapid Emergency Defense 5") button that ordered a "SCRAM" – a shutdown of the reactor, fully inserting all control rods, including the manual control rods that had been incautiously withdrawn earlier. It is unclear whether it was done as an emergency measure, or simply as a routine method of shutting down the reactor upon the completion of an experiment (the reactor was scheduled to be shut down for routine maintenance). It is usually suggested that the SCRAM was ordered as a response to the unexpected rapid power increase. On the other hand, Dyatlov writes in his book:

Prior to 01:23:40, systems of centralized control … didn't register any parameter changes that could justify the SCRAM. Commission … gathered and analyzed large amount of materials and, as stated in its report, failed to determine the reason why the SCRAM was ordered. There was no need to look for the reason. The reactor was simply being shut down upon the completion of the experiment.

The slow speed of the control rod insertion mechanism (18–20 seconds to complete), and the flawed rod design which initially reduces the amount of coolant present, meant that the SCRAM actually increased the reaction rate. At this point an energy spike occurred and some of the fuel rods began to fracture, placing fragments of the fuel rods in line with the control rod columns. The rods became stuck after being inserted only one-third of the way, and were therefore unable to stop the reaction. At this point nothing could be done to stop the disaster. By 1:23:47 the reactor jumped to around 30 GW thermal, ten times the normal operational output. The fuel rods began to melt and the steam pressure rapidly increased, causing a large steam explosion. Generated steam traveled vertically along the rod channels in the reactor, displacing and destroying the reactor lid, rupturing the coolant tubes and then blowing the lid off the reactor.  After part of the roof blew off, the inrush of oxygen, combined with the extremely high temperature of the reactor fuel and graphite moderator, started a graphite fire. This fire greatly contributed to the spread of radioactive material and the contamination of outlying areas.

At the time of the disaster, the plant's staff were not aware of the true radiation levels, which led to severe misassessments of the situation. The radiation levels in the worst-hit areas of the reactor building have been estimated to be 5.6 röntgen per second (R/s), which is equivalent to 20,000 röntgen per hour (R/h). A lethal dose is around 500 röntgen over 5 hours, so in some areas, unprotected workers received fatal doses within several minutes. However, a dosimeter capable of measuring up to 1,000 R/s was inaccessible due to the explosion, and another one failed when turned on. All remaining dosimeters had limits of 0.001 R/s and therefore read "off scale". Thus, the reactor crew could ascertain only that the radiation levels were somewhere above 0.001 R/s (3.6 R/h), while the true levels were 5,600 times higher in some areas.

Because of the fallacious low readings, the reactor crew chief Alexander Akimov assumed that the reactor was intact. The evidence of pieces of graphite and reactor fuel lying around the building was ignored, and the readings of another dosimeter brought in by 4:30 a.m. were dismissed under the assumption that the new dosimeter must have been defective. Akimov stayed with his crew in the reactor building until morning, trying to pump water into the reactor. None of them wore any protective gear. Most of them, including Akimov, died from radiation exposure within three weeks.

Fire containment

Shortly after the accident, firefighters arrived to try to extinguish the fires. The first one to the scene was a Chernobyl Power Station firefighter brigade under the command of Lieutenant Vladimir Pravik, who died on May 9, 1986. They were not told how dangerously radioactive the smoke and the debris were, and may not even have known that the accident was anything more than a regular electrical fire: "We didn't know it was the reactor. No one had told us." The fires on the roof of the station and the area around the building containing Reactor No. 4 were extinguished by 5 a.m., but many firefighters received high doses of radiation. The fire inside Reactor No. 4 continued to burn until the fire was extinguished by helicopters dropping materials like sand, lead, clay and boron onto the burning reactor.

The explosion and fire threw particles of the nuclear fuel and also far more dangerous radioactive elements like caesium-137, iodine-131, strontium-90 and other radionuclides into the air: the residents of the surrounding area observed the radioactive cloud on the night of the explosion.

Evacuation of Pripyat

After radiation levels set off alarms at the Forsmark Nuclear Power Plant in Sweden, the Soviet Union did admit that an accident had occurred, but still tried to cover up the scale of the disaster. In order to evacuate the city of Pripyat, the following warning message was reported on local radio, "An accident has occurred at the Chernobyl Nuclear Power Plant. One of the atomic reactors has been damaged. Aid will be given to those affected and a committee of government inquiry has been set up." This message gave the impression that any damage and radiation was localized, although it was not.

The government committee formed to investigate the accident, led by Valeri Legasov, arrived at Chernobyl in the evening of 26 April. By that time two people were dead and 52 were hospitalized. During the night of 26 April / April 27 — more than 24 hours after the explosion — the committee, faced with ample evidence of extremely high levels of radiation and a number of cases of radiation exposure, had to acknowledge the destruction of the reactor and order the evacuation of the nearby city of Pripyat.

The evacuation began at 14:00, 27 April. In order to reduce baggage the residents were told that the evacuation would be temporary, lasting approximately three days. As a result, Pripyat still contains personal belongings. From eyewitness accounts of the firefighters involved before they died (as reported on the CBC television series Witness), one described his experience of the radiation as "tasting like metal", and feeling a sensation similar to that of pins and needles all over his face. (This is similar to the description given by Louis Slotin, a Manhattan Project physicist who died days after a fatal radiation overdose from a criticality accident.)

Thermal explosion risk

The water that had hurriedly been pumped into the reactor building in a futile attempt to extinguish the fire had run down to the space underneath the reactor floor. The smouldering fuel and other material still suspended above was starting to burn its way through the reactor floor, mixing with melted concrete that had lined the reactor, and creating a radioactive semi-liquid material comparable to lava. If this mixture had burned through the floor into the pool of water, the burst of radioactive steam would have killed everyone on site and increased the severity of the fallout.

In order to prevent this, soldiers and workers were sent in as clean-up staff by the government. Two of these were sent in wetsuits to open the sluice gates to vent the radioactive water, and thus prevent a thermal explosion. They are thought to be engineers Alexei Ananenko (who knew where the valves were) and Valeri Bezpalov, accompanied by a third man, Boris Baranov, who provided them with light from a lamp, though this lamp failed, leaving them to find the valves by feeling their way along a pipe.

Debris removal

The worst of the radioactive debris was collected inside what was left of the reactor. The reactor itself was covered with bags containing sand, lead and boric acid thrown off helicopters (some 5,000 metric tonnes during the week following the accident). By December 1986 a large concrete sarcophagus had been erected, to seal off the reactor and its contents.

Many of the vehicles used by the "liquidators" remain parked in a field in the Chernobyl area to this day.

Possible causes of the disaster

 

Abandoned living blocks of Pripyat, with a surviving tree

There are two official theories about the main cause of the accident: the first, 'flawed operators theory', was published in August 1986 and effectively placed the blame solely on the power plant operators. The operators violated plant procedures and were ignorant of the safety requirements needed by the RBMK design. This was partly due to their lack of knowledge of the reactor's design as well as lack of experience and training. Several procedural irregularities also contributed to causing the accident. One was insufficient communication between the safety officers and the operators in charge of the experiment being run that night. It is also important to note that the reactor operators disabled every safety system down to the generators, which the test was really about. The main process computer, "S.K.A.L.A", was running in such a way that the main control computer could not shut down the reactor or even reduce power. Normally the reactor would have started to insert all of the control rods. The computer would have also started the "Emergency Core Protection System" that introduces 24 control rods into the active zone within 2.5 seconds, which is still slow by 1986 standards. All control was transferred from the process computer to the human operators who had very little or no experience with nuclear reactors.

The second 'flawed design theory' was proposed by Valeri Legasov and published in 1991, attributing the accident to flaws in the RBMK reactor design, specifically the control rods.

• The reactor had a dangerously large positive void coefficient. The void coefficient is a measurement of how the reactor responds to increased steam formation in the water coolant. Most other reactor designs produce less energy as they get hotter, because if the coolant contains steam bubbles, fewer neutrons are slowed down. Faster neutrons are less likely to split uranium atoms, so the reactor produces less power. Chernobyl's RBMK reactor, however, used solid graphite as a neutron moderator to slow down the neutrons, and neutron-absorbing light water to cool the core. Thus neutrons are slowed down even if steam bubbles form in the water. Furthermore, because steam absorbs neutrons much less readily than water, increasing an RBMK reactor's temperature means that more neutrons are able to split uranium atoms, increasing the reactor's power output. This makes the RBMK design very unstable at low power levels, and prone to suddenly increasing energy production to dangerous level if the temperature rises. This was counter-intuitive and unknown to the crew.
• A more significant flaw was in the design of the control rods that are inserted into the reactor to slow down the reaction. In the RBMK reactor design, the control rod end tips were made of graphite and the extenders (the end areas of the control rods above the end tips, measuring 1-metre (3 ft) in length) were hollow and filled with water, while the rest of the rod – the truly functional part which absorbs the neutrons and thereby halts the reaction – was made of boron carbide. With this design, when the rods are initially inserted into the reactor, the graphite ends displace some coolant. This greatly increases the rate of the fission reaction, since graphite is a more potent neutron moderator (a material that enables a nuclear reaction) and also absorbs far fewer neutrons than the boiling light water. Thus for the first few seconds of control rod activation, reactor power output is increased, rather than reduced as desired. This behavior is counter-intuitive and was not known to the reactor operators.
• The water channels run through the core vertically, meaning that the water's temperature increases as it moves up and thus creates a temperature gradient in the core. This effect is exacerbated if the top portion turns completely to steam, since the topmost part of the core is no longer being properly cooled and reactivity greatly increases. (By contrast, the CANDU reactor's water channels run through the core horizontally, with water flowing in opposite directions among adjacent channels. Hence, the core has a much more even temperature distribution.)
• To reduce costs, and because of its large size, the reactor had been constructed with only partial containment. This allowed the radioactive contaminants to escape into the atmosphere after the steam explosion burst the primary pressure vessel.
• The reactor also had been running for over one year, and was storing fission byproducts; these byproducts pushed the reactor towards disaster.
• As the reactor heated up, design flaws caused the reactor vessel to warp and break up, making further insertion of control rods impossible as the heat deformed them.

Both commissions were heavily lobbied by different groups, including the reactor's designers, power plant personnel, and by the Soviet and Ukrainian governments. The IAEA's 1986 analysis attributed the main cause of the accident to the operators' actions. But in January 1993, the IAEA issued a revised analysis, attributing the main cause to the reactor's design.

A variant theory holds that the operators were not informed about problems with the reactor. According to Anatoliy Dyatlov, the designers knew that the reactor was dangerous in some conditions but intentionally concealed this information. In addition, the plant's management was largely composed of non-RBMK-qualified personnel: the director, V.P. Bryukhanov, had experience and training in a coal-fired power plant. His chief engineer, Nikolai Fomin, also came from a conventional power plant. Dyatlov, deputy chief engineer of reactors 3 and 4, had only "some experience with small nuclear reactors", namely smaller versions of the VVER nuclear reactors that were designed for the Soviet Navy's nuclear submarines.

The effects of the disaster

 International spread of radioactivity

 

A monument to the victims of the Chernobyl disaster at Moscow's Mitino cemetery, where some of the firefighters who battled the flames and later died of radiation exposure are buried.

The nuclear meltdown provoked a radioactive cloud that floated not over just the modern states of Russia, Belarus, Ukraine and Moldova, but also Turkish Thrace, Macedonia, Croatia, Bulgaria, Greece, Romania, Lithuania, Estonia, Latvia, Finland, Denmark, Norway, Sweden, Austria, Hungary, the Czech Republic and the Slovak Republic, The Netherlands, Belgium, Slovenia, Poland, Switzerland, Germany, Italy, Ireland, France (including Corsica^[18]) the United Kingdom and the Isle of Man.

The initial evidence that a major exhaust of radioactive material was affecting other countries came not from Soviet sources, but from Sweden, where on April 27 workers at the Forsmark Nuclear Power Plant (approximately 1,100 km (684 mi) from the Chernobyl site) were found to have radioactive particles on their clothes. It was Sweden's search for the source of radioactivity, after they had determined there was no leak at the Swedish plant, which led to the first hint of a serious nuclear problem in the western Soviet Union. The rise of radiation levels had at that time already been measured in Finland, but a civil service strike delayed the response and publication.

Contamination from the Chernobyl accident was scattered irregularly depending on weather conditions. Reports from Soviet and Western scientists indicate that Belarus received about 60% of the contamination that fell on the former Soviet Union. However, the 2006 TORCH report stated that half of the volatile particles had landed outside Ukraine, Belarus and Russia. A large area in Russia south of Bryansk was also contaminated, as were parts of northwestern Ukraine. Studies in countries around the area say that over one million people could have been affected by radiation.

In Western Europe, measures were taken including seemingly arbitrary regulations pertaining to the legality of importation of certain foods but not others. In France some officials stated that the Chernobyl accident had no adverse effects.

Radioactive re