Why Is PU-239 Preferred in Nuclear Weapons Over Other Fissionable Isotopes?

[The Prince]

Hey there ,

1-I wonder why PU-239 is basically used in nuclear destructive weapons, while U-33, U-35,U-38 (with Energy more than 1 Mev) are fissionable materials as well??!


_ The Prince .

 

[Morbius]

Hey there ,

1-I wonder why PU-239 is basically used in nuclear destructive weapons, while U-33, U-35,U-38 (with Energy more than 1 Mev) are fissionable materials as well??!


_ The Prince .

The Prince,

As far as using U-238; as you point out, it is "fissionable" [ fissions for neutrons above
a 1 MeV threshold], instead of "fissile". So you don't get U-238 to go critical by itself.

As far as U-235 vs Pu-239; both can be used to make nuclear weapons, and have been.
The Little Boy bomb that destroyed Hiroshima was fueled by U-235.
The Fat Man bomb that destroyed Nagasaki was fueled by Pu-239.

The reason for using one over the other has to do with the cost of making the stuff.
In order to make a weapon with U-235, it has to be highly enriched. Natural Uranium
is only 0.7% U-235; the other 99.3% is mostly U-238.

In order to get a highly enriched Uranium; one that has a very high percentage of U-235;
you must separate the U-235 and U-238 isotopically. You can't do that by chemistry,
since they are both Uranium. It is difficult to separate the two isotopes.

The way you make Plutonium, is to fuel a reactor with a mix of U-235 and U-238. The
U-235 fuels the reaction, and the U-238 is a fertile material that can turn into Pu-239
when it absorbs a neutron. After irradiation in the reactor, one needs to separate out
the Pu-239. However, because the desired product is Plutonium and not Uranium;
it is chemically different, and chemical means can be used to separate out the Pu-239.

Dr. Gregory Greenman
Physicist

 

[tehfrr]

Pu-239 also gives you a smaller critical mass due to more neutrons/fission (Nu) and a larger neutron capture cross section

 

[entropiax]

And the U-233 is too radioactive...

 

[entropiax]

Other aspect.
Enrichment of uranium is very difficult, and breeding of plutonium also.
I think the theory of hungarian guys is dangerous (other topic), because that process open possibility to breeding of plutonium with spallation neutrons, without uranium enrichment.

 

[Morbius]

Pu-239 also gives you a smaller critical mass due to more neutrons/fission (Nu) and a larger neutron capture cross section

tehfrr,

Yes - however that is offset by the larger neutron background of Plutonium vis-a-vis
Uranium. You have to assemble Plutonium faster than Uranium.

Of course that is a solved problem.

Dr. Gregory Greenman
Physicist

 

[mathman]

One advantage that U-235 has over Pu-239 is that the bomb making is easier. U-235 bomb can be set off by slamming two pieces together, while Pu-239 requires a much more sophisitcated technique (implosion).