Is nuclear energy clean?

"Clean" bundles two separate questions: how much greenhouse gas nuclear power emits over its full life cycle, and what happens to its radioactive waste. They are different axes with different numbers. A third axis, operational safety — how severe an accident was — is measured by the INES scale and is not the subject here.

The metric that makes generation technologies comparable is life-cycle greenhouse gas emissions, in grams of CO₂ equivalent per kilowatt-hour (gCO₂eq/kWh): not just what comes out of a smokestack, but also the mining and manufacture of materials, plant construction, fuel and decommissioning. The IPCC's Fifth Assessment Report (2014), Working Group III, compiled the published literature for each technology in its Annex III and reported, for each one, the minimum, median and maximum found.

The gap between fossil fuels and everything else is close to an order of magnitude: pulverized coal has a median of 820 gCO₂eq/kWh and combined-cycle natural gas, 490. Nuclear, wind and solar all have medians between 11 and 48 gCO₂eq/kWh, in the same order of magnitude as each other given the spread across the underlying studies. Hydropower has the widest range in the table, from 1.0 to 2,200 gCO₂eq/kWh: the upper end comes from some reservoirs in tropical regions, where flooded vegetation decomposes and releases methane, a case that is not representative of hydropower generation as a whole.

The second question — what happens to the waste — is not answered with an emissions figure but with a volume, a classification by activity level, and the state of final disposal; that classification, and the detail of how spent fuel is stored and disposed of, are covered in the entry on radioactive waste, and the origin of that fuel, in the one on the nuclear fuel cycle. According to the IAEA's reference report on spent fuel and radioactive waste management, based on data from 38 countries as of 31 December 2016, the world inventory of solid radioactive waste is about 38 million m³: 81% (30.5 million m³) has already been disposed of permanently and the remaining 19% (7.2 million m³) is in storage awaiting a final destination. Over 98% of that volume is very-low- or low-level; almost all the rest is intermediate-level. The proportion reverses for radioactivity: about 98% of the inventory's total radioactivity is concentrated in intermediate- and high-level waste, even though it occupies a tiny fraction of the volume. This split is over the cumulative inventory as of 2016, which includes waste already disposed of permanently; the entry on radioactive waste gives its split by category over a plant's typical generation, a different basis, which is why the percentages do not match between the two entries. High-level waste — unreprocessed spent fuel or vitrified high-level waste from reprocessing — adds up to about 29,000 m³ in storage worldwide, and no country yet has a geological repository in operation to receive it: Onkalo, in Finland, is the most advanced and, as of September 2026, is still awaiting its operating license.

These are two independent numerical answers, not one. In life-cycle greenhouse gas emissions, nuclear power is in the same order of magnitude as wind and solar, well below coal and gas. In radioactive waste, the high-level fraction is a small volume but with no final disposal yet in operation in any country. This entry does not weigh which of the two axes matters more: it gives the numbers for each, separately.