Sunday, May 08, 2005

Joining the Club

There has been an interesting discussion going on over at LGF about nuke weapons. The discussion was prompted by this Tech Central Station article.

My comments:

I love nuke stuff. So much so that I was a Naval Nuke. A rod yanker so to speak.

About five or eight years ago we shipped 20 TONS of plutonium to Japan. In theory the Japanese were going to "dispose" of it in a specially designed reactor. Right.

BTW that is enough Pu for at minimum 1,000 weapons. (more like 8,000 actually)

So I'd add Japan to the nuclear club for the same reason Israel is in the club. Material and know how.

The #1 reason for China to put the brakes on the Norks is the knowledge that the Japanese are an unannounced nuclear power.

It seems to me that the Nork's testing of rockets by sending them in the direction of Japan is very unwise. For the Norks.

Europe has been sending nuclear material to Japan for some time. Greenpeace made their usual stink about the danger. If they only knew.

You can't ship big chunks (not over a pound likely per container). The container needs to be large enough and strong enough to keep it away from other chunks despite colision, fire, and a flooding hold. And that is just the obvious stuff.

Here is an article on the shipments from 1999. Straight from the horses mouth Greenpeace.

Weapns grade plutonium shipments to Japan.

And it was 40 tons total not twenty. i.e. Japan is now a world class nuclear power with a weapons capability second only to the USA.

The initial shipment was 450 kg said to be enough for 50 bombs. (more like 200 if well designed and fusion boosted)


The US Japanese nuclear co-operation goes back further than I thought. It could have been policy since Regan.

The U.S.-Japan agreement provides for the shipment of plutonium extracted from spent fuel rods removed from Japanese commercial reactors. The origin of the fuel rods is the United States. The agreement requires Japan to notify the United States State Department before each transfer of plutonium and to provide the United States with a transportation plan that includes a "threat assessment and a contingency plan."
American Plutonium shipments to Japan
The General Accounting Office, Congress' auditor, has weighed in with its report on the first shipment of plutonium to Japan made under the 1988 Agreement for Cooperation Between the United States and Japan Concerning Peaceful Uses of Nuclear Energy. Senator John Glenn, chairman of the Governmental Affairs Committee, asked the GAO to prepare the report, which was released in June 1993.
You have to scroll about half way down the page for the Japan stuff.

Interestingly enough South Korea is doing the reprocessing.

No wonder they do not see the Northern Nuke as such a threat. That would explain a lot.

Looks to me like if you are a reliable ally America will provide nuclear materials.

My guess is that all of the major Asian powers in the American orbit are nuclear armed.

That seems to be policy.


So how hard is it to design and build a weapon?

My understanding of modern plutonium based nuclear weapons is that they are all thermonuclear.

Our initial designs were developed with computers that had less power than a TRS-80. The lowest cost desktop machine of today is at minimum 1,000 times as powerful. Which says that the computations that would take a year in 1950 could be done in 8 hours or less today. And of course if you wanted to do paper tests faster of different designs just buy more computers.

FabioC adds a link to a modern nuke weapon with general design outlines, history, and lots of general guidelines for bomb design. More is easily found on the www. And there are links galore at the site.

How hard is it to design and build a nuke? You give me the Pu (say a couple of ounces accurately machined). 1,000 or 2,000 smoke detectors (to make a neutron source).

A NC machine tool capable of holding .0001" tolerance over a 5" distance (not too exotic these days). An interferometer based measuring tool.

Some test eqpt. o-scopes capable of recording around 1E9 samples a second ( again not too exotic these days).

Some test eqpt. (x-ray detectors, neutron dectectors etc)

I could work out all the required constants.

From there I buy an optics and a mechanical dynamics program and I'm off and running.

With really top notch people (don't forget this stuff was originally done with slide rules, IBM punch card machines capable of about ten calculations a second, and vacuum tubes. Feynman ran the IBM machine. It is a fascinating story.) I could get by with an eqpt budget of $20 to $40 million. Peanuts really.

The codes even done in Basic ought not be too hard. Or if you want to get really fancy Mathematica. Some one who understood x-ray grazing optics optics, stress/strain curves, partial differentials and cubic splines ought to do the trick. You go to a university and find the best politically motivated engineers and mathematicians. A team of five ought to be more than adequate. Did I mention that you need an explosive forming expert?

The reason such a big team was needed for the first one was the uncertainty and the lack of off the shelf eqpt. Everything was custom. Now a days all you pay for is the marginal cost. The R&D has already been done.

Now my training was in reactors. Weapons were mentioned in passing only from the standpoint of critical mass. But almost any engineer from open sources could figure out the requirements. Or just go to any good history of Los Alamos. (I have none to suggest).


Anonymous said...

The Greenpeace article actually uses the terminology, "weapons usable plutonium." The operative word here is "usable" in contrast to the description of "weapons grade plutonium" that is often used in error.

The difference in "grade" and "usable" has to do with the percentage of the Pu-241 isotope in the plutonium mixture. Plutonium produced in power reactors has very high percentages of Pu-241 (because the low enriched uranium fuel is left in the power reactor for long periods of time for economic reasons) whereas plutonium produced in weapons reactors such as those that were operated by the U.S. at Hanford and Savannah River has relatively low percentages of Pu-241 by design.

Now you may ask, "What's the difference? It's all plutonium and it all goes boom doesn't it?" Well yes, it does all go boom. However, Pu-241 has a bad characteristic for nuclear weapons purposes: it undergoes spontaneous fission. This is definitely NOT good for nuclear weapons design because spontaneous fission means spontaneous emission of neutrons - thus the more the amount of Pu-241 in the mixture, the more the number of these spontaneous neutrons whizzing around the plutonium core.

Now you might reasonably ask, "Well so what, we need neutrons anyway to start a nuclear chain reaction. What's the big deal with these spontaneous neutrons emitted from Pu-241?" The problem is timing. We indeed want a flood of neutrons to initiate the chain reaction in our plutonium weapon, but we want them ONLY at the precise time during the assembly of a supercritical mass. But, before that precise time, we ideally want NO neutrons present (or at least as few as possible.)

This proclivity for preignition by spontaneous neutrons emitted from the Pu-241 isotope of the plutonium "pit" being driven to a supercritical mass can be overcome - to a degree. But the difficulty in overcoming these spontaneous neutrons rises rapidly with increasing amounts of Pu-241 present.

Thus the reactor-grade plutonium that Japan and others possess is far from the optimum fissile fuel for a nuclear weapon. If you're LANL or LLNL, you're equipped to solve this problem. But of course, if you're LANL or LLNL, you don't fool around with reactor grade Pu. You just use weapons grade Pu and avoid the problem in the first place.

M. Simon said...

Thank you for the update.


Very interesting.

I'm sure if LNLL could figure out an answer so could any competent engineering group.

At minimum some type of gaseous diffusion or liquid chromatography might be required to get the ratios right.

Another possibility might be a carefully measured amount of neutron absorbing material that would be saturated once criticality is reached. Of course that might mean more regular maintenance of the bomb but it would be a viable answer for a country with a small nuclear inventory.

Or they might just settle for lower yield weapons.

Joining the yacht club may be more important than having the best yacht.

M. Simon said...

From an engineering standpoint the critical (pun intended) part of the information is that the Pu has been reprocessed.

Not much radiation danger, making the stuff much easier to work with.

AMac said...

"Anonymous" sent me Googling, and I quickly came to Global Security's monograph on plutonium isotopes and weapons design.

Shorter John Pike:

Irradiate uranium for a short time in a reactor, and then withdraw the fuel. (Screw electricity production.) Chemically extract plutonium, and you end up with 93% (+/-) of plutonium being Pu-239, the ideal bomb-making isotope.

On the other hand, you let fuel burn in a reactor for 3 or 4 years if you're trying to produce electricity economically. Reprocess that fuel, and you end 50% to 60% of the plutonium being Pu-239, with the remainder being isotopes 240, 241, and 242.


All three higher isotopes are bad for bomb-making, per anonymous' 07:25:46 PM comment. You get a bomb that emits a lot of radioactivity, bad for workers' health. The heat generated by the shorter-lived isotopes makes it harder to mate explosives with the machined Pu metal. The shelf life of inventory goes way down. And the yield drops--though even a "fizzle" packs one-third the punch of Hiroshima.

So if you start with reactor-grade Pu, you'll be denied an elegant bomb. But not necessarily a workable bomb.