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The process of nuclear fission is akin to splitting an atom. Think of it as chopping a log of wood, but on an atomic level. Massive atomic nuclei, like that of certain isotopes of Uranium, are bombarded with neutrons. When a nucleus captures a neutron, it becomes unstable and splits into two smaller atoms, releasing a significant amount of energy as well as additional neutrons. These freed neutrons may go on to strike other nuclei, causing a chain reaction.

In a nuclear reactor, this chain reaction is carefully controlled to produce energy steadily, which can then be harnessed to generate electricity, for instance. Understanding this intricate dance of atoms helps us grasp the potential power, and also the challenges, involved in safely harnessing nuclear energy.

Naturally occurring radioactive materials (NORM) can be found all around us. From the ground we walk on to the bananas we eat, small amounts of radioactive substances are part of our everyday life. These materials encompass a variety of isotopes, which are versions of elements with different numbers of neutrons in their nuclei, leading them to have different properties.

Some NORM, like Uranium and Thorium, have isotopes that are fissile, meaning they can be used to fuel a nuclear fission reaction. A fascinating aspect of these materials is how they've been part of our planet since its formation, and understanding them is key to fields ranging from nuclear energy to environmental science.

Within the tapestry of elements, Uranium-235 (U-235) stands out for its ability to undergo fission naturally. Although it's just one of uranium's isotopes, its natural abundance is vital for the feasibility of nuclear power. Found in trace quantities within uranium ore, U-235 is present at about 0.7%—which may sound small, but it's enough to fuel the world's nuclear reactors.

The rarity of U-235 compared to its more plentiful sibling, Uranium-238, necessitates the process of enrichment to increase the U-235 content for reactor fuel. A deeper understanding of U-235's natural abundance is not only central to the nuclear industry but also to the discourse on sustainable energy resources and geopolitical energy independence.