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Poor man's nuclear energy?

by truhaht
Tags: energy, nuclear, poor
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Morbius
#19
Jan25-09, 12:27 PM
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Quote Quote by truhaht View Post
And now our enemies have the recipe they've longed for?
truhaht,

It's a pretty OBVIOUS thing to do. Additionally, Pu-238 production doesn't give one the recipe for
a bomb. One needs fissile Pu-239 for a nuclear weapon - not radioactive Pu-238.

In fact, Pu-238 is something you want to LEAVE OUT OF a nuclear weapon - the Pu-238 doesn't
help in the fission reaction - and its radioactivity and heat just complicate matters.

Dr. Gregory Greenman
Physicist
vanesch
#20
Jan25-09, 12:48 PM
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Quote Quote by Morbius View Post
In fact, Pu-238 is something you want to LEAVE OUT OF a nuclear weapon - the Pu-238 doesn't
help in the fission reaction - and its radioactivity and heat just complicate matters.
I understand why one wants to leave out Pu-238, but nevertheless, the k_inf is more than 2.5 in its own fission spectrum. It is not fissile in a thermal spectrum, but it is in a fast spectrum.
Morbius
#21
Jan25-09, 03:13 PM
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Quote Quote by vanesch View Post
It is not fissile in a thermal spectrum, but it is in a fast spectrum.
vanesch,

Actually, Pu-238 IS fissile. At room temperature 0.025 eV;
Pu-238 has a (n,fission) microscopic cross section of about 15 barns.

That is in comparison to about 700 - 800 barns for Pu-239.

So Pu-238 is fissile - but it isn't going to help much - and the
negatives are overwhelming.

A nuclide can't be not "fissile" in a thermal spectrum but fissile in
a fast spectrum- by definition.

The word "fissile" means that the nuclide will fission with thermal neutrons.

If a nuclide like U-238 that will only fission with neutrons of energy above
a certain threshold - like with fast neutrons - those nuclides are termed "fissionable".

U-238 is NOT "fissile" - but it is "fissionable".

Dr. Gregory Greenman
Physicist
vanesch
#22
Jan26-09, 03:07 AM
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Quote Quote by Morbius View Post
A nuclide can't be not "fissile" in a thermal spectrum but fissile in
a fast spectrum- by definition.

The word "fissile" means that the nuclide will fission with thermal neutrons.

If a nuclide like U-238 that will only fission with neutrons of energy above
a certain threshold - like with fast neutrons - those nuclides are termed "fissionable".

U-238 is NOT "fissile" - but it is "fissionable".

Dr. Gregory Greenman
Physicist
You are right concerning terminology. I should have said fissionable, not fissile.

I just wanted to point out that a big enough lump of Pu-238 will undergo just as well a fast power excursion (if you see what I mean) as a big enough lump of Pu-239, as its k_inf value is far above 1. (which is not the case for U-238 btw).
Morbius
#23
Jan26-09, 08:21 AM
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Quote Quote by vanesch View Post
You are right concerning terminology. I should have said fissionable, not fissile.

I just wanted to point out that a big enough lump of Pu-238 will undergo just as well a fast power excursion (if you see what I mean) as a big enough lump of Pu-239, as its k_inf value is far above 1. (which is not the case for U-238 btw).
vanesch,

For high energy neutrons, the fission cross-sections for Pu-238 and Pu-239 are quite comparable.

The problem is that you are not going to be able to get the Pu-238 to the same densities that one
can with Pu-239.

The problem with looking at just k-infinity is that infinite sized devices are really tough to transport
to the target.

Dr. Gregory Greenman
Physicist


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