By the end of August 1945, there had been a total of three plutonium cores created in the entire world. Everyone knows about the first two. The first was put into the Gadget and detonated at Trinity in July 1945. The second was put into the Fat Man and detonated over Nagasaki in August 1945. The third, however, has been largely overlooked. The third core was the one that was destined to be the Third Shot dropped on Japan, had there been a Third Shot. Instead, it has a different story — but it was still not a peaceful one.

One of the questions I got from people regarding the “Why Nagasaki?” post I wrote last week was “When would the Third Shot really have been ready?” The reason for the question is that since the Third Shot was unlikely to have been ready by the time Hirohito announced Japan’s acquiescence to the American surrender demands (August 15), that satisfies the question of why another one wasn’t used. In a very practical sense, it does, but it ignores the fact that Truman actually put a “stop” on all further atomic bombings on August 10 — when the effect (if any) of the bombs on Japan’s high command was yet unknown. (He did not, it is worth noting, put a stop on firebombing: huge B-29 raids continued up until the surrender announcement.)

But still, it’s an interesting question to consider. There are two components to it: when did they think the third core would be ready, and when was it actually ready? On the first question, we know that on August 10, General Groves wrote to General Marshall that:

The next bomb of the implosion type had been scheduled to be ready for delivery on the target on the first good weather after 24 August 1945 . We have gained 4 days in manufacture and expect to ship from New Mexico on 12 or 13 August the final components. Providing there are no unforeseen difficulties in manufacture, in transportation to the theatre or after arrival in the theatre, the bomb should be ready for delivery on the first suitable weather after 17 or 18 August.

It was on this document that Marshall scrawled, “It is not to be released on Japan without express authority from the President” — the Truman “stop” order. But we also know, from the Seeman-Hull document I discussed in an earlier post, that Marshall was still interested in the atomic production rate on Monday, August 13, 1945. At that time, Seeman claimed that:

Seeman: There’s one ready to be shipped now — waiting on order right now. […] The whole program is phased according to the best production. There is one of them that is ready to be shipped right now. The order was given Thursday [August 9?] and it should be ready the 19th. Hull: If the order is given now, when can it be ready? Seeman: Thursday [August 16] would be its readiness; the 19th it would be dropped. Hull: In other words, three or four day advance notice before it can be shipped, and six days after that when it can be dropped.

So that’s a pretty interesting conversation — it tells us that the core was in some kind of almost-finished state by August 13. In a 2012 interview, physicist Lawrence Litz told Alexandra Levy of the Atomic Heritage Foundation that:

Levy: What was—how did—do you remember working on casting the plutonium for the third bomb? Litz: The particular day that remembers—that remains in my memory was the day that we cast the plutonium for the third bomb because we weren’t sure that the Japanese would surrender even after the second bomb was dropped. We had to cast the atmospheres for the third, and because time was short we had to cast the two hemispheres at the same time. But it was dangerous to cast them in the same laboratory at the same time so we set up two adjacent laboratories with the high vacuum apparatus and the—so we could cast one hemisphere in each one of the two labs. Levy: How long did that take to cast? Litz: About twenty-four hours and we had to work straight through.

Which gives some indication of the tenor of the day, and the fact that Truman’s “stop” order didn’t mean that they weren’t expecting to potentially keep atomic bombing. (As does the Seeman-Hull conversation.)

How much plutonium was on hand in August 1945? I’ve been hunting around for anything that would give me some hard numbers on this, and finally, basically when I’d given up on the effort, I was surprised to stumbled across a document that did:

“49 Interim Processing Program No. 24,” dated August 30, 1945, indicates that by that date that Los Alamos had, by their assessment, received 26.136 kg of plutonium from Hanford. Figuring out what was done with all of that requires a little decoding of the terminology. 12.292 kg of the material is listed as having been transferred to the US Army with the notation “HS-1, 2, 3, 4; R-1” after it. I haven’t seen this notation before, but I think it’s almost certain that “HS” means “hemisphere,” i.e. half of a sphere of plutonium. So two full spheres worth were transferred to the Army and were at that time “non-usable,” along with “R-1.” R-1 is almost certainly an “anti-jet” ring developed for use in the Fat Man core (and not present in Trinity’s core). So HS-1+HS-2 were the Trinity core components, and HS-3+HS-4+R-1 was the Fat Man core. The first two cores were “non-usable” because they had been detonated.

So we can see from the document that HS-5, HS-6, and R-2 had already been cast and were in the hands of Quality Control at the lab (QC). HS-7 and R-3 had been already cast by then, but still needed hot pressing and nickel coating. HS-8 was scheduled to be pressed on August 31, and finished by September 5. Which is the finest-grain look at the early nuclear production schedule that I’ve ever seen. (And as you can tell I’m quite proud of myself for finding it and deciphering it!)

But the story of the third core doesn’t end there.

The core was cast sometime around August 13th, but still likely needed to be pressed and coated, ergo the need to take until August 16th to finalize. By August 15th, it became clear that it wasn’t going to be needed in the war. So it was kept at Los Alamos.

What it was doing between August 15th and August 21st, I don’t know. But I do know that on August 21st it was being used for critical mass experiments — “tickling the dragon’s tail.” The experiments in question involved surrounding a full 6.2 kg core with tungsten carbide, getting information about the effect that different tamper arrangements had on criticality. (The tamper reflects neutrons back into the core, thus increasing the overall neutron economy and thus lowering the effective critical mass.)

The experimenter in question was 24-year-old physicist Harry Daghlian, Jr. To quote from a report on the experiment:

[Daghlian] was carrying one brick [of tungsten carbide] in his left hand over the assembly, to place it in the center of the fifth layer. While he had this brick suspended over the assembly, he noticed (from the instruments) that the addition of this brick would have made the assembly supercritical if placed on top of the assembly. Having realized this, he was withdrawing his left hand and the brick from over the assembly and while doing so the brick slipped out of his hand and fell immediately onto the center of the assembly. Knowing that this brick would made the assembly dangerous, he instinctively and immediately pushed this brick off the assembly with his right hand. While doing this, he stated that he felt a tingling sensation in his right hand and at the same time noticed a blue glow surrounding the assembly, the depth of the blue glow being estimated to be about two inches.

Daghlian was estimated to have received a 510 rem dose of ionizing radiation — a usually lethal dose. He died after an agonizing month. This, incidentally, appears to have been why at the time of the August 30 audit, the core was in Quality Control: they were checking to make sure it had not undergone any “dimensional changes” as a result.

One might think that someone involved with the investigation of the Daghlian accident would be especially cautious around using such a core in further critical mass experiments, even if only for superstitious reasons.

But exactly 9 months later, one of the co-authors of the above-cited report, Louis Slotin, would himself receive a lethal radiation dose from the exact same core in the process of yet another (different) critical mass experiment. Slotin knew the experiment in question was dangerous, and had been told by Enrico Fermi that he would be “dead within a year” if he continued to work with such bravado. Like Daghlian, his hand faltered at a literally critical juncture: he was holding a neutron reflector above the core with a screw driver when his fatal slip occurred, lowering the reflector just a fraction of an inch, releasing a stream of neutrons and the characteristic blue glow. Slotin died 9 days later.

The third core, by now nicknamed the “demon core” for having taken two lives, would not go out with a whimper. By some accounts, it found its final disposition in the first postwar nuclear test, shot “Able” of Operation Crossroads, on July 1, 1946, just under a year after it had been first cast, in that all-night session, in the closing days of World War II. (UPDATED BELOW) Encased in a “Fat Man” assembly with “GILDA” stenciled on its hull, it was finally dropped from a B-29, as it was originally intended to be, and it detonated over a fleet of empty ships in the Bikini atoll, with a yield of 21 kilotons. Alas, the journalists who saw it, with perhaps higher expectations for their first atomic bomb test, incorrectly dubbed it a flop.

That a single plutonium core could go through so much may seem remarkable. But it is a reflection of a time when such cores were extremely rare commodities. And so a single core could simultaneously be the one originally destined for the “third shot,” and also be the subject of two fatal criticality accidents, and also still be the first core consumed by postwar nuclear testing. It is a potent reminder of how paltry the American nuclear arsenal once was — when there were less than a dozen pieces of cores, much less cores themselves.

UPDATE: The third core was not used at Crossroads, after all! Learn what happened to it, here.

Tags: 1940s, Bomb design, Fat Man, Japan, Los Alamos, Manhattan Project, Nagasaki, plutonium, Trinity