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Geo-engineering to me means man as a species doing something to change the whole world. It is of interest because it has been suggested that perhaps we could use geo-engineering to either mitigate or delay the impacts of climate change caused by our proliferate use of fossil fuels. Proposals range from the simple such as painting all roofs white to reduce the earth's albedo. To the grandiose of deploying large mirrors in space to reduce the the amount of solar radiation reaching the earth.
In this blog I wish to suggest nuclear power be used to undertake geo-engineering. I would like to think the proposal contained in this blog is at the simpler end of the geo-engineering scale. The proposal is to use a nuclear reactor to produce electricity that in turn would power Biorock coral reef growth and restoration. From Wikipedia - "Biorock, also known as Seacrete or Seament, is a trademark name used by Biorock, Inc. to refer to the substance formed by electro-accumulation of minerals dissolved in seawater." The nuclear power plant (NPP) would be the source of the electricity in this process.
In this proposal, a NPP would be located near the coast and provide electricity for the electro-accumulation. The wikipedia article suggests "that one kilowatt hour of electricity will result in the accretion of about 0.4 to 1.5 kg (0.9 to 3.3 lb) of biorock, depending on various parameters such as depth, electrical current, salinity and water temperature." The main components of biorock are mainly calcium carbonate and magnesium hydroxide, again as provided by the Wikipedia article.
The chemical formula for limestone, a major component of biorock is Calcium Carbonate (CaC03). Therefore one mole of CaCO3 weights (40g + 12g + 3*16g) = 100g. I don't know the typical ratio of calcium carbonate and magnesium hydroxide in biorock but let me guess it is 50% calcium carbonate and 50% magnesium hydroxide. Assume that 1 kw-hr of electricity will produce 0.4 kg of biorock which converts to 0.2 kg Calcium Carbonate. Therefore each 0.2 kg of Calcium Carbonate contains 24g of Carbon (Chemical symbol "C").
Now assume we build a NuScale SMR which has a nominal output of 45Mw electric with 90% availabilty and typical carbon lifecycle output of 16g CO2 per kw-hr which converts to 4.4g Carbon per kw-hr (4.4g = 16g *12/44). Therefore each kw-hr of electricity can remove 19.6g (24g - 4.4g = 19.6g) of Carbon from seawater. The NuScale reactor produces 45,000 * 0.9 = 40,500 kw electric over the life of the reactor. Therefore each year a NuScale reactor would remove (40,500 * 24 * 365)kw-hr * 19.6 g per kw-hr = around 7,000,000,000 grams or 7 million kg or 7000 tonnes of carbon per year. It is also expected that the new or repaired reefs will sequester further Calcium Carbonate by biologic means as corals reestablish
Is this worth doing? It turns out that according to Tesco the average British person has a carbon footprint of 15 tonnes of CO2 (around 4 tonnes carbon per year). Therefore, 1 NuScale plant will offset the carbon emissions of 1750 people. On this basis this doesn't seem a very sensible idea. That seems to me to be a large effort to offset the emission of 1750 Brits or 0.003% of the population. This shows just how hard it is to remove carbon from the world once we have dumped it by burning fossil fuels.
On the other hand some low lying topical islands might consider this a reasonable idea if it were to make their communities less vulnerable to storm surges or rising sea levels. The NuScale reactor would allow the production of around, 40,500 * 24 * 365 * 0.4 / 1000 = 141,912 tonnes of biorock per year. The typical density of limestone is around 2.5 tonnes per cubic metre. I will assume that biorock has the same density. Therefore, the NuScale reactor would allow around 56,000 cubic metres of biorock to be produced in a year. If the biorock were grown in a strip 100m wide and 1m thick each year around 560m of coastline could be protected.
The above is a very simple calculation with simple assumptions. I recognise that the above has not considered the carbon input required for the metal used to make the initial structure. It is my understanding that the biorock process can continue for many years as the biorock accumulates. There are probably other carbon inputs that I have missed. On the other hand some of the assumptions above are conservative. Two conservative assumptions are the production of biorock per kw-hr and the availability factor of 0.9 for the NuScale reactor. Both numbers could well be larger.
The next time I write about geo-engineering with nuclear power I will look at biochar.
Have a nice day.