This is a guest post on behalf of ACW reader and occasional contributor Chris Camp.
The first Atomic bombs, Trinity, Hiroshima, and Nagasaki, hold an outsized place in our perceptions of what a nuclear weapon should be. Certainly they were notable as the first bombs, the only ones used in anger, and the most famous devices in a subject shrouded in secrecy, but times have moved on while perceptions largely have not. When we talk about cars people don’t think of the Benz Patent-Motorwagen, when we discuss airplanes the Wright Flyer isn’t the first thing that comes to mind, yet when you mention an atomic bomb odds are that one of the WWII devices is what people will think of.
This perception came up on Arms Control Wonk in an article on the 1973 Yom Kippur war by Avner Cohen in which he states:
[I]t is plausible that on the eve of the 1973 War Israel had a small nuclear inventory of weapons, say, between ten to twenty first-generation fission (PU) weapons (roughly, Nagasaki-type). One could speculate further that most of the inventory was in the form of aerial bombs (probably configured for the Mirage) and some were early prototypes of missile warheads for the Jericho I (which in October 1973 was apparently not yet operational).
This led to a discussion on what the Israeli arsenal might have actually looked like and whether a “Nagasaki type” bomb was in fact a reasonable assumption for a fledgling nuclear weapons state in 1973.
The bombs used by the United States in World War two were very much a product of their times and the environment in which they were created. They were not the best design Los Alamos could produce, even when they were built, but they were the quickest ones that could be gotten out the door. The “Little Boy” bomb dropped on Hiroshima was officially designated a Mark 1 bomb and the Nagasaki bomb, while not having an official designation, can be thought of as a Mark 3 prototype (The Mark 2 was a plutonium gun design that turned out to be unfeasible due to pre-detonation of plutonium 240). I say prototype because the production Mark 3 that formed the basis of the first US stockpile had several improvements over the actual wartime weapons. Both the Mark 1 and Mark 3 were complex, dangerous and wasteful designs that were largely obsolete before they were ever used, but were products of the state of both the weaponeers art and also of American nuclear industry at the time. To put it another way, they were products of the circumstances in which they were created, and those circumstances would not apply to any nation building a bomb since then.
I’m going to talk primarily about the Mark 3 prototype designs used at Trinity and Nagasaki since the Mark 1 was literally a one-off device which has never been replicated, though many of the points apply to it as well.
The Fat Man device was 60 inches in diameter, 128 inches long, and weighed 10,300 lbs. It used about 6 kilograms of plutonium in the form of a three piece sphere (Two hemispheres and a sort of wedge shaped equitorial piece) with a spherical cavity about half an inch in diameter in the center for a Polonium – Beryllium initiator. Surrounding the plutonium was a two part sphere of natural uranium tamper, and then around that was layers of different types of explosives totaling about 18 inches thick. The outermost layer of explosives was covered in an array of 32 detonators which were hooked to a firing mechanism called an X-unit which set them all off with great precision. The fusing was accomplished by four radar fuses, called “Archies” which were modified prototype tail warning radars for fighter aircraft. The whole device was powered by lead acid batteries which lasted 2 days on a full charge, and the entire bomb had to be disassembled to charge or replace them. The casing was made from 3/8” thick steel and accounted for almost half the weight of the finished bomb. At the back was a large boxy, high drag tail which was necessary because the ballistic shape of the bomb was massively unstable and it tended to tumble so much while falling that the radar fuses couldn’t see the ground. Finally there was a set of 4 impact fuses on the nose which were meant more to destroy the bomb if the radar fuses failed and it hit the ground rather than to cause an actual nuclear detonation. Once assembled, the Mark 3 prototypes were very dangerous and almost certainly would have suffered a low order nuclear detonation in the event of an aircraft crash. Assembling the bombs took a crew of 50 people close to 18 hours and once assembled the bomb had to be dropped within 48 hours or the batteries would die and the entire device would have to be disassembled. The bomb makers art has come a long way since the dark days of 1945 and many of these advances would be incorporated by any nation building a bomb for the first time. Here are some of those advances and how they apply to the nature of a first device.
Levitated pits. The biggest improvement in weapons efficiency came from a concept that was well understood at Los Alamos before the war ended, but didn’t make it into the wartime bombs. This was the concept of a “Levitated” pit that greatly increases compression and therefore efficiency. The levitation concept as it applies to nuclear weapons is a bit unintuitive, so let me use an analogy pioneered by Hans Bethe after the war. When you go to hammer a nail do you put the hammer on the nail and push, or does it work better to swing the hammer and get some momentum going before you hit the nail? The Mark 3 design was the equivalent of putting the hammer on the nail and pushing. All of the components of the core were in physical contact and there was no room for the implosion to build up momentum before it hit the core. In a levitated pit design there is an air gap between the uranium tamper and the plutonium pit which gives the tamper room to accelerate and build up momentum before striking and compressing the pit. Not only does it allow more energy to be delivered to the pit, it also tends to smooth out irregularities in the shock wave, both leading to increased efficiency. The first levitated pit design, the Mark 4, had the pit suspended on wires inside the tamper, but later designs used a stand made of thin aluminum to support the pit. To visualize this, think of the plutonium core as the ice cream on an ice cream cone, except the cone is upside down and the ice cream sits on the pointy end.
Explosives. The Mark 3 prototypes used an outer fast-burning layer of Composition B high explosive over a middle layer of slow burning Barotol and then another layer of Comp B. There were 32 of these “explosive lenses” and the whole system was designed to create a perfectly spherical shock wave that would evenly compress the core allowing the fission reaction to take place. One early avenue of post war research was into more efficient high explosives which could generate more compression with less weight and bulk. Modern high performance explosives can deliver greater performance in a layer only 1-2 inches thick and weighing on the order of tens of pounds. In addition, Insensitive High Explosives (IHE) have been developed which are much more difficult to detonate accidentally, making for much safer bombs in the event of an aircraft crash. IHEs are used in most US nuclear and conventional aircraft bombs, and the guidance given to crash rescue crews is that even if the bomb is fully engulfed in fire, it is safe to attempt to extinguish the fire for 10 minutes before there is a chance of detonation.
Another improvement to weapons design that came about in the late 1940s was increasing the number of detonators which led to a smoother shock wave and more efficient detonation. This was one of the strategies used during the design of the Mark 5 bomb which was the first of the “small” nuclear weapons (only 39 inches diameter and 3200 lbs) designed for use by tactical aircraft. In order to compensate for the efficiency lost by using less explosives, Los Alamos increased the number of detonators, first to 60 and then to 92 (32+60). This allowed for a bomb that produced about the same yield in a smaller size than it’s contemporary 60 inch brother, the Mark 4.
Initiators. The early nuclear weapons used Polonium – Beryllium (Po-Be) initiators at the center of the core to produce a burst of neutrons to get the fission chain reaction going once the core had been compressed by the high explosives. These initiators were small spheres a little less than half an inch in diameter that rested at the very center of the core. When hit by the shock wave they were squeezed, which mixed the two materials and produced neutrons. This system worked fairly reliably, but had several significant drawbacks. The biggest one is that polonium has a half life of only 138 days and so in order to maintain a stockpile of initiators you must have a continuously functioning nuclear reactor to keep making more material. This was an immense headache in the early post war years when the Hanford production reactors were encountering numerous technical difficulties. At one point it was planned to shut one of the three reactors down completely and run the other two at reduced capacity so that even if they wore out and could no longer produce plutonium for new bombs, there would be at least one available to produce polonium to keep the existing arsenal operational. Polonium initiators are also difficult, dangerous, and expensive to build as the element is extremely toxic.
The other downside to Po-Be initiators is that they “fire” when they are squeezed, which is not the same as being when the core reaches maximum compression, typically a few milliseconds later. The premature burst of neutrons tends to cause a little bit of pre-detonation and reduces efficiency of the whole system. It’s a marginal loss, but one that becomes critical when someone starts designing weapons with very tight tolerances, such as thermonuclear triggers.
The solution to the initiator problem came in the 1950s in the form of solid state neutron generators which were basically portable (sort of) linear accelerators. The first ones were far too large to be incorporated into a deliverable bomb, but eventually they shrank down to about the size of a fist. In addition to being safe, cheap, and easy to build and store, these new neutron sources could be precisely timed to fire at peak compression of the core, reducing pre-detonation and increasing efficiency. Finally, because they could now be located outside the core they made storage assembly and maintenance of the bombs much simpler. This technology is also used in the oil field industry for well logging and so is readily understood and available for any nation building a bomb.
Delivery systems and casings. The Mark 3 bomb was 60 inches in diameter and 128 inches long because that’s how big the bomb bay on a B-29 was. The delivery system dictated the upper bound on the size of the finished weapon. Likewise, delivery systems have always driven weapons design. Any nation seeking to design a nuclear weapon will need to think about how they plan to deliver it, and, to put it simply, not many nations bother with strategic bombers anymore. Only the US B-52 and the Russian TU-95 strategic bombers can carry a weapon the size and weight of a Mark 3 type device, and both of them were designed in the 1950s to do exactly that. No nation is going to build a bomb they can’t deliver, and no one builds an aircraft that can deliver a bomb of this size.
Finally, there is the story of the casings on the Mark 1 and 3 weapons. Both weapons were built with 3/8” thick steel casings weighing over two tons each. The reason for this is that the bombs themselves were designed to be armored so as to survive flak and machine gun bullet impacts during the ride to the target. In both cases this armor accounted for around half of the total weight of the weapons. Nearly all post war designs dispensed with this armored casing design and instead use lightweight aluminum or steel casings optimized for aerodynamic efficiency. I’ve always wondered if the Air Force requested this “feature” on the early bombs or if it was something that the Los Alamos came up with on their own. Certainly by the late 1940s the Air Force had decided that it was unnecessary and asked that it be removed from future weapons.
Given all the reasons why an initial capability would look different from what it did in 1945, can we extrapolate what it might look like? In the same conversation on Israel in 1973 that led to this piece, John Schilling posited that something similar to a US Mark 12 was a good guess. The Mark 12, introduced in 1954, utilized most of the improvements I’ve mentioned. It was a 22-inch diameter, 1200 lb bomb which used a 92 point detonation system around a levitated core, producing a 12-14 kiloton yield, and could be carried by tactical aircraft at supersonic speeds. This was one of the last pure fission weapons developed before the widespread adoption of thermonuclear designs. Something on this order is certainly a reasonable guess as to what a first bomb might look like. Such a device is also in the size and weight range for carriage by a missile warhead.
While it’s hard to say what the first weapon from a newly minted nuclear weapons state might look like, we can be pretty sure that it won’t look like the bombs that were dropped on Hiroshima and Nagasaki in World War Two. Just as technology has advanced in every other field, so it has in the art of nuclear weapons. What hasn’t advanced is how many people think of these weapons, and this antiquated thinking clouds the conversation of weapons in the 21st century.