Joseph Rey Tweets

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Josep Rey, who describes himself on Twitter as "@JosepRey_ Estudiante de ingeniería eléctrica especializándome en sistemas eléctricos de potencia, nuclear y renovables. Escritor de varios artículos de divulgación."


The Chernobyl accident was the biggest nuclear accident in history, and one of the biggest industrial accidents. In this thread I Will explain the most relevant aspects of it, such as the actions that led to the accident, its consequences and context.

The idea is inspired with “EVERYTHING about radioactive waste” written by my mentor @OperadorNuclear. The intention is that Twitter has a referenced information source about the accident, increasing the value of this social network.

At the same time, I have written this thread into an article with all references and texts that I have written into this thread, so that its reading becomes less tedious.

Link: (Not available yet)

Due to the length of the thread, it will be divided into mini internal threads where certain topics will be covered, or that the reader may directly go to parts of its interest.


This thread is not made to read it all at once in such platform (Twitter), but with the intention that the interested reader will be able to directly go to the chapter of his interest. Some chapters are linked between them and it is important to read them.

For example, it is more difficult to understand the chapter “health consequences” without previously reading “radioactive contamination” and “dose” chapters.

References from each individual chapter will be at the bottom of it. All tweets will be uploaded with a picture or reference, to make the reading more light and to provide the direct reference to some statements.

At the “REFERENCES” chapter there will be links to my Google Drive, with all PDF documents adequately highlighted for an easier and faster read.

Tweets written in capital letters will be the title of different sections. To go back to the main thread (index) there will be a tweet at the end of the chapter, following the references.

THANKS TO Special thanks to @OperadorNuclear and @maesetote, as they have given me some of the references to make this thread possible by giving me enough information regarding the accident.


The Chernobyl accident had its origins in the philosophy of the Soviet Union around (but not only) the nuclear industry: Savings in construction costs, low quality of materials, secrecy, constant denial of the industry’s problems...

Chernobyl - Rey - History - 1.jpg flaws, lack of regulation and normative, failures in proceedings, lack of a safety culture and a long list ended up causing the worst accident in the nuclear industry.

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After startup of the first three nuclear power plants in the Soviet Union in the fifties, more development was halted as there was plenty of fossil fuels. But, this amount was overestimated and in the sixties a nuclear program started.

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The US, France and England already had a reactor online, and the soviets decided to design their own PWR (VVER) of domestic use. Because there was a hurry, they looked after the cheapest design, being able to be built in many units with the same investment.

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The intention of the soviets was to supply about 40-45% if their energy needs with nuclear energy by the end of the eighties, closing their fuel cycle with breeder reactors.

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In fact, electricity production was just one of many uses of nuclear technology

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Anyways, there was a problem: The soviet metallurgical industry couldn’t supply so many components as was demanded by nuclear plants build (vessels, steam generators…), so it was decided to design another type of reactor (BWR), cheaper to build.

Not only the industry couldn’t supply the necessary components, even some of the components were used without knowing their technical specifications.

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This way, the RBMK appeared. A graphite moderated, water cooled, boiling water reactor, based on pressure tubes that could be designed as powerful as the costumer wanted it to, as there was no pressure vessel, there was no physical limitation when installing high capacities.

It could be built in 1000, 1500 or even 2400 MWe blocks. The fact that it could be reloaded while in operation gave it even more interest in the industry.

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This design was the product of the soviet union’s philosophy of building fast, cheap and high capacity reactors to cover their energy needs, leaving nuclear safety aside. The lack of a containment building in 28 reactors built in the USSR is an example of many.

If western safety standards had been applied in the first place, the RBMK reactor would have never appeared in the first place. Its construction would simply make the containment building very expensive and difficult to build. Security issues aside.

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Other examples are poor construction work standards, such as low quality welds, inadequate terrains, flawed instrumentation, plans that did not correspond with reality, etcetera.

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Kola NPP operated with low quality welds at the reactor feed water lines. Reason is that the worker was rewarded for its welding speed, and not for its quality.

This USSR philosophy regarding construction works was held intact until Finland bought two of their reactors, but on one condition: They had to have a containment building equipped with them. These reactors ended up with higher operation results than the soviets.

Some people questioned nuclear safety. An example is that safety systems can be manually deactivated, without any safety system as backup. In our western plants not only is this forbidden, but impossible.

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RBMK reactors are unstable by design, and the lack of experience in their operation (exclusively domestic experience), added up to the lack of simulators where operators would learn to differentiate normal from abnormal situations, would prepare the formula for a disaster.


The KGB was who was in charge of some of the construction Works at Chernobyl, so their reports present a high grade of reliability.
History of nuclear energy in the USSR


A few months after the startup of the first RBMK, in Leningrad-1, design flaws in control and stability of the reactor were evident, making its operation very difficult. Changes were put in place that would help solve the problem, but the operator had to be permanently alert.

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In 1975, a year after starting its commercial operation, there was an accident that melted a fuel channel, and also damaged other 30 pressure tubes. The reason was a combination of reactor instability and negligence linked to a poor safety culture.

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In 1982, at Chernobyl’s unit 1 reactor a similar accident to the one at Leningrad 1 in 1975 occurred. After some maintenance works, the reactor was started up while water flows to some fuel channels of the reactor were still being adjusted.

Similar to the 1986 accident, flow of water was cut to one of the channels, and with a lack of cooling, they had a local power surge. Fuel ended up breaking the pressure tube. A mix of steam, water and fuel fragments entered the graphite cavity and washed part of it.

Operators did not stop the reactor until 20 to 30 minutes after knowing that something was wrong. They didn’t consider it necessary until they knew what was going on. Repair works would take one week, then one month. Finally, that part would be permanently out of service.

The accident was not made public, although the consequences were wide: Places at 15 km were contaminated, and Iodine levels in a liter of air reached 444 GBq per litre.

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In 1983, Iganlina (In Lituania) was started up. This plant featured a more powerful version of the reactor, a RBMK-1500. The same year of its startup, an unknown design flaw was discovered, involving the emergency shutdown system.

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Under some circumstances (see: REACTOR), an emergency shutdown could make the control rods produce a power surge during the first moments of insertion, and not a decrease in power, as it was designed.

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The only reason why the reactor at Ignalina didn’t blow up was that control rods did not get stuck, and thus could reach the bottom of the reactor at their end position while the reactor continued to be cooled.

When the design was under review, it was concluded that it was very difficult to meet all requirements to such a power surge to occur, and, that if proceedings were followed, such requirements would never be met. Time proved they were wrong.

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The study of the rest of accidents in other RBMK plants show proof that the Chernobyl accident in unit 4 was a combination of both accidents at Lenignrad-1 (repeated in Chernobyl-1) and the near accident at Ignalina-1.

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Nothing was done to correct this and many other design flaws until 1988, more than a year after the Chernobyl accident, that could have been prevented if upgrades had been applied after the events in Leningrad and Ignalina.

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The soviet union’s secrecy led to a complete trust with technology. Operators and personnel from nuclear plants couldn’t learn from other’s mistakes nor own mistakes even among units from the same plant.

Internally, as serious accidents in all reactors were covered, led to acknowledge that they were infallible and couldn’t ever fail. “Nothing had ever gone wrong, nothing can ever go wrong”. This ended up giving a sense of total security.

It wasn’t until 1988 that an upgrade was tested to prevent power surges, among many other upgrades that would significantly increase reactor safety.

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It is important to point out that this reactor was designed by two different departments, with little communication between them. One was the department of energy, while the other one was a military department.

The latter means that, if a design flaw was found, if its correction put difficulties in the military use of the reactor, it wouldn’t be recognized as a design flaw, but as a normal feature, and the design just continued.


The KGB was who was in charge of some of the construction Works at Chernobyl, so their reports present a high grade of reliability.


In 1970 Pripyat started to be built, a city that would house workers from the Chernobyl nuclear complex, that was going to start to be built in 1972. The chosen design would be graphite moderated boiling water reactors.

Chernobyl was built in blocks of two, commissioning the first two in 1977 and 1978. Blocks three and four would be commissioned in 1981 and 1983 respectively. Two more blocks were being built separated from the other four. They would never get finished.

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In 1971, a design flaw was found in the design of the emergency core cooling system, that would prevent enough water flow going into the reactor if a pipe break was equivalent to a circumferential break of 300 mm of diameter in the primary circuit.

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This deficiency is insufficient in a 39%, and although it was notified to the Hydroproekt institute (where the reactor was designed), units one and two were finished without any correcting measures taken.

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Construction continued with units 3 and 4, commissioned in 1981 and 1983 respectively, without either correcting the problem. With blocks 5 and 6 there neither was intention of correcting it.

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Units 3 and 4, considered 2nd generation RBMK, had improvements respect units 1 and 2 due to experiences in Leningrad and Kursk. The most evident change was the design of the reactor building.

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In 1979, reports regarding deviation and violations of norms were found when building unit 2, and similar reports appeared involving unit 1. Both units would already be in operation when such reports were made.

In 1984, a report from the KGB affirmed existing problems at the Chernobyl nuclear power plant. A document specifies there are deficiencies in units 3 and 4, some of them, involving a low quality instrumentation.

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In February 1986, an Ukrainian energy minister commented “The possibilities of an accident are one in ten thousand”. A month after this, an engineer from the power plant stated that a ticking bomb is being created “whose consequences will be payed for decades”.

The 26th of the next month, the disaster would occur.

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The reactor design is very different to the rest of reactors already in operation, particularly for its height, necessary to equip the fuel loading machine. The kind of neutron moderator and the absence of a reactor vessel made it distinct from the rest.

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The RBMK was allocated on the ground, with the heads of the pressure tubes (some containing fuel, some containing control rods) of the reactor. Coolant circulated individually through the pressurized tubes, producing steam.

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Another particularity was the online refueling capability that the reactor had as a feature, being able to continually supply electricity onto the grid, although the process was complex and required of specialized machinery.

The demonstration of RBMK refuelling process

The operating principle of a soviet RBMK is similar to the one in boiling water reactors, where water is allowed to boil in the reactor circuit, to then directly go to the turbine building. This steam is then condensed and put back into the reactor.

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Safety systems are similar to those in BWR’s, based on an “Emergency Core Cooling System”, but with some deficiencies that made it insufficient under certain circumstances, that would later end up producing an accident.

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During the first construction stages of units 1 and 2 of Chernobyl and Leningrad, deficiencies in the ECCS were found. More precisely, when there was a pipe break equivalent to a break of 300 mm in diameter.

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This problem persisted 10 years later,with reactors 5 and 6 at Chernobyl under construction and without having taken any preventive measures in reactors 1, 2, 3 and 4 already in operation, plus other 13 groups distributed in Kursk (4), Smolensk (3), Leningrad (4) and Ignalina (2)

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The neutron moderator, whose purpose is reducing neutrons’ speed after coming from the fission reaction so that the reaction can continue, was made of graphite blocks. A total of 2400 tones of graphite were used for each unit.

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A particularity from graphite, is that it has a great thermal inertia. Contrary to water, the former absorbs less neutrons and moderates (reduce their speed) them at lesser ratios, thus increasing neutron flux and using a more light enrichment. (2%).

A disadvantage of graphite is the room it needs, thus creating the need for a larger reactor than a light water reactor for the same rated power. Not to mention that graphite is a combustible material.

Reactor’s armor consists on an upper and lower biological shield, protecting people around the reactor. Reactor boundaries were made of concrete, sand and water.

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The RBMK has a total of three different models, with similar features among two of them. Two were built, while the third one didn’t go further than a theoretical design.

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Furthermore to models, there were also different reactor generations (do not confuse with generation of nuclear reactors). 2nd generation reactors had improvements in safety compared to the first, while 3rd generation included upgrades after the Chernobyl accident.

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1st and 2nd generation RBMKs were upgraded to reach the same level of safety as the 3rd generation units, after the Chernobyl accident to prevent it from happening again.

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When the first reactor of this kind started up, problems were found in operating the reactor regarding power control, making it necessary that operators continuously check values and correct deviations.

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When too much water boils in the reactor, its neutron absorption capacity is lost, while graphite continues moderating it. This causes a net power increase, making the reactor in a inherently unstable one.

The former effect is called “positive void coefficient”, and played an important role at low power outputs, that is the mode at which the reactor in Chernobyl was operating before blowing up.

The void coefficient is very sensible as it relies on the water temperature entering the reactor inlet, as, if it is not properly cooled, can start boiling in the lower parts of the reactor.

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Other problems involving power management are due to the big volume of the reactor, making it difficult to map all neutron fluxes, leaving operators practically blind in the lower yield of both power and zones of the reactor, where sensors did not reach.

To partially solve this problem, operation below 20% of power was forbidden, although operation proceedings stated that it was not recommended, but not forbidden. This leaves the operators the last word regarding operation parameters.

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Power management of the reactor in such a volume was so difficult that the reactor could even operate as the sum of different individual reactors inside of it.

As it was mentioned, the only reason why this reactor appeared was for industrial convenience, as the soviet industry could not supply so many reactor components for VVERs, so it was decided to go for a more simple design.

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The reactor shutdown system consisted on 211 control rods, with an actuation time of 18 seconds starting from the upper position until it reached its total insertion. Besides its slowness, there were flaws at the technical and proceedings level.

In normal conditions, insertion of control rods produces a power decrease at the moment of actuation, until rods are fully inserted (18 seconds) and stop the reactor.

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Due to the fact that control rods are fitted with graphite tips, under certain conditions their insertion can produce the contrary effect and increase power temporarily instead of decreasing it.

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The reason is, that when graphite displaces water (a good neutron absorber and moderator) only the absorption is lost, producing a surge in neutron population and, thus, power. The problem wouldn’t be accepted and corrected until 1988.

This effect was seen for the first time at the first unit of Ignalina in 1983, but they considered it wasn’t necessary to include any modification because the probability of meeting all conditions to make it happen were too small.

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It was alleged that, if reactivity margins were kept at 15 or more fully inserted control rods worth, a margin would be kept. There was no consideration on its physical configuration. After the accident, this number would be increased to 45.

All design flaws mentioned were found to be more severe in more powerful versions, such as the RBMK-1500.



The safety test that would later lead to the worst nuclear accident in mankind’s history, would consist on give enough inertia to the plant’s generator so that after switching it off would still power the pumps to continue cooling the reactor.

The reason to make this test would be to make sure that, in case of an electric failure, reactor would still be cooled while diesel generators started up.

Steam turbines have an inertia coefficient that factories give to the customer as a feature of them, but in the soviet union, turbines were supplied without carrying out any tests before delivery.

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This would mean that it would be necessary to test the inertia experimentally, in a nuclear power plant. Former tests proved that inertia was not enough to keep the pumps running, and no one thought about dealing with the problem from the other side: upgrading diesel generators.

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On 25th April 1986, a power decrease in unit 4 started to set the plant to the necessary conditions for the safety test. But, the test would be delayed until midnight due to high power demand.

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The necessary power needed for the test was from 700 to 1000 thermal MW, a test that had already been successfully done in other RBMKs, so in principle, the test by itself wasn’t dangerous.

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But, at 00:28 AM on 26th April, power output would be of just 30 MW due to a reactor operator mistake. This operator would advice to abort the test to its superior, as at such power levels there weren’t reliable readings of power levels. The advice was ignored.

It is important to mention that operation proceedings weren’t clear, being able to consider as valid notes that were previously crossed. The operators were the ones who would interpret what was or wasn’t correct.

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Nuclear reactors, when operated at low power yields after long periods at high power, get poisoned with Xenon-135 (semi disintegration period of 8 hours), a strong neutron absorber that make it difficult to increase power until it disappears.

Xe-135 poisoning would be another reason to abort the test and stop the reactor. Again, the supervisor would ignore it and go on with the test.

Operators would try to get power back to objective values, but such intentions would be frustrated by a combination of loss of void in the reactor, graphite overcooling and xenon poisoning.

To compensate these effects, more control rods than allowed by proceedings would be extracted (minimum reactivity margin of 15 rods) leaving just 6 of the 211 control rods in the reactor. Operating with less than 30 was forbidden.

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It is important to highlight that operators did not know about the positive reactivity effect of voids in the reactor, nor did about the “positive SCRAM” that could occur under certain circumstances. Finally, they could stabilize power at 200 MW at 01:03 AM.

Knowing that 200 MW was a very low power, operators started two auxiliary pumps to avoid an overheating. Such action was counterproductive: overcooling would reduce the amount of steam that would later turn the turbine.

During previous moments before the test, at the bottom of the reactor there was power being accumulated, but sensors did not register all of it. For the head of the control room, the risk of having an accident was just theorical.

At 1:23:03 AM the test would start closing valves to feed the turbine, feeding with electricity four water feed pumps while Diesel generators started up.

The power surge would concentrate at the bottom of it, where sensors did not reach. Reduction of water flow, added up to an increased temperature at the reactor water inlet, would produce boiling at the bottom, increasing power.

This increase in power would also eliminate the remaining Xenon, producing another power increase. Witnesses saw how the 350 kg pressure tubes heads rose individually, in the reactor hall.

The former would indicate rupture of pressure tubes and release of steam into the reactor’s atmosphere, increasing power – normally inert in He and N –.

Control panels indicated trouble in both power and water flow to the reactor, so at 1:23:40 AM the emergency shutdown button was pressed while power slowly continued to increase.

Operators did not know about the positive effect in power the reactor could have when trying to stop it under these conditions. Three seconds later, power jumped to 530 MW, while continued to increase as boiling occurred.

It is calculated that power could have reached 100 times nominal reactor power.

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The thermal shock produced by the fast overheating of the reactor weakened pressure tubes, making them more fragile and susceptible to breaking down and leak.

The reactor operator tried to disengage control rods from their motor mechanism to let them fall by gravity into the reactor, but they did not move as they got stuck inside the tubes due to dilatation of the channels where they should have gone in.

As tube channels broke, pressure in the reactor cavity increased with no possibility of shutting down the reactor. At 01:24 AM a steam explosion would occur, that would break many of the reactor’s pressure tube heads.

This explosion would allow for Oxygen to get into the reactor’s atmosphere, that would later ignite graphite inside the reactor, producing a massive fire in it and around the reactor after the second explosion.

The second explosion would be produced by the insertion of control rods into the reactor, producing an explosion because of the sudden power surge. This would lift the reactor cover of 2500 kg, destroy the reactor building and eject graphite, fuel and fission products outside.

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The force of the explosion would leave the reactor completely exposed to the atmosphere with a graphite fire inside. The inventory of graphite was of 2500 tones, which, burning at a rate of a ton per hour, if no measures were taken, such fire would last 3 months.

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The reactor explosion would directly kill 2 people, plus other 29 due to thermal burns. One of the two bodies would never be recovered, assuming it was buried under the ruins of the reactor.

Emissions from the reactor would vary through 10 days, with Iodine and noble gases being the first emissions, to then expel solid particles because of the graphite fire.

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Immediately after the accident, firefighters from the nearest stations went to the power plant to put off the fires. They were not told about the radiation danger and went with the usual equipment.

Reactor number three, next to reactor four, stopped its operation as a precaution. Afterwards, a fire from reactor 4 would also fire up the roof of reactor 3. Reactors 1 and 2 continued with normal operation.

After putting off the main fires, the USSR government organized a committee to manage the accident. They were sent to Pripyat, 3 km from the plant with the only information that there was an accident with class I, II, III and IV dangers.

Classification of dangers correspond to an incendiary danger, explosive danger, chemical danger or radiological danger. Experts flew over the power plant nearly a day after the accident, seeing an incandescent red color on the destroyed reactor.

The first task was to find out if the destroyed reactor continued to operate, this is, if the nuclear chain reaction didn’t stop although it was destroyed.

For this task, neutron monitors would be used, but measures would not reveal any accurate data as radioactivity was producing a too high error in measures.

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Finally, samples of the gases expelled by the reactor were taken to confirm absence of fission products with very short half-lives, indicating that the reactor was stopped.

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After all fires were put off, it was criticized that some of the firefighters were kept waiting in the turbine hall, and to receive an additional radiation dose for no reason.

But, its task was very important: if a new fire broke out in the turbine hall and approached the generator, this one could explode as it was cooled by Hydrogen.

On the 26th April night, all attempts to put off the fire in the reactor were useless: Water was filtering through adjacent corridors in buildings, or was causing higher radioactive emissions due to evaporation.

The task of putting off such fire was not easy: the reactor has in its interior 2500 tones of graphite, that burning uniformly at 1500ºC and a rate of one tone an hour, could burn up to 3 months.

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The next morning, 27th April, a new attempt would be tried by dropping Boron from helicopters, a strong neutron absorber.

No one had ever fought against a fire of such characteristics, so it was decided to use materials that absorbed the highest amount of heat possible when changing from one phase to another, so it could be possible to decrease temperature until the fire was suffocated.

The first idea was to use Iron stored in the power plant, but unfortunately it was heavily contaminated as the radioactive cloud flew over all these materials just after the accident.

Another problem regarding Iron is that it wasn’t exactly known if the reactor’s temperature would be high enough to make it melt, and absorb heat from the fire. This material would be discarded.

This material would be the only one to be discarded. Next propositions would be Lead and Dolomite, both of them with a very low melting point. Those materials had the advantage that once they solidified, would create an external radiation armor.

But, it was the possibility that fire’s temperature was above 1500ºC, and as Lead boils from 1600-1700ºC, it could boil and produce further pollution aside from the radioactive one.

Using thermographic cameras, attempts to know the reactor’s temperature were taken in order to make a decision, but as they were made using semiconductors, radiation made the readings fuzzy

Trying to attack the problem from another side, samples of air around the reactor were taken, and by knowing the ratio of CO2 and CO, the temperature of the reactor could be approximately known.

With all data together, it was found out that most parts of the reactor were below 300ºC, with few points reaching 2000ºC. With this, the decision to dump 2400 tons of Lead was taken.

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Dolomite was also dropped with the task of absorbing heat from zones with higher temperatures, so that Magnesium oxide was created and absorbed Oxygen from the fire to suffocate it. Plus, it was a good thermal conductor, making it easier for the heat to distribute.

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Later, sand and clay would also be dropped to cover the reactor and build a protective layer. Finally, other materials would be used to create a layer at the surface of the reactor, creating a film to avoid dust dispersion.

All these decisions, including the one of creating a layer to prevent dust from escaping from the reactor, were taken the night of the 26th, a day before the operations were started. Such plans continued until 2nd May.

That same night, it was discussed what should be done with the population in Pripyat, even that radiation levels were not yet too high, it was clear that the situation was not going to improve in the short term.

At 11:00 PM, 21 hours and half after the accident, it was decided that the evacuation of Pripyat would be taken at 2:00 PM from the following day, 36 hours and a half after the accident.

In just 2 hours and a half, the city was completely deserted, except the essential personnel for the following days operation. It was estimated that the 50 000 evacuated people did not absorb a very high dose, and that their personal objects wouldn’t be too contaminated.

The USSR commission to manage the accident would stay in Pripyat 2 days more, to then be relocated to the city of Chernobyl, 20 km away from the plant. The 2nd may, it was decided to evacuate not only Pripyat, but any population in a 30 km radius from the plant.

The next problem to appear, is that it was suspected that the reactor’s explosion, the fire, and the dropping of materials from high altitudes could have weakened the building’s structure, where just below it there was a suppression pool.

If a part of the melted fuel penetrated this pool, a rapid vaporization of water would cause a new steam explosion, worsening the situation and polluting the zone even more.

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On 9th May, a tunnel from unit 3 was started, to build a concrete base in the suppression pool (previously emptied) with an integrated cooling system, just in case the molten reactor filtered.

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KGB documents: … Chernobyl: Assessment of Radiological And Health Impacts. 2002…