In the 1940s, scientists worldwide worked to create the first nuclear weapons. It was difficult to design a functional and combat-ready warhead that met strict criteria. This article examines the various methods that emerged during that time and offers valuable insight into the development of nuclear weaponry.

The Cannon Scheme

In 1945, the American military used a bomb called “Little Boy” to attack Hiroshima. This type of nuclear weapon used uranium and had an explosive power of 15-18 kilotons of TNT. To make the production process more efficient, they used the cannon method. This method was developed as a result of the start of nuclear warfare.

Evolution of Nuclear Warhead Designs
Schematic diagram of the bomb “Kid” gun scheme. The blocks of uranium are marked in red. Graphics Wikimedia Commons

The containment vessel for this design looked like a series of artillery barrels. Inside the barrels were two uranium-235 blocks that were not fully critical at either end.

One of these blocks was mobile and equipped with a powder charge, designed to be propelled towards the second block upon initiation, facilitating a nuclear chain reaction. Additional neutron sources were incorporated to enhance reliability.

Although the cannon scheme was uncomplicated, it was not very efficient as a significant amount of the main charge dispersed into space during the explosion, causing only 1% of the uranium to participate in the reaction. There was also a risk of spontaneous detonation. Nevertheless, the United States still utilized the cannon scheme early on by producing 35 Little Boy devices and artillery shells. However, with the invention of more advanced designs, the cannon scheme eventually became obsolete and faded into history.

The Squeeze Effect

In July 1945, the US tested a nuclear weapon called “Little Boy.” Before that, they had tested another weapon called “Gadget” at the Alamogordo test site. The gadget was made with plutonium-239 and used an implosive design instead of a cannon design because plutonium couldn’t be used with a cannon.

Evolution of Nuclear Warhead Designs
Model of the implosion warhead used in the Gadget and Fat Man items. Numerous fuse control wires are visible. Photo by Wikimedia Commons

The focus was on a plan that involved a spherical plutonium core with a smaller mass than needed. The core was surrounded by layers of regular explosives, each with evenly distributed fuses. The fuses were timed to detonate the layers simultaneously, compressing the core and initiating a nuclear chain reaction.

Evolution of Nuclear Warhead Designs
Soviet nuclear bomb RDS-4 of the implosion scheme, adopted for service in 1954. The world’s first tactical nuclear weapon. Photo by Wikimedia Commons

The implosive scheme, though more complex, offered superior reliability and efficiency. It served as the blueprint for the “Fat Man” bomb dropped on Nagasaki in August 1945. The United States continued to develop and employ various models of implosive charges, while the Soviet Union adopted this scheme as the basis for its early nuclear weapons, starting with the RDS-1.

Development Options

Although the implosive scheme had its advantages, it wasn’t very efficient in early samples. They only achieved a 13-15% reaction of fissile material. This made people start looking for new ideas and solutions to make nuclear devices safer and more reliable.

One of those new ideas was a nuclear booster. It used a small amount of thermonuclear fuel, like deuterium or tritium, as a neutron source. When they were compressed, these substances produced high-energy neutrons. This effectively started the chain reaction in the main charge and made the weapon more efficient and safer.

During the 1950s, a nuclear weapons design called the “Swan” scheme was developed. It featured a subcritical mass core that was spherical in shape and an intricate initiating charge with a curved design. The purpose of this charge was to distribute the shock wave in the most effective way possible to ensure that the plutonium sphere was properly compressed. In addition, extra fuses could be used to interrupt the shock wave’s path, thus stopping a chain reaction from occurring.

Fundamentally New Technologies

During the 1940s and 1950s, various basic nuclear charge schemes were developed and modified. However, in the mid-1950s, the focus shifted towards creating thermonuclear weapons based on nuclear fusion.

These efforts redirected scientists’ and engineers’ focus toward thermonuclear weaponry, relegating “conventional” nuclear charges to serve as the first stage in a thermonuclear system.

Despite this shift, work on improving “conventional” nuclear devices continued. Variations of the implosive scheme were predominantly employed to meet specific project requirements. This approach remains relevant today, aligning with the objectives at hand and enabling the creation of weapons with desired characteristics.

A Solid Foundation

Nuclear weapons entered the arsenals of major nations in the mid-20th century. The scientific and engineering community explored diverse design options, ultimately achieving success in increasing the weapons’ effectiveness. While the ideas from that era continue to shape modern nuclear weaponry, advancements in technology and materials have further harnessed the potential of these longstanding schemes.

These age-old developments continue to play a vital role in bolstering our nuclear defenses and providing strategic deterrence. The evolution of nuclear warhead designs is a testament to human innovation, even in the most perilous fields.