Transport of Neptunium Through Yucca Mountain Tuffs
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TRANSPORT OF NEPTUNIUM THROUGH YUCCA MOUNTAIN TUFFS I. R. TRIAY, B. A. ROBINSON, A. J. MITCHELL, C. M. OVERLY, AND R. M. LOPEZ Los Alamos National Laboratory, Los Alamos, NM 87545
INTRODUCTION Neptunium has a high solubility in groundwaters from Yucca Mountain [ 1]. Uranium in nuclear reactors produces 2 37 Np which has a half-life of 2.14 x 106 years. Consequently, the transport of 23 7Np through tuffs is of major importance in assessing the performance of a high-level nuclear waste repository at Yucca Mountain. The objective of this work is to determine the amount of Np retardation that is provided by the minerals in Yucca Mountain tuffs as a function of groundwater chemistry.
EXPERIMENTAL The experiments conducted consist of batch sorption measurements and column experiments with crushed tuff and pure mineral separates. Batch sorption experiments consisted of pre-conditioning solid phases with groundwater (in the ratio lg to 20 ml) for two weeks. Solutions of 2 37 Np in groundwater were added to the pre-conditioned solid phases (in the ratio 1 g of solid to 20 ml of solution) and equilibrated for three weeks. After sorption, the two phases were separated by centrifugation and the amount of Np in each phase was determined by gamma spectrometry. The column experiments consisted of eluting tritiated water through columns of crushed tuff to determine the column's hydrologic parameters. This step was followed by eluting 2 37Np solutions in groundwater through the columns and determining Np retardation by the tuff. Neptunium solutions were prepared with two different groundwaters from Yucca Mountain: water from the wells J-13 and UE-25p #1. The Np solutions were prepared by adding an aliquot of a well-characterized Np(V) acidic stock to groundwater filtered through a 0.05 gm filter. The chemical composition of the filtered groundwaters at Los Alamos, NM is given in Table I. Both of these waters are oxidizing with an Eh larger than 200 mV. The Np solutions are approximately 10-6 M.
Table I. Groundwaters Chemical Composition
Si Mg Ca Na K Li TOC
J-13 20. 1.6 13. 47. 5.4 0.04
UE-25p #1 14. 32. 8.0 150 11. 0.5
0.6
2.
CO 3-2 HCO 3FClI SO 4 -2 NO 3pH
-
Concentrations in mg/L J-13 1.5 120 2.2 7.2 27. 8.6
UE-25p #1 31. 300 4.0 25. 160 0.5
8.4
Mat. Res. Soc. Symp. Proc. Vol. 294. `c1993 Materials Research Society
9.0
798
The solid phases used for the batch and column experiments were tuffs from the Topopah Spring Member (G4-275) and Calico Hills (G4-1530) and the minerals quartz, clinoptilolite, montmorillonite and hematite. Tuff G4-275 consists of 61% feldspar, 24% tridymite, 11% cristobalite, 1% hematite, 1% quartz, and traces of smectite and mica. Tuff G4-1530 consists of 55% clinoptilolite, 16% opal-CT, 12% mordenite, 7% quartz, 7% feldspar, and 2% smectite. The size range of the tuff particles utilized is from 75 to 500 gim. The surface area of both of these tuffs is 4.3 m2 /g. The properties of the minerals and tuffs utilized are given in Tables Ila and Ilb. Table Ila. Properties of Minerals for Batch S
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