# The difference between fusion and fission?

i'm new to nuclear power and i wanted to know some things ANY help/ insight would be appreciated. im still in highschool and im just interested in nuclear power
1. does fusion always involve just 2 atoms? i know that fusion and fission both require huge amounts of energy so, is there no way to "bundle" fusion reactions togther to make them more practical?
2. how much U-235 is used in a fission reaction on average?
3. someone asked me to compare the yields of fusion and fission... any explainations?
4.i found this on the web "For fission of U or P, energies released are around 200 MeV or so. The energy per event is greater (in these examples) in fission, but the energy per nucleon (fusion = about 7 MeV/nucleon, fission = about 1 Mev/nucleon) is much greater in fusion." is this always the case? even when fission reaction are done in large quantities are they always less than fusion reactions(per nucleon)?

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1. There might probably be fusion cases where more than 2 atoms are involved, but for calculation purposes, it wouldn't matter. Input/output of the overall would still be the same.

2. You can use any amount. Fission of U-235 is self-propagating, all you'll need is the initial bombarding neutron.

3. Because the energy you can get per mass of initial is different, and one produces more than the other. Fusion is more efficient, by the way.

4. It's probably a comparison between uranium and hydrogen. And of course when fission reactions are done in large quantities there would be more reactions than fusion. However, they usually compare it per mass.

Drakkith
Staff Emeritus
1. does fusion always involve just 2 atoms? i know that fusion and fission both require huge amounts of energy so, is there no way to "bundle" fusion reactions togther to make them more practical?
For all intents and purposes for use on earth, only 2 nuclei are involved. Only rarely do more than 2 nuclei fuse together at once.
3. someone asked me to compare the yields of fusion and fission... any explainations?

4.i found this on the web "For fission of U or P, energies released are around 200 MeV or so. The energy per event is greater (in these examples) in fission, but the energy per nucleon (fusion = about 7 MeV/nucleon, fission = about 1 Mev/nucleon) is much greater in fusion." is this always the case? even when fission reaction are done in large quantities are they always less than fusion reactions(per nucleon)?
A nucleon is a proton or neutron, since they occupy the nucleus. The energy released in each Fission event is much greater than Fusion events. (An event is one fission or fusion of nuclei) However, if you look at Uranium, it has around 47 times the mass of the easiest fusion fuels, Tritium-Deuterium. (5 in D-T Fusion, Deuterium has a mass of 2 and Tritium has a mass of 3, compared to 238 in Uranium)

This means that if you use 10 kg of fuel for fission and for fusion, the fusion will give you 47 times the amount of energy that the fission one. (Assuming you used Uranium for fission fuel)

in the equation where U^235 SPLITS UP INT0 B0R0N and KRYPT0N....masses on both sides of the equation are same in nuclear fission reaction....then which mass deficient cause the release of high amount of energy?????can anyone please explain this???

QuantumPion
Gold Member
in the equation where U^235 SPLITS UP INT0 B0R0N and KRYPT0N....masses on both sides of the equation are same in nuclear fission reaction....then which mass deficient cause the release of high amount of energy?????can anyone please explain this???
The difference in mass comes from the binding energy of the nucleus. Basically, the sum of the parts weighs more than the individual parts. When you fission a heavy nucleus, the leftover products weigh less than the original nucleus. The difference in mass is converted into heat and radiation.

Disconnected
Gold Member
thanks
Note that the mass loss per fission is minute - a very, very small percentage of the mass of the original U235.

"You can use any amount. Fission of U-235 is self-propagating, all you'll need is the initial bombarding neutron."

not correct...you need a "critical mass" to sustain a nuclear reaction...enough material to capture neutrons and release new ones to keep the reaction going. A subcritical mass is a mass of fissionable material that does not have the ability to sustain a fission reaction. That's one reason "suitcase" nuclear devices are really big trunk sized affairs....You can't carry that kind of thing around by hand.

see here for more:

http://en.wikipedia.org/wiki/Critical_mass_(nuclear [Broken])

Nuclear fission is a process where a large atomic nucleus (such as uranium) is split into two smaller particles; In fusion, multiple smaller nuclei join into a more massive nucleus.

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