Uranium is a mineral that occurs naturally as a mix of two isotopes: uranium-238 (99.3%) and uranium-235 (0.7%). The proportion does not vary in nature. Current nuclear reactors can only derive energy from the uranium-235 part.
If the whole world was powered only by nuclear fission, we'd start to run out of uranium-235 in less than 4 years:
Uranium 2020: Resources, Production and Demand ('Red Book')
"The total recoverable identified resources to $260/kg U is 8.070 million tonnes U."
Citation: Key World Energy Statistics 2020 (IEA report) - Page 73 - Glossary - Nuclear
Does not include the fuel used in generating electricity. See [energy.tes] for that.
Citation: "Key World Energy Statistics 2020" IEA
- Page 47 - Simplified energy balance table - World energy balance, 2018
We'd run out even faster if all nations were developed.
- Using nuclear power only as a baseload (in other words, only for the electricity that has to be available 24/7). Other energy could come from solar and wind.
- Extracting uranium from seawater (but the viability of this is questionable (like other trace minerals), because the concentration is extremely low).
- Breeder reactors could make use of uranium-238 (and also thorium which is somewhat more abundant than uranium).
If uranium gets into the wrong hands, it can be used to make thermonuclear bombs. Nuclear power plants need strong redundant security measures to mitigate this risk.
Nuclear fusion is not currently viable for generating power. Scientists are working on it - because in theory, it could power the world for millions of years, from only tiny amounts of abundant material, and with almost no pollution. Fusion has the potential to completely solve the energy crisis, but it is nowhere near ready yet (despite some misleading news headlines). It's certainly worthwhile to invest time & money into fusion, but we can't "put all our eggs in one basket". Climate change is near a tipping point, so we need clean energy sooner than fusion might be available.