Adsorption of Carbon-14 on Mortar
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ADSORPTION OF CARBON-14 ON MORTAR
Junko MATSUMOTO, Tsunetaka BANBA and Susumu MURAOKA Department of Environmental Safety Research, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-11, Japan.
ABSTRACT The sorption experiments of carbon-14 on the mortar grain (grain size: 0.50 - 1.0 mm) focused on the chemical form of the carbon-14 were carried out by the batch method. Three kinds of carbon-14 chemical form were used for the experiments : sodium carbonate 14 (Na 14 CO ) as the inorganic radiocarbon, and sodium acetate (CH COONa) and acetalde14 hyde (14C3 CHO) as the organic radiocarbons. 0.30 gram sampes of mortar were soaked in the solution with carbon-14 at 15"C for periods of up to 160 days. At the end of each run, carbon-14 concentrations in the supernatants were determined before and after centrifugation (3,500 rpm., 1 hr.). In the mortar-sodium carbonate system, the retention process of carbon-14 related to reaction on the surface of the mortar was speculated as follows. First, 3CaO-SiO and 2CaOSiO of the mortar components contact with water and produce Ca(OH)2 . Ca(OA)2 produces 2 Ca2ý and OH- in the solution. Then, calcite forms from Ca + and CO 2- in the solution. Thus, the sorption ratio of carbon-14 onto mortar will be high until mortarhas been completely carbonated because Ca 2+ is rich in the mortar and the solubility of calcite is low. In the mortar-organic carbon system, the soluble organic carbon-14 is hardly sorbed on the surface of the mortar. Therefore, the cementitious materials may not inhibit the release of organic radiocarbons from the low-level radioactive wastes, contrary to the case of inorganic radiocarbon. INTRODUCTION Carbon-14, which has a radioactive half-life of 5,730 yr., might give a significant contribution of the long-term biological hazards from low-level radioactive wastes. According to the estimation of Grahlke and coworkers [1], analysis of the activity level of carbon-14 contributed from the fuel and non-fuel cycle sources to low-level radioactive waste repository sites information: 2,100 of the United States in the period from 1985 - 2004 provides the following 13 Bq) from institutional Ci (7.8x10 13 Bq) from the nuclear fuel cycle ; 1,650 Ci (6.1x10 13 wastes; 2,150 Ci (8.0x10 Bq) from industrial wastes. Chemical form of the carbon-14 in the fuel cycle waste approximates 75% inorganic (carbonate), 20% activation metal, and 5% organic radiocarbons. The chemical form of the institutional and industrial carbon-14 waste is Similar estimations have seemed to be essentially 100% organic radiocarbon compounds. been also reported by Swedish Nuclear Power Inspectorate [2], and Atomic Energy Research Establishment, Harwell, United Kingdom [3]. The average chemical composition of organic radiocarbons in the coolant of Japanese pressurized water reactors (PWR) is estimated to be approximately 40% as acetaldehyde, 20% as methylalcohol, 30% as ethylalcohol and acetone, 2% as acetic acid [4]. It is generally assumed that most of carbon-14 discharged as alcohol and acetone would
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