Survey on current status of laboratory test method and experimental consideration for establishing standardized procedure of material containing bentonite. Report of collaboration research between JAEA and CRIEPI (Joint research)

In the current concept of repository for radioactive waste disposal, compacted bentonite as well as bentonite based material will be used as an engineered barrier mainly for inhibiting migration of radioactive nuclides. In most cases, properties of bentonite, such as low permeability etc., are obtained by laboratory tests. However, results of laboratory tests of bentonite often vary considerably even if index parameter, such as effective clay density, is constant. One of the causes of the variability is considered to be lack of standardized method of laboratory test for bentonite. Thus standardization of laboratory test methods for bentonite is needed. So, investigation for establishing standardized laboratory test method of bentonite is conducted based on the results of survey on current status of laboratory test method for bentonite. In particular, the literature survey as well as laboratory tests were conducted to find factors affecting the results of laboratory tests for bentonite and to estimate their degree of influence. The following conclusions are obtained through this study. (1) Hydraulic conductivity test. According to the results of literature survey, it is revealed that constant pressure permeability test and consolidation test are currently used for measuring hydraulic conductivity of bentonite and that (a) hydraulic gradient, (b) local seepage flow between lateral surface of the specimen and lateral wall of the container, (c) water pressure which is applied to the specimen, (d) degree of saturation and (e) size of the specimen possibly affect the results of the constant pressure permeability test, while (f) friction between lateral surface of the specimen and lateral wall of the container accompanied by deformation of the specimen, (g) consolidation pressure together with factors (d), (e) affect the results of the consolidation test. Literature which describes that factors (a), (b) and (e) affect the results of the constant pressure permeability test is not found. In the constant pressure permeability test, the effect of factor (c) can be avoided by setting applied water pressure difference for seepage flow smaller than the swelling pressure. In the constant pressure permeability test, the effect of factor (d) is also avoided by infiltrating water into the specimen one-dimensionally so that air does not remain in the specimen and by applying back pressure during seepage flow test. In the consolidation test, a correction method for the effect of friction is needed because the effect of the factor (f) is inevitable. It is revealed that one-dimensionally infiltration of water into the specimen is difficult because of the structure of the consolidation test apparatus. There is no literature which describes the effects of factors (e) and (g) on the results of the consolidation test conducted for bentonite specimen. According to the reasons mentioned above, it is currently desirable to use the constant pressure permeability test for compacted bentonite. (2) Swelling pressure test. According to the literature survey, confined type testing apparatuses and apparatuses which are similar to the consolidation test apparatuses are used for measuring swelling pressure. Factors affecting results of swelling pressure tests are saturation of the specimen, size of the specimen and difference of apparatus. Saturation of the specimen set in confined type testing apparatus can be raised easily by one-dimensional infiltration of water through the specimen and by applying backpressure. It is revealed that size of the specimen affects the test results if effective clay density is larger than 1.6 Mg/m{sup 3}. Though swelling pressure measured by the confined type test apparatus is larger than that by apparatuses which are similar to the consolidation test apparatuses, further study is needed to clarify the cause of the difference. (3) Thermophysical properties measurement. There seems no problem in measuring thermal conductivity and thermal diffusivity by the current four methods for uniformly mixed specimens of sand-bentonite mixture. However, heterogeneity of sand content and moisture content in the specimen affects the results of measurement. Therefore, procedure for mixing bentonite and admixture uniform and procedure for prohibiting migration of water content by covering the specimen by wrapping are necessary. (author)