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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 April 27, just after the accident in Chernobyl. Blue, indicates its almost worldwide distribution until the 6th of 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.

How bad was it? Could it happen again?

Some quotes (in italics) from NZ Herald articles.

John Roughan: Chernobyl was not that bad after all [broken link] (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.

Blair haste fuels fears of waste hot zone [broken link]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 [broken link]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 [broken link] – Former Chernobyl director Viktor Bryukhanov said official investigations into the cause of the disaster had been a whitewash designed to exonerate the nuclear industry.

 

Yes, 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).