Chernobyl Did Chernobyl divers prevent a multi megaton explosion?

[dangderang2000]

Judging the TV series by what is written about it isn't fair, and if you haven't seen it you're missing out. It's actually a brilliant psychological drama, one of the best I've seen. It's filmed in a highly realistic way, when it would be all too easy to go over the top with special effects. The themes are mostly about human behavior, lies, ambition, oppression, bravery, sacrifice.

Reactor physics is not the main topic, but many of the key characters are physicists - both good and bad. Some aspects seem exaggerated, like the one OP brought up. But even though it's a drama, it's more scientifically sound than 90% of any popular science stuff on TV about physics - be it quantum computing, solar cells or nuclear power. It's 5 hours long after all.

I'm reminded of some of Kubrick's classics like Dr. Strangelove and 2001. The main message I felt, is that humans, including scientists and engineers, are not perfect nor rational. Combining this with the potency of nuclear fission creates a very dangerous mix. The lesson learned should be to remain humble and accept that we can't foresee every technical possibility or every human behavior. That's why it's so important to watch for scientists.

Towards the end, there's also a not so subtle reference to the fight about the truth around climate change. That's a current topic where the scientists, much like the Soviet ones, are fighting for the truth while the powers at be try to silence them.

 

[artis]

The show is good but don't be too overtaken by it, the truth is that in fact most scientists knew the flaws of the RBMK reactor as well as many of the engineers that worked on it even though they were not told about them, they found out by simply being smart and seeing the reactor in actual working conditions.

The truth as always is a bit more hidden and trivially rotten, fission by itself is not dangerous , in fact nothing truly is. But take any thing and combine with secrecy , neglect, systematic pressure and a utopian ideology driven bureaucracy and you get a bad bad result. Most people don't realize this but an awful lot of things had to be neglected and done wrong for a long time until the "lottery" was finally won in 1986.
From this point I agree with the show that lies and laziness can make many things go wrong.

In fact we can see potential technical problems or bad human behavior, many saw it in the Soviet Union , the problem is that once they saw what they saw and tried to do something about it they were stopped or simply silenced.
The same year 1986 few months before Chernobyl, the Challenger disaster happened and it was almost like an omen to Chernobyl, the designers and builders/contractors of the NASA space shuttle also knew their o-ring system had modes where it could fail (temperature etc) but never fully disclosed this to the managers at NASA which not being physics geniuses decided to go with the launch even though the parameters (air temp) was lower than deemed safe by the designers of the boosters but since it was their product they were not keen on reporting the potential problems that could arise in some very rare situations much like the designer of RBMK thought the operators would never put the reactor in a low power state in which the reactor was known (by some) to be dangerously unstable.

Quoting Bob Marley from his Redemption song :"Have no fear for atomic energy,
Cause none of them can stop the time"

I am not afraid of nuclear reactors or spaceflight but I am afraid of idiots and egomaniacs being in positions of power approving reactors or building and operating them.

 

[jim hardy]

That's a current topic where the scientists, much like the Soviet ones, are fighting for the truth while the powers at be try to silence them.

Oh Pu-lease, don't restart that GW debate here.

 

[PeterDonis]

Towards the end, there's also a not so subtle reference to the fight about the truth around climate change. That's a current topic where the scientists, much like the Soviet ones, are fighting for the truth while the powers at be try to silence them.

This is off topic for this thread, and it is also a topic where PF has strict rules for discussion. Please be advised.

 

[artis]

A recent Sky short documentary about their own created tv show, but go to 22:45 and here I give them credit for being realistic with respect to the importance of the Chernobyl divers which is also the title of this thread.
disregard the short intro from the Chernobyl tv series and pay attention from the moment the woman voice narrator starts saying that they wanted to drain the water because there was a risk of an explosion which could have blown up the entire plant itself and so destroy or damage the neighboring 3 intact reactors. Then one of the divers start telling his recount of the events.

I think that this was the most probable scenario considered even back then, the idea that the core meltdown material won't create a H bomb like blast(which is impossible and scientists knew that even back then with limited info I think) but instead might create a blast strong enough to destroy the structures surrounding the blast area. Now this scenario was entirely real as the unit 3 reactor was located just besides a few walls from the 4rth unit, these walls had already been shocked and vibrated with the strong blasts from the original explosion so I think they took no chances and no risks.

Just as a sidenote , can someone with knowledge say how does the critical mass requirement changes for reactor grade fuel when it is fused together in a lava mass instead of when it is set apart in individual fuel pellets packed inside fuel rods which are then separated by some distance and each fuel assembly is separated from the next by an even larger distance when it is under normal operating core conditions,
I would assume that bringing the fuel together in a molten ball like shape would increase its reactivity when a moderator is introduced? This IIRC is also the reason why molten salt reactors needed less fuel mass for criticality right?

 

[anorlunda]

When the fuel is diluted with other materials, it makes it harder or impossible to get a critical mass. A whole mountain of uranium ore is far from critical. Yet, there is evidence of one location that had naturally occurring critical reactions in nature.
https://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor

 

[jim hardy]

I would assume that bringing the fuel together in a molten ball like shape would increase its reactivity when a moderator is introduced?

While that's quite plausible it's as yet speculation for the reason @anorlunda mentioned - dilution of the fuel by whatever else got melted into the ball.

Dropping such a ball of hot molten metal and fuel into water would increase its reactivity all right,
but i believe the mechanism would be more by reflecting neutrons back into the mass from its periphery than by water migrating into that hot molten mass and remaining in liquid form to do its moderating.. .

Here's a picture of an experimental reactor that uses reflectors , but they're solid not liquid. And they're made from heavy atoms so as to reflect neutrons without reducing their energy.
Recall from high school physics that in elastic collisions, energy division between the bodies is proportional to their masses - that's why a Volkswagen will bounce off a Buick but will dominate a motorcycle.
So uranium atoms make a better reflector than water molecules .

so a return to criticality just long enough to make another big steam explosion is i believe not implausible.
Would it have happened ? I surely don't know.

old jim

 

[Rive]

so a return to criticality just long enough to make another big steam explosion is i believe not implausible.

In general, this kind of recriticality events are expected to be a kind of slow/pulsing reactions like in Tokaimura: as the water boils, the reactions subsides, then with the water cooling down it starts again.
Given that any kind of criticality there would have rendered that area inaccessible the chance that it actually happened is low.

 

[Astronuc]

I would assume that bringing the fuel together in a molten ball like shape would increase its reactivity when a moderator is introduced? This IIRC is also the reason why molten salt reactors needed less fuel mass for criticality right?

Not necessarily, and more unlikely, to the first question. No to the second question.

In the case of a molten core, the molten mass of corium includes neutron poisons, e.g., boron, or silver-indium-cadmium, and some fission products, although likely little or no Xe-135, and largely excludes the moderator (water). So it should not go critical even if immersed in water, yet that scenario must be considered, which is why some of the emergency cooling water contains high concentrations of boric acid.

In an MSR, criticality depends on the enrichment and neutron energy spectrum, i.e., whether the system relies on fast vs thermal (moderated) spectrum. Fast reactors require higher enrichments, or more Pu-239, -241 than U-235. In order to compare the masses of fuel required for criticality and power density in different systems, one must perform detailed calculations using a full neutron energy spectrum from thermal energies (~0.01eV) up to fast energies (10 MeV).

In the Flat Top slide in jim hardy's post, it refers to U-235 to Pu-239 cores, which take to mean better than 90% or essentially fully enriched. The more highly enriched U or Pu, the less mass it takes to become critical, and such a system is far different from a power reactor.

 

[artis]

@Astronuc I guess I was wrong as it has been some time since I read about MSR theory.
So a MSR reactor on average needs either a higher enrichment or a larger total fuel mass in order to reach criticality than a conventional solid state reactor?

 

[Astronuc]

So a MSR reactor on average needs either a higher enrichment or a larger total fuel mass in order to reach criticality than a conventional solid state reactor?

Basically, yes. There are no simple comparable examples readily available, but if an MSR neutron energy spectrum has a higher fast flux component, i.e., if the fissions are more in the fast flux region, then the enrichment must be greater, since the fission cross-sections are less in the fast energy range, keV to MeV.

The original MSRE used enrichments of better than 30%, up to 93%, but that was in the early phase (and not a good example), and it was a small core. The necessary enrichment depends on the fraction of the core that is U/Pu/Th vs that which is salts of LiF, NaF, KF, BeF₂, ZrF₄, or chlorides. Natural chlorine has a relatively high thermal neutron absorption cross-section, and Cl-35 has an issue with transmutation by n,α reaction to P, which decays to sulfur, which causes issues.

I've seen another theoretical cycle that uses about 10% enrichment, which is twice the current LWR limit of 5%, although there is some interest in possibly increasing LWR enrichment to 6%, and possibly up to 7%.

However, MSR technology is off-topic. The OP relates to RBMK, or water-cooled, graphite-moderated reactors.

 

[Aufbauwerk 2045]

Here's a link to the relevant British documentary. You can see Gorbachev, scientists, and others speaking about the accident.

Speaking of stuff blowing up in water, here's an interesting passage from a WW2-era book on nuclear physics. This book was copyrighted in 1942. The authors reveal a knowledge of the ongoing large-scale production of U235.

In this passage they are explaining the chain reaction process, in the context of some experiments by Fermi.

"In order to use the neutrons efficiently, they would first have to be reduced in energy by adding some hydrogen-containing material such as water. But on being slowed down a majority of the neutrons will be quickly gobbled up by the more numerous U238 atoms present, and from this capture only sedate U239 atoms will result. Now, if someone could succeed in isolating a few pounds of U235 and the whole were to be submerged in water, very interesting developments would almost certainly follow. The separation of the uranium isotopes in quantity lots is now being attempted in several places. If the reader wakes some morning to read in his newspaper that half the United States was blown into the sea overnight he can rest assured that someone, somewhere, succeeded." Pollard and Davidson, Applied Nuclear Physics, Copyright 1942, 8th printing 1946, p. 196. Published by John Wiley & Sons.

I am not a "green energy" enthusiast, but I think it's reasonable to be very concerned about nuclear energy safety. On the other hand, we obviously have not seen half the USA blown up, so I must wonder what these guys were thinking. How does this relate to Chernobyl? Maybe an expert can clear this up for us.

 

[Drakkith]

I am not a "green energy" enthusiast, but I think it's reasonable to be very concerned about nuclear energy safety. On the other hand, we obviously have not seen half the USA blown up, so I must wonder what these guys were thinking. How does this relate to Chernobyl? Maybe an expert can clear this up for us.

How does what relate to Chernobyl? The physics behind fission Fermi mentioned, or something else?

 

[PeterDonis]

we obviously have not seen half the USA blown up, so I must wonder what these guys were thinking.

The authors of the book or the people actually working on the Manhattan Project?

The authors of the book were uninformed about what had actually been discovered about uranium fission in 1942, because they weren't involved with the Manhattan Project, and all knowledge about uranium fission had been highly classified for several years. So they are not good sources of information for what the actual risks were.

Also, "half the United States blown into the sea overnight" is a huge exaggeration even of what the public understanding of the possible range of risk was (which was, as above, uninformed by all the actual knowledge that had been gained in secret for the last few years) at that time. No reputable scientist ever thought that was possible, nor did any reputable estimate of the possibilities ever indicate that it might be.

The people actually working on the Manhattan Project were taking a series of carefully planned steps to build a working fission bomb that would only go off when it was told to. The first step was actually to make a controlled fission reaction; Fermi and his group did that in 1942:

https://en.wikipedia.org/wiki/Chicago_Pile-1
Note that this first controlled fission reactor operated at very lower power, about 1/2 watt. That was because Fermi and his group didn't want to try building a more powerful reactor until they understood more about how fission actually worked in practice.

In other words, the people actually working on the Manhattan Project were simply not going to take the kind of risks that the authors of that nuclear physics textbook were describing, even after we correct what the textbook says for hyperbole, as above. That would have been stupid.

 

[PeterDonis]

The quote says what would happen if a few pounds of U235 was submerged in water.

No, it doesn't. It's a combination of huge exaggeration and lack of knowledge on the part of physicists who did not have security clearances in 1942. See my previous post.

 

[PeterDonis]

what is the calculation for this scenario

The amount of energy that can be released by fission of a given mass of U-235 is easily found online. The amount of energy it would take to blow half of the United States into the sea can be easily estimated: for example, use the rule of thumb that one ton of TNT is roughly enough to blow up one city block (that's why 2000 pound bombs in WW II were often called "blockbusters"), and estimate how many city blocks the area of the half the United States is.

This will actually be an underestimate if "blowing into the sea" is the goal, since that requires more explosive energy than just leveling whatever is on the surface, but the number of orders of magnitude of difference between the energy in a fission explosion and the number of tons of TNT needed to "blockbust" half the area of the United States is already so huge that it's not necessary to go into such fine details.

You say that "all knowledge about uranium fission had been highly classified for several years". This must have been no earlier than 1939

It was in 1939, yes, when what became the Manhattan Project was starting up.

You say they had no security clearance in 1942. Do you know that for a fact?

Neither of them are on any list I can find of scientists who worked on the Manhattan Project, and if they didn't, they weren't cleared for that information at the time.

Also, if they had been working on the project in 1942, they would have known better than to make the statement they made about half the US being blown into the sea, even as exaggeration. Scientists on the project by that time were well aware of the available fission reactions and the energy that could be yielded from them. What they didn't yet understand was how to trigger those reactions in a controlled manner.

I must wonder how they knew about large-scale of production of U235 taking place,

I don't think they knew in the sense of knowing the specifics of what the Manhattan Project was doing. They say "in several places", which is vague. I believe that the bare fact that several countries were working on uranium isotope separation was public knowledge at the time.

 

[Aufbauwerk 2045]

Hey thanks for the replies. But I deleted my post because I could see I was getting overly anxious about this topic and beginning to ramble. Sorry about that. I think I'll sign off for another year at least.

P.S. it was interesting to be on this forum again.