Marine Radionuclides (Summary)

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  • 　We are inviting Dr. Hyoe Takada of the Institute of Environmental Radioactivity, Fukushima University, as an external lecturer for the specialized course "Marine Biogeochemistry" of the Faculty of Fisheries. Part of the contents of environmental radioactivity that I would like to ask Professor Takada to give a lecture on was provided to the LASBOS Moodle course.

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• Abundance of natural radionuclides in the ocean

  Many natural radionuclides are soluble in seawater, so here we calculated the abundance of nuclides that are soluble in seawater.
  

  　The figure below shows the abundance when the amount of seawater is 1.4 billion km³. Units are PBq (10¹⁵ becquerels) or trillions of becquerels. The most common of these is potassium 40 (40K), 10²² becquerels (Bq). Compared to rubidium 87 (⁸⁷Rb) and uranium 238 (²³⁸U), the amount of becquerel is about two orders of magnitude higher. Potassium 40 (⁴⁰K) weighs about 40 billion tons. Uranium-238 (²³⁸U) is about 4 billion tons. It can be imagined that seawater as a whole contains a considerable amount of natural radionuclides.

  
  

  

  海洋における主な天然放射性核種の存在量

  Abundance of major natural radionuclides in the ocean

  地球上の海水量:約14億km³

  Amount of seawater on Earth: Approximately 1.4 billion km³

  天然放射性核種の存在量

  Abundance of major natural radionuclides
  

  千兆ベクレル 1e15 Bq
  

  
  

  
  

• Marine radionuclides (natural vs. man-made)
  • 　Finally, we compare natural and artificial radionuclides. In the figure below, natural radionuclides are light brown and artificial radionuclides are pink.
    

    Since tritium (³H) and carbon 14 (¹⁴C) have sources from both sources, they were calculated separately.

    After all, you can see that the abundance of natural radionuclides is overwhelming. However, tritium (³H) seems to be more artificial. This is because artificial radionuclides released by atmospheric nuclear testing remain. Estimates are for the year 2000 AD, so in 2020 we can calculate that the ratio of natural and artificial will be about half and half. As I mentioned about tritium, tritium (³H) is also produced from nuclear power plants, but it is characterized by the overwhelming amount produced by cosmic rays. As for carbon 14 (¹⁴C), the ratio of man-made to natural is about 1%, so if it is used for dating, the error is only a small amount.

    
    

    
    

    天然VS人口natural vs population　天然放射性核種natural radionuclides　人工放射性核種artificial radionuclides　圧倒的にK40が多いThere are overwhelmingly many K40
    

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    　A comparison like this might lead to the argument that "natural radionuclides are overwhelmingly abundant, so artificial radionuclides are not a problem," but that is not necessarily the case. As long as the radionuclides are evenly dispersed in the seawater of the ocean, it seems that there is no problem whether it is natural or artificial. However, radionuclides are dangerous when they are concentrated in a certain place at a certain time. It is important to judge this area carefully.