Migration Behavior of Uranium Series Nuclides in Altered Quartz-Chlorite Schist
- PDF / 440,515 Bytes
- 8 Pages / 420.48 x 639 pts Page_size
- 41 Downloads / 158 Views
MIGRATION BEHAVIOR OF URANIUM SERIES NUCLIDES IN ALTERED QUARTZ-CHLORITE SCHIST
T. Y.
OHNUKI, T. KOBAYASHI
MURAKAMI*,
K. SEKINE,
N. YANASE,
H.
ISOBE and
Department of Environmental Safety Research, Japan Atomic Energy Research Institute, Tokai, Ibaraki, 319-11, Japan Presently Australian Nuclear Science and Technology Organisation, PMB 1, Menai, NSW 2234, Australia ABSTRACT The migration behavior of uranium series nuclides in an altered quartz-chlorite schist has been studied by utilizing data on the activity distributions of uranium series nuclides at Koon arra in the Northern Territory of Australia. The variation 2 34 of ?3°Th/ U activity ratios (ARs) with distance along surface (2 m depth), intermediate (16 m) and deep (31 m) layers show different trends in the three layers parallel to the water flow. The relationship between the 23 4U/ 23 8U and 23 0 Th/ 23'U ARs reveals that the mobility of the uranium series nuclides is in the order 23 8 U, 234U and 23 0Th, and that the retardation factors of 23 4U are greater than those of 238U by a factor of 1.1, 1.9 and 1.0 in the surface, intermediate and deep layers, respectively. X-ray diffraction patterns show different mineral assemblages, which are the alteration products of chlorite at the three layers. These results imply that the migration behavior of uranium series nuclides at each depth would be related to the alteration of chlorite. INTRODUCTION Part of the primary uranium ore deposit, at Koongarra in the Northern Territory of Australia, has been weathered to form a secondary ore deposit [1]. The secondary ore deposit is located in the "weathered zone", which is present below the surface for a depth of about 20 m (Fig. la). As the extent of weathering increases, uraninite, the primary uranium mineral at Koongarra, is changed to uranyl silicates, and then to uranyl phosphates [I]. Uranium is also associated with clay minerals and iron minerals downstream from the secondary ore deposit [2,3]. The clay minerals and the iron minerals are altered products of chlorite, which is a major rock-forming mineral of the quartzchlorite schist, the host rock of the ore deposit. These observations suggest that chlorite alteration may play an important role in the migration of uranium series nuclides, and that the redistribution of uranium should be examined in terms of its relationship to the alteration. The uranium migration at Koongarra is a good testing sample to evaluate actinide element migration following the disposal of high level radioactive waste. Host rocks around disposal sites cannot be considered to be static but are dynamic over a long, geologic timescale, analogous to the case at Koongarra. The rock-forming minerals are changed structurally and chemically with time by water-rock interactions. The alteration products
Mat. Res. Soc. Symp. Proc. Vol. 176. ©1990 Materials Research Society
608
are usually metastable with equilibrium being delayed by as much as 107 years [4]; longer than the time required for the safety assessment of the high level waste d
Data Loading...
