The implosion design of the atomic bomb is considered the ultimate secret triumph of Los Alamos. Unlike the relatively simple gun-type design, the implosion design required innovation on a whole manner of scientific fronts: nuclear physics, metallurgy, chemistry, ordnance engineering, electronics... the list goes on. Making explosive lenses that could precisely compress a solid sphere of plutonium into a supercritical state wasn't easy.
The very idea of implosion -- much less the specifics of its implementation -- wasn't declassified after the bombings of Japan. As I've mentioned previously, it wasn't until 1951 (that is, well after the Soviets had demonstrated their own ability to do it) that a bare-basics idea of implosion was declassified, as part of the evidence in the Rosenberg trial.
But that isn't to say that people hadn't thought that it was perhaps safe to declassify it earlier than that. This week's document is a memo from then-Commodore William S. Parsons (USN) to Norris Bradbury, scientific director of Los Alamos, from late October 1945 on this very subject. Parsons you will remember as the weaponeer on the Enola Gay, and a truly key figure at multiple junctions in the development of the practical ordnance engineering of the atomic bomb. In this memo, Parsons was arguing against the declassification of implosion -- something he felt needed to be done because there were a considerable number of folks who were arguing for its declassification:1
As you've probably picked up on by now, I like anything that helps me get inside the head of a classifier (or declassifier) and see how they saw the world in their time and place. It's the grist for my historical mill: it's how I understand nuclear secrecy as a never-quite-stable category, one that is always evolving, one whose logic is always up for grabs and thus needs to be articulated and re-articulated repeatedly.
Parsons wrote the memo because at various meetings he had noticed that a number of Los Alamos scientists had come to believe that implosion would be "obvious" to other people working on nuclear weapons design, and thus ought to be declassified fairly readily. Parsons disagreed with this strongly:
It would be impossible to settle now, but I would be willing to bet that except for the stubborn refusal of Neddermeyer to give up working on implosion, and the brilliant thinking and arguments of von Neumann, implosion would have been put on the shelf here by Christmas of 1943 (perhaps to be taken off later when "240" appeared).2 It is, of course, impossible to establish the probability of an uninformed group discovering the principle of implosion and following up this method in spite of the discouragements we encountered. ...
I rate the chances as very good that if we collectively kept our mouths shut about bulbous bombs, shaped charges, high explosive assembly, lenses, equations of state of materials at several times normal density, and implosion itself; and perhaps released a little more than Smyth said about the gun, a competitor seeking to assemble U-235 would not ever arrive at implosion assembly. If he ran into our difficulty the case of plutonium, my guess is that he would be sufficiently stumped to lose a great deal of time while investigating the problem and attempting to obtain our solution by espionage or an international horse trade.
American physics will for many years include many able, imaginative people who know what densities can be achieved by the implosion process. If physics can be advanced by the use of implosion to achieve these densities for a few fleeting microseconds, we might trust that this possibility will be realized, and the experiment and the implosion theory perhaps declassified at that time.
My present feeling is that implosion is an art and a science naturally focused on the production of an atomic bomb, of no obvious use for any other prupose, and that it should be classified on this basis.
There's a lot going on here. First, I think Parsons is right that those who came onto the project after implosion had been pursued would over-estimate its obviousness. This is a fairly common phenomena; I'm reminded of Edward Teller's over-estimation of the obviousness of the Teller-Ulam design after it had finally been worked out. It's hard to put yourself into the perspective of the ignorant.
I also think it's pretty interesting that Parsons thinks that implosion could stay secret perpetually if a nuclear power only pursued a gun design. This seems too far the other direction unless you really did assume that potential proliferators would exclusively pursue uranium-235, and never pursue reactors for anything. Seems unlikely. I like, though, that he considers putting out more information on the gun as a way to encourage this: information overload as a form of distraction.
It's also a complicated point he's making about the fact that people who know about implosion are going to be circulating within the "open" scientific world, and the question of whether implosion could at any point be used within those fields. He seems to be holding open the door for that -- in the future implosion might have an "open" usage, but not yet.
One last observation: Parsons is pretty down on the idea of disconnected scientists figuring out implosion independently. He would, curiously enough, find out that this was far from impossible. In 1946, a group of physicists not in any way connected to nuclear weapons development would try to publish a book on Nuclear Fission and Atomic Energy. Their publisher sent a copy to the Manhattan Project declassification office, where a chapter on implosion (they even used the term, which got them accusations of having a secret source) got them into hot water. It took years of wrangling with the MED officers and the Atomic Energy Commission for the book to come out, sans any mention of implosion. (In the copy linked to previously, note how chapter 11 ends abruptly.) The officer sent to make liaison with the book physicists was, coincidentally... William Parsons. But the full story of that is for the book, so I'll just leave it at that for now.
- Citation: William S. Parsons to Norris E. Bradbury, "Declassification of Implosion," (30 October 1945), copy in the National Security Archive, George Washington University, Chuck Hansen Papers, Box 11, "1945-1949," Folder 3. [↩]
- As is well known, the discovery that reactor-bred plutonium was contaminated with the isotope Pu-240 led to a radical reorientation of the Los Alamos lab, since Pu-240 precluded the use of plutonium in a "gun-type" design. [↩]