87 grams critical mass for plutonium-239

[Nick]

This is a wonderful document and a pleasure to read:

ANOMALIES OF NUCLEAR CRITICALITY, REVISION 6
E. D. Clayton
Edited by
A. W. Prichard, B. M. Durst, D. G. Erickson and R. J. Puigh
February 2010
Pacific Northwest Laboratory
Richland, Washington 99352

Apparently netiquette requires that its URL not be revealed, but
anyone can Google for it if they are so inclined. On page 152 the
author discusses use of beryllium hydride BeH2 as a moderator.
Moderation reduces critical mass as does use of a neutron reflector.
The author presents a table with three candidate neutron reflectors,
H2O, BeH2 and BeO. For obvious reasons of nuclear security the author
does not discuss the geometric configuration considered.
With the use of BeO neutron refector critical mass of plutonium-239 is
reduced to 87 grams.

Elsewhere in this treatise, the author discusses Lithium-6 Deuteride
as a neutron moderator. This is odd because a neutron moderator needs
to be squashed right next to or mix in some mechanical arrangement
with a fissile material. For obvious reasons Li6D can not be a good
neutron reflector.

My question is this: is there any better document than this, in the
open source literature for thermonuclear weapons design?

[Bill Baker]

Post by Nick
Apparently netiquette requires that its URL not be revealed, but
anyone can Google for it if they are so inclined. On page 152 the
author discusses use of beryllium hydride BeH2 as a moderator.
Moderation reduces critical mass as does use of a neutron reflector.
The author presents a table with three candidate neutron reflectors,
H2O, BeH2 and BeO. For obvious reasons of nuclear security the author
does not discuss the geometric configuration considered.
With the use of BeO neutron refector critical mass of plutonium-239 is
reduced to 87 grams.

More likely he doesn't bother to discuss geometries because there
aren't any practical ones that allow you to achieve prompt criticality
with such a small mass of Pu. Likely the mass of needed moderator is
so high and the rate of neutron propagation is so slow that you get
disassembly long before achieving any useful efficiency. Notably, both
the test shots of designs using thermal neutrons were fizzles.

[Carey]

Post by Nick
Apparently netiquette requires that its URL not be revealed, but
anyone can Google for it if they are so inclined. On page 152 the
author discusses use of beryllium hydride BeH2 as a moderator.
Moderation reduces critical mass as does use of a neutron reflector.
The author presents a table with three candidate neutron reflectors,
H2O, BeH2 and BeO. For obvious reasons of nuclear security the author
does not discuss the geometric configuration considered.
With the use of BeO neutron refector critical mass of plutonium-239 is
reduced to 87 grams.

More likely he doesn't bother to discuss geometries because there aren't
any practical ones that allow you to achieve prompt criticality with
such a small mass of Pu. Likely the mass of needed moderator is so high
and the rate of neutron propagation is so slow that you get disassembly
long before achieving any useful efficiency. Notably, both the test
shots of designs using thermal neutrons were fizzles.

I haven't attempted ("bothered to" might be more accurate) find the URL
mentioned, but if you look at this abstract:
http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=5384935
you will see work published in 1983 about minimum mass critical
configurations that has the minimum at 105 grams, for both BeH2 and BeO
and Pu-239, so this claim is 18% reduction - interesting, but not too
dramatic.

This is of course, as Bill Baker suggests, a thermal critical
configuration useful for a tiny reactor perhaps - but useless for
creating a significant explosion.

[Carey]

Post by Nick
Apparently netiquette requires that its URL not be revealed, but
anyone can Google for it if they are so inclined. On page 152 the
author discusses use of beryllium hydride BeH2 as a moderator.
Moderation reduces critical mass as does use of a neutron reflector.
The author presents a table with three candidate neutron reflectors,
H2O, BeH2 and BeO. For obvious reasons of nuclear security the author
does not discuss the geometric configuration considered.
With the use of BeO neutron refector critical mass of plutonium-239 is
reduced to 87 grams.

More likely he doesn't bother to discuss geometries because there aren't
any practical ones that allow you to achieve prompt criticality with
such a small mass of Pu. Likely the mass of needed moderator is so high
and the rate of neutron propagation is so slow that you get disassembly
long before achieving any useful efficiency. Notably, both the test
shots of designs using thermal neutrons were fizzles.

I haven't attempted ("bothered to" might be more accurate) find the URL
mentioned, but if you look at this abstract:
http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=5384935
you will see work published in 1983 about minimum mass critical
configurations that has the minimum at 105 grams, for both BeH2 and BeO
and Pu-239, so this claim is 18% reduction - interesting, but not too
dramatic.

This is of course, as Bill Baker suggests, a thermal critical
configuration useful for a tiny reactor perhaps - but useless for
creating a significant explosion.