Greenhouse gases are emitted in all stages of the nuclear energy chain

Greenhouse gases are emitted in all stages of the lifecycle of a nuclear reactor: construction, operation, fuel production, dismantling and waste disposal. Leaving out any of these five stages will bias estimates towards lower values.

The last two contributions, dismantling and waste disposal are particularly difficult to estimate. Not many commercial reactors have been fully decommissioned. Also there is still no scientific or political consensus on the approach to be used for the long-term storage of waste.

The fuel preparation contribution is also problematic. Considerable amounts of carbon are released in the mining, milling and separation of the uranium from the ore. Also the carbon emitted is very dependent on the concentration of uranium in the ore.

It’s important to appreciate that these three problematic contributions, fuel production, dismantling and waste disposal are either non-existent or small contributions in the case of electricity generation by renewable technologies. Estimates of the carbon footprint of renewably generated electricity therefore should be much more reliable than those for nuclear.

A False Solution Why Nuclear Power is Not “Low Carbon”, CounterPunch,  by KEITH BARNAM, 5 Feb 15  The UK government is committed to massively subsidising new nuclear reactors, based on the claim that they generate ‘low carbon’ electricity.

But what is the carbon footprint of nuclear power? I have trawled the literature and found that there is no scientific consensus on the lifetime carbon emissions of nuclear electricity.

Remarkably, half of the most rigorous published analyses have a carbon footprint for nuclear power above the limit recommended by the UK government’s official climate change advisor, the Committee on Climate Change (CCC).

According to the CCC, if we are to avoid the worst effects of climate change, by 2030 all electricity should be generated with less than 50 grams of carbon dioxide emitted for each kilowatt-hour (50 gCO2/kWh).

Since all new generators have lifetimes well over 20 years, I believe this limit should be imposed on all new electricity supply systems here and now – and all the more so for those with lifetimes spanning many decades.

Note that thanks to long construction times for the EPR design and a forthcoming legal challenge, it’s entirely possible that the planned Hinkley C reactor will not be completed until 2030 or beyond. It will then be subsidised for the first 35 years of its projected 60 year lifetime – taking us through until 2090.

What is the carbon footprint of renewable electricity?

When comparing the carbon footprints of electricity-generating technologies, we need to take into account carbon dioxide emitted in all stages in the life of the generator and its fuel. Such a study is called a life cycle analysis (LCA).

There are other gases such as methane that are more dangerous greenhouse gases than carbon dioxide. The most reliable LCAs take all greenhouse gases into account and present equivalent carbon dioxide emissions.

In a recent paper in Energy Policy, Daniel Nugent and Benjamin Sovacool critically reviewed the published LCAs of renewable electricity generators. All the renewable technologies came in below the 50 gCO2/kWh limit.

The lowest was large-scale hydropower with a carbon footprint one fifth of the CCC limit (10 gCO2/kWh). A close second was biogas electricity from anaerobic digestion (11 gCO2/kWh). The mean figure for wind energy is 34 gCO2/kWh, and solar PV comes in a shade under the 50g limit, at 49.9 gCO2/kWh. Bear in mind that rapidly evolving PV technology means that this last figure is contantly falling.

What’s the carbon footprint of nuclear power?

There have been nearly three hundred papers on the carbon footprint of nuclear power in scientific journals and reports in recent years. Two peer-reviewed papers have critically assessed the literature in the way Nugent and Sovacool compared renewable LCAs.

The first was by Benjamin Sovacool himself [1]. He reviewed 103 published LCA studies and filtered them down to 19, which had an acceptably rigorous scientific approach. The carbon footprints ranged from 3 to 200 gCO2/kWh. The average carbon footprint was 66 gCO2/kWh, which is above the CCC limit.

In 2012, four years after Sovacool’s paper, Ethan Warner and Garvin Heath found 274 papers containing nuclear LCAs [2]. They filtered them down to 27 for further consideration. These yielded 99 estimates of carbon footprints which the authors describe as “independent”.

Their data for carbon emissions ranged from 4 to 220 gCO2/kWh. They did not report an average but rather a median value: half the estimates were below 13 gCO2/kWh.

These two reviews of the published literature, often called meta-analyses, produced conflicting results. One suggests the carbon footprint is above the CCC limit, the other well below.

Looking in more detail at the Warner and Heath meta-review it becomes clear that their 99 estimates are not all ‘independent’ – in the sense of independent from each other – as they come from only 27 papers.

In fact, a significant number of their 99 estimates come from LCAs that have varied one or more of the assumptions in their calculations: for example the concentration of uranium in the ore, the geographic location of the reactor or the type of reactor.

Treating these variations as ‘independent’, as Warner and Heath do, gives undue weight to studies that report a large number of different assumptions for the same LCA model. We will see that these are mainly analyses which report low carbon footprints.

It is therefore not surprising that Warner and Heath’s median is so much lower than Sovacool’s average. I believe it more appropriate to present these different estimates from the same LCA as an average and a range of values. This I have done for the most rigorous of the LCAs.

The five stages of nuclear electric generation

Another concern with the Sovacool and Warner-Heath reviews is that among their selected LCAs there are, in both reviews, some analyses that do not include all the five stages in the nuclear lifecycle.

Greenhouse gases are emitted in all stages of the lifecycle of a nuclear reactor: construction, operation, fuel production, dismantling and waste disposal. Leaving out any of these five stages will bias estimates towards lower values.

The last two contributions, dismantling and waste disposal are particularly difficult to estimate. Not many commercial reactors have been fully decommissioned. Also there is still no scientific or political consensus on the approach to be used for the long-term storage of waste.

The fuel preparation contribution is also problematic. Considerable amounts of carbon are released in the mining, milling and separation of the uranium from the ore. Also the carbon emitted is very dependent on the concentration of uranium in the ore.

It’s important to appreciate that these three problematic contributions, fuel production, dismantling and waste disposal are either non-existent or small contributions in the case of electricity generation by renewable technologies. Estimates of the carbon footprint of renewably generated electricity therefore should be much more reliable than those for nuclear.

Selecting the most rigorous analyses

I have reviewed the LCAs of all the light water reactors and pressurised water reactors that passed the selection procedures of either the Sovacool or the Warner-Heath meta-analyses. I have further refined their selection by excluding any LCA that does not estimate a carbon footprint for all five stages of the life cycle. Only eight LCAs survive.

The figure shows the carbon footprints of the eight LCAs that pass this more rigorous test. All eight LCAs considered different assumptions that resulted in a range of estimates for the carbon footprints indicated by the vertical error bars. The circles show the average carbon footprint in the range of estimates.

The most important point to notice in the figure is that four of the circles fall below the horizontal broken line at 50 gCO2/kWh and four above. Half the most rigorous of the published LCAs are below the CCC limit and half are above.

The conclusion from the eight most rigorous LCAs is therefore that it is as likely that the carbon footprint of nuclear is above 50 gCO2/kWh as it is below. The evidence so far in the scientific literature cannot clarify whether the carbon footprint of nuclear power is below the limit which all electricity generation should respect by 2030 according to the CCC.

Another important point to notice in the figure is that the spread of the estimates is smallest for the two lowest LCAs (references 4 and 5). These two LCAs only considered a small range of assumptions in their LCA models. However they considered a large numberof different assumptions (13 in the first reference and 9 in the second) compared with many of the higher carbon footprint LCAs.

Warner and Heath consider these 22 estimates as ‘independent’. This is why the median (13 gCO2/kWh) of their 99 estimates is so low compared to Sovacool’s average. I am surprised that the peer-reviewers for the Journal of Industrial Ecology did not object to this seriously flawed methodology.

Why is there such a large spread of LCA results?…..http://www.counterpunch.org/2015/02/05/why-nuclear-power-is-not-low-carbon/

 

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