Chernobyl Did Chernobyl divers prevent a multi megaton explosion?

[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.