Problems with Problems With The Orion Spacecraft #9: Stress

In article .com,
Jack Linthicum wrote:

You must not be conversant with Ted Taylor. TRy Jamie McPhee's The
Curve of Binding Energy. Taylor goes from a source the size of a
softball (not specified whether the 12, 13 or 20 inch circumference)
page 108, 20 pounds weight page 109, a small cantelope page 148, one
gram was converted to energy in the Nagasaki bomb, page 163, home-made
about the size of a golf bag page 193, the gum reference is to a stick
of U-235 that size which 10% of the energy contained therein would
bring down the World Trade Center, when it was still standing, page 15.

All the nuke people hate Taylor as he kind of makes their brain strains
into easy exercises.

It's at least moderately marvellous that there was a person in the
world who was to nuclear weapons design what the master-smiths of
Japan were to swords; the kind of person whose intuitive understanding
of implosion made him a living national treasure.

It's possibly also marvellous that there was but one, he is dead, and
his like unexpected to return.

[though Aldermaston is very strongly hiring at the moment, if anyone
here's a security-clearable Brit with an interest in very large-scale
computing and not concerned about building what can only be either
exercises in engineering masturbation or tools of genocide; I'm afraid
I match only #1 and #2, and #3 is something of a deal-breaker]

Tom

In article ,
Derek Lyons wrote:

With a higher velocity gun, and better ability to calculate the
effects of pre-detonation, all manner of things become possible.

OTOH, there have been references to 'supergrade' plutonium, I.E.
having less -240 than is the norm. I don't know if the -240 is
separated out or if the Pu is produced using methods that create less
-240.

If you have vast amounts of lightly-enriched uranium (or, I suspect,
even fully-depleted uranium if you've already got a reactor producing
decent neutron flux), you can cycle it through the reactor fairly
quickly; you don't get _much_ 239-Pu in the result, so it's
inefficient in terms of uranium usage and, more importantly, reactor
and reprocessing capital cost, but you get proportionally
substantially less 240-Pu. I suspect that's easier than going for
actual enrichment.

Tom

On Mon, 24 Oct 2005 12:30:54 -0500, Pat Flannery
wrote:



Paul F. Dietz wrote:


Well, either the Orion team made a major slip in their report, or by
1964 we knew how to separate Pu-239 and Pu-240.


... or maybe that pre-reacting is perfectly acceptable
in the low yield situation here?


From what I read, what would happen is that the plutonium would go
molten and start to vaporize before it could be detonated- you would
probably end up with a dirty bomb more than a true nuclear bomb. I
wouldn't want to be the guys who had to retrieve the pusher plate, as it
would probably have been sprayed with plutonium if that happened.
I was always suspicious of that stated inability to separate Pu-239 and
Pu-240; our isotope separation technology is a lot more advanced
nowadays than in W.W. II.

Pat

The "Super" grade Pu produced at Hanford during WWII was some of the
most pure reactor Pu produced, largely due to the rapid fuel cycles
they used at the time. It is a real balancing act, the longer you
leave the U-238 slugs in the reactor, the more Pu-239 will be bred,
but more Pu-240,241, and 242 will be produced as well. You can get
fairly pure Pu-239 if you change the slugs out quickly, but then the
costs go up dramatically, as you need to shut down the reactor,
extract the slugs and put them through chemical seperation each time.

This "Super" grade Pu was just barely on the edge for an extremely
large scale gun weapon, in which two sub-critical masses would be
fired at one another. It was huge, heavy and was still very suceptable
to a fizzle. This was why the whole idea was discarded as it would not
have lead to a practical weapon, and would have been horribly
inefficient. Not the kind of device I could see being used on an Orion

Standard seperation techniques can be used to seperate Pu240,241,242
out of Pu239, but because the mass ratio is much smaller it is much
harder to do. I suspect the AVLIS could probably be used to great
effect here. This part of the movie "The Manhattan Project" (The one
where a kid builds an a-bomb), wasn't that far off.

Kelly McDonald

In sci.space.history Thomas Womack wrote:
[though Aldermaston is very strongly hiring at the moment, if anyone
here's a security-clearable Brit with an interest in very large-scale
computing and not concerned about building what can only be either
exercises in engineering masturbation or tools of genocide; I'm afraid
I match only #1 and #2, and #3 is something of a deal-breaker]

Well, its not preceicely a secret why their budget nearly doubled even if
the PM will not publicly say it yet.


Tom




--
Sander

+++ Out of cheese error +++

In article , Pat Flannery says...



John Schilling wrote:

Or they were paying attention to the *Very Low Yield* part.

This is not something we heard here first. It has been publicly known
for about sixty years now, that what happens if you try to build a Pu
gun is that it predetonates, resulting in a very low yield. Normally,
this is undesirable behavior and we thus don't build a plutonium gun.
If a very low yield is what you actually *want*, go ahead and buuld
the gun - it's a simple and reliable, if somewhat heavy, way to get
a very low nuclear yield.

Wouldn't you end up with very inefficient fission and a lot of
unfissioned plutonium getting sprayed around?

Yes, precisely. Now, just what were you imagining a very low yield
nuclear explosion would be, if *not* "very inefficient fission an a
lot of unfissioned plutonium getting sprayed around"?

Well, OK, the early Orion proponents imagined that they'd get fusion
explosions of whatever yield they needed from the Fission-Free Hydrogen
Bombs That Were Going To Be Invented Any Day Now, Really!, but that
seems to have not worked out real well. The techniques that actually
work to produce nuclear explosions start with a critical mass[1] of
highly enriched uranium and/or plutonium, and a full critical mass
efficiently fissioned results in a high-yield explosion.


[1] An imprecise term that incorporates lots of assumptions about things
like geometry, compression, and tamping, but is measured in kilograms,
not grams, for any currently plausible arrangement of these.


--
*John Schilling * "Anything worth doing, *
*Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" *
*Chief Scientist & General Partner * -13th Rule of Acquisition *
*White Elephant Research, LLC * "There is no substitute *
* for success" *
*661-951-9107 or 661-275-6795 * -58th Rule of Acquisition *

On Mon, 24 Oct 2005 18:29:41 GMT, (Derek Lyons)
wrote:

Pat Flannery wrote:

You heard it here first- a gun assembly nuclear device employing
plutonium, not uranium, as its fissile material.
That was supposed to be impossible due to a unstable isotope of
plutonium (Pu-240) that would cause it to pre-react if gun assembly was
used, and which was supposed to not be separable from the Pu-239.

It was impossible in terms of the crash program during WWII to build a
bomb that would fit into a B-29.

Well, either the Orion team made a major slip in their report, or by
1964 we knew how to separate Pu-239 and Pu-240.

With a higher velocity gun, and better ability to calculate the
effects of pre-detonation, all manner of things become possible.

OTOH, there have been references to 'supergrade' plutonium, I.E.
having less -240 than is the norm. I don't know if the -240 is
separated out or if the Pu is produced using methods that create less
-240. (The amount of -240 produced is IIRC, sensitive to irradiation
rates.) However, it's pretty scarce and expensive stuff, and is used
(again IIRC) in special applications and as a 'sweetener'.

D.

"Super" grade is produced by shortening the fuel cycle when breeding
reactor Pu. Most of the stuff made in WWII was "super" grade because
of how often they extracted the slugs out of the Hanford piles.

The longer you leave the U-238 in the reactor, the more Pu-239 breeds
(increasing the efficiency of the process), but the more
Pu-240,241,242 is also produced (at higher ratios the longer you leave
it in there). This is why Pu produced in commercial nuclear reactors
isn't highly valued for building a bomb. The Pu is highly contaminated
with Pu-240,241 and 242.

Kelly McDonald

: Thomas Womack
: If you have vast amounts of lightly-enriched uranium (or, I suspect,
: even fully-depleted uranium if you've already got a reactor producing
: decent neutron flux), you can cycle it through the reactor fairly
: quickly; you don't get _much_ 239-Pu in the result, so it's
: inefficient in terms of uranium usage and, more importantly, reactor
: and reprocessing capital cost, but you get proportionally
: substantially less 240-Pu. I suspect that's easier than going for
: actual enrichment.

Of course, if you cut short before you yield much pu239 (that is, before
there's enough for much of it to pass along to pu240), then it's
easier to separate the pu from the u, and you can just put the u
back in for another run.

The issue I see being that it'd be easier to separate substances
with different chemistry, than to separate substances with exactly the
same chemistry which differ only in mass. So presumably, several separation
runs with chemistry is better than several *more* runs with centrifuge or
gas diffusion or something.

Or so I conjecture.


Wayne Throop http://sheol.org/throopw

In article ,
(Derek Lyons) wrote:

You heard it here first- a gun assembly nuclear device employing
plutonium, not uranium, as its fissile material.

Well, from what the thread has been saying, it sounds as if this was a
static testing device, not a Orion driver bomb.

One might suspect the existence of devices like LENS for use in the
warhead hardening program. You need to simulate the effects of an ABM
warhead going off a mile or two away? A deliberate fizzle from something
parked next to the thing you're trying to test is going to be much easier
to arrange - underground in Navada - than setting up a thoroughly
realistic test,

That was supposed to be impossible due to a unstable isotope of
plutonium (Pu-240) that would cause it to pre-react if gun assembly
was used, and which was supposed to not be separable from the Pu-239.

The initial studies of Pu had been done using very small quantities
transmuted in a cyclotron, which had essentially no Pu240. As soon as they
got Pu240 made in a reactor, they found that it had significant amounts of
Pu240, which has a high spontaneous fission rate, which means there are
enough neutrons around an any significant mass of reactor-made Pu that a
gun device made from it is very liable to predetonation.

It was impossible in terms of the crash program during WWII to build a
bomb that would fit into a B-29.

It's bloody hard in any case: 3000 feet/sec is still serious velocity for
a gun, and that was thoroughly inadequate. And separating Pu239 from Pu240
is going to be fairly difficult however you do it. Maybe there's a useful
resonance you can hit with a laser or something, but centrifuges would
still struggle today.

OTOH, there have been references to 'supergrade' plutonium, I.E.
having less -240 than is the norm. I don't know if the -240 is
separated out or if the Pu is produced using methods that create less
-240. (The amount of -240 produced is IIRC, sensitive to irradiation
rates.) However, it's pretty scarce and expensive stuff, and is used
(again IIRC) in special applications and as a 'sweetener'.

The whole of the Fat Man assembly was essentially "supergrade", with very
little Pu240. You make it by not irradiating the U238 very much, so that
not many of the Pu239 nuclei formed when a U238 nucleus absorbs a neutron
get a chance to absorb another and become Pu240. Rate isn't really
relevant; what matters is the amount of neutrons per nucleus.

"Weapons-Grade" plute is stuff with a small proportion of Pu240. One
advantage is that it doesn't generate so much heat, so you don't need such
a powerful cooling system to prevent your warheads getting too hot. It's
also more forgiving of imperfect implosions, because of its lower
spontaneous fission rate.

You can make perfectly good nukes out of "reactor-grade" plute, which
comes from fuel rods that have been used in a power-generating reactor
until they had so many fission products in them that they were impeding
reactivity. Such plute has quite a bit of Pu240, plus Pu241 and Pu242 as
well. It's harder to get an implosion to go well with such stuff, but
there is nothing impossible about it.

---
John Dallman, , HTML mail is treated as probable spam.

In sci.space.history John Schilling wrote:

Well, OK, the early Orion proponents imagined that they'd get fusion
explosions of whatever yield they needed from the Fission-Free Hydrogen
Bombs That Were Going To Be Invented Any Day Now, Really!, but that
seems to have not worked out real well. The techniques that actually
work to produce nuclear explosions start with a critical mass[1] of
highly enriched uranium and/or plutonium, and a full critical mass
efficiently fissioned results in a high-yield explosion.


[1] An imprecise term that incorporates lots of assumptions about things
like geometry, compression, and tamping, but is measured in kilograms,
not grams, for any currently plausible arrangement of these.


Hmmm.... I though concentrated solution of Plutonium Nitrate in a good
reflector would be critical near a kilogram ? No idea if it would be
usable in a weapon or if you would need a special geometry.

--
Sander

+++ Out of cheese error +++

With the initial microgram samples of cyclotron-generated plutonium,
which was almost pure Pu-239, the plutonium gun was considered
difficult but possible by Los Alamos. It would have required a gun
velocity of around 3000 m/s, which would have required a long and heavy
device.

With the first reactor-generated plutonium, the Pu-240 problem became
apparent. That was made in research reactors with lower burnups than
planned for the production reactors under construction at Hanford. The
Hanford plutonium was around 0.9% Pu-239. Modern 'supergrade'
plutonium is around 1.5% to 3.0% Pu-239, with ordinary weapons grade
plutonium at around 6.5%.

Increasing gun velocity only gives you a proportional improvement in
how much Pu-240 you can accept, while the gun weight is going up as
something like the fourth power. A double-gun arrangement can only get
you a factor of two in effective velocity. And at some point you run
into a speed-of-sound limitation that requires moving to a light gas
gun. That might get you a total factor of 5 over the marginal 3000 m/s
device. I estimate that would allow the use of plutonium with about
0.1% Pu-240 with a 10% predetonation probability. That isn't very
practical, but not completely ridiculous.

An implosion device can guarantee a decent minimum yield even with
predetonation. This can be large enough to ignite fusion boosting,
after which
predetonation isn't really a problem. If a gun device fizzles, odds
are that you get no significant yield.

I don't see why anyone would use isotope separation to eliminate
Pu-240. If you want to use a gun, why don't you use U-235? It is much
easier to separate, due to the larger mass difference. You don't need
to get it anywhere near as pure, since 80% enriched uranium is better
than 100% Pu-239 for predetonation. Yes, plutonium is a more efficient
fissile material, but you give up any mass advantage for that when you
try to use a gun.

I am surprised that this would come up in connection with Orion. There
you want small devices, which are already hard to make efficient.
Without the compression of implosion, efficiency drops dramatically.
You need a lot of devices, which means you want light weight and low
cost. This certain doesn't qualify.

Orion also wants highly reliable devices. A dud screws up the
resonance of the shock absorption on the pusher plate, putting
dangerous stress on the system. Devices vulnerable to predetonation
are not appropriate.

Orion also puts design constraints on the arrangement of tamper in the
device, to try to directionalize the thrust. I would expect that the
design constraints of a gun would interfere with that.