Nuclear Power

Nuclear fusion is a reaction where two different atomic nuclei are combined to form one or more different nuclei and subatomic particles. The reaction can potentially release energy and be a more powerful and cleaner source of energy than nuclear fission.

Overview
Although the idea of harnessing power from a fusion energy has been proposed since the 1940s, the only "practical" use for these weapons has been the development of thermonuclear weapons, with yields several times greater than fission-based weapons. It wasn't until the 21st century and the energy crisis that the United States finally developed a practical way to harness nuclear fusion for energy generation. The first fusion cell was revealed in the Summer of 2066, as an offshoot of the US power armor project. The technology developed rapidly, becoming the cornerstone of many civilian and military technologies. Although the process of incorporating nuclear fusion into the general US infrastructure began in 2066, nuclear fusion proved to be difficult to scale to industrial-level power generation and by 2077 only a few sections of the United States relied on fusion power for base load generation.

It's important to note that nuclear fusion relies on the capture of radioactive energy from microfusion reactions, rather than generating power by using heat energy to drive turbines, as with nuclear fission. The hot fusion reaction thus created is distinct from sustainable nuclear reactions achieved through electrochemical processes at or near room temperatures (so-called cold fusion), which are considered to be impossible to achieve. This impossibility did not stop Vault-Tec from referring to the fusion generator at the core of the Garden of Eden Creation Kit as a cold fusion generator.

Uses
The discovery of nuclear fusion has revolutionized life and industry in the United States, slowly supplanting nuclear fission as the premiere power source. Its uses were many and varied, including:
 * Power armor: The TX-28 micro fusion pack developed by West Tek was capable of producing the power necessary to make power armor a practical weapon. Using fusion cores as reaction starters and sources of fuel, the fusion-powered power armor revolutionized modern warfare.
 * Rayguns: The introduction of the micro fusion cell, a medium sized energy production unit and a self-contained fusion plant allowed for the creation of directed energy weapons. The MFC became the de facto industry standard and became a common choice for large energy weapons of all stripes, such as laser and plasma rifles. MFCs could also be turned into improvised satchel charges by removing safety catches. Further innovation on the MFC came in the form of the recharger rifle, improved upon by the recharger pistol, utilizing a special microfusion breeding technology.
 * Nuclear propulsion: After the introduction of fission-powered cars, fusion batteries and engines started to dominate the market. Unfortunately, they also suffered from the same design flaws that caused aging car engines to become unstable - explosively so.
 * Nuclear weapons: One of the oldest forms of practical nuclear fusion. One of the peculiarities of United States technological development is the fusion pulse charge, which can be used to turn working fusion reactors into thermonuclear weapons.
 * Portable power generation: The dominant form of power generation were portable fusion generators, such as the ones produced by GDA Fusion for household power generation. This kind of technology was also developed after the Great War by certain wasteland organizations, like Rivet City.
 * Microfusion power units are also sometimes referred to as batteries, especially when installed in appliances. These have the same effect and can provide a lasting supply of energy and keep the device independent of the existing energy grid.
 * Robotics: Fusion piles were used to power certain advanced robot types, such as the Robobrain and Assaultron combat robots.
 * Fusion core: A standardized high-grade, long-term nuclear battery used in a variety of military and civilian applications.
 * Large scale energy generation: One of the major potential uses of fusion energy, pre-War united states experienced some limited success with scaling the technology up to fusion generators, capable of powering entire buildings and providing backup power for Vaults. Nuka-Cola Corporation paired multiple fusion reactors to meet the energy demands of Nuka-World, the corporation's flagship amusement park. However, efforts to further expand the power generation have proven problematic. Mass Fusion was developing an experimental reactor before the Great War that could provide power at a large enough scale to replace conventional fission power plants. However, their work was never completed. The key obstacle was the energy required to trigger the reaction and make it self-sustaining. Mass Fusion's chief achievement was the creation of the beryllium agitator, which would allow the fusion reaction to be jumpstarted much more easily. It's possible to start the reactions conventionally, although doing so requires prohibitively large amounts of energy: Robert House would require the power output of the restored Hoover Dam to restart Lucky 38's reactor. Another issue, especially with systems used in non-standard role like the Prydwen, is cooling.  A sustained reaction requires a sustained supply of coolant, which can be in short supply.
 * The military used fusion power to a great extent. The Appalachian Automated Launch System in particular reduced on a combination of onsite fusion reactors and megacells to provide power to its nuclear silos, with the most common types used being military-grade Class-VIII and Class-IV fusion reactors connected in a closed circuit, capable of serving as the silos' primary power source indefinitely. Vault-Tec profited from its role as a strategic defense contractor, obtaining compact fusion power systems from the military for use in its facilities.