Nuclear energy is a non-renewable energy source because it relies on the finite, naturally occurring element uranium, which is not replenished at a rate that matches its consumption. 

Non-renewable resources, like fossil fuels (coal, oil, natural gas), are finite and cannot be replenished by natural processes within a human timescale. While nuclear energy is a clean energy source (not producing greenhouse gases like fossil fuels), it's not considered a renewable alternative because the fuel source (uranium) is not replenished naturally.