Energy Balances and CO2 Implications
Updated March 2014
- Lifecycle analysis, focused on energy, is useful for comparing net energy yields from different methods of electricity generation.
- The amount of energy inputs to the nuclear fuel cycle has implications for carbon dioxide emissions, and in any scenario nuclear power has negligible emissions.
The economics of electricity generation are important. If the financial cost of building and operating the plant cannot profitably be recouped by selling the electricity, it is not economically viable. But as energy itself is sometimes seen as a more fundamental unit of accounting than money, it is useful to know which generating systems produce the best return on the energy invested in them. This is part of Life Cycle Analysis (LCA). A subset of this addresses CO2 or similar implications.
In the 1970s a lot of attention was given to analysing the energy inputs to different parts of the nuclear fuel cycle, and some of the data available today still depends on that work. In recent years some utilities generating electricity have undertaken Life Cycle Analysis (LCA) studies as part of their social accountability. Also mining companies have been publishing their energy use as part of broader environmental or social responsibility disclosure – part of product stewardship – and this feeds into broader LCA figures. Both kinds of results have been audited and published.
Comments on methodology and some greater detail is in the paper on Energy Analysis of Power Systems in this series.
As well as energy costs, there are external costs to be considered – those environmental and health consequences of energy production which are quantifiable but do not appear in the financial accounts. Beyond these, and less readily quantifiable in the same way, are the costs involved with global warming. Where emissions trading schemes put a direct cost on carbon dioxide emissions, that can be added in too.
The principal focus of LCA for energy systems today is their contribution to global warming. There is an obvious linkage between energy inputs to any life cycle and carbon dioxide emissions, depending on what fuels those inputs. LCA includes mining, fuel preparation, plant construction, transport, decommissioning and managing wastes.
In the last few years it has been asserted that known uranium supplies are critically limited and that it will become increasingly difficult and expensive to recover due to decreasing ore grades, and will thus require undue amounts of energy to mine, largely negating any low-carbon advantage over fossil fuels. This notion is based on a misunderstanding of mineral resources generally, and on academic abstractions rather than published data. In fact, ongoing mineral exploration increases known mineral resources (not simply for uranium) faster than they are depleted.
Energy balance from LCA
Life cycle analysis for Vattenfall's Environmental Product Declaration for its 3090 MWe Forsmark power plant for 2002 yielded some detailed energy data which is reasonably up to date (though including some diffusion enrichment which is no longer used) and also certified. It shows energy inputs over 40 years to be 1.35% of the output.
Related to this is the question of carbon dioxide emissions, which for Forsmark were 3.10 g/kWh.
More typical data is tabulated in the Energy Analysis paper referred to above. Here, conservative assumptions have been made, but centrifuge enrichment assumed, and the estimate of lifetime inputs is 1.6% of output.
If very low grade ore of 0.01% U is envisaged – as has been said to make mining uneconomic – the input figure rises to 3.7% of output.
All of these suggest a very favourable energy balance, by any criteria.
Forsmark: On basis of PJ (thermal) per 1000 MWe (1 GWe) of capacity over 40 years the input figures* are:
|Mining & milling
|Build & decommission plant
* Electrical inputs in PJ have been multiplied by 3 on the assumption that they might have come from thermal steam plant (though most were hydro). Details: Re mining: 42% of U comes from Rossing (0.025%U), 37% from Olympic Dam (0.042%U), 21% from Navoi (ISL).
Enrichment: 20% Eurodif (diffusion), 60% Urenco, 20% Tenex (both centrifuge) – over 90% of energy input for it is from nuclear.
The output of Forsmark is 7.47 TWh/yr per GWe. Over 40 years: 299 TWh or 3226 PJ (factor of 10.8 at 33% thermal efficiency).
Input is thus 1.35% of output.
More typical data tabulated in the Energy Analysis paper, with conservative assumptions, 1000 MWe operating over 40 years:
|Mining & milling
|Build, operate & decommission
With output of 7.5 TWh/yr this gives 300 TWh over 40 years or 3240 PJ.
Input is thus 1.6% of output.
The mining figure in this data is as published for the Ranger mine in Australia – ore head grade in 2008 averaged about 0.26% U and energy used was 273 GJ/t U3O8, 322 GJ/tU.
Figures for Beverley ISL operation 2004-05 were 187 GJ/t U3O8 or 221 GJ/tU, slightly less than the Ranger data, and would reduce the Mining & milling figure in the table above to 1.4 PJ. So no real change in the energy input percentage.
Published figures for nuclear fuel cycle with centrifuge enrichment range 1.7 to 2.3% for inputs as percentage of outputs. Related to this is energy payback time for building a nuclear power plant: at 25 PJ for a 1 GWe plant it is obviously about four months.
If very low grade ore (of 0.01% U) is envisaged – as has been said by critics to make mining uneconomic – this would give 70 PJ total for mining & milling in the above table for an operation like Ranger, hence total 120 PJ for the whole fuel cycle, so input 3.7% of output. So though the energy inputs for mining and milling would increase significantly, total inputs would still only be under 4% of outputs for the full fuel cycle.
In Vattenfall’s 2014 EPD statements, Forsmark had life cycle energy inputs of 3.8% of output (2.0% being energy use in the plant to give net output), for Ringhals inputs were 4.2% of output (exactly half of this being energy use in the plant to give net output), hence EROI of 56 and 50 respectively (or 26 and 24 on net power basis). These figures compare with inputs of 6.3% of output for Vattenfall’s wind farms, split fairly evenly onshore and offshore, hence EROI of 16 on either basis.
It is difficult to compare these figures with coal, since so much of the coal energy input (beyond the fuel itself) is often in transport, which varies from very little to a lot, and figures of 3.5% to 14.0% are published. For natural gas the figures again depend on transport to point of use, and published figures range from 3.8% to 20%.
Another question which arises in this connection is energy payback time. If 25 PJ is taken as the energy capital cost of setting up (Other published figures for building a 1 GWe nuclear power plant range from 2 to 24 PJ) including enrichment of the first fuel load, then at 7 billion kWh/yr or 75 PJ/yr output the initial energy investment is repaid in 4 months at full power. Construction time for nuclear plants is 4-5 years.
Life cycle analysis: external costs and greenhouse gases
The principal concern of life cycle analysis for energy systems today is their likely contribution to global warming. This is a major external cost, though not the only one.
The ExternE study (1995) attempted to provide an expert assessment of life cycle external costs for Europe including greenhouse gases, other pollution and accident potential. The European Commission launched the project in 1991 in collaboration with the US Dept of Energy (which subsequently dropped out), and it was the first research project of its kind "to put plausible financial figures against damage resulting from different forms of electricity production for the entire EU". A further report, focusing on coal and nuclear, was released in 2001.
The external costs are defined as those actually incurred in relation to health and the environment and quantifiable but not built into the cost of the electricity to the consumer, and therefore which are borne by society at large. They include particularly the effects of air pollution on human health, crop yields and buildings, as well as occupational disease and accidents. In ExternE they exclude effects on ecosystems and the impact of global warming, which could not adequately be quantified and evaluated economically.
The methodology measures emissions, their dispersion and ultimate impact. With nuclear energy the low risk of accidents is factored in along with high estimates of radiological impacts from mine tailings (since shown to be exaggerated) and carbon-14 emissions from reprocessing (waste management and decommissioning being already within the cost to the consumer).
The report shows that in clear cash terms nuclear energy incurs about one tenth of the costs of coal. Also, the external costs for coal-fired power were a very high proportion (50-70%) of the internal costs, while the external costs for nuclear energy were a very small proportion of internal costs, even after factoring in hypothetical nuclear catastrophes. This is because all waste costs in the nuclear fuel cycle are already internalised, which reduces the competitiveness of nuclear power when only internal costs are considered. The external costs of nuclear energy averages 0.4 euro cents/kWh, much the same as hydro, coal is over 4.0 cents (4.1-7.3 cent averages in different countries), gas ranges 1.3-2.3 cents and only wind shows up better than nuclear, at 0.1-0.2 cents/kWh average.
The EU cost of electricity generation without these external costs averages about 4 cents/kWh. If these external costs were in fact included, the EU price of electricity from coal would double and that from gas would increase 30%. These particular estimates are without attempting to include possible impacts of fossil fuels on global warming. See also ExternE website.
Turning to carbon dioxide, if all energy inputs are assumed to be from coal-fired plants, at about one kilogram of carbon dioxide per kWh, it is possible to derive a greenhouse contribution from the energy input percentage of output. However, as the Forsmark data quoted above show, many energy inputs are not fossil fuel, giving it the very low CO2 emission figure of 3.1 g/kWh. The 2005 Environmental Product Declaration for British Energy's Torness 1250 MWe power station shows 5.05 g/kWh (reference year 2002).
In 2014 the US National Renewable Energy laboratory (NREL) published LCA figures for nuclear power based on 1980-2010 data which had been harmonised for greater consistency. The median lifecycle greenhouse gas emissions for PWR and BWR power plants were 12 and 13 g/kWh respectively. (This compared with coal on the same basis at 1000 g/kWh overall, but IGCC at about 900 and supercritical coal at about 800.)
In France to about 2010, despite energy-inefficient enrichment plants which were run by nuclear power, the greenhouse contribution from any nuclear reactor using French-enriched uranium was similar to a reactor elsewhere using centrifuge-enriched uranium – less than 20 g/kWh overall. That would now be no greater, as centrifuges have taken over in France.
Figures published in 2006 for Japan show 13 g/kWh, with prospects of this halving in future.
The UK Sustainable Development Commission report in March 2006 gave a figure of 16 g/kWh for nuclear, compared with 891 g/kWh for coal and 356 g/kWh for gas.
Older figures published from Japan's Central Research Institute of the Electric Power Industry give life cycle carbon dioxide emission figures for various generation technologies. Vattenfall (1999) published a popular account of life cycle studies based on the previous few years experience and its certified Environmental Product Declarations (EPDs) for Forsmark and Ringhals nuclear power stations in Sweden, and Kivisto in 2000 reports a similar exercise for Finland. They show the following CO2 emissions:
||1170 (peak-load, reserve)
|gas combined cycle
||10 - 26
The Japanese gas figures include shipping LNG from overseas, and the nuclear figure is for boiling water reactors, with enrichment 70% in USA, 30% France & Japan, and one third of the fuel to be MOX. The Finnish nuclear figures are for centrifuge and diffusion enrichment respectively, the Swedish one is for 80% centrifuge.
Other published figures are consistent with the above for nuclear power, showing it to have around 1-2% of the carbon dioxide emissions of coal-fired power (ie under 20 g/kWh). If extremely low grade ores are envisaged, the figure would rise by a further 1% in line with the energy inputs, making it about 3% of coal (ie about 30 g/kWh) or perhaps 6% of gas - still a very substantial margin where carbon constraints are increasingly needed.
see Energy Analysis and Energy Subsidies and External Costs Papers.
Sims, R.E.H., Rogner, H-H, et al, 2013, Carbon emission and mitigation cost comparisons between fossil fuel, nuclear and renewable energy resources for electricity generation, Energy Policy 31: 1315-1326.