I will take the guess you are asking the minimal technological requirements that a human society would need to develop a nuclear reactor.
But first of all, you'll need the theory.
Building a functional nuclear reactor is an incredibly difficult task and you have no chance to discover it 'by accident'. Even natural occurences would be explained by magic or other unknown forces if your civ encounter some and they would be unable to reproduce and exploit it.
Let's retrace the nuclear fission history to identify the key events you could make happen earlier.
1 - Matter is made of atoms.
Requirements - just a brain and freedom of thought.
The atomistic theory itself dates back from the ancient greeks with Democritus. Even though, its conception of matter as atom-based was more philosophical than practical.
2 - Atoms are chemical elements
Requirements - complex glass apparatus to contain, burn, distill and observe natural extracts. Precision scales to measure material weights before/after reactions. Writing materials to take notes. One genius to understand how are atomic elements agenced.
Decomposing the matter in base elements is a fundation of both occidental and oriental alchemies, and they also brought the idea of transmutation - turning an element into another. You'll have to wait the brink of the French Revolution with Lavoisier's experiments to reconect with the idea of a particulate matter, and the fortunate intuition of Mendeleiev in the 1860's to arrange the elements in the periodic table. This new organisation brought the idea of yet unknown, missing elements that researchers around the world started to hunt frantically.
3 - Some elements are radioactive
Requirements - Photographic paper or phosphorous screen. Even finer glassmaking techniques and vacuum pumps to build these screens.
Among these elements hunters, Sklodowska-Curie and Becquerel discovered the radioactive properties of uranium and other isotopes at the brink of the XIXth century. At this time, radioactive material where considered as harmless phosphorescent materials and started to be used as a paint on watches or to dye fancy glass.
4 - Radioactivity can generate tremendous amounts of energy
Requirements - Same as above, more experiments and Albert Einstein.
The characterisation of particles and especially the neutron by Chadwick took 30 more years. Szilard suggested soon to use the neutron products of radioactive decay to trigger more reactions and get a huge energy from it, building on Einstein's most famous equation e=mc².
Formulation of the chain reaction theory could be considered in your setting as the rationale for building a nuclear reactor.
Then, you need a huge amount of material and human resources to locate, mine, extract, refine, assemble and make work even a tiny nuclear reactor. Note also that nuclear reactor where developed first as a way to generate the plutonium required for atom bombs, and without this urgent requirement petrol and charcoal were already fulfilling much of the population's needs in energy.