Blasts via nuclear fission

[Amaton]

When discussing atomic bombs, there's always been the notion of "splitting the atom" and "undoing" the strong interaction. Is this truly the sole force behind the most feared weapon in history?

 

I've always wondered if the fission just sets up a much larger reaction of combustive materials... Or if it's the actual fission itself that does most (or all) the damage.

[swansont]

The reaction is not combustion.

 

The energy released in fission occurs primarily because you are reducing the electrostatic repulsion; the nuclear attraction has saturated for the nuclei in question. Fusion reactions, OTOH, occur for small nuclei where the interaction has not saturated, so the energy released is from increased nuclear binding, which is larger than the increase in electrostatic repulsion.

[Moontanman]

The energy come from fission, not combustion...

 

Oppps swansonT cross post...

Edited January 21, 2013 by Moontanman

[Jacques]

The H bomb use dynamite to ignite a fission reaction, that ignite a fussion reaction.

The first explosion compress the fissible material (Uranium or plutonium) over the critical mass where the chain reaction start. That fission explosion compress and heat some hydrogen or deuterium that start fussioning.

Edited January 23, 2013 by Jacques

[Amaton]

Thank you for the responses.

 

The energy come from fission, not combustion

 

Okay. Though I thought I've read somewhere that there is some kind of chemical combustion involved, apart from the obvious fission mechanism.

 

Fortunately, I was able to find the following thanks to somewhat meticulous link searching...

 

Wikipedia:

 

The immediate energy release per atom is about 180 million electron volts (MeV), i.e. 74 TJ/kg. Only 7% of this is gamma radiation and kinetic energy of fission neutrons. The remaining 93% is kinetic energy (or energy of motion) of the charged fission fragments, flying away from each other mutually repelled by the positive charge of their protons (38 for strontium, 54 for xenon). This initial kinetic energy is 67 TJ/kg, imparting an initial speed of about 12,000 kilometers per second. However, the charged fragments' high electric charge causes many inelastic collisions with nearby nuclei, and thus these fragments remain trapped inside the bomb's uranium pit and tamper until their motion is converted into heat. This about a millionth of a second (a microsecond), by which time the core and tamper of the bomb have expanded to plasma several meters in diameter with a temperature of tens of millions of degrees.

 

This is hot enough to emit black-body radiation in the X-ray spectrum. These X-rays are absorbed by the surrounding air, producing the fireball and blast of a nuclear explosion.

 

So the blast is effectively an indirect result of the energy released via fission.

 

swansont

The energy released in fission occurs primarily because you are reducing the electrostatic repulsion; the nuclear attraction has saturated for the nuclei in question.

 

Which is to say the nuclear attraction reaches a maximum, resulting in its overcoming and the fission of the nucleus? Just making sure I'm getting this.

[swansont]

Okay. Though I thought I've read somewhere that there is some kind of chemical combustion involved, apart from the obvious fission mechanism.

 

There will be conventional explosions triggering the bomb, as Jacques mentioned. You have to move the subcritical masses together.

 

 

Which is to say the nuclear attraction reaches a maximum, resulting in its overcoming and the fission of the nucleus? Just making sure I'm getting this.

 

Once the nucleus reaches a certain size, each nucleon added feels the same attraction, because the nuclear force has a finite range. We say that the force has saturated when that happens. But the electrostatic force has infinite range, so all the protons repel each other, and more protons means more repulsion. What's happening in fission is the nucleus is in an excited state from the neutron absorption and it acts like a drop of water that has extra energy - it jiggles and ceases to be a sphere. That deformation allows it to form two lobes, which now repel each other and it splits apart. Since each part is now more tightly bound (from the reduction in electrostatic repulsion) the reaction releases energy.

[Amaton]

Once the nucleus reaches a certain size, each nucleon added feels the same attraction, because the nuclear force has a finite range. We say that the force has saturated when that happens. But the electrostatic force has infinite range, so all the protons repel each other, and more protons means more repulsion. What's happening in fission is the nucleus is in an excited state from the neutron absorption and it acts like a drop of water that has extra energy - it jiggles and ceases to be a sphere. That deformation allows it to form two lobes, which now repel each other and it splits apart. Since each part is now more tightly bound (from the reduction in electrostatic repulsion) the reaction releases energy.

 

Nice and clear explanation, thanks. A little confused with the last sentence. Where exactly did the released energy come from?

[swansont]

Nice and clear explanation, thanks. A little confused with the last sentence. Where exactly did the released energy come from?

 

The reduction in electrostatic repulsion is negative work, i.e. released energy.

[Amaton]

The reduction in electrostatic repulsion is negative work, i.e. released energy.

 

Ah, I understand now. Considering I have little experience in physics and was able to follow that easily, I'd say your explanations were great. Thanks a lot