Energy sources and their trade-offs
Fossil fuels are stored sunlight, and every source, fossil or nuclear or renewable, trades emissions, waste and reliability against each other.
A worked example
A generator taking in 20 000 J and delivering 17 000 J of useful output is 85% efficient, and the missing 3 000 J leaves as heat.
The mistake people actually make
Nuclear fission is carbon-free while it runs but leaves long-lived radioactive waste, so calling any source simply clean or dirty misses the trade-off.