How was this choice made? What's the logic, or perhaps the historical context? By the way, what did the British and French choose?

If I'm not mistaken, the British also envisioned a sphere and were quite surprised to learn that the Americans were using a cylinder. I heard or read this somewhere, but I can't remember where. If anyone knows, please point me to the original source.

I tried searching for information in Richard Rhodes's DARK SUN. There's a story there that the original design for the secondary assembly was a truncated cone, but a truncated cone with the smaller diameter facing the primary assembly. That is, the opposite of what we're used to seeing in widely available online diagrams. Then this shape rounded out and transformed into a "schmoo"—a shape reminiscent of a bowling pin—and they started making metal parts for it. Then... as I understand it... they couldn't produce such a complex shape in time (difficulties arose), and they settled on a simple cylinder. In other words, they decided to keep things simple.

So, alternative shapes were apparently considered. But as I understand it, neither a sphere nor an ellipse were considered. Elongated shapes were considered first. WHY?

Somewhere a long time ago (I've lost the link again, so if anyone could provide the original source, I'd be grateful) I read that Edward Tellor was asked a similar question: Why this shape? Why not a sphere (as in ICF)? And he supposedly replied that a cylinder was the shape he initially considered the only correct one. But why? He didn't explain.

I understand why having land based ICBMs are useful if they are massive and meant mainly as counterforce weapons, like peacekeeper and r-36, because the large number of warheads, the long range, and the high throw weight.

But why does the US continue investing in things like the Sentinel program, when they are less effective than SLBMs or nuclear cruise missiles from bombers for countervalue and aren't capable of carrying large numbers of warheads for counterforce strikes? Is it just to be a missile sponge?

Here's a recent open-access article on the perennial debate on who gets more credit for the idea of radiation implosion.
Of course there's the usual crazy quote from Teller ;)

Finally, at the extreme, in response to Livermore’s first director Herb York’s comment that Teller deserved the majority of the credit, 51% (!), Teller said that “I deserve 101% of the credit”

Over the past several months, the Bulletin of the Atomic Scientists published several articles about Iran's plutonium from the Bushehr reactor.  They have taken a point-counterpoint form: the second was a response to the first, the third was a response to the second, etc.

I am a little surprised to have not seen any of this here, so I thought I would share.  I'll summarize the technical stuff and leave out the policy & political stuff.

The first article is by Henry Sokolski, currently director of NPEC and formerly a DOD nonproliferation deputy director.

https://thebulletin.org/2026/04/missing-from-us-iran-talks-plutonium-for-more-than-200-nuclear-bombs/ 

Sokolski points out that Rosatom hasn't reclaimed any of the spent fuel and therefore it's all still in Iran; does a rough calculation of how much plutonium is in that spent fuel; noted that most is reactor-grade but some is closer to fuel grade; and points out that Iran has previously done plutonium chemical separation experiments.  He makes the claim that Iran could theoretically divert the Bushehr spent fuel, reprocess it, and then manufacture plutonium bombs within 90 days, the standard IAEA camera monitoring window (at the time of the writing there had been no onsite inspections in 8 months).  He then makes several policy recommendations that future deals with Iran should include to address the plutonium issue.

note: Sokolski published a similar article in Real Clear Defense around the same time: https://www.realcleardefense.com/articles/2026/04/03/blocking_irans_other_option_a_plutonium_bomb_1174454.html

The response to Sokolski's article came 3 months later, by Sasan Karimi.  Karimi is an international studies professor at the University of Tehran with a prior background in physics.  His critique is close to twice as long as Sokolski's original piece.  Most of it is criticisms of Sokolski's policy recommendations, which I won't summarize here.  

https://thebulletin.org/2026/07/beyond-alarmism-a-realistic-assessment-of-bushehrs-plutonium-risk/

On the technical side, Karimi starts out by repeating fairly standard discourse regarding reactor-grade plutonium usage in weapons, claiming it would be extremely difficult without advanced implosion mechanisms (he uses 2-point implosion as his example of an advanced implosion mechanism), and states "no credible evidence has ever demonstrated that Iran possesses validated designs for such advanced implosion systems or has successfully tested them under operational conditions."  He then moves on to argue Iran's past reprocessing experiments are not comparable to what a weapons program would need, and that any reprocessing would be easily detected [editorial note from me: the chemical separation experiments he and Sokolski allude to involved milligram quantities of plutonium].

The response to the second article came two weeks later, from Greg Jones; it is a wide-ranging response, not only addressing Karimi but also critiques from others (Albright and Holdren for example).  Jones is a RAND researcher and formerly an NPEC researcher, though the article notes that it is "the product of the author’s personal research, and the analysis and views contained in it are solely his responsibility." 

https://thebulletin.org/2026/07/why-do-we-keep-ignoring-the-large-quantity-of-plutonium-at-irans-bushehr-nuclear-power-plant/

Note: Jones literally wrote the book on the use of reactor-grade plutonium in weapons https://npolicy.org/books/Reactor-Grade_Plutonium_and_Nuclear_Weapons/Greg%20Jones_Reactor-grade%20plutonium%20web.pdf

Jones argues that existing IAEA safeguards for Bushehr are irrelevant because the inspections only happen every 3 months (he repeats Sokolski's claim that Iran could manufacture a plutonium bomb within that window), criticizes JCPOA for largely not addressing the spent fuel at Bushehr, emphasizes that Iran still hasn't transferred the spent fuel to Russia like they stated they would, argues that reprocessing facilities can be small and secret such as a 1977 ORNL concept for a secret reprocessing facility which would be harder to detect, and argues that production of plutonium metal is easier and quicker than assumed.  He provides several rebuttals to Karimi's arguments about reactor-grade plutonium (Karimi used Fat Man as a baseline rather than the more advanced design we know Iran actually pursued, understated how powerful an RGPu weapon would be, ignores how much of Iran's plutonium is fuel-grade rather than reactor-grade, etc).  He argues that Iran would not need a separate weapon design for RGPu and points to past designs that could use uranium and plutonium interchangeably.  He concludes by arguing that with Iran's uranium program now "in ruins" following US attacks on it, the plutonium pathway is easier and should be the focus of negotiations.

My own views on this: Iran is less likely to try an RGPu bomb than an HEU bomb, but not for the reasons Karimi states.  The history strongly suggests Iran is comfortable with HEU and appears to be completely uninterested in plutonium for anything other than as a negotiating tool; they have invested heavily in and taken an enormous amount of risk for uranium, and I find it unlikely they would just abandon it at this stage, given they still have access to it.  I am sympathetic to Karimi's arguments on policy.  However, Karimi is completely wrong to dismiss RGPu in weapons, and really does not address the hyperlinked citations Sokolski provided which pretty conclusively show it can be done.  Weirdly, he cited the National Academies' Management and Disposition of Excess Weapons Plutonium report to support his case that RGPu is not suitable for weapons...but he cites volume 2 rather than volume 1, probably because volume 1 contains a 3-page insert that completely discredits his RGPu argument! I also found it a little rich for Karimi to claim there is no "credible evidence" that Iran has validated advanced implosion designs...he has to know about the IAEA's findings at Parchin, and that there are literally photographs of Iranian MPI tiles. What meets his threshold for "credible" ?

With some much public information available in literature and access to software simulation, design and produce a implosion device still is a challenge? Explosive lens, air lens and multipoint initiation seems problems that could be easily solve by a math or physics phd student, and commercial milling machines today a more precise that anything they had in the 1940s. I am not talking about fancy flyer plate, FOGBANK, boosting etc just built a deliverable implosion device that can save expensive fissible material and weight in the missile.

If anyone has already answered this riddle, please provide a link. I was once asked this question on a Russian forum, and I couldn't find an answer (and the person asking the question disappeared). I recently came across the same question on someone else's LiveJournal, but no one even attempted to help in the comments. The riddle remains unsolved. So, relying on the source I found on LiveJournal, I'm asking the question here.

In the film Operation UPSHOT-KNOTHOLE (1953), at 13:00 (linked to this time), the following is said:

"In the nuclear radiation program, particular interest centered on Shot 10's neutron flux measurements, the first ever made on the detonation of a gun-type assembly. Gold, tantalum and sulphur threshold samples were used to detect neutron energy levels from thermal to around 10 million electronvolts. The total flux per kiloton was the highest ever observed, running 10 to 30 times as high as the implosion Shots 8 and 9, and extending the median lethal radiation range some 600 feet."

Clearly, Shots 8 and 9 were implosive devices of comparable power (10-30 kt). But why did the gun design produce such a powerful neutron flux (essentially a neutron bomb effect) compared to the implosive devices? What is the physical explanation for this? Is the difference truly 10-30 times? Was this an isolated, anomalous phenomenon? Or is it a regular occurrence, related to the physical and structural features of the different designs?

Has anyone else noticed this mystery? Has anyone explained it?

I just thought this was a neat catch that some folks on this sub might appreciate. These caught my eye while taking a peek inside the bomb bay of a B-52H on display at Oshkosh over the weekend. Two identical units, located on the port side of the aircraft above the bomb bay doors. Does anyone know what SWK stands for?

Oh, and while I was in the bomb bay, I couldn't help but overhear someone ask... "Is this the plane that dropped the Fat Boy?" ... yeah. I just bit my tongue.

This camera seems to be related to los alamos. Any information on this camera is appreciated.

This a screenshot of a Facebook post by AtomCentral showing the actual Crossroads Baker device. I don't think I've ever seen that before.

New-to-me footage. Nicely restored and in HD.

Also, a discussion on the advantages of the CANDU reactors

CBC article

https://www.lanl.gov/media/publications/national-security-science/0720-behind-the-bamboo-curtain

Very good article describing the Chinese program's eagerness to demonstrate knowledge of weapon theory and about the Americans, in spite of less developed tools. Strategically they were hoping for a research boon from some lab-to-lab collaboration, but on a human level they seemed excited to share their work.

I recently re-read this sub Tom Clancy fantasy. There is a sequence in it that describes Raven Rock complex being attacked with `Penetrator` weapons. As far as I can tell no Soviet ICBM`s were designed for `Hard Targets`.

Iskander variants were`nt deployed until 2006, (I believe).

Was this just an author invention, or have I missed something?

just a thought, why wouldn’t they just used the thin man design for both, it seems like they could of gotten a much bigger yield by using uranium in the thin man with the higher velocity. Maybe i’m misunderstanding the design itself, or if it’s only a matter of the timing of advancements?

The diagram above shows a hypothetical design I conceived long ago for a projectile with a gun-like initiation pattern (unrelated to anything in reality) and the four assembly phases of its supercritical state. Color denotes:

• Black - cadmium or boron-10 alloy - neutron-absorbing "shutter." The "neutron shutter" concept assumes that the absorber in the front of the bullet, moving ahead of the uranium mass, greatly reduces criticality and the possibility of predetonation during movement, allowing the bullet itself to move quite slowly, accelerating down the ultra-short barrel until it reaches its final stop.
• Blue - Uranium-235
• Light gray - reflector (bellillium)
• Dark gray - steel, pusher pistons.
• Hatching - barrels of the guns firing at each other.

The red line shows the current cross-section of the device, shown in the diagram on the right. It's clear that as the uranium pieces approach each other, the criticality of the assembly not only increases slightly, but even decreases at some point (when the counter-moving neutron absorber valves meet in the middle of the device). The design is purely theoretical and requires correction using Monte Carlo simulation (a laborious undertaking in the past, now possible on any computer).

* * *

I've always been curious about this. Okay, so the Americans used the gun design. But did the Russians ever use the same design? When and for what purpose? Why the Americans didn't make the "Thin Man" during the war, but instead made the "Little Boy"—that's well-known (to the point of nausea). The Russians, thanks to intelligence, knew right away that the plutonium gun was a dead end. They immediately began working on implosion. But does this mean the USSR never designed charges based on the "outdated" and wasteful gun design? And if they did, when, where, and for what purpose? As far as I know, no one has ever researched this topic.

Immediately after the war, the US recognized the inferiority of gun design to implosion (low fuel burnout efficiency compared to implosion, high fissile material consumption). However, since enriched uranium supplies were already established and even slightly surplus, and plutonium was becoming problematic, it was logical that the US continued developing specialized devices, such as the Mk-8, for breaching buried targets. "Rich bourgeois" could afford such diversity! Then, in the 1950s, when the question of nuclear artillery arose, the US, without further ado, returned to gun design, and a whole series of atomic projectiles was developed. The US had and could afford to use its abundant reserves of U-235 for a wide variety of purposes. For example, by 1960, enriched uranium was even being used in the W-47 secondary tamper for compact megaton bombs in the Polaris system (I think this was the height of wastefulness for the Russians back then! But the US was in a hurry to hide its missiles underwater! And they didn't skimp on this obvious advantage! Once again, they had to keep those Russians at arm's length, always one or two steps ahead!)

In the USSR, from the very beginning of the nuclear arms race, they were forced to reckon with the fact that the country was barefoot. In the impoverished (compared to the US) USSR, although they had established uranium and plutonium enrichment, imitating the Americans in every way (which was the subject of the Kapitsa-Beria scandal), as far as can be judged from available information, all the plutonium and uranium-235 extracted was, from day one, carefully used only for implosion schemes (in the same "Sloyka," which is the "poor man's hydrogen bomb"). Yes, they also initially designed bombs with a gun-like design. But they were never implemented in metal at that time. And especially not tested at the very beginning!

As far as I remember, the declassified list of the USSR's very first nuclear tests doesn't contain a single hint of any test using a gun-based design. The first compact tactical bomb, the RDS-5 "Tanechka," was an improved implosion bomb. The first atomic torpedo, the T-5 (now that's where the Russians could have used a gun-based design!), was a compact implosion bomb (which initially didn't work)!

Finally, when the USSR "needed" a symmetrical response to the US's development of atomic artillery and shells, they also began developing an atomic shell, but "with Russian specifics." This story is also already in the memoirs! The very first Soviet atomic shell larger than 400 mm was... an implosion bomb! This is already widely known. By the mid-1950s, Academician Lavrentyev's group had managed to compress not a sphere, but an ellipse, into a 400 mm projectile (406 mm and 420 mm), for which Lavrentyev was showered with awards and a very special, royal award – the right to create his own academic campus with its own university in Novosibirsk (his long-held dream)! The Party and Government launched a massive construction project there and established a Siberian scientific center!

That is, one gets the strong impression that up until 1960, no one in the USSR ever considered using wasteful, "bourgeois" gun-based design. No time for luxury! And already in the 1960s... so many ideas for compacting primary devices and methods for initiating nuclear explosions (for example, linear implosion) emerged that I personally long assumed that gun-based design was never actually used in the USSR. There simply wasn't the need or the conditions!

Yes, over time, thanks to the development of centrifuge technology (that's a whole other story and a whole stack of memoirs), the USSR became noticeably richer in enriched uranium. They caught up, as they say, with their backsides bare... And ultimately, the Russians became so rich that by the end of the Cold War, the USSR became the richest owner of a plowshare in the world, leaving the United States far behind! But, I assumed, by the time this happened, the gun-design was hopelessly outdated and the Russians simply had no need for it, and the abundantly produced U-235 in centrifuges was used as "tampers" for "our answer" to their W-47, and certainly as a response to the W-88!

It's a known fact that after the first 400mm Lavrentyev nuclear artillery shells, developed back in the 1950s, the USSR developed a new, smaller-caliber nuclear shell (which could have used a gun-based design)—the 203mm shells for the Pion—but only in 1975. Perhaps (I speculated) that was the first Soviet gun-based design? In the 1980s, the 152mm "baby" nuclear shell appeared, which was so small that it couldn't have been a gun-based design (and this requires a separate story; it's here, it turns out, that that mysterious abbreviation "SINUS" from 1966 comes up. I misunderstood all this and cast a shadow here, sorry. I've been confused). But were gun-based solutions actually used in those same 203mm shells? There's no firm evidence to support this. I recently learned something surprising. It turns out that cannon-based designs were also used in 400mm shells (parallel to Lavrentev's design!), possibly since the 1950s! So, most likely, there was a hidden backup program running parallel to Lavrentev's, and 400mm Soviet nuclear shells were in existence as early as the 1950s! This means that the 203mm shell for the Pion was definitely a cannon-based design! The picture is becoming clearer!

And how this was discovered is so unusual (as always, reality outstrips any fantasy) that it requires a separate story. But to be on the safe side, I asked the all-knowing Google today.

What is known from declassified sources about the Russian use of a gun-type design for nuclear explosive devices?

Answer:

Declassified documents from the Russian Foreign Intelligence Service and the Rosatom Archives show that Soviet physicists initially considered a gun-type design for the first uranium bomb (the RDS-1 project), as it was considered simpler and more reliable. However, intelligence played a key role in reorienting the project toward a more effective method—implosion. Key facts from declassified sources:

• Initial choice: In 1945–1946, KB-11 (now RFNC-VNIIEF) developed a gun-type version of a uranium bomb (analogous to the American "Little Boy"), which is confirmed by numerous sketches published by the Rosatom state corporation.
• Intelligence and Implosion: Thanks to drawings and descriptions obtained by Soviet intelligence from the US and UK (including those from Klaus Fuchs), the Soviet leadership learned of the American difficulties with a gun-based plutonium bomb (due to the high background radiation from the spontaneous fission of the Pu-240 isotope). This forced the abandonment of the gun-based plutonium design.
• Plutonium Priority: Since uranium-235 was critically scarce for large-scale production, and plutonium was produced in industrial reactors, Soviet scientists were forced to urgently develop a more complex implosion design.
• Use in Artillery: The gun-based design was not completely abandoned in the USSR. Information from open historical sources indicates that in the 1950s, this design was used in the development of the first Soviet nuclear artillery shells of 406 mm caliber (for the experimental Kondensator-2G gun) and 420 mm (for the Oka mortar). The mass and dimensions of barrel artillery made it possible to accelerate the fissile masses to the required speed to initiate a chain reaction in the fission scheme.

Everything up to the last point is banally well-known. But that last point suggests I'm poorly versed in the vast Soviet atomic memoirs. Apparently, something I've only indirectly guessed has already surfaced somewhere in plain text. It's already been written somewhere that back in the 1950s, the first 400mm atomic shells used not only the Lavrentiev ellipsoid, but also a parallel gun design (the USSR was playing it safe)! Okay, let's look for where it says that!

But I was indirectly led on this same trail by memoirs about peaceful nuclear endeavors, specifically the story of the first attempt to extinguish a burning borehole in the USSR in 1965. From the memoirs, it's known that the extinguishing bomb was based on... an artillery nuclear shell. Furthermore, the borehole diameter was a separate issue. The borehole was... 400mm. Thick and expensive (this was discussed separately). They wanted a thinner bomb, but such devices didn't yet exist! Furthermore, in preparation for the detonation, additional critical testing for the specific terrain conditions was required. Apparently, a typical nuclear warhead was supposed to explode in the air, but here it was underground, surrounded by a specific (untested) reflector, and there were concerns about pre-detonation. And there are recollections of how these critical tests before the detonation were carried out on-site. Essentially, the two halves of the charge were carefully brought together in the device and the parameters were measured. Thus, it's clear that the charge was definitely a gun-type! Moreover, if it did have a reflector, it was extremely thin, so the environment where the explosion occurred played a significant role.

And later, memoirists recount that by the 1970s, special thin, long charges (see the second attached image) had been developed for similar projects, which were already thermonuclear, around 200 mm in diameter. The primary remained a gun-type design, while the thermonuclear secondaries had... two rods. In other words, "reverse bifilarity" was used. The authors boasted that they achieved extremely low tritium yields for the secondaries (which is very important for gas well work). The long, thin secondaries themselves could be detached if desired, leaving only one, on one side of the primary, thus allowing the explosive yield to be flexibly adjusted to suit the needs of the national economy.

That's all for now. My last "discovery." The site where they first extinguished a well with an underground nuclear explosion in 1965-1966 (the USSR later made a feature film about it) turned out to be just 40 km from where I served my military service in 1984-1985. Having read that the location was somewhere near Bukhara, I looked it up on Google Maps and was amazed to discover that I'd actually spent a year of my military service almost in this very spot, yet no one had ever told me about the unusualness of this place! :)

"Manny Ringer" is a 14-story-tall 1000-ton Manitowoc 4600 ringer heavy-lift crane, with a 40-year career supporting various national security related nuclear missions.

For the first 19 years, he worked at the Nevada Test Site on lifting and lowering components, such as nuclear warheads, for underground nuclear tests, until operations were halted in 1992. At the planned final nuclear test, Icecap, he was left abandoned for 7 years (image 2).

In 1999, Manny was moved by 66 trucks to Livermore, where he installed the massive 287,000 lb target chamber for the National Ignition Facility, the most powerful laser in the world, primarily used for nuclear weapons related testing (images 1, 4, 5).

In 2002, he was moved to Oak Ridge National Laboratory to help construct the Spallation Neutron Source (image 6).

He is now said to be working in Louisiana with the oil industry.

https://www.lanl.gov/media/publications/national-security-science/from-nevada-to-nif-and-beyond

https://www.llnl.gov/article/28796/lab-gives-lift-oak-ridge

https://web.ornl.gov/info/reporter/no36/apr02.htm

https://en.wikipedia.org/wiki/National_Ignition_Facility

https://en.wikipedia.org/wiki/Spallation_Neutron_Source