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Chernobyl Meltdown

On 26 April 1986 one of the reactors at the Chernobyl Nuclear Power Plant in northern Ukraine suffered a cooling failure and produced a steam explosion, spreading nuclear radiation around the world.

Spread of radiation

The radiation cloud from Chernobyl spread around the world: "Red indicates the extent of the radiation cloud on 27 April, just after the accident in Chernobyl. Blue, indicates its almost worldwide distribution until 6 May."

One person's colloquial (and possibly inaccurate) summary of the events

They were going to change the fuel rods in the #3 reactor. Of course to do that you have to power down the reactor. So as they were reducing the power in the reactor they decided to do some testing to see how "slow" they could get it to run without stopping the reaction altogether.

So they started slowing down the reactor, which reduces power output, and the reactor automatic control systems are like "hey, the reactor is going too slow" so it tried to bring up the power. The controllers were like "Hm. The computer is interfering with our experiment" so they crawled behind the panel and disconnected the safety system.

So they're slowing down the reactor again, which reduces the power output, and the reactor starts cooling off a little. So the other reactor safety system reduces coolant flow to the core, thinking "hey, the core is too cool, so that must mean there is too much water in there." So the water flow goes down, and the reactor warms up again and the power output goes back up again. So the guys disconnect that automatic system too and again reduce power.

So now the reactor is slowing down, water flow is being reduced, which reduces water *pressure* in the core... as the pressure drops, suddenly it flashes to steam, which is much less able to transport heat out of the core, so the core starts heating up, and fast. But the automatic water system is disconnected now, so the heat is going up and the reaction rate is going up, which makes the heat go up faster. The controllers go "oh s**t" [but presumeably in Ukrainian or Russion] and hit the emergency control-rod release, which should allow the control rods to free-fall into the reactor core and stop the reaction. Except that due to the now very high heat the fuel rods have expanded and the control rods only get in a little ways before they get stuck.

So the heat is going up and up, and the middle of the core starts to melt and drip into the bottom of the reactor vessel, which causes some chemical reaction and hydrogen starts being created from other molecules that are there. Lots of it. Meanwhile the controllers hit the "emergency core flood" button, which is supposed to allow thousands of gallons of water to flow into the reactor vessel... except that the high pressure in the vessel basically keeps that water out. [I also heard that the low power output of the reactor meant there wasn't enough electricity to run the water pumps.] Some kind of spark in the vessel ignites the hydrogen and explodes the vessel, blowing the roof off the reactor building and the rest is history.

The first part of this experiment lasted a few hours... the runaway part only took about 30 seconds from the guys going "hmm... something's not quite right" to the explosion.

A timeline of the Chernobyl events

25 April 1986, events leading to the Chernobyl accident (when explosion and fire occurred in a graphite core reactor) began as part of a test procedure.

01:06, the scheduled shutdown of the reactor started. Gradual lowering of the power level began.
03:47, lowering of reactor power halted at 1600 MW(t).
14:00, the emergency core cooling system (ECCS) was isolated (part of the test procedure) to prevent it from interrupting the test later.
4:00, the reactor's power was due to be lowered further; however, the controller of the electricity grid in Kiev requested the reactor operator to keep supplying electricity to meet demand. Consequently, the reactor power level was maintained at 1600 MW(t), and the experiment was delayed.
23:10, power reduction recommenced.
24:00, shift change.

26 April 1986, preparation for the test resumed.

00:05, power level had been decreased to 720 MW(t) and continued to be reduced. It is now recognised that the safe operating level for a pre-accident configuration RBMK reactor was about 700 MW(t) because of the positive void coefficient.
00:28, power level was now 500 MW(t). Control was transferred from the local to the automatic regulating system. Either the operator failed to give the "hold power at required level" signal or the regulating system failed to respond to this signal. This led to an unexpected fall in power, which rapidly dropped to 30 MW(t).
00:32 (approximate time), in response to the drop in power, the operator retracted a number of control rods in an attempt to restore the power level. Station safety procedures required that approval of the chief engineer be obtained to operate the reactor with fewer than the effective equivalent of 26 control rods. It is estimated that there were less than this number remaining in the reactor at this time.
01:00, the reactor power had risen to 200 MW(t).
01:03, an additional pump was switched into the left hand cooling circuit in order to increase the water flow to the core (part of the test procedure).
01:07, an additional pump was switched into the right hand cooling circuit (part of the test procedure). Operation of additional pumps removed heat from the core more quickly, which reduced the water level in the steam separator as more steam was generated.
01:15, automatic trip systems to the steam separator were deactivated by the operator to permit continued operation of the reactor in this mode.
01:18, the operator increased feed water flow to the cooling loop in an attempt to address the low water level problems in the cooling system.
01:19, some manual control rods were withdrawn to increase power and raise the temperature and pressure in the steam separator. Operating policy required that a minimum effective equivalent of 15 manual control rods be inserted in the reactor at all times. At this point it is likely that the number of manual rods was reduced to less than this (probably eight). However, automatic control rods were in place, thereby increasing the total number.
01:21:40, feed water flow rate reduced to below normal by the operator to stabilize steam separator water level, decreasing heat removal from the core.
01:22:10, spontaneous generation of steam in the core began as heat was not removed from the core fast enough.
01:22:45, indications received by the operator, although abnormal, gave the appearance that the reactor was still stable.
01:23:04, the test began. Turbine feed valves were closed to start turbine coasting. This was the beginning of the actual test.
01:23:10, automatic control rods were withdrawn from the core. An approximately 10 second withdrawal was the normal response to compensate for a decrease in the reactivity following the closing of the turbine feed valves. Usually this decrease is caused by an increase in pressure in the cooling system and a consequent decrease in the quantity of steam in the core. The expected decrease in steam quantity did not occur due to reduced feedwater to the core.
01:23:21, steam generation increased to a point where, owing to the reactor's positive void coefficient, a further increase of steam generation would lead to a rapid increase in power.
01:23:35, steam in the core began to increase uncontrollably.
01:23:40, the emergency button (AZ-5) was pressed by the operator. Control rods started to enter the core, but the insertion of the rods from the top concentrated all of the reactivity in the bottom of the core.
01:23:44, reactor power rose to a peak of about 100 times the design value.
01:23:45, fuel pellets started to shatter, reacting with the cooling water to produce a pulse of high pressure in the fuel channels.
01:23:49, fuel channels ruptured.
01:24, two explosions occurred. One was a steam explosion; the other resulted from the expansion of fuel vapour.

And that's about it for the event itself. But the results didn't end there.

How bad was it?

Some quotes (in italics) from NZ Herald articles.

John Roughan: Chernobyl was not that bad after all [broken link; it still (May 2019) turns up as a search result but that link doesn't work either] (opinion column) – We drove to the town of Pripyat just 3km from ground zero. It was deserted. Homes were closed up, lawns were long and grass grew through the cracks in concrete paths.
I was warming to my story and asked the guide how many had died here.
"I don't think anybody died here," he said.
Right. What was the death toll all told?
About 30, he thought.
He didn't mean 30,000? No, 30.

At best this is disingenuous, and John Roughan deserves contempt for misleading New Zealand Herald readers about the extent of this disaster. His article heading is also simply wrong.

Blair haste fuels fears of waste hot zone [now relinked]Twenty years ago the reaction inside the Soviet power plant at Chernobyl was allowed to race out of control, causing the worst nuclear disaster in history. It is now estimated to have caused 100,000 deaths. British land is still contaminated and children here have cancer as a result of the fallout.

Chernobyl death toll underestimated, says Greenpeace [now relinked]Environmental group Greenpeace said today the eventual death toll from the Chernobyl nuclear disaster could be far higher than official estimates, with up to 93,000 cancer deaths attributable to the accident.
Based on research by the National Academy of Sciences of Belarus, the report said that of the 2 billion people globally affected by the Chernobyl fallout, 270,000 would develop cancers as a result, of which 93,000 would prove fatal.
(Emphasis added.)
Gregory Haertl, a spokesman for Geneva-based WHO [World Health Organisation], said it stood by its figures. He said the predicted eventual number of extra deaths in the hardest-hit areas of Ukraine, Belarus and Russia was estimated to be 4000.
The Greenpeace report said that a further 200,000 people in Russia, Ukraine and Belarus could have died as a result of medical conditions – such as cardiovascular diseases – attributable to the disaster, but that there was no accepted methodology to calculate deaths from such diseases.

The report said the incidence of cancer in Belarus had jumped 40 per cent between 1990 and 2000, with children not yet born at the time of the disaster showing an 88.5-fold increase in thyroid cancers.
The relocation of hundreds of thousands of people has put further strains on the population.
"The Chernobyl accident disrupted whole societies in Belarus, Ukraine and Russia," Greenpeace concluded.

Chernobyl boss says true cause of disaster hidden [now relinked] – Former Chernobyl director Viktor Bryukhanov said official investigations into the cause of the disaster had been a whitewash designed to exonerate the nuclear industry.

A more recent (2019) article, Chernobyl: You've seen the TV series, now understand the awful reality refers to a 2009 study which "covers more than 5000 medical and epidemiological papers from the Ukraine, Russia, Europe and Britain. ... concludes some 985,000 people died prematurely, mainly of cancer, as a result of the Chernobyl accident." The article mentions some of the cancers people have been getting since the disaster.

Could it happen again?

Yes. Indeed, something similar has happened again. From Wikipedia:

Fukushima disaster cleanupThe Fukushima disaster cleanup is an ongoing attempt to limit radioactive contamination from the three nuclear reactors involved in the Fukushima Daiichi nuclear disaster which followed the earthquake and tsunami on 11 March 2011. The affected reactors were adjacent to one another and accident management was made much more difficult because of the number of simultaneous hazards concentrated in a small area. Failure of emergency power following the tsunami resulted in loss of coolant from each reactor, hydrogen explosions damaging the reactor buildings, and water draining from open-air spent fuel pools. Plant workers were put in the position of trying to cope simultaneously with core meltdowns at three reactors and exposed fuel pools at three units.


No strontium was released into the area from the accident; however, in September 2013 it was reported that the level of strontium-90 detected in a drainage ditch located near a water storage tank from which around 300 tons of water was found to have leaked was believed to have exceeded the threshold set by the government. Decommissioning the plant is estimated to cost tens of billions of dollars and last 30–40 years. Initial fears that contamination of the soil was deep have been reduced with the knowledge that current crops are safe for human consumption and the contamination of the soil was not serious; however, in July and August 2013, it was discovered that radioactive groundwater has been leaking into the sea.


Japan's economy, trade, and industry ministry recently (as of 2016) estimated the total cost of dealing with the Fukushima disaster at ¥21.5 trillion (US$187 billion), almost twice the previous estimate of ¥11 trillion (US$96 billion). A rise in compensation for victims of the disaster from ¥5.4 trillion (US$47 billion) to ¥7.9 trillion (US$69 billion) was expected, with decontamination costs estimated to rise from ¥2.5 trillion (US$22 billion) to ¥4 trillion (US$35 billion), costs for interim storage of radioactive material to increase from ¥1.1 trillion (US$10 billion) to ¥1.6 trillion (US$14 billion), and costs of decomissioning reactors to increase from ¥2 trillion (US$17 billion) to ¥8 trillion (US$69 billion).

Yes. Russian authorities have not learned anything about openness from the Soviet-era Chernobyl disaster in Ukraine, and are still not open about incidents. A 2017 incident released 100 times the amount of radiation into the atmosphere that the Fukushima disaster did.

An international team of researchers has traced an unusual 2017 radioactive release that blanketed a large part of Europe to Russia.


Russian authorities had repeatedly denied responsibility for the release of the Ruthenium-106 isotopes and the delay in identifying the suspected origin site has robbed scientists of crucial evidence it would need to help prevent another massive leak.


Steinhauser and others said that Russia's refusal to take responsibility for the nuclear release remains troublesome, even if nobody was harmed. Scientists rely on rapid evidence gathering after such an event to help formulate plans that could prevent a similar occurrence elsewhere.


The 2017 incident has triggered memories of the Chernobyl nuclear accident in Ukraine – at the time part of the Soviet Union – in 1986. Soviet officials initially remained silent.

Another article highlights the covering-up that Russia still clearly engages in, and outlines what probably happened to cause a leak.

A senior official at the Mayak Chemical Combine has taken a small step toward admitting that a radioactive isotope discovered over Europe last month could have come from his facility – something Russian state nuclear corporation Rosatom continues to deny.


Russia’s federal weather service, Rosgdormet, subsequently confirmed – and quickly downplayed – that it had found levels of ruthenium measuring nearly 1,000 times normal in a village neighboring the nuclear facility.

Both Rosatom and the administration of Vladimir Putin have repeatedly denied that the ruthenium came from Mayak, and Rosatom has endorsed a conspiracy theory that the radioactive pollution measured by the Europeans came from a western government’s spy satellite crashing to earth.


Bellona’s general director and nuclear physicist Nils Bøhmer has suggested that the most likely cause for the ruthenium leak was a filtration error at an oven Mayak uses to bake nuclear waste into glass – a process called vitrification, which is a routine procedure that Mayak regularly performs.

During vitrification, ruthenium 106 can become volatile and change form, and these changes determine the kind of filter that’s required. Without the right kind of filter, gaseous ruthenium can end up in the atmosphere – which is exactly where the Europeans found it.

It would appear that Rosatom found it there too: On November 29, three weeks after the ruthenium was detected in Europe [actually about eight weeks after it was first detected in Europe], the corporation quietly announced a tender for contractors willing to clean up after a radiation incident at Mayak’s vitrification plant.


In the coming days, it’s conceivable that Mokrov will find himself chastised like Rosgidromet, but until then the ruthenium business follows the time-honed Soviet handbook of dealing with radiation accidents – blanket denial, followed by a kaleidoscope of conspiracy theories blaming the West, preceding accidental admissions of the truth, all counting down to the big humiliating reveal.

It bears recalling that it took the Soviet government two weeks and four days to admit that the high radiation levels western countries were picking up were coming from an exploded reactor at Chernobyl.

A third article gets into some of the details about how Russia's "conspiracy theory" explanation of the leak being caused by a spy satellite is wrong.

On July 26, a large group of experts led by Olivier Masson from the Institute of Radioprotection and Nuclear Safety in Durance, France, and Georg Steinhauser of the Institute of Radioecology and Radiation Protection in Hanover, Germany, published a paper definitively attributing a radiation cloud that spread over Europe in 2017 to an undeclared accident in the Southern Urals region of Russia, likely at the Mayak nuclear facility near Chelyabinsk. Two years ago, Russia promised to investigate where the cloud could have come from, but a commission formed on Russia’s initiative failed to produce a clear conclusion. At the same time, Mayak and Rosatom, the state-owned nuclear power company to which it belongs, have denied that any accident took place.

In late September 2017, several Russian regions and then a number of European countries detected high concentrations of radioactive ruthenium-106 in the atmosphere. As the contamination traveled, more countries noticed it and published reports. The first Russian denials, from the authorities of the Sverdlovsk and Chelyabinsk regions in the Urals, came a week later. Even after the Russian state weather service Roshydromet reported ruthenium-106 pollution in the South Urals, a Rosatom-led commission insisted the contamination wasn’t the result of an accident and suggested that it could have been caused by a defunct satellite burning up in the atmosphere.

The Masson-Steinhauser group ruled out the satellite theory: Such an incident would have caused higher concentrations of ruthenium at high altitudes, which was not the case. Instead, the researchers argued that only a nuclear reprocessing facility could have caused this particular type of contamination. A number of Western and Russian facilities had been responsible for ruthenium-106 releases in the past. By analyzing the weather patterns and ruthenium measurements, including those from Roshydromet, the international team placed the source close to the location of Mayak, the erstwhile scene of a major nuclear disaster – the 1957 “Kyshtym Accident” that forced the evacuation of 10,000 people in the area and caused long-term consequences for residents’ health.


... the Masson-Steinhauser paper discusses a possible link between the accident and Mayak’s work on a cerium-144 source for an Italian neutrino experiment; Mayak canceled the order soon after the ruthenium release. But the headline carefully avoids mentioning Mayak or Russia.

The right thing for Mayak and Rosatom to do would be to share with international experts any data on that work, including, of course, whatever could have led to the ruthenium release.

Yes. Russian authorities have not learned that reducing risk is not the same as completely eliminating risk. They have just launched a floating nuclear power station.

Rosatom argues the floating station has higher safety standards than land-based plants and says any allusion to Chernobyl is like "comparing a 100-year-old automobile to one today".

And yet we still have fatal car crashes. The consequences of nuclear accidents can be far more serious, with effectively permanent consequences due to the very long half-lives of radioactive contaminants.

What should we conclude?

Nuclear energy is not a clean energy source.

  • Nuclear reactors do not produce carbon dioxide (or pollutants such as sulphur dioxide etc), but this does not mean nuclear power is clean.
  • Nuclear reactors produce radioactive decay products that are dangerously harmful for tens of thousands of years. These are products that trees cannot convert into wood like they do with carbon dioxide.
  • The radioactive decay products require safe storage for tens of thousands of years.
  • We struggle to maintain safe storage of radioactive decay products for tens of years.
    • A report on Stuff.co.nz says a nuclear waste storage site on Runit Island in the South Pacific was capped with 45 cm concrete in 1980, and indicates it may have been leaking after just 33 years (emphasis added):

    According to a 2017 report by the Australian Broadcasting Corporation, among the fallout material was plutonium-239, an isotope that is one of the world's most toxic substances, and one with a radioactive half-life of 24,100 years.


    According to the Guardian, a 2013 report by the Energy Department admitted radioactive material may have already begun to leak from the dome, but cautioned the health risks were likely low.

Nuclear power is not a cheap way to produce electrical energy in the long term.

  • A cost analysis of producing a nuclear power plant can be made to show that on a large scale, electricity can be generated cost effectively, but these costings fail to take into consideration the costs of storage of nuclear waste, and costs involved with containment and cleanup of accidents (which can be global in their scope).
  • The costings of course fail to consider the non-financial impact of accidents.

Nuclear power is non renewable.

  • Uranium ore has to be mined, and is not continually being created or replenished.
  • Mining has a serious effect on the environment, especially once easily accessible uranium ore is mined out.

Accidents at nuclear power stations do not simply go away.

  • Accidents at nuclear power stations can spread radioactive contamination over extremely widespread areas. The image at the top of this page shows radiation from the Chernobyl meltdown travelled around the world in just a day.
  • The isotopes in the radioactive contamination can have half-lives in the tens of thousands of years (plutonium-239 has a half-life over 24,000 years).
  • The most prevalent isotope in both the above incidents is now caesium-137 with a half life of 30 years. It will be at dangerous levels for hundreds of years. Because of the Cs-137 spread around Europe, 26 years after the Chernobyl meltdown some reindeer and sheep in Scandinavia were over the legal limit for radioactivity.
  • Children are the worst affected. Cancer rates and birth defects are increased for multiple generations of huge numbers of people over large areas.
  • When the chance of an accident occuring is very low but the consequences are serious, we take precautions to protect ourselves. When travelling in a car we wear a seatbelt. When cycling we wear a helmet. We do these things not because we're masochists who like being less comfortable, or simply to obey a law. We do it because doing these simple things could prevent our death, permanent brain injury, or our eyes being smashed through a jagged broken glass windscreen in the very unlikely event that something goes very wrong while travelling.
  • The consequences of a nuclear disaster are very much more serious than car crashes and last for inconceivably longer, effectively meaning permanent radioactive contamination.

It would be extremely naive to think that further serious accidents will not occur.

  • Saying "nuclear power stations are better designed now, so accidents will not happen" makes as much sense as saying that aeroplanes are much better designed now, so we won't have plane crashes. We still have plane crashes.
  • Aeroplanes these days are more fuel efficient, more reliable, quieter, and more comfortable. They are designed to be very safe, so plane crashes generally happen when multiple things go wrong at once. Multiple things do go wrong at once, and not just with aeroplanes.
  • Cars are much better designed now. We still have road fatalities.
  • The Chernobyl and Fukushima nuclear disasters described above are not the only accidents which have happened with nuclear reactors. See International Nuclear Event Scale for a list of numerous other nuclear accidents that have occurred. It includes three incidents which occurred as recently as 2017 and 2018 but does not include an iodine-131 cloud that drifted south over Europe in January 2017, or a ruthenium-106 cloud which originated in Russia in late September 2017 and drifted across Europe in the first half of October 2017.
  • Some countries have recognised the likelihood of problems, and have started reducing their dependence on nuclear power. From en.wikipedia.org/wiki/Nuclear_power_phase-out:

Following the March 2011 Fukushima nuclear disaster, Germany has permanently shut down eight of its 17 reactors and pledged to close the rest by the end of 2022. Italy voted overwhelmingly to keep their country non-nuclear. Switzerland and Spain have banned the construction of new reactors. Japan’s prime minister has called for a dramatic reduction in Japan’s reliance on nuclear power. Taiwan’s president did the same. ...

As of 2016, countries including Australia, Austria, Denmark, Greece, Ireland, Italy, Latvia, Liechtenstein, Luxembourg, Malaysia, Malta, New Zealand, Norway, Philippines, and Portugal have no nuclear power stations and remain opposed to nuclear power. Belgium, Germany, Spain and Switzerland are phasing-out nuclear power.