CEEF居住実験における目的と計画に関する考察

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  • Some Considerations on Purpose and Schedule of the Human Habitation Experiment in CEEF.

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Closed Ecology Experiment Facilities (CEEF), an experimental “mini-earth,” is composed of several closed chambers physically separated and sealed from outside except the flows of energy and information. CEEF was designed to house plant, animal, and human ecosystems in the separate chambers and to maintain a constant environment needed to sustain all living organisms in these chambers by circulating CO2, O2, water and other necessary elements for living organisms between chambers. Therefore the overall ecosystem planned in CEEF makes it scientifically a mini-earth model. The behavior of 14CO2 released from nuclear reprocessing sites into the natural environment can be simulated by tracking 14C dynamics in the artificial mini-earth. In the nuclear power generation process, 14C is produced from 13C, 14N, 15N, 16O, and 17O atoms by neutron collision and radioactive decay. Oxygen atoms in uranium oxides and nitrogen atoms contained in the binder used for solidifying fuel rods are changed to 14C, and 14CO2 is also produced during reprocessing and is released. In the natural environment, percentages of carbon isotopes are 12C: 98.892%, 13C: 1.108% and 14C: 0.05%. The decay constant (half life) of 14C radioactive material is about 5570 years. Therefore it is inadvisable to put 14C in the habitat of CEEF, so instead, the stable isotope 13C is used to simulate the accumulation quantities of 14C in each ecosystem and atmosphere of CEEF. The ability of plant leaves to take in CO2 seems to change a little because of the mass difference of 14C and 13C carbon isotopes. This discrimination ratio between 14C and 13C can be measured using the radioisotope laboratory facility located near CEEF. The quantity of 14C taken in by plants is estimated using this discrimination ratio. On the other hand, animals and human eat foods coming from plant biomasses and digest them without any discrimination. And the stored carbon in their bodies can be estimated by analyzing the foods, expiratory gases, and feces and urine excreted. In order to proceed with this radioactive trace experiment, it is necessary to establish the complete circulation system of metabolic materials produced from living organisms including human being. The habitation experiment is on the way to this target and its schedule is presented.

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