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    • 　We are inviting Dr. Hyoe Takada of the Institute of Environmental Radioactivity, Fukushima University, as an external lecturer for "Marine Biogeochemistry" , the specialized course 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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  • Origin of uranium series and thorium series, solubility and abundance in the ocean

    • 　Uranium and thorium, which are explained below, are elements that exist in the earth's crust. It dissolves in river water etc. and flows into the sea. Alternatively, it undergoes radioactive decay in the earth's crust and becomes gaseous radon, which escapes into the atmosphere. I will explain the nuclides (uranium radioactive decay series and thorium radioactive decay series) produced by the radioactive decay of uranium and thorium.


    • 【Description of the uranium series】

      　Uranium-238 (238U), the parent of the uranium family, is uniformly present in seawater at a concentration of 3 micro(10-6) grams/liter. Expressing this in becquerels gives a concentration of 0.04 becquerels (Bq/liter). For example, the concentration of radioactive cesium 137 (137Cs), which was released in large quantities by atmospheric nuclear tests and the Fukushima Daiichi Nuclear Power Plant accident, in seawater along the coast of Japan is 0.001 to 0.003 Bq/liter, so
      Uranium 238 (238U) has a higher concentration.

      　After that, uranium-238 (238U) undergoes alpha decay (emits two protons and neutrons each), and becomes daughter nuclides such as thorium-234 (234Th), radium-226 (226Ra), and radon-222 (222Rn). It becomes lead 206 (206Pb), a stable nuclide.


      【Description of the thorium series】

      　Unlike uranium-238 (238U), the parent nuclide, thorium-232 (232Th), is extremely insoluble in seawater, so its concentration in seawater is as low as about 0.3 nano(10-9) grams/liter. Expressing this in becquerels is a very small amount of 0.0000012 becquerels (Bq) per liter.

      　After that, thorium-232 (232Th) undergoes alpha decay in the same way as uranium-238 (238U), becoming radium-228 (228Ra) and radon-212 (212Rn), and finally becomes stable nuclide lead-208 (208Pb).

      Radium 228 (228Ra) generated from thorium 232 (232Th) has a short half-life of about 6 years. On the other hand, radium-226 (226Ra) generated from uranium-238 (238U) has a long half-life of 1600 years. So by looking at the ratio of these isotopes, it is possible to distinguish whether it is open ocean seawater or coastal seawater (you can see the history of the influence of rivers). For example, both radium 228 (228Ra) and radium 226 (226Ra) flow out to the coast from rivers, and the 228Ra/226Ra ratio exceeds 1. On the other hand, open ocean waters have very little radium-228 (228Ra), resulting in a very low 228Ra/226Ra ratio.


      崩壊系列を持つ核種Nuclides with decay series　ウラン系列Uranium series　アクチニウム系列Actinium series　濃度concentration　半減期half-life　1億年100 million years

    • 【Description of the actinium series】

      Uranium-235 (235U), the parent nuclide, is uniformly present in seawater, like uranium-238 (238U). Moreover, the ratio of uranium-238 and uranium-235 is now constant at about 9:0.7. Expressing this in becquerels is 0.00016 becquerels (Bq) per liter. It is characterized by a concentration difference of nearly 1,000 times that of uranium-238 (238U).

      　This is because the specific radioactivity of uranium-238 and uranium-235 are different. The specific radioactivity can be expressed as the number of becquerels per gram, but this involves the half-life. The longer the half-life, the lower the specific radioactivity.

      　The specific radioactivity (Bq/g) of uranium-235 is 80,000. On the other hand, uranium-238, which has a half-life that is six times longer, is 12,400. If you show the difference in mass, you will see a difference in concentration.

      　Uranium-235 (235U) is an essential nuclide for nuclear power plants. This is because uranium-235 (235U) easily undergoes nuclear fission when hit by neutrons, and a large amount of thermal energy can be obtained from this.

      Uranium-238 produces plutonium-239 when hit by neutrons, and cannot obtain much heat energy. Some nuclear power plants enrich uranium-235 and use it as fuel.



    • 【The Past and Future of Uranium-235 and Uranium-238】

      　As mentioned above, the current ratio of the two uranium isotopes is about 9:0.7, but has the ratio been the same since the birth of the earth?

      　The figure below shows the isotope ratio of uranium in the past (2 billion years ago), the present, and the future (2 billion years from now).

      　Since the two isotopes have different half-lives, their proportions change over time. Uranium-235 (235U) now makes up 0.7% of all uranium, compared with 3.67% in the past. After another 2 billion years, it is estimated to be 0.14%. Carbon 14 (14C), which I talked about before, is always produced by cosmic rays, but uranium has hardly been produced since the birth of the earth, and stable uranium has not been confirmed.



      崩壊系列decay series　ウラン系列Uranium series

      時系列的に、存在量と235U/238Uが変化Changes in abundance and 235U/238U over time

      20億年前2 billion years ago　現在the present　20億年後2 billion later


    • 　【Mysterious movements of the uranium series】

      The figure below explains the daughter nuclides produced by the decay of uranium-238 (238U).

      Black arrows: indicates that they are easily removed from seawater (become gas or deposit in seafloor sediments)

      Blue arrows: indicates daughter nuclides that are soluble in seawater

      　Various daughter nuclides enter seawater from the atmosphere, rivers, and seafloor sediments, or are removed from seawater through decay. For example, when uranium-238 (238U) in seawater becomes thorium-234 (234Th), it is removed from seawater by adsorption to particles, but thorium-234 (234Th) has a half-life of several tens of days, so it quickly becomes uranium-234 (234U) and dissolves in water. It also turns into thorium-230 (230Th) and is removed from seawater by adsorption on particles. This time, when it becomes radium 226 (226Ra), it dissolves in seawater again. When radon 222 (222Rn) is formed, it becomes gas and moves into the atmosphere.


      　Although the movement is somewhat complicated, by skillfully using these properties, it is an excellent tool that can estimate the material circulation of seawater, the degree of influence of rivers and the atmosphere, and the age of water. Successful analysis of the decay series is a valuable source of information for oceanography that tells us about time and displacement.


      娘核種の性質によって海水へ溶けたり、除去されたりします

      Depending on the properties of daughter nuclides, they are dissolved in seawater or removed.