The Use of 222 Rn as a Flow Path Monitor for Studies of Radionuclide Transport in Fissures
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THE USE OF 222Rn AS A FLOW PATH MONITOR FOR STUDIES OF RADIONUCLIDE TRANSPORT IN FISSURES
J. HINES, D. COHEN, S. FRIED, A. M. FRIEDMAN Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439
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ABSTRACT A technique has been developed using 222Rn as a flow path monitor. Its principle advantage over other radiotracers or dyes is that while the Kd of Rn gas in water solution is zero and hence follows the water path, its 210Pb daughter is retained strongly by rock. The immobilized 210Pb is not subject to diffusion, an important consideration in the proposed Nevada Field Test. To evaluate the technique, a series of laboratory scale experiments have been performed. Artificial fissures consisting of glass plates and prepared flat surfaces were coupled to insure well characterized fissures. Water solutions of radiotracer were metered through these fissures and the discharge monitored to produce breakthrough curves as a function of flow rate. The rock surfaces were analysed for radionuclide concentration and the rock sectioned for diffusion profile into the rock.
INTRODUCTION Tracers have been extensively used as water-front monitors in geological measurements. In essence their purpose has been to distinguish the water pumped into the matrix as part of the experiment from the native ground water, usually to determine the time of arrival of the water-front. To do this the tracer must meet certain requirements. It should respond to the host matrix in the same manner as water, that is, no retardation in the matrix by chemical or physical processes, ion-exchange, precipitation, etc. The tracer should be easily assayed in the low concentrations typical of field tests. Radioisotopes are ideal for this application, especially in anionic form. Both 131 and 125 iodine as iodide ions have been extensively used. Tritiated and deuterated water have also been used but are more difficult to assay. A second need is for a tracer which will delineate the flow path of the experiment, and which will also serve to measure the thickness of a fissure if the flow takes place by this means. We will call these flow path tracers. The flow path tracers have to mark the path without interfering with the absorption of other species and should deposit an amount of radioactivity which can be related to the thickness of the fissure. These two requirements appear to be mutually incompatable since the first implies an absorption coefficient of zero and the second can only be met if the coefficient is large. Our method of solving this conundrum is to use a radioactive flow path tracer which is not absorbed and whose daughter is strongly absorbed. The Nevada Field Test experiment has been structured in a way to preclude the use of only conventional water-front tracers. The purpose of the field test is to evaluate tuff as a candidate material for potential repository usage. *This work was performed under the auspices of the Department of Energy, Energy and Conservation.
188 Tuff is a very porous rock, cont
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