# Reactor Fission VS. Weapon Fission

1. May 27, 2009

### jmatejka

Perhaps some semantics are involved here, Wikipedia and some others describe nuclear reactor material as going through a slow "fission" process. I have always thougt of it more as "natural decay".

I visualize fission in a weapon, as the "breaking" of the atom, which creates new atoms and releases energy.

If we argue these are BOTH fission,(and we used identical radioactive elements), are the decays the same?(just occuring over a different span of time?)(my guess is not), my guess is the weapon,(nuclear detonation),produces different decay particles than the "natural decay" of the reactor ever would.

Perhaps "fission" is a more broad term than I thought, any clarification is appreciated. Thanks, John

2. May 27, 2009

### fatra2

Hi there,

Nuclear power plant

Assuming that we use a mixture of uranium, it would be stupid to wait for the spontaneous decay to happen, as it would take very very long. The decay of, let's say U-235, has many different possibilities, which one is spontaneous fission, where the parent nucleus fissions into two daughter nuclei (and a big bunch of energy). Since, we are very busy people, and we don't have time to wait many hundreds of billions of years, we found a way to induce this fission of heavy nuclei. We just need to send a neutron to collision with the heavy nucleus, and boom the fission happens within a few splits of a second, freeing two or three neutrons along the way. These neutrons can then induce some more fission, and so on, having a CHAIN REACTION.

The process of induced nuclear fission is a quite fast one ($$10^{-6}s$$ if I remember right!!!), therefore I would not call it a slow process. But, since every fission frees at least two neutrons, the growth is tremendous. To "slow" down this growth, "nuclear poison" is inserted into the reactor core (either control rod or soluble poison), having a sustain chain reaction for many years if necessary.

Nuclear weapons

In the case of dirty bombs, radioactive materials is included in the bomb. The radioactive material is spread around where the bomb explodes. But, I suspect that you were not thinking of dirty bombs in your question.

In a nuclear bomb (also called H bomb), plutonium P-239 is used in the nuclear head. This plutinium, being very radioactive fissions spontaneously, and quite fast, creating alot of energy. This energy is confined into a very small area filled with hydrogen, heating the gas. When the gas reaches a certain temperature ($$\sim 10^8 K$$), the hydrogen can start fusing, liberating some more energy.

In this idea, the atom bomb recreates a small sun on Earth.

Cheers

P.S. my explanation are very general. I did not go into anymore details, because I just did not feel like it.

3. May 27, 2009

### mgb_phys

Fission is one, fairly rare, mode of radioactive decay.
An atom spontaneously splits in two releasing more than one neutron, those neutrons split other atoms and so on. If each splitting atom goes on to split more than one other atom you have a chain reaction which will ultimately release a lot of energy.
The rate of splitting depends on how you arrange the material, put a lot of it close together in a bomb and each decay will cause many other atoms to split giving a rapid release of energy. For a bomb you generally add an extra source of neutrons to get things going.
In a reactor the fuel is more spread out and so each decay produces less other decays. Designing a reactor is a balance between keeping the chain reaction going and not making a runaway reaction.

A reactor also contains a moderator to slow neutrons but this is used to INCREASE the reaction rate. Slow neutrons are more effective at splitting an atom than fast ones ,it seems odd but think of a fast one going straight through an atom without having time to dump any energy.

Last edited: May 27, 2009
4. May 27, 2009

### QuantumPion

Fission just means a nucleus splits into two or more parts. Technically, alpha decay is a form of fission.

You are correct that the fission by neutron absorption of Uranium produces different fission products then the alpha decay of Uranium. Alpha decay of U-238 produces Th-234 + He. Fast fission of U-238 produces a spectrum of possible products such as Cs and Tc.

5. May 27, 2009

### fatra2

Hi there,

In the definition of it, you are absolutely right. Fact is that in practice, radioactive decay can occur thorugh three types of emission (alpha, beta, gamma), and sometimes through spontaneous fission.

6. May 27, 2009

### jmatejka

Much good information, Thanks!

7. May 27, 2009

### mathman

One point that has not been discussed in comparing reactors to bombs is that in reactors, neutrons are slowed down by the moderator. The fission cross-section for the fissionable material is much higher for thermal neutrons as compared to fast neutrons (fast neutrons are what comes out of the fission process).

This has important ramifications, such as what is happening in Iran. Both reactors and bombs (when using U235) need enriched (higher U235 % than natural in U used) material. However the enrichment needed for a bomb is much higher than that for a reactor. Iran asserts that they are enriching only to reactor grade, but the rest of the world is concerned that they could continue enriching until they get bomb grade.

8. May 27, 2009

### Bob S

Fission requires a neutron to initiate it, so it is not natural radioactive decay. But fission is also natural. The natural reactor at the Oklo site in Gabon (Africa) ran for about 1 million years. See
http://www.ocrwm.doe.gov/factsheets/doeymp0010.shtml [Broken]

Last edited by a moderator: May 4, 2017
9. May 27, 2009

### mgb_phys

No, you can have spontaneous fission it's just that the rate is rather low.
IIRC for U235 the half life is 10^9 years and you only get a few neutrons/hour/kg

10. May 27, 2009

### Bob S

The number of alpha decays per hour is over 1011 per kg (=2.5 x 1024 nuclei), so the branching ratio is rather low.

11. May 28, 2009

### mathman

The reason that this occurred was that the U235 concentration was high enough so that it was able to function as a reactor. U235 has a shorter half life than U238 so natural Uranium mines cannot function as reactors today.

Last edited by a moderator: May 4, 2017