Difference between fusion and fission

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1. Oct 18, 2014

cl_steele

Hi there,
Sorry if thisis a stupid question its just been bugging me for a while and im not a scientist as such so need help with the answer...
nuclear fission is where two atoms collide and release energy yes?
nuclear fusion is where two atoms combine to release energy and a new isotope however.. this is my question;
these two methods of nuclear reaction same the basic core principle of two atoms colliding how does the differentiation occur when the method as best i see is the same?
Thank you :)

2. Oct 18, 2014

Staff: Mentor

No, fission is when a single nucleus splits into two smaller ones plus maybe some "small change" like a couple of neutrons.

Where did you see that fission is when two nuclei (atoms) collide?

3. Oct 18, 2014

cl_steele

Because atoms dont tend to just split in half if i remember rightly?
And School, College and a friend who studies physics albeit the wrong branch

4. Oct 18, 2014

cl_steele

i should point out im talking in an atomic weapons stand point

5. Oct 18, 2014

Staff: Mentor

That's true, something has to "trigger" the fission. In the classic example of uranium or plutonium fission in A-bombs, the trigger is a slow neutron. To start the chain reaction, the trigger neutron comes from some external source, or maybe it's just a stray neutron from the environment or the fissionable material itself. (I'm not an expert on the exact engineering details of these things.) Then the neutrons released in the fission process (the "small change" I referred to) trigger fission in more nuclei.

6. Oct 18, 2014

Drakkith

Staff Emeritus
Radioactive elements do not need a trigger, they will spontaneously split into less massive elements. However, a good hit by a neutron or other particle can cause them to undergo fission.

7. Oct 18, 2014

SteamKing

Staff Emeritus
The principal elements to which fission and fusion reactions occur lie at different ends of the periodic table. Fission reactions occur more frequently for heavier elements, like uranium, which have high atomic numbers (uranium has the highest atomic number of the naturally occurring elements). Fusion reactions occur more readily (albeit with great difficulty) when the nuclei of two lighter elements combine. The element with the lowest atomic number is hydrogen, and isotopes of this element are the ones used in H-bombs.

8. Oct 18, 2014

Chronos

In fusion, the process is essentially reverse that of fission. In the core of a star hydrogen fusion occurs when two hydrogen atoms fuse together. The immense heat inside the core allows the hydrogen atoms to acquire sufficient kinetic energy to overcome their mutual electrostatic repulsion and binding energy. The 'waste' product of this reaction is a helium atom, which is less massive than the two hydrogen atoms from which it formed. This missing mass is converted to energy. Hydrogen fusion releases more energy than fission and the waste product is not toxic.

9. Oct 19, 2014

Staff: Mentor

Individual nuclei of fissionable isotopes (e.g. 235U) don't fission by themselves, but if you have enough of them close together, stray neutrons will trigger fission and a chain reaction. This is where the term "critical mass" originally came from.

During the Manhattan Project, they had to do experiments to study this in the context of bomb design, by bringing two chunks of enriched uranium closer and closer together, while watching radiation detectors. People died of radiation poisoning as a result of these experiments.

http://en.wikipedia.org/wiki/Louis_Slotin

10. Oct 19, 2014

Drakkith

Staff Emeritus

11. Oct 19, 2014

Staff: Mentor

OK, with a half-life of 7x108 years. I remember seeing that page before, but I forgot about it.

You definitely need to goose that rate up in order to make a usable bomb!

12. Oct 19, 2014

Drakkith

Staff Emeritus
Oh yeah, absolutely.

13. Oct 19, 2014

Staff: Mentor

Actually, the half life of 7x108 years includes all modes of decay. According to

http://atom.kaeri.re.kr/ton/nuc11.html

the usual decay mode is alpha decay to 231Th. The branching ratio (fraction of decays) for spontaneous fission (SF) is 7x10-9.

If 235U decayed only via spontaneous fission, the half-life would be 1017 years.

14. Oct 19, 2014

Staff: Mentor

Augh! That page gives the branching ratio as a %. As a fraction it would be 7x10-11 which makes the SF-only half-life 1019 years. oo)

15. Oct 19, 2014

Staff: Mentor

Still enough to occur frequently in a chunk of uranium - and even a single one of them can start a chain reaction (with induced fission then). In nuclear bombs, this can start the chain reaction too early (before all the material is close together) and lead to a significantly weaker explosion. That issue makes all the weapon designs more complicated (especially for plutonium, where spontaneous fission is much more frequent) just to reduce this risk.

16. Oct 19, 2014

Staff: Mentor

I get an average of about one spontaneous fission every three minutes in a 1-kg chunk of pure 235U. That's a nice little exercise.

17. Oct 19, 2014

Staff: Mentor

238U has more spontaneous fission (5*10-7 or 9*1015 years) - even in highly enriched uranium, this is dominant.
Little Boy (the Hiroshima bomb) had 70 spontaneous fissions per second in ~70kg uranium. Wikipedia says 10% risk of an early detonation - without source, but I saw a similar number in a source before.

18. Oct 19, 2014

snorkack

Does lithium produce energy by fusion or fission?

Reactions
2Li-6->C-12+$\gamma$
and
2Li-6->3$\alpha$
both produce energy. But I suspect that the first reaction is harder to pull off (electromagnetic process, not strong). So, since the count of nuclei increases in the second reaction, is it fission? But one lithium nucleus does not produce energy by fission...

19. Oct 19, 2014

Staff: Mentor

Both processes involve the strong interaction, but the second one looks more likely if the energy is sufficient.
The definition of fission and fusion is a bit tricky if you look at reactions like those.

20. Oct 20, 2014

e.bar.goum

Unless you're very very very lucky (i.e. you populate the Hoyle state, and are in the <1% of that state that decays via a $\gamma$), most of the time,

2Li-6->C-12->3$\alpha$ will happen.

Not the first reaction.

In any case, I'd describe the latter reaction as "fusion-fission" (a fusion event happens first, followed by a fission). But as mfb rightly said, the definition becomes tricky with very light reactions like these.

For instance, is

7Be + d -> 8Be + p

transfer or fusion-fission or fusion-quasi-fission? It depends on whether there is an intermediate 9B state, which will depend on the energy and angular momentum you bring in with the beam. Reaction dymanics are a lot more complex than just fusion and fission!