a. Initiator
b. Active material
c. Tamper
d. Aluminum layer
e. Explosive
f. 32 explosive lenses
g. Detonating device
h. Duralumin shell
i. Armor steel shell
j. Fins
All the above-listed parts of the bomb, except for the fins, detonation device and external steel shell are hollow balls, inserted into each other. For instance, the ball of active material itself is inserted into the tamper (moderator), which is also a hollow ball. The ball of the tamper in its turn is inserted into another hollow ball, made of aluminum, which, in its turn, is surrounded by a spherical layer of explosives.
The layer of explosives, which also contains the lenses, is surrounded by the duralumin shell, to which the detonation device is attached, and which is covered by the outer shell of the bomb, made of armored steel.
There followed a systematic discussion of each of the important parts of the bomb, beginning with the initiator. David Greenglass had described an initiator with shaped-charge cones machined into its beryllium shell to facilitate shock-wave mixing of its beryllium and polonium; the document Beria received in October 1945 described another initiator design, “a hollow beryllium ball with wedge-like grooves on the internal surface of the ball” with “the axes of all grooves… parallel to each other.” This design, the document noted, was called “Urchin.” Physicist Rubby Sherr at Los Alamos had nicknamed the grooved initiator the Urchin. Because of its grooves, its other nickname was the “screwball.” The Trinity implosion device used an Urchin initiator. So did the Fat Man exploded over Nagasaki; so would all US atomic bombs for half a decade to come. The October 18 document gave the Urchin's precise measurements and described its operation in detail — its two parts, its gold and nickel plating, the way its various layers and parts interacted to generate neutrons to start the chain reaction. The collapsing grooves produced a “Munroe jet,” the document noted, referring to the Munroe effect that David Greenglass was later to cite as the principal mechanism by which initiators mix beryllium with polonium. Of the “active material” the document noted:
2. ACTIVE MATERIAL
The active material of the atomic bomb consists of the element plutonium of delta phase with specific weight of 15.8 [sic: the modern value is 15.7]. It is manufactured in the form of a hollow ball consisting of two halves, which like the outer ball of the initiator are pressed in an atmosphere of nickel-carbonyl. The external diameter of the ball is 80–90 mm. The weight of the active material, initiator included, is 7.3-10.0 kg [the Fat Man core without initiator weighed 6.2 kg]. A gasket of corrugated gold 0.1 mm thick is located between the halves of the sphere, which prevents penetration to the initiator of high-speed jets moving along the junction planes. These jets might otherwise prematurely activate the initiator.
There is an opening 25 mm in diameter for the purposes of inserting the initiator into the center of the active material, where it is fixed on a special bracket [the initiator was levitated]. After the initiator is inserted, the opening is closed with a plug also made of plutonium.
The gold foil was a significant detail. Metallurgist Cyril Stanley Smith had developed it to resolve a jetting problem. The foil served to true the surfaces of the two plutonium hemispheres after plating and improve the fit. In retirement, Smith would sometimes exhibit a spare foil to visitors — a small circular sheet of pure gold with a large hole in the middle which he kept at home in a plain white cardboard jewelry box. After his death in 1992, the National Museum of American History acquired Smith's spare and placed it on permanent exhibit.
The Merkulov document next discussed the tamper — the heavy shell of natural uranium that surrounded the core and served as both a neutron reflector and an inertial restraint on explosive disassembly. Besides the tamper's composition and dimensions, and the fact that it also was plugged for core insertion, the espionage report included a significant detail that is invariably omitted in US official descriptions of the Fat Man design: “The external surface of the tamper is covered with a layer of boron which absorbs thermal neutrons emitted by the radioactive materials of the system that are capable of causing predetonation.” The muddled description of the implosion bomb that David Greenglass had passed to Julius Rosenberg in September had included this neutron-absorption system, although Greenglass had misidentified it as barium, not boron. At his trial in 1950 he called the boron layer a “barium plastic sphere”; in one of his confessions the previous July he had correctly positioned the boron layer, which he called “a plastic shield,” “between the plutonium [sic uranium tamper] and the high explosives.”
The Merkulov document described the Fat Man high-explosive configuration in detail, but omitted crucial information about the precise curve of the HE lens assemblies:
5. THE LAYER OF EXPLOSIVES AND LENSES
The layer of explosives consists of 32 blocks of special form. It follows the layer of aluminum. The internal surface of the blocks, facing the center, is spherical and has a diameter equal to the external diameter of the aluminum layer. There are special slots in the external surface of the blocks which provide for the insertion of 20 lenses of hexagonal and 12 lenses of pentagonal shape. A felt lining Vi6 of an inch thick is located between the surfaces, perpendicular to the axis of the sphere. Voids between radial surfaces are filled with blotting paper. The air gap between layers of explosives and lenses shouldn't be greater than V32 inch, since a larger gap could lead to an increase, or, alternatively, a decrease of detonation depending on the orientation of these gaps. Each lens consists of two types of explosives — one fast-burning, another slow-burning. The lenses are cast in special casings made of cellulose acetate. The lenses are installed so that the fast-detonating part contacts the layer of explosive. Total weight of explosives is around 2 tons.
One detonator is attached to each lens, which for higher reliability is provided with two electric fuses. There are 64 wires in total, divided into 4 quadrants, 16 wires in each. Two wires lead to each lens, but from separate quadrants.
The document concluded with a brief description of the bomb's duralumin shell and a careful discussion of its assembly, noting in particular that “since plutonium and the radioactive substances of the initiator generate heat and warm themselves to temperatures higher by 90 degrees centigrade than the temperature of the environment, they are transported to the bomb assembly site in special containers fitted with cooling systems.”
This historic document was evidently specially prepared for Lavrenti Beria, who had broad experience in industrial management but no scientific training; it was not a verbatim transmission of the more detailed information Klaus Fuchs had passed in June and late September 1945-
Beria was vulnerable. He had been assigned a vital project which had been given the highest priority of the state, but he lacked the knowledge necessary to judge its progress. He was at the mercy of scientists, intellectuals, people he viscerally distrusted. “With all of Beria's apparent power,” writes a Russian historian, “he understood nothing about physics and he remained silent when the subject came around to uranium, plutonium, the separation of isotopes, ‘items'… And the success of the work… also meant the destiny of the leader's adviser himself, who bore personal responsibility for the creation of nuclear weapons under Stalin.” “At first all the problems were solved through Kurchatov,” says Yuli Khariton. “[Eventually] [Beria] was forced to pay attention to us.”
So Beria sought ways to decrease his vulnerability. He sent security officers to Japan to film the destruction at Nagasaki. He began developing a stable of “backup” scientists — with fewer Jews among them — whom he might call upon to replace the Kurchatov team if it proved to be treacherous.
Already on October 3, Peter Kapitza had written Stalin boldly complaining of Beria's leadership of the atomic-bomb project. “Is the position of a citizen in the country to be determined only by his political weight?” Kapitza had asked rhetorically, adding, “It is time… for comrades of Comrade Beria's type to begin to learn respect for scientists.” Beria first tried to smooth over the disagreement. After Stalin called him about Kapitza's letter, he telephoned Kapitza and invited the physicist over for a talk. When Kapitza refused to visit Beria at the Lubyanka, Beria sent Kapitza an elegant gift, a Tula shotgun. Kapitza was not assuaged, however, and the argument
