Sources of Artificial Radionuclides

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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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• What is a radionuclide
  • The type of atom classified by the number of protons and neutrons in the nucleus is called nuclide. Radioactive nuclides are called radionuclides (reference: ATOMICA).　
    The type of atom is determined by the number of protons. Even with the same atomic species (same number of protons), there are things with different numbers of neutrons. The nuclide distinguishes them.
    
    For example, naturally occurring potassium with an atomic number of 19 (19 protons) are three types, potassium 39 (K-39) with an atomic weight of 39 (19 protons and 20 neutrons), and potassium with an atomic weight of 40 (19 protons and 21 neutrons).  and potassium 41 (K-41) with an atomic weight of 41 (19 protons and 22 neutrons), and these are called isotopes. Of these, potassium-39 and potassium-41 are called stable nuclides because they are not radioactive, while potassium-40 is called a radionuclide because it is radioactive (reference: ATOMICA).
    
  • In this course, radionuclides include man-made radionuclides and natural radionuclides found in nature.

    Artificial radionuclides include:
    
    ① Those derived from nuclear tests (mostly atmospheric nuclear tests)
    
    ② Derived from accidents at nuclear-related facilities
    
    ③ Derived from planned releases under the control of nuclear-related facilities
    
    there is. Radioactive cesium-137 is explained below for ① to ③.

• (1) Atmospheric nuclear test
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    Release of ¹³⁷Cs into the ocean due to atmospheric nuclear testing

    　Since 1945, about 500 atmospheric nuclear tests have been conducted. A partial nuclear test ban treaty was signed in 1963 because nuclear tests in the atmosphere would disperse large amounts of radionuclides into the environment. Primarily, atmospheric nuclear testing, conducted between 1945 and 1963, is the largest source of artificial radionuclides in the ocean.
    
    
    

    
    

    The graph below shows the radiation dose of radioactive cesium (¹³⁷Cs) in the oceans (North and South Pacific, North and South Atlantic, and North and South Indian Oceans).

    The unit is PBq (petabecquerel). Peta stands for 10¹⁵. Becquerel is a unit that expresses the number of atoms (radioactivity) that a radioactive substance decays in one second.

    
    

    IAEA(2005) TECDOC-1429を基に作成Created based on IAEA (2005) TECDOC-1429

    単位Unit　世界地図World map
    

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    The amount of ¹³⁷Cs released by nuclear tests is more than 10 times that of the Chernobyl accident and more than 40 times that of the Fukushima Daiichi Nuclear Power Plant accident. In 2010, one year before the Fukushima Daiichi Nuclear Power Plant accident, ¹³⁷Cs was detected in seawater off the coast of Japan. This is because even after more than 50 years have passed since the nuclear test, it still remains.
    

• ② Accidents at nuclear facilities
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    Below is a summary of past accidents at nuclear facilities.

    1957,Mayak Nuclear Technology Facility (Soviet Union)

    1957, Windscale Reactor (UK)

    1979, Three Mile Island Nuclear Power Plant (U.S.)

    1986, Chernobyl Nuclear Power Plant (Soviet Union) 　

    2011, TEPCO Fukushima Daiichi Nuclear Power Station (Japan)

    
    

    In the past accidents, the Chernobyl nuclear power plant accident and the TEPCO Fukushima Daiichi nuclear power plant accident have known the amount of artificial radionuclides released into the environment (the red part in the table below).

    In the Chernobyl nuclear power plant accident, you can see that the amount of radioactive iodine ¹³¹I released is outstanding.

    The Fukushima Daiichi Nuclear Power Plant accident also accounted for a large proportion of the release of radioactive iodine, but it is characterized by a high proportion of radioactive cesium ¹³⁷Cs.
    
    

    In addition, since the Fukushima Daiichi Nuclear Power Plant accident released contaminated water containing radionuclides into the ocean, a certain amount of radioactive nuclides were also released into seawater.
    
    
    

    
    

    核種nuclide　半減期half-life　放出量Emission amount　核実験Nuclear test　チェルノブイリ事故Chernobyl accident　東電福島第一原発事故TEPCO Fukushima Daiichi Nuclear Power Plant Accident　日Day　年Year　海水sea ​​water　大気atmosphere
    

    
    

    
    

  • 　These nuclides (¹³⁷Cs and ¹³¹I) are derived from fission product elements created during the extraction of nuclear energy at nuclear power plants. These nuclides are easily produced in nuclear reactors and accumulate in the human body, so they are also the nuclides of greatest concern with regard to their effects on humans.

    　The rate at which a particular material is produced (fission yield) when the uranium-235 in nuclear fuel fissions is shown below.

    　　  ¹³¹Ｉ：２.878％　

    　　 ¹³⁷Cs ：6.2％（Together with beta-decay origin from other nuclides）

    　Although ¹³⁷Cs appears to be more abundant in terms of fission yield, the higher radioactivity (Becquerel number) released by ¹³¹Ｉis due to the difference in half-life. (Becquerel is the number of atoms that fission in one second. Half-life is the time it takes for fission to reduce the number of atoms to half of their original number). The shorter the half-life, the higher the Becquerel number. Compared with the amount produced and accumulated in a nuclear reactor, the following is a comparison.

    　　Amount of ¹³¹Ｉproduced ：Approx. 50g

    　   Amount of ¹³⁷Cs produced: approx. 7,000 g

    Thus, when compared in terms of the amount produced and accumulated, ¹³⁷Cs, which has a longer half-life, is more abundant. In addition, the proportion of ¹³⁷Cs with a long half-life increases with the number of years a nuclear power plant has been in operation.

    
    

    The Chernobyl nuclear accident shows that the amount of radioactive iodine ¹³¹I released is outstandingly high.

    This is due to the difference in the operating life of nuclear reactors. Nuclear power plants obtain energy by burning large amounts of nuclear fuel (causing nuclear fission), and the longer the fission period, the more radioactive products accumulate.

    　 ¹³¹I has a half-life of 8 days, so even if it is produced, it decays quickly and accumulates little over a long period of operation.

    　On the other hand, ¹³⁷Cs has a half-life of 30 years, so the longer a nuclear reactor is in operation, the more it accumulates.

    　In other words, the ratio of cesium to iodine gradually rises in a nuclear reactor as it is operated for a longer period of time. The accident at the Chernobyl nuclear power plant occurred when the plant was in operation for a relatively short period of time. Therefore, the amount of ¹³⁷Cs accumulated in the reactor was low, and when radionuclides in the reactor were released into the atmosphere due to the accident, the amount of ¹³¹I released was significantly higher than that of ¹³⁷Cs.

    
    

    
    

    　Look at the other table. The release of a radionuclide called xenon (Xe) 133 is also included. In fact, ¹³³Xe is the most common radionuclide released in nuclear accidents. Comparing the total amount of radionuclides listed in the table below, including ¹³³Xe, we can see that the Fukushima Daiichi accident released much more radionuclides than the Chernobyl accident. However, the release of ¹³³Xe has not been regarded as a major problem. Why is this?
    

    　This is because xenon (Xe) is a noble gas and does not accumulate in the bodies of living organisms or in certain places in the environment. What adversely affects the human body is when radionuclides accumulate in the human body and undergo radioactive decay in the body (internal exposure). Another is the exposure to radiation that accumulates in a certain location in the environment and is then exposed to a person in the vicinity (external exposure). ¹³³Xe is ignored because these effects are likely to be very small.

    　Also, the same nuclides are released in different amounts in different nuclear power plant accidents: in the Chernobyl accident, the fuel blew away with each fuel, whereas in the Fukushima Daiichi accident, a steam explosion and radioactive nuclides dissolved in the cooling water that came into contact with the nuclear fuel, and some of that contaminated water flowed underground and into the ocean. Since no nuclear fuel was blown up at Fukushima, the amount of ¹³⁷Cs and ¹³¹I released is thought to be less than at Chernobyl.
    

    
    

    
    

    核種nuclide　半減期half life　沸点boiling point　融点melting point　環境への放出量Amount released to the environment　チェルノブイリChernobyl　福島第一Fukushima Daiichi　キセノンxenon (Xe)　ヨウ素iodine (I)　セシウムcaesium　ストロンチウムstrontium (Sr)　プルトニウムplutonium (Pu)　日day　年year　事故発生時に炉心に蓄積されていた放射性核種の環境へ放出された割合Percentage of radionuclides accumulated in the reactor core at the time of the accident that were released into the environment　約approximately
    

    Source: Ministry of the Environment HP（https://www.env.go.jp/chemi/rhm/h28kisoshiryo/h28kiso-02-02-05.html）2020.8.12 reprinting

    
    
    

• ③ Planned releases from nuclear facilities
  • What is a nuclear fuel reprocessing facility?

    　Nuclear power plants use fuel rods containing uranium dioxide fuel pellets (equivalent to coal in thermal power plants) to evaporate water (boil water) using the heat energy emitted when uranium 235 fission occurs, and use the steam to turn a turbine to generate electricity.

    
    

    
    

    火力発電thermal power generation　原子力発電nuclear power generation (of electricity)　燃料fuel　石炭coal　核燃料atomic fuel　熱エネルギー源Heat energy source　石炭を燃やすBurning coal　核分裂nuclear fission　電力源Power Source　お湯を沸かして発電タービンを回すBoils water and turns power turbine
    

    　In nuclear power generation, fuel rods are replaced with new fuel rods when they are used up. Used fuel is called "spent fuel rods".Unlike thermal power generation, however, spent fuel rods contain a large amount of unburned plutonium, which is produced from unreacted uranium-235 and uranium-238. In other words, there is a considerable amount of unreacted uranium-235 and plutonium produced from uranium-238 in the spent fuel rods, so a "reprocessing plant" is a place to extract them and make fuel rods again.

    ＊For more information, please visit the Federation of Electric Power Companies of Japan HP：https://www.fepc.or.jp/nuclear/cycle/recycle/index.html

    
    

    
    

    
    

  • Why are radionuclides released from nuclear fuel reprocessing facilities?

    　Spent nuclear fuel used in nuclear power generation is sheared and dissolved in a reprocessing plant, then separated and refined, and separated into nuclear fuel material and fission products. In this process, gases and liquid waste are generated. These wastes contain fission products (¹³⁷Cs, ⁹⁰Sr, etc.) and are released into the environment under the concept of keeping the dose to the surrounding public below the legal limit and as low as reasonably achievable (ALARA: as low as reasonably achievable), while managing them.

    　The amount of emissions is determined by the facility's own control targets (limit values) in accordance with the laws of each country.

    For more information, go to ATOMICA's Radiation from Reprocessing Facilities page.（https://atomica.jaea.go.jp/data/detail/dat_detail_09-01-02-06.html）

    
    

    
    

  • In Europe, nuclear fuel reprocessing facilities are in operation, and artificial radionuclides are being systematically (and controlled) released from these facilities.

    The two reprocessing facilities are located in La Argues, France and Sellafield, England.
    

    The figure below shows the annual releases (1970-1998) of a typical reprocessing plant in Europe (shown on the right).

    　During this period, Sellafield released approximately 40 PBq of ¹³⁷Cs into the ocean. In 1976, the maximum amount reached 5.2 PBq/year (1.4 times higher than the direct leakage immediately after the Fukushima Daiichi Nuclear Power Plant accident), but this amount decreased to 0.008 PBq/year in 1998.

    　The amount of ¹³⁷Cs released from La Argues is less than 3% of that from Sellafield when compared to the total amount of ¹³⁷Cs released, but this is also due to the difference in the amount of spent fuel processed annually.

    
    

    
    

    セラフィールド、イギリスSellafield, United Kingdom

    ラアーグ、フランスLa Argues, France
    

    
    

    Below we show the year-to-year changes in the amount of radioactive cesium released from nuclear fuel reprocessing plants in Europe.

    
    

    
    

    年間放出量Annual emission　福島第一原発事故直後の海洋への直接漏洩量Amount of direct leakage to the ocean immediately after the Fukushima Daiichi Nuclear Power Plant accident
    

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