Since questions arose, just before I declared a moratorium,
concerning the conditions under which a nuclear weapon will explode I thought I
would say a few words of explanation. I am no kind of physicist but back in the
day when I spent a good deal of my time as part of the Campaign for Nuclear
Disarmament I had to learn some of this stuff up and it sort of stays with you.
The crucial point to remember is that the mass of an atom is
determined both by the number of protons and neutrons it contains and also by
the amount of energy, in the form of mass, required to hold it together. Some
atoms either in their standard form or in their isotope form are sufficiently
unstable to break down periodically, giving off an amount of energy determined
by the famous formula e=mc2. Since the constant c stands for the
speed of light which is 300,000 km/s, the energy equivalent of even a very
small bit of mass is obviously very large.
Very heavy atoms like uranium and plutonium are unstable and
break down into their component parts regularly without external prodding.
Sometimes when one of those atoms spontaneously breaks down, it gives off an
alpha particle that hits another atom of the same material and causes it to
break up as well. But since, contrary to intuition, most of supposedly solid
matter is actually empty space, the likelihood of this happening is rather
small. If you cram enough uranium or plutonium into a small enough space you
can raise the likelihood of a series of such break ups, called a chain
reaction.
All of this was well understood theoretically at the time
during the second world war when both the allies and the axis had a go at
creating a weapon out of nuclear materials. All the physicists involved knew
that the amount of energy that could be released by such a chain reaction was
enormous but the engineering problem was how to get the chain reaction to start
and carry on for some period of time (a microsecond, actually) before the
explosion produced scattered the material and stopped the chain reaction.
The amount of energy produced by even a primitive so-called
atomic bomb was beyond anything human beings had ever produced. The super big
conventional bombs carried by Allied bombers in the air raids over Germany were
as large as 1 ton or 2000 pounds of TNT. That is a monster big bomb but the
atom bombs dropped on Hiroshima and Nagasaki were rated not in equivalent
thousands of pounds of TNT but in equivalent thousands of tons of TNT – hence
the invention of the term kiloton.
The secret of making the damn thing work turned out to be
creating a sheath of conventional explosive around two components of uranium or
plutonium, neither one large enough all by itself to produce a chain reaction,
and then exploding the sheath inward, or imploding it, so that the two
components were jammed together by the explosion and held there just long
enough to produce a chain reaction of enormous explosive power.
Blowing up a functional atomic bomb by hitting it with a missile would scatter the material harmlessly (setting aside for the moment the
effect of the radiation produced by the components). The likelihood that such a
missile attack would literally trigger the explosive mechanism and cause the
atomic bomb to explode as intended is virtually nil.
So what on earth is a hydrogen bomb? Well, it turns out that
if you arrange all the elements in order of their atomic weight from the very
lightest, which is hydrogen, to the very heaviest which is uranium or
plutonium, in each case the heavier atom is a trace heavier than the combined
component atoms resulting from a breakup, the additional weight consisting of
the mass form of the energy holding the parts of the atom together.
Except one case. If you break up one atom of helium, which
has an atomic weight of four, which is to say two protons and two neutrons,
into two atoms of so-called heavy hydrogen, which is to say an isotope of
hydrogen with one proton and one neutron, the two atoms of heavy hydrogen weigh more than the one atom of helium! This means that if you could find a way of
combining two atoms of heavy hydrogen into one atom of helium, energy would be
released. The amount of energy, it turned out, is enormous even by atom bomb
standards. But there is a problem: this process of combining hydrogen atoms
into helium – the same process, by the way, that generates the energy of the
sun – requires enormous temperatures. But the yield is vast – measured not in
thousands of tons of TNT equivalent but in millions of tons. Hence the new term
“megaton.”
Heavy hydrogen exists in nature in very small quantities in
ocean water and if you are willing to take the trouble of processing huge
amounts of that ocean water you can extract heavy hydrogen and use it to make
an H – bomb, with an A-bomb trigger. During the second world war the scientists
gathered at Los Alamos undertook to create a uranium bomb, and they solve the
problem of how to trigger it with a covering of conventional explosive (this
was of course an international group of scientists and it turns out that the
shape of the conventional explosive that is optimally effective is roughly that
of a soccer ball!) The Germans made the mistake of trying to create a fusion
bomb, not realizing that it would require an atomic bomb trigger. They had a
big plant processing ocean water at Peenemünde, which was raided by the allies
in an important air raid.
Well, that just about exhausts my understanding of these matters but readers of this blog might find this interesting.
7 comments:
A few minor corrections seem in order. The chain reaction in an atomic bomb has to do with neutrons, not alpha particles. A slow neutron hitting a U235 or Plutonium nucleus causes it to split apart and give off more than one neutron in the process. Thus the exponential explosion. Stellar fusion in really big stars works for elements up to iron, not just hydrogen to helium. Finally, the spherical form of the A-bomb is used for Plutonium bombs, and needs rather exact timing all around the surface. U235 bombs are much easier and can work with a linear gun arrangement. Thus the classic phrase "Fat Man and Little Boy" for Plutonium and Uranium bombs. The spherical form needed testing, so the Trinity explosion in NM was run. The Manhattan Project folks were so sure the U235 bomb would work, and U235 was so hard to produce, that the very first U235 bomb was used on Hiroshima, with no prior test.
I have long believed that a U235 bomb could be built by reasonably handy teenagers in shop class, given a sufficient supply of highly enriched Uranium. That's why the Iran agreement was so valuable, because it got Iran's stock of HEU out of the country. The risk, imho, is not that Iran's leaders would attack Israel themselves, because they are evil but not suicidal, but that they would carelessly or deliberately allow U235 to fall into the hands of a terrorist organization which could easily turn it into a working bomb.
Thank you so much for the corrections. They were very welcome. I knew I was beyond the limits of my knowledge I wrote that post but I thought it was worth the effort.
This was a very informative read; thank you. I am left with a question. How could it be that each of the two components of uranium are insufficiently large for there to be a significant probability of a chain reaction on their own, even over weeks or months of storage, but sufficiently large to create one within microseconds merely by being put together? Why is it that merely doubling the number of atoms would make the chance of a change reaction jump from virtually nil to virtually certain? If the bomb somehow put together trillions of small pieces of uranium this might make intuitive sense, but it is counterintuitive that a merely doubling would make that difference.
https://en.wikipedia.org/wiki/Critical_mass
To what extent, in your view, was JFK responsible for the Cuban missile crisis? Is it merely that the Soviet Union didn’t consider Latin America a safe-space for Communism until the Bay of Pigs (as someone suggested to me)? Or was JFK a cause of the crisis in a more direct way?
It was a drawing of the design of the implosion device which Julius and Ethel Rosenberg were accused of transmitting to the Soviets for which they were charged with treason and executed. They were convicted based on testimony by Ethel Rosenberg’s brother, David Greenglass, implicating Julius. After Greenglass’s death, information disclosed in Walter Schneir’s “Final Verdict,” indicated that Julius Rosenberg did not transmit the drawing, because the KGB had already taken him out of the loop when the drawing was transmitted. Ethel Rosenberg was for all intents and purposes unaware of her brother’s and husband’s involvement with the Communist Party, and was innocent.
One bit about legal unethical conduct by the judge, Irving Kaufman, who presided over the trial and who issued the death penalty. It was revealed after Judge Kaufman’s death that he had had inappropriate ex parte meetings with the prosecutor, outside of the presence of the defense counsel, in which the prosecutor advocated for the death penalty. Judge Kaufman, who was Jewish, acceded to the prosecutor’s recommendation, in his effort to remove what he regarded as the stain of Jewish treachery.
One more bit of trivia related to the Rosenbergs. Abel Meeropol, the songwriter who wrote “The House I Live In,” denouncing discrimination and made famous by Frank Sinatra, as well as “Strange Fruit,” the song made famous by Billie Holiday about the lynched corpses of African-Americans hanging from trees, adopted the two sons of the Rosenbergs after they were executed.
In response to the comment by Anonymous on 12/12 at 9:42 A.M., there is another definition of the word “entropy” in addition to the technical reference to thermal energy. Theoretical physicists used the term to refer to the presence of disorder and chaos in the universe. Entropy increases as the disorder and chaos increases. Formation of the various galaxies required/represented a reduction in entropy. My reference to the Paris jigsaw puzzle which Prof. Wolff and his wife completed was meant as a compliment, that by finishing puzzle and imposing order on a discombobulation of jigsaw puzzle pieces, they had succeeded in reducing the amount of entropy in the universe. Unfortunately, however, theoretical physicists believe that, overall, the amount of entropy is increasing in the universe and eventually will result in the dissolution of the galaxies, including our own. The concept is also used to explain what physicists refer to as “the arrow of time,” i.e., why does time appear to move “forward” rather than “backward”? For an excellent discussion of these and other subjects, I recommend Brian Greene’s “The Fabric of The Cosmos.”
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