In the spring of 2043, Karl Oslow was an electrical engineer for Poseidon Energy. During this career, he dreamed of revolutionizing the energy industry, with limitless power being provided to countless homes from a single power plant. In 2049, he left Poseidon and founded Mass Fusion to realize his vision. Lead-lined fusion distribution boxes were placed in homes. The company insisted that while the devices gave off a trace amount of radiation, there were no adverse effects. Around a decade of research later, the Cleanpower Initiative was put in motion throughout Boston, which was embraced by the population. By 2070, Mass Fusion became Massachusetts' primary power provider. The company had plans to construct ten more power plants within the next several decades.
With the proceeds from an enormously lucrative line of fusion wells, engines, and cores, the Mass Fusion building, the corporate headquarters, was constructed, becoming the tallest building in Boston. By 2077, the company's research into cold fusion and the beryllium agitator were complete. The agitator was used to power the nuclear reactor beneath their Boston headquarters, and in October that year, Oslow decided to send the agitator to Washington, D.C. in order to hand it over to the United States Armed Forces.
The company also owned disposal sites and containment sheds throughout Massachusetts. They were accused of environmental irresponsibility and illegal waste dumping, such as into lakes, before the Great War.
Mass Fusion appears in Fallout 4.