低中水平放射性废物浅地层处置安全评价方法研究

STUDY ON SAFETY ASSESSMENT METHODOLOGY FOR SHALLOW LAND DISPOSAL OF LOW AND INTERMEDIATE LEVEL RADIOACTIVE WASTE

中国辐射防护研究院和日本原子力研究所合作开展了为期 5年 (1988年 1月～ 1993年 1月 )的“低中水平放射性废物浅地层处置安全评价方法研究”,以建立一套低中放废物浅地层处置安全评价技术和方法 ,包括参数、模式和程序。本文主要介绍在黄土包气带中核素迁移规律、水分运移行为研究及其相关参数测定的方法和试验结果 ,以及试验场址主要特征和开发的核素迁移模式与计算程序为开展现场核素迁移示踪试验和实验室核素迁移模拟实验 ,建立了野外试验场、喷淋试验大厅和环境模拟实验室 ;开发了现场核素迁移直接测量系统 ;研制了实验室模拟装置和原状土取样设备。包气带核素迁移示踪试验 ,从 1989年 5月开始到 1991年 8月结束 ,试验在天然 (试验期间年均降雨量为 4 38mm)和喷淋 (喷淋强度 15mm/ d,相当于降雨量 5.4 8× 10 3 mm/ a)两种条件下进行。实验室模拟实验 ,喷淋强度为 0 .796 mm/ d(4号土柱 )和 0 .6 56 mm/ d(2号土柱 ) ,历时约 1年。示踪核素为 60 Co、85Sr和 13 4Cs(或13 7Cs)。还与现场试验同步开展了试验场水分运移研究 ,及用 3 H作示踪剂的水分运移研究。得到以下主要结果 :(1)对 85Sr,喷淋条件下 2年的现场试验期内 ,浓度峰迁移了 13cm,天然条件下迁移约 2 cm;在 1年的实验室模拟实验期内 ,取?

China Institute for Radiation Protection (CIRP) conducted a five year (from Jan. 1988 to Jan. 1993) project entitled “Study on Safety Assessment Methodology for Shallow Land Disposal of Low and Intermediate Level Radioactive Waste” in cooperation with Japan Atomic Energy Research Institute (JAERI). The project was aimed to develop a set of techniques and methods, including parameters, models and computer codes, for safety assessment of shallow land disposal of low and intermediate level radioactive waste. This paper describes the pattern of nuclide migration in loess aerated zone, study on moisture movement, determination of related parameters and the results. Site characteristics and development of relevant models and codes are also described. In order to carry out the field tracing test and the laboratory simulation test of radionuclide migration, a field test site (CIRP's Field Test Site), a test hall equipped with sprinkler, and an environmental simulation laboratory were established. A direct measurement device used for field radionuclide migration test, laboratory simulation setup for radionuclide migration and an intact soil sampler were developed. The loess aerated zone radionuclide migration test started in May 1989 and finished in August 1991. Tests were carried out under two rainfall conditions, natural rainfall (438 mm/a) and articicial sprinkling (with sprinkling density of 15 mm/d, corresponding to annual rainfall of 5 480 mm). The laboratory simulation ran about one year with daily sprinkling density of 0.796 mm for soil column 4 and 0.656 mm for soil column 2. The tracer nuclides are 60 Co, 85 Sr and 134 Cs and (or 137 Cs ). Field moisture movement study and a test using 3H as tracer for moisture movement were performed simultaneously. The major conclusions are summarized as follows: (1) Under the articicial sprinkling condition, peak of 85 Sr moved about 13 cm in the loess aerated zone during about two years, while under the natural condition, it moved less than 2 cm and in the laboratory simulation test about 8 cm movement was observed in one year under the articicial sprinkling condition. Under the both rainfall conditions of the two year field test, and the one year laboratory simulation test, peaks of 60 Co and 134 Cs had no significant movement, and both migration (under field test) are less than 1 cm. All these imply that the loess has very strong adsorption capacity to Co and Cs,and relatively strong adsorption capacity to Sr. (2) It is found from the results of both field aerated zone radionuclide migration test and the laboratory simulation test that there is a minor fraction of each of the tracing radionuclides moving fast. This finger flow phenomenon was also found along the horizontal direction. (3) Double peak of 85 Sr concentration distribution was observed under the articicial sprinkling condition when using mixed tracers of 60 Co, 85 Sr and 134 Cs. Both the double peak distribution and the finger flow as in (2) are important for the safety of LILW disposal. (4) Distribution coefficient K d derived from the field test of 60 Co, 85 Sr and 134 Cs in loess areated zone are 8.8×10 2～1.5×10 3,0.5～70 and 1.1×10 3～1.5×10 3 mL/g respectively; while in the laboratory bath experiment, they are 5.4×10 3,82 and 7.4×10 3 mL/g respectively. The latters is remarkably higher than the former, which implies that the site specific condition and measurement conditions must be taken into account when using the laboratory K d data. The results from the field aerated zone radionuclide migration test showed that retardation factor of Sr in the unsaturated loess increases with increasing water content in the soil. (5) NESOR, a non equilibrium adsorption model developed in this project, taken into account of non equilibrium adsorption and desorption process, has good agreement with the measured results from the field test. (6) It was