Question on nuclear fusion?

Question

Guys look in nuclear fission involves in breaking of a heavier nucleus into lighter nuclei and in doing this the binding energy of nucleus is lost and hence enormous amount of energy is lost and i can understand that, but in nuclear fusion two or more nuclei combine to form a heavier nucleus and hence the binding energy should be acquired but rather its released, howcome this happens and i know that nuclear reactions occur in stars and the temperature required for the reaction is found to be of order 10 power 6 and during the beggining of formation of star, how did it attain that high temperature and it is easy to understand high temp. of stars coz of the energy released during fusion, however it is and one more doubt give me an example of fissile material apart from Uranium - 235 and 100% to points for the best answer. Thanks for answering if answered and hope for good answers coz i wud like to understand the subject.

Answer 1

When there are a low number of nucleons (approximately under 63), nuclei can combine with a net release of energy. In other words, a more stable and less energetic configuration is one with MORE nucleons. In this range, the binding forces are much stronger than the Coulombic forces that push the protons away from each other. Thus, if enough energy is added to overcome the initial Coulombic repulsion, the binding forces will be so strong that the nucleons will combine and release more energy than was added. This is what occurs in nuclear fusion.

Around 63 nucleons, this trend reverses. There become so many nucleons in the nucleus that the binding force (that decays very quickly) cannot reduce the tension due to the Coulombic forces enough to make adding another nucleon release any energy. The result is that more energy has to be added in order to increase the number of nucleons. This additional energy is what is released in nuclear fission.

It may help to visualize a nucleus as a ball whose radius increases with the number of nucleons. As the radius increases, the strong force holding the nucleus together quickly decays. As it decays, the Coulombic repulsion begins to dominate. Thus, the strong force is king up through 63 nucleons and the Coulombic force becomes king after 63 nucleons.

That being said, in theory any material with less than 63 nucleons could be a candidate for exothermic nuclear fusion and any material with more than 63 nucleons would be a candidate for exothermic nuclear fission. You might be interested to know that the binding energy per nucleon makes a huge leap between hydrogen and helium and then actually decreases immediately after that. A similar spike happens between Berilium and Carbon. In other words, a Helium atom is extremely stable.

Finally, the temperature in a star (in fact, its low entropy in general) is a result of the initially high temperature (and extremely low entropy) at the moments around the time of the Big Bang. The early universe was extremely hot and, at some moments, was very uniform. Because gravity causes things to clump, this uniform configuration was extremly energetic. Eventually gravity caused regions of matter to clump together, and this clumping converged that energy into heat. It is this sort of clumping and heating that has the ability to ignite stars.

Answer 2

As another answerer wrote:

"Typical fusion reaction:

H1 + H1 -> He2 + energy"

However, the sum of the masses for the two H atoms is actually greater than mass for a single He atom. That's why, to balance the reaction relationship, we get the released energy on the right hand side of the relationship. That released energy comes from E = mc^2, where m is the amount of loss mass equal to the difference in the sum of masses for the two hydrogens and the mass of the one helium. And, as mentioned by another answerer, the enormous energies of the sun, nuclear weapons, etc. come from the huge number of atoms involved in the fusion reaction.

Although hydrogen fusion is feasible, deuterium fusion is the most useful for nuclear fusion energy. A deuterium is a hydrogen isotope containing one proton and one neutron, rather than just the one proton of ordinary hydrogen. [See source.]

Answer 3

Its all to do with the neutrons.

For small atoms the residual strong force (attractive) does a very good job of overcoming the electrostatic force between protons. This means that smaller atoms are tightly bound. Binding energy per nucleon (bit of the nucleus) increases the bigger you get the nucles because of this until you reach iron.

From iron onwards the electrostatic force wins, and you cannot add enough neutrons to compensate. Binding energy per nucleon gets lower after iron.

So you can release energy by fusing nucleons smaller than iron, and by breaking up (fission) nucleons larger than iron.

Answer 4

It's all to do with binding energies per nucleon.

If the two atoms you are trying to fuse have a binding energy greater than that of the fused atom then energy will be released as a result of the fusion.

Iron has the largest binding energy per nucleon and so all atoms lighter than Iron can release energy via fusion. All atoms heavier than Iron will release energy via fusion.

Answer 5

Typical fusion reaction:

H1 + H1 -> He2 + energy

Two Hydrogen atoms fuse to form one Helium atom. The sum of the binding energy of the two Hydrogen atoms is less than the binding energy of the single helium atom. so the excess energy is released.

This is a tiny amount of energy for individual atoms; but multiply by Avogadro's number (6.023 x 10^23) and you get a very big bang.

Answer 6

http://science.howstuffworks.com/nuclear-bomb.htm and http://science.howstuffworks.com/nuclear-power.htm and http://science.howstuffworks.com/nuclear.htm might be helpful links.

Also, Before I figured this out, I never understood how stars could be so dense if we don't have elements on Earth that are similarly dense. A spoonful of lead will not be as heavy as a spoonful of star... The secret is that the immense heat causes the repellent force of the electrons to break down and the huge empty spaces between the nucleus and the electrons now can be filled with other nucleii from surrounding atoms.

Hopefully this helps!

Answer 7

Kelvin-Helmholtz (Sp?) Radiation?