Friday, April 26, 2013

Nuclear Power and Carbon Emissions:

Doing the Numbers

A recent article caught my attention because it was titled "Nuclear is NOT a Low-Carbon Source of Energy."  Now, this puzzled me.  In the first place, this is not a new argument, and in the second place, I thought it had been addressed a long time ago.
I remember that, some years ago, the claim emerged that nuclear power only looked clean because the last step was clean--if you took the other steps into account, the argument said--especially the enrichment--nuclear power wasn't so clean at all.  (Actually, the argument was originally made in terms of net energy produced as opposed to carbon emissions, but the point is the same.)  At the time, I did a back-of-the-envelope calculation on the enrichment step that convinced me that the energy used per unit of energy produced was reasonably small, even with the gaseous diffusion enrichment process, and subsequent research convinced me this calculation was right. 

So why, I wondered, was I reading the same thing all over again, this time focusing on carbon emissions.  When I reviewed the recent article, I saw right away that it was really all over the place in its opposition to nuclear power.  It dredges up every anti-nuclear argument, and even starts with the fact that scientists are not always right!  There is far more in this article than I want to tackle in a single post, but what stood out for me is that some of the points made are qualitatively valid (that is, they have identified emissions that do exist), but there is no quantification--and even more important, there is no comparison with other energy sources.

Furthermore, the article quotes sources that claim that, "The energy put into mining, processing, and shipping uranium, plant construction, operation, and decommissioning is roughly equal to the energy a nuclear plant can produce in its lifetime. In other words, nuclear energy does not add any net energy."  Now, this statement is bizarre on the face of it.  Even among people who believe that government and industry are the Evil Empire, most must find it a stretch to believe that so many people could have successfully pursued a strategy that didn't add net energy for such a long time.

Tracing back some of the arguments in the recent article led me to reports done around 2006 that detail some of the sources of carbon emissions from the nuclear fuel cycle (which in turn draw from a 2005 report by Storm and Smith).  Again, the arguments presented are qualitatively correct.  Mining, transportation, enrichment, construction, etc., for the nuclear fuel cycle are all energy intensive processes.  

BUT, most (although not all) of these same steps exist for other energy sources as well.  You can't capture the energy from wind or the sun efficiently and at large scales without using exotic materials such as rare earths, which have to be mined and transported.  (And some of which, by the way, may be in short supply, or may make us dependent on other countries, or may yield some quantities of hazardous waste.)  Perhaps the quantities required are smaller than the amount of uranium that must be mined, but all energy sources require materials and the mining and processing of all materials required for each technology must be taken into account if different energy sources are to be compared fairly.

Enrichment, of course, is unique to nuclear power, and is energy intensive but as noted above, is not so energy intensive as sometimes claimed.  And gas centrifuges, which are far more efficient than diffusion, are supplanting the diffusion process. 

What really took me by surprise, however, was to see the article mention the steel and concrete needed for nuclear power plants (which, of course, are made using fossil fuels).  The reason I was surprised is that recent reports have noted the much larger requirement for construction materials for the many windmills that would be needed to replace one fossil or nuclear power plant.  So raising this issue only draws attention to an measure that doesn't seem favorable for wind. 

The real point is that any such numbers must be compared to the numbers from other energy sources.  There are, in fact, a number of organizations that have done just that, and their results show that 1) nuclear power does indeed produce net energy, and a lot of it, even when the whole fuel cycle is considered, and 2) the energy use/carbon emissions of the nuclear fuel cycle is less than that of most other forms of energy when a fair comparison is made.

Some reliable sources of such studies include a study done by Yale University of life cycle greenhouse gas emissions of nuclear electricity generation, and a review conducted by the Intergovernmental Panel on Climate Change (IPCC).  Although I regard these sources are reliable, I can't claim to have reviewed every one in detail and verified all the data and calculations.  I must leave that to others.  A Wikipedia page on life-cycle greenhouse-gas emissions of energy sources provides references to other studies.  Finally, while some might consider the World Nuclear Association a biased source, their energy analysis of power systems includes a useful discussion of areas for which comparisons are straightforward and other areas for which comparisons may be more difficult.

Another concern I have is that many studies of net energy production assume that the future is rather static.  It is, of course, one thing to give credit for technology advances that have not yet been achieved, but it is another thing to fail to give credit for substitutions by technologies that already exist.  For example, an enrichment facility clearly could draw power from a nuclear plant, so the carbon emissions associated with enrichment are not a law of nature.  Rather, they are a consequence of the way things are done now.   

When looking farther ahead, we must recognize that advances are possible in all areas--but they are not to be counted on.  We all know that a major shortcoming of solar and wind systems is their intermittency.  Solutions are often proposed that involve either advanced storage technologies and/or advanced grids (and backup power sources of some variety).  While there are promising developments in both areas, they are not yet reality, and furthermore, the costs and the environmental footprint of any of these options must be incorporated into future studies.

Another example involves the possibility of using small, modular reactors (SMRs) to supply needed power for mines.  This could, of course, substantially reduce emissions associated with mining operations for a variety of energy sources.  This scenario has actually been considered by mining operators, such as for mining oil sands in Canada, so it would seem that this is not a far-off vision.  Nevertheless, SMRs are not yet a fully developed technology, and must be treated as speculative in any projections of carbon emissions from different fuel cycles.

Likewise, as already noted in some of the more negative assessments of nuclear power, the implications of using lower-quality uranium ores, should that be necessary, must be considered.  At present, it appears the supplies of uranium are adequate, and further exploration plus the potential for using a thorium fuel cycle could push the need to tap lower-quality ores far into the future.  However, whenever we speculate about such possibilities as achieving a limitless resource by extracting uranium from seawater, we must both assess the state of the technology to do that, plus add the higher energy use necessary to extract uranium from such sources.  While we may anticipate advanced technologies that would be more efficient, we should recognize that they are still in developmental stages.  


1 comment:

  1. Great points.
    This new publication might interest you "Energy intensities, EROIs (energy returned on invested), and energy payback times of electricity generating power plants"