How “green” is nuclear energy’s carbon lifecycle?

Both surging interest and investment in nuclear energy as a way to meet net zero goals and to diminish the worst effects of climate change, as well as the US commitment at COP28 to triple nuclear energy capacity by 2050, are reasons to pause and examine how “green” nuclear power really is. It is important to consider the carbon life cycle of a typical, light-water reactor nuclear power plant (NPP). If nuclear energy is not truly lower in greenhouse gas (GHG) emissions over that life cycle than other power sources, then nations choosing it as a way to meet legally-binding Paris Agreement NDCs regarding GHG reduction requirements may open themselves up to climate litigation.

The International Energy Agency (IEA)’s 2050 Net Zero transition pathways all involve some amount of nuclear power, as the IEA considers nuclear a low-carbon energy source. While some research supports the IEA view, other studies discovered that the carbon intensity of nuclear energy is much higher than that of a low-carbon source, and still others concluded that nuclear energy does not reduce fossil fuel use. Why is there so much controversy around a relatively established energy production method? The issues have to do with which of the myriad factors nuclear life-cycle analysis (LCA) studies take into account, and which they don’t. In addition, some factors are simply difficult to pin down or quantify. As a result, it is nearly impossible to compare “apples to apples” when it comes to nuclear life-cycle research. However, one thing that is clear is that the argument that nuclear energy is “clean” rests on shaky ground: “Despite its heat and electricity generating stages not causing any greenhouse gas emissions, nuclear energy is not a zero emissions energy source. Its extensive system of upstream supply stages requires energy inputs throughout, and given that in practice, a substantial part of these energy inputs are provided by fossil fueled sources, nuclear energy indirectly involves the emission of greenhouse gases.”

A few studies have tried to demonstrate all the components of nuclear energy’s carbon footprint in a life-cycle analysis (LCA). The various elements are represented in the graph below:

Upstream, or “Front-end” and “Construction, and operation” emissions come from material manufacturing and component manufacturing, construction, fuel cycle (uranium extraction/production/processing/conversion/enrichment). Downstream, or “Back-end,” emissions come from dismantling and decommissioning efforts. There are also additional considerations like the impact on landscapes that might have acted as carbon sinks prior to construction: “The direct carbon loss results from vegetation deterioration during preliminary [reactor] construction, while the potential carbon loss is the carbon sequestration potential of the destroyed vegetation during the operation period of NPPs (60 years). The total carbon emissions resulting from vegetation loss during NPP construction and operation reached 842.6 t CO2, 84.91% of which was the potential carbon loss during the operation period.”

Construction elements account for so much of the overall emissions because of the fact that the wholesale inventory for a nuclear plant “likely exceeds millions of individual products and components, and designs are sensitive information.” In addition, NPP construction requires massively heavy equipment, millions of tons of carbon-emitting concrete, and longer and more technically challenging construction designs overall, ensuring more GHG emissions and “a significant delay before the embodied carbon invested can be repaid through greenhouse gas (GHG) emission reductions during the operational phase.”

The process of moving uranium from the ground into fuel rods is also extremely carbon-intensive. Mining technique and the energy mix of the mine are central factors. With any extraction method, there are additional factors such as heat from diesel or propane and electricity from diesel generators or the grid on some occasions; any extraction done with renewable energy would have a lesser carbon footprint. One important note is that poorer quality ores require greater resources to mine, mill, convert and enrich, thus as the world’s current nuclear fleet exhausts high-quality ore and eventually turns to poorer-quality ore, the carbon intensity of mining that ore will increase. After milling or in situ leaching, uranium is converted into gaseous UF6 in order to enable enrichment.

For the majority of nuclear power plants, it must be further enriched by various methods, each of which uses differing levels of energy. Again, based on the percentage of renewable energy in a given country’s energy portfolio, the overall impact of uranium enrichment on the total GHG of the NPP will vary. To produce energy, uranium-packed pellets are placed into fuel rods inside the reactor. During the lifetime of an NPP, fuel rods grow depleted and the plant must be refueled (40 days every 18 months), using energy. Afterwards, nuclear waste must also be stored after removal from the rods, a process which requires energy for encasing and steel for storage canisters.

Once a NPP is ready to be decommissioned, there are options regarding the extent to which the site will be remediated, but on average this phase makes up about 35% of the life cycle GHG emissions. For the ‘‘environmentally responsible” option, which includes safeguarding, clean up, demolition, dismantling, packaging and permanent disposal, costs of up to 200% of the construction costs may be incurred, indicative of GHG emissions involved in the transport and storage of contaminated materials. An NPP creates radioactive waste that must be stored in cooling pools for around five years, and then typically in above-ground overpacks until a deep geologic repository or other long term storage solution arrives. At the end of its life–up to 80+ years–a typical nuclear reactor has created about 10,000 tonnes of medium to high level radioactive waste, 10,000 tonnes of low to medium level radioactive waste, and 100,000 tonnes of non-active materials. There is currently no permanent (deep geologic repository) storage for this commercial waste in the U.S., but any effort to build one would be highly carbon-intensive.

However, aspects of the NPP lifecycle may be gradually improving. For example, some countries, such as France and the UK, reprocess the fuel into mixed oxide (MOX) fuel, which could influence the overall LCA. Like any technology, nuclear energy production continues to evolve. For example, a LCA will change as electricity generation technology and uranium supply chain evolve, as new reactor designs are studied more thoroughly, and as the use of reprocessed spent fuel becomes more common.

In its most recent study, the National Renewable Energy Laboratory asserted that nuclear has a similar overall carbon footprint to solar, concentrating solar, and geothermal energy. However, it is not clear if the study considered the major factors listed above that indicate how truly carbon-intensive nuclear may be. If that is the case, the Biden Administration could face legitimate questions and legal challenges to its plans to increase the share of nuclear energy in the US portfolio to meet its Paris Agreement obligations.