RE: What is Weapons Grade - RGPu NOT bomb tested

["Franta, Jaroslav" <frantaj@aecl.ca>]

Michael C. Baker [mailto:mcbaker@lanl.gov] wrote on Wednesday August 16,
2000 1:33 PM

Another reference you may want to take a look at is,

J. Carson Mark, "Explosive Properties of Reactor-Grade Plutonium," Science 
& Global Security, Vol. 4, pp. 111-128, 1993

His conclusions include the following:

1.  Reactor-grade plutonium with any level of irradiation is a potentially 
explosive material.

2.  The difficulties of developing an effective design of the most 
straightforward type are not appreciably greater with reactor-grade 
plutonium than those that have to be met for the use of weapons-grade 
plutonium.

J. Carson Mark was the director of the Theoretical Division at Los Alamos 
National Laboratory (1947-1972)

Mike
<><><><><><><><><><><>

Comment :

Thanks for posting this reference Mike.
J. Carson Mark started out his nuke weapons career way back in the early
1940s in Canada's secret "Montreal Laboratory" before transfering to the
Manhattan Project's Los Alamos lab. You can read about the early days in a
very good article in the special centenary issue of the journal PHYSICS IN
CANADA, March/April 2000 (Vol. 56, No.2, pp. 123-131), in Philip R.
Wallace's article, "Atomic Energy in Canada: Personal Recollections of the
Wartime Years."
But Mark's publications on RGPu bombs are rather glib, particularly on the
subject of problems caused by RGPu self-heating inside a large mass of
chemical high explosive (HE) which acts as an excellent thermal insulator -
to the point that temperature would rise above the HE melting point in
minutes. He gives no hint he's given more than cursory attention to the
heating and radiation problem, and his calculation of decreased yield are
extremely rudimentary - but most readers wouldn't realise that that's the
case without being forewarned & given additional clues. Specifically,
there's NO mention at all of extra neutron emission & extra heating as a
result of subcritical (thermal neutron) multiplication -- he uses a straight
watts/kilogram factor. This is a truly sophomoric error - both heating and
emission rates could easily be 8 to 100 times the "basic" rate !!! ( based
on k0 = 0.88 to 0.99, which corresponds to about a 1.3kg Pu mass difference
- on a total of about 3.5kg to 4.5kg, depending on whether its RGPu or WGPu
- alpha-phase, more for delta-phase ). This is WAY above anything he's
listing in his Table 5: Selected properties of various grades of plutonium (
and doesn't include neutrons from the nat-U tamper either ).
I can't imagine how an inteligent & knowledgable person such as Mark could
make such an obvious omission - unless there was intent to deceive.  
Both alpha decay of Pu-238 and the heavier Pu isotopes and Am-241, as well
as fission energy release due to both spontaneous fission and to
background-neutron-induced fission generate heat inside the bomb core, which
must somehow be removed. A 6kg BARE sphere of alpha-phase RGPu would
generate about 50W, which would heat it to over 100°C. If you pack a uranium
tamper and explosives around it, the temperature would rise - due to
increased insulation - to well above the Pu transition point to delta phase
(115°C). This is a much less dense metal, with a correspondingly higher
critical mass - on the order of 22kg versus 15kg for alpha-phase Pu. A
larger mass of delta-phase RGPu would produce about five times as much heat,
some 250W for a bare sphere. Its a vicious circle. Then when you add the
tamper and explosives around the bare sphere, the neutron background rises
due to subcritical multiplication, as does the resulting fission heating -
to something like 500W to several kilowatts. This is MORE than enough to
melt the entire device within minutes if continuous cooling is not provided
from the moment its assembled to the time of the explosion. The whole RGPu
bomb pit manufacturing process would be comlicated by the need to carry it
out in an oven that would maintain the RGPu above the delta-phase transition
point, because otherwise a bomb pit manufactured in apha-phase would crack
up and deform upon transition to delta-phase, once installed in the bomb.

Carson Mark suggested that a type of provision can be devised to cope with
the heating problem.   Aluminum, with its excellent thermal conductivity,
might theoretically be used in a kind of "thermal bridge." Using a "bridge"
with a cross-section at the surface of the core of about one cm^2 could
halve the temperature increase induced by a small amount of reactor-grade
plutonium. But with several hundred watts of heat generation, you would need
more than one cm^2. The hot HE next to the metal will have a different
characteristic propagation velocity than cold HE farther away. Also, the
inert metal would cause all sorts of reflections & refractions of the
shockwave as it converges towards the core - aluminum is not only a great
"thermal bridge," it is also a great "shockwave bridge." Specifically, the
high-velocity shock which "ignites" the low-velocity lens explosive
immediately below, travels tangentially in a relatively thin layer along the
bomb's surface; if it encounters an inert metal object, the shockwave
reflects & refracts, screwing up the uniform surface ignition of the
low-velocity explosive, which in turn destroys the spherical geometry of the
wave that hits the final, much thicker layer of high-velocity HE implosion
charge, where distortions get further amplified.... This is a perfect
prescription for failure, because the RGPu pit in the center gets blasted to
bits instead of being compressed in a perfect spherical geometry.

To illustrate the above problem, I have prepared a color animation showing
the traveling shockwave in a simplified RGPu bomb, called "Mark's RGPu dud."
It shows the famous Carson Mark aluminum "thermal bridge," and how it
conducts a part of the shockwave towards the center more rapidly than the
surrounding material (due to locally modified "refractive index" of the
lens), thus destroying the spherical implosion geometry & causing a
"fizzle." "Mark's RGPu dud" is an AVI animation composed of ~16kB jpegs
compressed into ~1 MB zip file. If anyone is interested I can send it to
you, BUT PLEASE DON'T reply on RADSAFE - contact me personally at
frantaj@aecl.ca 

Jaro
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