Did Chernobyl divers prevent a multi megaton explosion?

[Jeremy Thomson]

TL;DR Summary

If the molten corium of chernobyl had breached the water tanks, would anything more dramatic than what we see when lava flows reach the ocean happen?

There appears to be much myth about the 'divers' that swam through radioactive water to drain the water underneath reactor 4 at Chernobyl. This History channel link alludes to some of the myth https://www.history.co.uk/article/the-real-story-of-the-chernobyl-divers. It also links to a youtube video where Vasili Nesterenko talks of a possible 3 to 5 megatons explosion if the corium reached the water. The 2nd episode of the Chernobyl min-series has a Belorussian Physicist (a composite character? played by Emily Watson) warning of a 30-40 megaton explosion.

I'm not a physicist I just find the physics of reactors and weapons interesting, I have only high school physics from 40 years ago and what I've learned through Wikipedia and the internet. I could see how a steam explosion might cause further damage to reactor 4, causing more mess. But to all three remaining reactors? I presume for such megaton class explosion some nuclear weapon type triggering must happen. But I don't see it. The corium isn't going to drop en masse into the water, it will initially drip, faster and faster. At some point the evaporating water is going to form a crust on the corium like the lava flows from kilauea when they meet the ocean.

I'm guessing that someone is theorising that the pressure from a sudden steam explosion is going to compress the corium causing it to detonate like an atomic device. Perhaps... perhaps, a steam 'explosion' might compress and increase the fission rate of the corium. But it would be a poor bomb, the enrichment levels are very low. A RBMK reactor might work only natural (unenriched) uranium. Megaton class explosion is in the H-Bomb range, did someone think the 0.0156% of the hydrogen in the water that is deuterium might start a fusion reaction?

Does anyone have a realistic thought out explanation for what would have happened if the corium had reached the water tanks?

Jeremy Thomson

 

[Drakkith]

The 2nd episode of the Chernobyl min-series has a Belorussian Physicist (a composite character? played by Emily Watson) warning of a 30-40 megaton explosion.

Ridiculous. A nuclear reactor, even when severely damaged, is not a nuclear weapon and will not explode with anything close to this kind of energy. If for no other reason than the fact that the fuel will blow itself apart into tiny fragments before enough reactions have occurred to generate a multi-megaton explosion.

I'm guessing that someone is theorising that the pressure from a sudden steam explosion is going to compress the corium causing it to detonate like an atomic device.

I can't see how. Nuclear weapons using an implosive strategy require an extremely well timed series of explosive blasts placed around the core. Any deviation in the timing of these explosives causes the core to become deformed when imploding (a bit like trying to squeeze a balloon) and you don't get anything but a fizzle.

A steam explosion is simply not going to do this.

 

[Xforce]

It’s not possible for a nuclear fission reactor to explode. Nuclear fission reactors usually use Uranium-238 at relative low concentration, and the half life of it is very long. Even large chunks of uranium 238 will not go off even without graphite to slow down neutrons. Nuclear weapons, however, uses a highly unstable isotope, U-235, when it receives a neutron, it turns into U-236 and split apart, releasing more neutrons and cause a chain reaction. An U-238 atom won’t turn into U-236 or U-235 naturally. So it is impossible for a reactor to explode. In fact, in most cases, turning a nuclear reactor into a nuclear weapon is pretty much ridiculous. We sometimes see ideas of overloading reactors to cause a mass destruction in sci-fi movies, but in reality, it won’t do more than melting down and cause a catastrophic radiation leak, just like what happened in Chernobyl.
For fusion reactors, they are perfectly safe. If you want to disable the magnetic field, the reaction just stops. Nuclear reactors never explode. The exploding reactors in sci-fi are likely to be antimatter reactors, and we don’t have them right now because we don’t have enough antimatter.

 

[Rive]

I presume for such megaton class explosion some nuclear weapon type triggering must happen. But I don't see it. The corium isn't going to drop en masse into the water, it will initially drip, faster and faster. At some point the evaporating water is going to form a crust on the corium like the lava flows from kilauea when they meet the ocean.

You are correct. For example in case of Fukushima it did happen, and while it likely had effect on the internal pressure of the reactor, the explosions happened due a complete different reason (and likely: different time).

As I recall there was two (real) dangers involved in this story: one is that the corium might reach criticality when submerged in water - this could have rendered the whole basement inaccessible for a long time. Second is, that the corium could have flushed the water out of the basement. Also, it is still possible that they feared the possibility of a big explosion. With that much knowledge about the actual status of structure... Well, even we can't dig out the truth of the matter, so how could they?

 

[artis]

Seems like with the HBO's new drama "Chernobyl" and other news articles Chernobyl is back in business as a cool scare story for the average person.

They also dug a long tunnel underneath the whole reactor building, this as well as most other things were done purely to contain and save what was left in order for there not to be an even bigger contamination of the surroundings including freshwater.
I kind of doubt that the top Soviet nuclear physicists that were all called in on duty with respect to Chernobyl were so illogical that they really thought that tons of low enriched Uranium mixed with concrete, steel, sand and dirt(all of which only minimize the chances of fission) all combined into a lava could make a large let alone megaton range nuclear blast. This is probably folklore and there is plenty around this event, simply because thousands of people were involved and millions more have heard about it.

 

[HowlerMonkey]

They were worried that a huge steam explosion could propel the already pulverized fuel debris further than the initial explosion.

A 2% enriched fuel could never cause a nuclear explosion.

 

[artis]

exactly , they did the things that they could do in order to contain as much of the debris inside as they could which involved not only covering the blast area with neutron absorbers and fire retardants but also making sure the contamination doesn't go underground, surely water was to be cleared in the way because otherwise the water that was already radioactive would have become extremely radioactive and steam would carry contamination further inside the building probably.

just in case for someone reading if they did not know, the reactors 1,2,3 were later reopened and kept running for years so obviously they tried to salvage as much as possible because the reactors had a rather high net electric capacity (1000MW each) and they did not want to lose that.But you know it's always like that, those few who are truly interested know the facts while the rest can't be bothered to read further than a few lines in VICE or elsewhere, surely Chernobyl has become a meme , just a cliche , a word much like Jesus or The Beatles, most of the ones who use these words know very little behind them, others use them for their own benefit.
My own personal experience shows that History channel is just another "fake news" source. I don't like that most of their narratives use voices that exaggerate the importance of the facts presented while at other times they dramatize simple events. I personally enjoy history without interpretation.

 

[davenn]

My own personal experience shows that History channel is just another "fake news" source. I don't like that most of their narratives use voices that exaggerate the importance of the facts presented while at other times they dramatize simple events.

Agreed, the same thoughts here. I don't get pay TV at home, so it's usually at a motel somewhere on holiday.
Some of the things I have seen on various programs of theirs have left me stunned in disbelief at the inaccuracies.Dave

 

[HowlerMonkey]

Chernobyl Notebook.
https://apps.dtic.mil/dtic/tr/fulltext/u2/a335076.pdf

 

[Nik_2213]

Perhaps they were afraid of a hydrothermal or phreatic eruption ?

 

[artis]

@davenn

Agreed, the same thoughts here. I don't get pay TV at home, so it's usually at a motel somewhere on holiday.
Some of the things I have seen on various programs of theirs have left me stunned in disbelief at the inaccuracies.

Exactly, and in the case of Chernobyl such poorly made documentaries that aim for drama and entertainment usually interview "experts" who where actually some pipe welders without university diploma at best and then they answer complicated questions like levels of contamination in different places and possible risks involved and they make this concerned looking face and in the background depressing classical music plays ever louder.
@Nik_2213 I don't think so, it is wrong to imagine the red hot lava being thrown into a large pool all at once which surely if a large hot mass gets suddenly introduced to large amounts of cold water can cause a violent steam explosion, just because the RBMK did not have a western style reinforced concrete dome around it doesn't mean it was built from paper and plastic exactly.
The reactor still had a thick reinforced concrete so called "biological shield" around the graphite/pipe channel structure which was filled with sand and lead, altogether the thickness was few meters, underneath the reactor were the pipe inlets for each channel (imagine a large pipe network) underneath that was a reinforced concrete pool meant for suppressing a leak if one or two of the channel pipes burst. The hot lava was not introduced to the bottom of the reactor all at once , it moved slowly just like you imagine a large hot mass breaking through concrete and steel , in the process it cooled off a little and mixed with all kinds of crap.
At best it could have boiled the water below and caused radioactive steam to spread in all direction that's about it. But compared to the amounts of steam and ash already blown into atmosphere this would be just a minor thing in the overall scheme of matters.

Notice how the lava has stopped and solidified within a steel diverter pipe which means that once the lava broke through layers of concrete and steel it was already cool enough and incapable of melting any more steel.

Basically the only real danger for the red hot fuel mass was that it was very toxic and radioactive so they only tried to contain it within the building and not let it out and that's all , everything else is speculation and fake news.

 

[artis]

Just a side point , notice the news articles , these were the papers a few days after the west got some information bits about what happened in the USSR the republic of Ukraine, only a few firefighters were dead at the time alongside a few of the staff of the 4th reactor that blew up, surely a number of people were transferred to specialized hospitals to treat acute radiation sickness and would later die but the numbers presented here were purely made up by the morning coffee table. So clearly the news media has been in the money earning and scare business even back then.I would suggest to the mods making a special thread and permanently attaching it in the nuclear subforums where legitimate pictures and documentaries could be attached with some historically accurate and physically correct explanations in order to save time because there will probably be a dozen more of these Chernobyl threads given HBO's new drama and other films about the events.

 

[Rive]

...everything else is speculation and fake news.

The problem with this kind of things that it is so easy to draw conclusions from facts known at a later point of time. However, the different questions has different answers: if somebody asks if they feared a big explosion (by any reason) that has absolutely nothing to do with later simulations or any scientific basis. To answer that question the only thing needed to know is if they feared it or not.

It might be kinda' like the question 'did people fear the End of the World within ten years in 990?'. Yes, they did. If you want to do any type of historical stuff about that era then you just can't omit that part, even in you already know that the world did not end that time and all those fears were without basis.

So, about those divers: the only fact known that they did dive there and opened some valves so the water in the basement and/or in the tanks in the basement could be removed before the corium poured in. The attached historical context is that it was to prevent things getting worse, possibly by explosion or further contamination. The scientific context according to our present time is that explosion was unlikely to happen, especially anything nuclear.
And these three things can be true simultaneously.

 

[artis]

Ok , technically the workers and scientists at the time had no way of knowing everything so they simply assumed with their best calculations surely, before later they were able to drill holes in the walls that took them months and found out the reactor was gone for good.

But this still doesn't exonerate the media claiming victim counts without any proof or mass hysteria, much less History channel's and other news media dramatic actor voice-over narratives and twisting historical facts.

PS. as much as the Soviet government had lied in the past, in the second half of 1980's they were getting more honest about matters and being under pressure with respect to Chernobyl they actually released a correct count of the workers who died which was 31 about a month after the accident , another 20 or more folks died in the months after from cancer and overall psychological and physical damage.

 

[ElliotSmith]

I'm not a nuclear physicist, but I don't understand how molten nuclear fuel coming into contact with cold water could cause a nuclear explosion.

A steam explosion, yes, but a nuclear blast?

Here is a video of volcanic lava coming into contact with ocean water, and there is no explosion. The lava is a similar temperature to the molten fuel at Chernobyl.

 

[PeterDonis]

I don't understand how molten nuclear fuel coming into contact with cold water could cause a nuclear explosion.

Because the cold water is a very effective neutron moderator and can cause the fuel to go supercritical, which is what a nuclear explosion is--an uncontrolled supercritical chain reaction. That can't happen with lava. Whether it actually will happen with the molten nuclear fuel will depend on the details of its physical configuration.

 

[BWV]

the scientist in the show spoke of a 3-4 megaton explosion, not 30-40. The source for the writers appears to be from this documentary



"Our experts studied the possibility and concluded that the explosion would have had a force of three to five megatons," said Soviet physicist Vassili Nesterenko. "Minsk, which is 320 kilometers from Chernobyl, would have been razed and Europe rendered uninhabitable."

Vassili Nesterenko (2 December 1934 – 25 August 2008) was a Soviet and Belarusian physicist from Ukraine and a former director of the Institute of Nuclear Energy at the National Academy of Sciences of Belarus (1977-1987).[1]

https://en.m.wikipedia.org/wiki/Vassili_Nesterenko

Because the cold water is a very effective neutron moderator and can cause the fuel to go supercritical, which is what a nuclear explosion is--an uncontrolled supercritical chain reaction. That can't happen with lava. Whether it actually will happen with the molten nuclear fuel will depend on the details of its physical configuration.

So, hypothetically, immersion in water could have triggered a nuclear explosion? Only hydrogen bombs, to my limited knowledge, have generated blasts of this magnitude, so
a) is there some set of conditions where a melting reactor core could transform into a multi-megaton nuclear explosion? And b) did rightly or wrongly, did the Soviet authorities at the time believe this scenario to be probable?

 

[artis]

pardon my ignorance but something doesn't add up here. In H bombs the primary device is an A-bomb, usually plutonium (have there been U235 primaries?)
so the fusion which makes the yield stronger is made possible by the extreme radiation and heat from the A-bomb, but in case of U235 we are talking about clean precisely timed 90 something % enriched U235,

Correct me please but to the best of my knowledge the highest chance the RBMK fuel ever had to go boom was when it was still in the core at the moment when the reactivity got out of control and the core went "supercritical" ,but I guess because reactors work with slow neutrons the power couldn't climb fast enough (as in a bomb) and the core tore itself apart before the fuel had the chance to develop anything close to a nuclear explosion, after the first steam explosion the core was thrown apart , tons of graphite moderator were flying every direction landing on the roof and premises of the building.

The part where the scientists worry about the second explosion they essentially have a "dirty" molten fuel mixed with dirt and sand, boron, lead, concrete and metal , everything they threw from helicopters and everything the fuel itself melted along the way as it descended down through metal pipes and reinforced concrete structural elements,
so how can such fuel cause an explosion that would rank in the H bomb category?
either I fail to see something obvious here or something is exaggerated?

 

[artis]

PS. Could a reactor core, if it had the strength to hold itself together for long enough develop a nuclear type explosion under runaway conditions, with such low enrichment?

the molten fuel falling into water would probably be like a moderated core that unlike the real core could probably hold together for longer, probably as long as the water surrounds the fuel before it evaporates violently.

 

[Rive]

...or something is exaggerated?

Exactly. There is a deeply studied concern that core debris might go critical when submerged in water, but that is not like an explosion and this possibility depends on many things, like enrichment of the fuel and additional materials in the debris/corium pile. While it is considered a possibility it does not have really high chance with the standard fuel used in commercial reactors these days.
If it happens then it is expected to boil the water and dance on the very edge of criticality till water lasts or enough boron is added, finally.
Such recriticality could have made the basement inaccessible due radiation and hot water/steam. But that's still not an explosion.

Again: this is a speculation based on knowledge available at a later point of time. What they did fear that time is primarily a historical question.

Ps.:

Could a reactor core, if it had the strength to hold itself together for long enough develop a nuclear type explosion under runaway conditions, with such low enrichment?

The definition of the 'nuclear type explosion' is a bit too vague, but in general it is a no. What they had in Chernobyl is more or less the biggest blast that a (commercial) reactor can possibly produce (and they had to work very hard to push that old reactor with that no longer even acceptable design to the edge of disaster - and then push it a bit further).

 

[Bandersnatch]

"Our experts studied the possibility and concluded that the explosion would have had a force of three to five megatons," said Soviet physicist Vassili Nesterenko. "Minsk, which is 320 kilometers from Chernobyl, would have been razed and Europe rendered uninhabitable."

What? Is he actually saying that? That doesn't make any sense.

 

[artis]

I too wonder why even back then with the limited knowledge they had about the details of the accident they would think in terms of megatons as the fuel was like 2%? enriched and at the moment of explosion already halfway burned, sounds rather weird given that in 1986 the Soviets already had a 40 year experience with both A-bombs and H- bombs in their arsenal, so their top scientists must have had a rather good knowledge of what it takes to yield certain explosive power.

Maybe they worried the explosion could be strong enough to completely destroy the already half destroyed structure and so take the neighboring 3rd reactor with it or something along those lines, who knows.

 

[russ_watters]

I too wonder why even back then with the limited knowledge they had about the details of the accident they would think in terms of megatons as the fuel was like 2%? enriched and at the moment of explosion already halfway burned, sounds rather weird given that in 1986 the Soviets already had a 40 year experience with both A-bombs and H- bombs in their arsenal, so their top scientists must have had a rather good knowledge of what it takes to yield certain explosive power.

How much of a "top scientist" does one need to be to know that a nuclear reactor isn't an A-bomb and an A-bomb isn't an H-bomb? I read "The Sum of All Fears" when I was in high school, does that count?

I can't fathom who that guy is or why he would say that, but one thing's for sure; I'm not watching the show to find out! And I even watched a little of the Tesla Death Ray show!

 

[artis]

If by "the show" you refer to HBO's Chernobyl then after watching all five episodes I must say the director has shown a lot of respect and care for the detail and deviated only in certain places for the sake of drama and tv.

I also listened to the podcast where Craig Mazin explains each episode and I must say he has done his research,

Both Dyatlov's rude and arrogant character and Toptunov's inexperience and mistakes as well as the positive void coefficient and xenon poisoning were all addressed quite accurately and that in itself is already a good sign.

Personally I did not knew before that in the court where Anatoly Dyatlov, alongside his colleagues in crime Viktor Bryukhanov (the station director at the time) and chief engineer Fomin were given the chance to say their position Dyatlov contrary to his colleague testimonies said that he did not give the directive to override safety instructions and warning from the SCALA computer because he was in toilet (yes I checked , turns out this was the excuse he came up with), so he said his colleagues decided for themselves to carry on. So he lied.
Also Akimov insisted that they shut down the reactor as they were approaching certain suicide but Dyatlov (apparently relying on the SCRAM to save him) dismissed him and carried on.
So basically Dyatlov's dance with the devil was rather calculated and reckless at the same time but he did not know the control rods had graphite tips, but he should have know that they take 22 (IIRC) seconds to fully insert( which wasn't a secret) which is too long for a reactor that is put to the point where the core has almost no neutron moderation and only relies on xenon poisoning (which can't be calculated real time) and coolant light water to absorb neutrons , so as the water boiled away (after Dyatlov ordered the pumps to be switched off) and apparently so did the xenon at the same time, the core "ran away" for good.I'd suggest @russ_watters watch the show, there is no hype or hysteria like in China Syndrome, mostly historical facts presented on screen with some minor drama.
As someone who has lived in the former USSR I can tell you this show unlike others is surprisingly accurate

 

[Astronuc]

I'd suggest @russ_watters watch the show, there is no hype or hysteria like in China Syndrome, mostly historical facts presented on screen with some minor drama.

What HBO’s “Chernobyl” Got Right, and What It Got Terribly Wrong - by Masha Gessen
https://www.newyorker.com/news/our-...obyl-got-right-and-what-it-got-terribly-wrong
Another criticism - What HBO's 'Chernobyl' gets right (and wrong) about the world's worst nuclear power plant accident
https://www.businessinsider.com/chernobyl-hbo-whats-true-myths-2019-5
Folks have asked me about the program, but I have not watched it. I thought it was a documentary, but apparently it is a dramatization based on an historical event, and according to Gessen, the writers/producers introduce commentary based on their imagination.

As for a multi-megaton explosion of a low enriched core with a moderate fission product distribution, it seems unlikely it would be in the megaton (1 MT = 4.184e+15 J) range. One would have to know the reactivity insertion and rate, then calculate the neutron flux (pulse) from which one would calculate the power and energy produced in the reactor core.

Designers do consider so-called hypothetical core disruption accidents, and basically we design core and systems to preclude such events. Chernobyl was a severe transient resulting in a steam explosion, which then led to dispersion of some of the core, melting of other parts of the core, and chemical reaction (oxidation) of still other parts.

One article puts the energy yield of the Chernobyl accident at 10t TNT, which is not even in the kT range.
https://link.springer.com/article/10.1007/s00024-009-0029-9

 

[artis]

Well just to be fair, the megaton explosion risk idea came from a rather unknown scientist not the HBO show, but it is in the show because this prediction historically happened and is not made up.

Also a historical fact is that they drained the lower suppression pool and I believe the main reason was not fears of megaton explosions but in order for there not to be any more contaminated steam causing explosions within the building itself and doing any additional damage.

As for the show , well it's a show so take it with facts and a grain of salt.
All I said was that compared to other shows with zombies and mutants this one is as close to reality as ever for a show.

 

[Bandersnatch]

this one is as close to reality as ever for a show.

You should watch Threads sometime.

 

[Rive]

...there is no hype or hysteria like in China Syndrome...

I have a feeling that what @russ_watters is annoyed about is the hysteria around the movie, and not about the hysteria in the movie... What is indeed getting really annoying. It is getting really hard to do any search for facts, and it is just started to spin... I've already started using time-limited search: any hit from the past half year excluded.

The movie is a psychological/societal drama based on really well gathered background, but adapted to TV/movie. I see less and less reason to discuss the content of the movie (!) in science topics.

 

[jim hardy]

Because the cold water is a very effective neutron moderator and can cause the fuel to go supercritical, which is what a nuclear explosion is--an uncontrolled supercritical chain reaction.

Cold water can also reflect fast neutrons back in.

A glob of molten fuel dropped into a pool of water might well go critical
produce an awful lot of heat
and make a huge steam explosion
and spew an awful lot of radioactive fission products
as has been described earlier in the thread.

But how does one put a number on the consequences that'll make sense to civilians?

I suppose were i a physicist
i'd estimate the amount of those fission products likely to be released in the steam explosion
and compare what size weapon would make a similar release
and tell the civilians they might expect consequences equivalent to that size weapon
although the actual energy released would be much smaller.

The reactor core has been fissioning uranium atoms since its last reload maybe months ago so it carries a tremendous inventory of fission products of varying radioactities
while the weapon only fissions for a fraction of a second.

Now the conversion from megawatt hours to tons of tnt is only 1.16 ,
that is it takes the same number of fissions to to produce the heat equivalent of either a ton of tnt or 1.16 megawatt hours
whether you do them all in a split second or stretch them out over months..

RMBK 1000 makes 3200 megawatt hours every hour, the heat equivalent of around 3200/1.16 = 2759 tons of tnt per hour.
So it builds up equivalent fission products at the rate of, let's round to 2.76 kilotons per hour.
That'd be 66.24 kilotons per day, which is 1987 kilotons in a 30 day month or 2053 in a 31 day month.

Can we just call it 2 megatons per month? 24 megatons per year?
How long had Chernobyl run in that fuel cycle prior to the explosion ?

Anyhow -
That'd be my guess
as to why a pyhysicist told them to expect consequences similar to a few megatons ,

even though the steam explosion would amount to orders of magnitude less.It's what i'd probably do..

Anything wrong in my reasoning? I just asked myself "Why would a rational man say that ?"

old jim

 

[artis]

I like your thinking Jim, indeed maybe we dismissed the rather unknown physicist too early thinking just in terms of the physical blast yield but not in terms of the fallout damage, indeed a well made modern A or H bomb to the best of my knowledge is "cleaner" (much cleaner?) than a Chernobyl type event,
even after the bombings of Hiroshima and Nagasaki we see that the population of those cities got back to normal only some 15-20 years after the blasts.

@jim hardy , hard to say how long the fuel was in the core given that RBMK have the capability to change individual fuel assemblies while online , which they routinely do, but overall the 4th reactor opened in 1983, so I would suspect that some of the fuel was still there from the beginning , anyhow although I can't find any direct info about this I'd bet that atleast half of the fuel assemblies were long into their useful lifetime.@Rive I am not discussing the show , just the parts that are historically accurate which i found interesting to add.

 

[jim hardy]

You should watch Threads sometime.

and Peter Watkins' 1965 "The War Game" ..

 

[Astronuc]

Can we just call it 2 megatons per month? 24 megatons per year?
How long had Chernobyl run in that fuel cycle prior to the explosion ?

Anyhow -
That'd be my guess
as to why a pyhysicist told them to expect consequences similar to a few megatons ,

Reactivity transients (and explosions) take place over millisecond time frames. In PWRs, some tests have pulse widths of 10 to 30 ms, so the megatons would be greatly reduced to tons. During a pulse, the power can be many times the steady state level, usually less than a factor of 10. However, I have not seen numbers for the pulse height (peak power level) or pulse width for Chernobyl 4.

Mikhail V. MALKO, The Chernobyl Reactor: Design Features and Reasons for Accident
http://www.rri.kyoto-u.ac.jp/NSRG/reports/kr79/kr79pdf/Malko1.pdf
Malko provides an estimate of 1.0 TJ, "equivalent to the energy of explosion of approximately 200 tons of the trinitrotoluene." (top of page 22, and reference 18).

It is difficult to determine the configuration of the core since those records are not readily available. We know the following:

First Criticality: 25 November 1983
First Grid Connection: 21 December 1983
Commercial Operation: 25 March 1984
Permanent Shutdown: 25 April 1986 (Catastrophically destroyed)
Ref:
https://www.world-nuclear.org/reactor/default.aspx/CHERNOBYL-4
Chernobyl 4 was a RBMK-1000. It had 1661 fuel channels, and it was capable to replace 2 fuel channels per day, so it would take approximately 830 days to replace all the fuel in the reactor, which is more than two years, but Chernobyl 4 reactor was operating for 2.3 years at the time of the accident. Various papers mention that the reactor was shutting down for routine maintenance, but they do not mention the frequency associated with routine, e.g., an annual schedule, which means that it the unit was shutting down for the second outage.

Malko mentions comments "blue light corresponds to the temperature about 6,000 K," which is true, however Cerenkov radiation is blue, so very likely the "blue flash" was due to Cerenkov radiation, or ionization of the air/steam/hydrogen after the initial steam explosion. One can observe a blue flash in a pulsed reactor.

Another good reference on the accident (See Table 1 and Figure 11, and Figure 10)
https://mragheb.com/NPRE 402 ME 405 Nuclear Power Engineering/Chernobyl Accident.pdf

 

[artis]

interesting to see the gamma ray interference when the camera is rather close to the core and the water thickness is not high enough, in the other research type pool reactors the camera is further away, the blue light still can be seen but there seems to be no destructive interference with the camera photo-electronics , would this indicate that a thick layer of water not only moderates/absorbs neutrons but to a large extent shields gamma radiation?I have no detailed information on the subject but my intuition tells me that they rushed the test so it can be finished (hopefully) before the planned maintenance shutdown of the reactor where probably also some (most?) fuel assemblies would be changed because the dates from commercial operation until spring 1986 suggest 2 years of full power which would seem as many fuel assemblies were at the end of their useful life so full of fission products.
PS. @Astronuc , even though a bit off topic, I wonder when they changed a fuel assembly while reactor is online, as the fuel assembly comes out, did they close the water flow of that specific channel beforehand? Otherwise I fail to see how this would not result in hot water/steam spewing everywhere.
What still intrigues me is how each assembly rod had a seal that could be tightened or loosened by the remote machine in order for the channel to be sealed before being put back into operation. I assume CANDU'S have similar things since their also channel type.

 

[jim hardy]

However, I have not seen numbers for the pulse height (peak power level) or pulse width for Chernobyl 4.

surely there's no direct measurement.

This statement from your second reference seems plausible
pdf page 17, right below the picture fig 14.

 

[Astronuc]

PS. @Astronuc , even though a bit off topic, I wonder when they changed a fuel assembly while reactor is online, as the fuel assembly comes out, did they close the water flow of that specific channel beforehand? Otherwise I fail to see how this would not result in hot water/steam spewing everywhere.
What still intrigues me is how each assembly rod had a seal that could be tightened or loosened by the remote machine in order for the channel to be sealed before being put back into operation. I assume CANDU'S have similar things since their also channel type.

I'm more familiar with the CANDU system, but I understand in both cases, the refueling machine locks onto the cooling channel, then the cap is removed, the spent fuel is retrieved and fresh fuel inserted, then the cap is returned and locked. I don't know if there are valves to block/stop the coolant flow into the channel, but perhaps there is.

CANDUs have horizontal channels, whereas RBMK are vertical. In CANDU systems, to refuel a channel, a pair of fuelling machines latch onto each end of the channel. New fuel bundles are inserted at one end, old fuel is pushed along the channel, and spent fuel is discharged into the other machine. Typically, four or eight fresh bundles are added at each fuelling. The RBMK must refuel from one end, the old fuel must be retrieved, then the new fuel installed.

See page 4/5 - https://canteach.candu.org/Content Library/20043404.pdf