Nuclear Fission and fissionable isotopes

[esmeralda4]

Hi,

My textbook says that only Uranium and Plutonium are fissionable but it doesn't say why.

Do smaller isotopes become bigger when they are bombarded by neutrons instead of spitting apart?

Thanks

 

[mfb]

There are more elements - most elements which are heavier than plutonium and some isotopes of lighter elements, too.
However, they have to be quite heavy to do that without external energy, otherwise the fissioned state would have a higher energy than the original nucleus.

If you bombard a nucleus with a neutron of sufficient energy, you can split every nucleus (apart from the single proton in hydrogen, haha). But if the energy is too low, this might be impossible (depends on the nucleus). In that case, the nucleus can absorb the neutron or it can do elastic or inelastic scattering.

 

[Drakkith]

Does this help? http://en.wikipedia.org/wiki/Fissionable#Fissile_vs_fissionable

 

[K^2]

Do smaller isotopes become bigger when they are bombarded by neutrons instead of spitting apart?

Some do. It depends on a lot of things. Take a look at the CNO Cycle for a neat example. It's proton capture, but principle is the same.

 

[esmeralda4]

Thanks for the replies.

So is the textbook wrong to suggest that uranium and plutonium are the only isotopes that are fissionable?

 

[LURCH]

These are the only elements humans split for enegy. Any element can be split, except maybe H. Any element heavier than iron can theoretically be split for a net release of energy (elements lighter than iron take more energy to split than they yield).

 

[mfb]

While the binding energy per nucleon has its highest value around iron-56 (26 protons, 30 neutrons): In which way would you split Zinc-70 (30 protons, 40 neutrons, mass 69.9253193(21)u) to release energy? Random example, works with all other nuclei in this range.
Iron-56 (mass 55.9349375(7)u) plus Beryllium-14 (14.04289(14) u)? Beryllium has an extremely low binding energy (compared to iron and zinc), which is much more important than the small difference between iron and zinc. Split it into two equal parts? Phosphorus 35 has a mass of 34.9733141(20) u, therefore this is impossible, too.

If you could split 56 Zinc-70 into protons and neutrons, make some beta decays and put it together to 70 iron-56 nuclei, it should release net energy. But that is not possible with fission.

(elements lighter than iron take more energy to split than they yield)

Well... Beryllium-8 likes to split into two alpha particles. Maybe some other nuclei can do fission, too.

 

[Drakkith]

Well... Beryllium-8 likes to split into two alpha particles. Maybe some other nuclei can do fission, too.

Beryllium 8 is a highly unstable isotope with a half life of 7×10^−17s. All stable isotopes of elements lighter than iron will not yield energy upon being split.

 

[mfb]

That is the reason why stable isotopes are stable and Beryllium-8 is not...
If fission can release energy, the nucleus is not stable - it could have some ridiculous large lifetime, but it would be unstable.

 

[Drakkith]

That is the reason why stable isotopes are stable and Beryllium-8 is not...
If fission can release energy, the nucleus is not stable - it could have some ridiculous large lifetime, but it would be unstable.

Of course.

 

[haruspex]

Thorium has been used in experimental reactors for many years. Some claim it is preferable to using uranium.

 

[QuantumPion]

Thorium has been used in experimental reactors for many years. Some claim it is preferable to using uranium.

Thorium can be transmuted into fissile Uranium-233 in a breeder reactor but natural Thorium cannot be used as fuel by itself.

 

[QuantumPion]

Thanks for the replies.

So is the textbook wrong to suggest that uranium and plutonium are the only isotopes that are fissionable?

U and Pu are the only (common) isotopes which are fissile. There are heavier elements which have fissile isotopes, such as curium, americium, californium, but they are only produced in minute quantities using reactors or accelerators.

There are other elements which are fissionable with fast neutrons, but with small cross sections. I know Thorium isotopes are fissionable, and IIRC Radium and Actinium have fissionable isotopes as well.

 

[esmeralda4]

Thanks again for all the replies. I have further question...

My textbook goes on to say...

"The fission neutrons need to be slowed down significantly to cause further fission of U 235 nuclei otherwise they would be traveling too fast to cause further fission."

To me this just reads "they need to be slowed down because they need to be slowed down".

Why do they have to be slowed down - if the neutrons are moving fast wouldn't that just make fission easier? Why is there a need for a moderator?

Thanks again.

 

[daveb]

I think (perhaps I'm wrong in my assessment) that you think when a neutron fissions nuclei, that the neutron hits the nucleus and it breaks apart, similar to how a billiard ball would break apart dozens of billiard balls that are glued together. This is called spallation. In fission, the neuron is actually absorbed into the nucleus, binds with it, but this becomes so unstable that it breaks apart (i.e., fissions). This all happens pretty quickly (on the order of 10^-25 seconds if I recall for the time it takes between absorption and fissioning, but don't quote me on the time).

If you take a look here at the section on Energetics, it explains pretty well the current model for how energy affects the fissioning for different isotopes.

 

[Astronuc]

Thanks again for all the replies. I have further question...

My textbook goes on to say...

"The fission neutrons need to be slowed down significantly to cause further fission of U 235 nuclei otherwise they would be traveling too fast to cause further fission."

To me this just reads "they need to be slowed down because they need to be slowed down".

Why do they have to be slowed down - if the neutrons are moving fast wouldn't that just make fission easier? Why is there a need for a moderator?

Thanks again.

A moderator is used to slow fast (MeV) neutrons to thermal energies (0.025 eV). This increases the likelihood (probability) that a neutron will cause fission in U-235. The books description is awkward and not strictly accurate.

Some of the commentary in this thread seems to confuse spallation for fission. Technically, an (n,α) reaction is spallation, not fission, although for light elements, the distinction is muddy or really, there isn't a distinction.

A good place to review nuclear reactions and their cross-sections is BNL site.
http://www.nndc.bnl.gov/sigma/index.jsp?as=235&lib=endfb7.1&nsub=10

If one picks an element, then an isotope/nuclide, one will find data for various nuclear reactions, including absorption and scattering, and fission for those isotopes that can fission.