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Behavior of Radioactive Elements during Thermal Treatment of Nuclear Graphite Waste Thermodynamic Model Analysis a
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Hee-Chul YANG , Hee-Chul EUN , Dong-Gyu LEE , Won-Zin OH & Kune-Woo LEE
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Nuclear Fuel Cycle R&D Group , Korea Atomic Energy Research Institute , P. O. Box 105 Yuseong, Daejeon , 305-600 , Korea b
Quantum Energy Chemical Engineering , University of Science and Technology , P. O. Box 52 Yuseong, Daejeon , 305-333 , Korea Published online: 05 Jan 2012.
To cite this article: Hee-Chul YANG , Hee-Chul EUN , Dong-Gyu LEE , Won-Zin OH & Kune-Woo LEE (2005) Behavior of Radioactive Elements during Thermal Treatment of Nuclear Graphite Waste Thermodynamic Model Analysis, Journal of Nuclear Science and Technology, 42:10, 869-876 To link to this article: http://dx.doi.org/10.1080/18811248.2005.9711038
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Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 42, No. 10, p. 869–876 (October 2005)
ORIGINAL PAPER
Behavior of Radioactive Elements during Thermal Treatment of Nuclear Graphite Waste Thermodynamic Model Analysis Hee-Chul YANG1; , Hee-Chul EUN2 , Dong-Gyu LEE1 , Won-Zin OH1 and Kune-Woo LEE1 1
Nuclear Fuel Cycle R&D Group, Korea Atomic Energy Research Institute, P. O. Box 105 Yuseong, Daejeon 305-600, Korea 2 Quantum Energy Chemical Engineering, University of Science and Technology, P. O. Box 52 Yuseong, Daejeon 305-333, Korea
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(Received March 24, 2005 and accepted in revised form August 15, 2005) Nuclear graphite waste retains various radioactive elements. This study investigated the behavior of the radioactive elements during a thermal treatment of the nuclear graphite waste, based on the thermodynamic equilibrium. Two typical thermal conditions were simulated: an excess air incineration and a hydrothermal oxidation. Tritium, 14 C and 36 Cl are expected to be present in the gas phase throughout the whole waste thermal systems. 133 Ba, 90 Sr, 22 Na, and 137 Cs were analyzed to be semi-volatile elements. Their behavior will be dependent upon the gaseous atmosphere as well as the operating temperature. Uranium species are expected to convert into a gas-phase UO3 at temperatures above 1,000 C under excess air incineration atmospheres. Other radioactive elements such as 59 Ni, 60 Co, 141 Ce, 152 Eu and 241 Am are non-volatile at temperatures up to 1,200 C, regardless of the gaseous atmosphere being simulated. KEYWORDS: waste nuclear graphite, radioactive elements, thermodynamic equilibrium, incineration, hydrothermal oxidation
I. Introduction Graphite has been used as a moderator and reflector of neutrons in many nuclear reactors. The old graphite moderated reactors are being shut down and the decommissioning of an old graphite reactor generates a huge amount of radioactive graphite waste. The existing processing technologies for nuclear graphite waste are based mostly on the isolation of the radioactive graphite from the environment. They are, however, not able to provide for a significant volume reduction.1) For this reason, the high-temperature thermal treatment technologies such as an incineration are considered as effective alternative technologies, since they provide a substantial reduction of the waste volume. However, the radioactive graphite wastes include various radionuclides such as tritium and 14 C, as well as corrosion/activation products (57 Co, 60 Co; 54 Mn; 59 Ni; 63 Ni; 22 Na, etc.), fission products (134 Cs, 137 C; 90 Sr; 152 Eu, 144 Ce, etc.) and a small amount of uranium and transmutation elements (238 Pu, 239 Pu; 241 Am, 243 Am, etc.).2) Therefore, while an incineration is considered as an effective tool for the integrated management of the nuclear graphite waste, the environmental acceptability of the emissions of the radioactive elements is a major criterion for the successful development of the incineration process. Radioactive elements cannot be destroyed by an incineration. Either they remain with the non-combustible portion of the waste or they vaporize, depending on the volatility of the elements. Any gaseous radioactive elements condense
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onto larger particles in the waste stream, which are removed by the off-gas cleaning system, or nucleate and form submicron aerosols of their own. These particles can often penetrate the off-gas cleaning system equipment. Emitted sub-micron radioactive particles are easily inhaled into the lungs. They can also be deposited onto the surface reservoirs or food crops. For these reasons, an incineration of the radioactive waste is often contested by the public as a viable treatment option.3) Atmospheric emissions of radioactive elements from a waste thermal plant depends on not only its thermal conditions but also on the chemical reactions that determine the distribution of radionuclides among the relevant chemical species. A better understanding of chemical process that results in the vaporization of the radionuclides will lead to better methods for a postfurnace control. This study investigated the vaporization and condensation of radioactive elements during a thermal treatment of nuclear graphite waste, based on the thermodynamic equilibrium. The purpose of this study is to establish the influence of the furnace operating parameters, such as the temperature and the gaseous composition, on the behavior of the radioactive elements in the nuclear graphite waste. The results of this work will be utilized to improve the system design and to optimize the operating conditions to minimize the emissions of radioactive elements during the thermal treatment of nuclear graphite waste.
II. Methods 1. Element Partitioning Model An important partitioning mechanism is the distribution of
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H.-C. YANG et al. Table 1 The proximate and the ultimate compositions of the waste nuclear graphite samples from Korea Research Reactor I (dry basis)
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Proximate analysis
Ultimate analysis
Volatile matter
Fixed carbon
Ash
C
H
O
Cl
>2:2%
99.8%
99:98%
