Wednesday, January 23, 2008

Carbon Dioxide Emissions from Nuclear Fuel Enrichment.

During the lecture on Tuesday, Nuclear Energy was presented as having essentially zero CO2 emissions.

I accept that the energy return per kg of Uranium is large enough that CO2 emissions from most of the nuclear fuel cycle can be neglected (i.e. mining, conversion to UF6, fuel fabrication, transportation, and reactor construction and decommision). It is interesting to note that a 1000 MWe nuclear power plant requires ~175 metric tons of yellowcake per year. Yellowcake is the 85-90 wt% uranium oxide material derived from processing the initial uranium ore, which only contains on the order of 0.1 wt% uranium. In contrast, a 1000 MWe coal plant requires ~10,000 metric tons of coal per day!

However, there may be non-negligible emissions during the enrichment stage. U235 is the fissile isotope, while the majority of Uranium is U238. Uranium ore generally contains ~0.7% U235, and this must be increased to ~3.0% in order to be used as a fuel.

There are two types of enrichment processes currently employed to increase the concentration of U235. One is by using gas centrifuges, and the other is through gaseous diffusion. Gas centrifuges are more capital intensive, while gaseous diffusion plants require about 20x as much energy to operate. A full-scale gaseous diffusion plant consumes ~2000 MW of power. Commercial scale plants of this type operate in the US, Russia, and France. This energy requirement is why I believe there is a non-negligible quantity of CO2 emissions related to nuclear power. Assuming that the enrichment plant operates regularly enough that its energy demands are met by baseload energy production, then the 2000 MW of power is currently supplied primarily by coal. Unfortunately, I do not know how many of these plants are currently operating, how much energy such plants consume yearly, or how many nuclear power plants are fueled from one diffusion plant.

In assessing the impact of additional nuclear power plants, it would be interesting to know what % of enrichment is currently via gaseous diffusion, whether further enrichment plants will be necessary, and what kind of enrichment plants would be built. I am hoping to find a decent article that crunches through these numbers and facts, and if anyone is aware of one, please post it. Perhaps, in the end, it will turn out that even 2000 MW of electricity is negligible...

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