You might want to look up answers to:
https://scifi.stackexchange.com/questions/241706/first-atomic-powered-transportation-in-science-fiction[1]
And:
https://scifi.stackexchange.com/questions/2742/earliest-story-about-civil-nuclear-power/43647#43647[2]
When scientists realized how much binding energy was in atoms, and when Einstein's theory of specal relativity included the equation E = mc2, showing that mass and energy were the same, it became natural to speculate whether it could be possible to release the energy of atoms somehow and produce vast amounts of energy.
So there were early science fiction stories using types of atomic energy, which seem more or less plausible - often much less plausible - according to modern knowledge, since the early 1900s at least.
One example is H.G. Wells's 1914 Novel The World Set Free.
Wells's knowledge of atomic physics came from reading William Ramsay, Ernest Rutherford, and Frederick Soddy; the last discovered the disintegration of uranium. Soddy's book Wealth, Virtual Wealth and Debt praises The World Set Free. Wells's novel may even have influenced the development of nuclear weapons, as the physicist Leó Szilárd read the book in 1932, the same year the neutron was discovered. In 1933 Szilárd conceived the idea of neutron chain reaction, and filed for patents on it in 1934.[8]
Wells's "atomic bombs" have no more force than ordinary high explosive[dubious – discuss] and are rather primitive devices detonated by a "bomb-thrower" biting off "a little celluloid stud."[9] They consist of "lumps of pure Carolinum" that induce "a blazing continual explosion" whose half-life is seventeen days, so that it is "never entirely exhausted," so that "to this day the battle-fields and bomb fields of that frantic time in human history are sprinkled with radiant matter, and so centres of inconvenient rays."[10]
Never before in the history of warfare had there been a continuing explosive; indeed, up to the middle of the twentieth century the only explosives known were combustibles whose explosiveness was due entirely to their instantaneousness; and these atomic bombs which science burst upon the world that night were strange even to the men who used them.[11]
https://en.wikipedia.org/wiki/The_World_Set_Free[3]
With the coming of pulp science fiction magazines beginning in 1926, stories about peaceful or warlike use of atomic power became rather common, such as E.E. Smith's The Skylark of Space (1928), begun in 1915.
So the phrase "atomic bomb" was used in some early stories.
For example, atomic bombs are used in Stanley G. Weibaum's short story "Redemption Cairn", Astounding Stories, March, 1936. Those were about the size of hand grenades, and much less powerful than real atomic bombs, and apparently were available for civilian as well as military use.
But nobody had any idea how to make atomic bombs in real life, until uranium fission was discoverd in 1938, and in the next few years various governments began projects to develop fission reactors for energy production and/or atomic bombs for war.