[paul cotter (OP)]

Andrei Sakharov who designed the largest thermonuclear bomb ever tested was tasked with producing a 100megaton device. Since this was to be tested over the territory of the then ussr he was concerned about fallout and suggested a 50mton as an alternative and this was accepted. The yield was~57mton and said to be 97% fusion derived and 3% fission derived and this I don't understand. In a thermonuclear device the initial fission reaction compresses and heats the lithium deuteride fusion fuel ; the fusion reaction releases energy, principally in the form of energetic neutrons and in most devices these neutrons are used to fast fission u238 either in the form of natural or depleted uranium. The fission of u238 is where the majority of the destructive energy comes from ( or so I thought ). If the u238 is left out one has an enhanced radiation or so called neutron bomb. So how did Andrei Sakharov produce a 97% fusion device? Or am I just misinformed.

[MikeFontenot]

Andrei Sakharov who designed the largest thermonuclear bomb ever tested was tasked with producing a 100megaton device. Since this was to be tested over the territory of the then ussr he was concerned about fallout and suggested a 50mton as an alternative and this was accepted. The yield was~57mton and said to be 97% fusion derived and 3% fission derived and this I don't understand. In a thermonuclear device the initial fission reaction compresses and heats the lithium deuteride fusion fuel ; the fusion reaction releases energy, principally in the form of energetic neutrons and in most devices these neutrons are used to fast fission u238 either in the form of natural or depleted uranium. The fission of u238 is where the majority of the destructive energy comes from ( or so I thought ). If the u238 is left out one has an enhanced radiation or so called neutron bomb. So how did Andrei Sakharov produce a 97% fusion device? Or am I just misinformed.

I also remember concluding at some point that all the modern "thermonuclear" bombs actually get almost all of their energy release from fission, not fusion, and that the only real bomb that got its energy release from fusion was that one huge bomb set off in Siberia.  But when I was a kid, I remember that the post-WW2 bombs were called "hydrogen bombs".

[paul cotter (OP)]

Thanks Mike, for your reply, I was beginning to think that no one was interested in the physics question involved given that this topic may not be flavour of the month with the current geopolitical situation. Your interpretation is the same as mine. Off topic, I believe Andrei Sakharov was a hero: as a brilliant young scientist he was twice asked to join the weapon program and twice declined, the third time he was given no choice. He became increasingly spooked by the nature of his work and together with the oppressive political atmosphere in the ussr he became a dissident. He and his wife Yelena Bonner were exiled internally and treated most cruelly by the authorities. Asked in later life how he felt about his technical achievements he said "truth is more powerful than any weapon". The lack of truth is, in my opinion, one of the greatest challenges facing man, if not the greatest.   

[MikeFontenot]

In a similar vein, long ago I was very surprised when I tried to see an actual conversion of mass into energy (a` la E = m c^2) in the WW2-type fission bombs.  I thought I should be able to look at all the particles before the reaction, then compare them with all of the particles after the reaction, and find at least one particle that was there before the reaction but was NOT there after the reaction.  I.e., I thought I should be able to identify at least one particle that was now missing, because it had been converted into energy ... "poof".  But ALL the original particles were still there after the reaction!  All that happens is that one or more of the particles just weigh slightly less after the reaction.  All the hype about that equation is overblown in my opinion.  (Particles CAN disappear in particle/anti-particle interactions, but not in interactions consisting of only particles, without any anti-particles.)

[evan_au]

ALL the original particles were still there after the [fission] reaction!

I guess you could say that there is the same number of baryons (protons+neutrons) after the reaction as before.

However, a nuclear fission reaction releases an immediate burst of neutrons from the Uranium/Plutonium atoms during the fission chain reaction.
- Plus gamma rays, X-Rays and light (which were not physically present in the original lump of fissile material)
- If not absorbed by another atom, the neutrons will decay into protons, releasing an electron and neutrino (which were not physically present in the original bomb)
- The daughter nuclei are also radioactive and will emit a variety of particles as they decay over time

Overall, the number of electrons and neutrinos increases after the explosion (there is a net conversion of neutrons into protons + electrons + neutrinos)
- And a change in the mix of quarks, as neutrons (1 up + 2 down quarks) gets turned into protons (2 up+1 down quarks)

The loss of mass comes from the nuclear binding energy, much of it in the form of kinetic energy of the daughter products in the explosion, plus gamma rays as the baryons shuffle into a more stable form
- If you ignore the gamma rays, and slow down the daughter products to a halt, you will find that the mass of the products is less than the mass of the original Uranium nucleus.
- It is this "missing mass" that is converted to energy