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Analytical method development
It was necessary to develop a variety of innovative analytical methods during FUNMIG in order to
be able to quantify the migration of highly-sorbing tracers in intact clayrock samples. The ‘hiresolution
abrasive peeling’ method developed by PSI consists of diffusing tracer into a flat clayrock
surface using a specially developed cell, for a given period of time (e.g. ~170 days for Eu(III)),
and then abrasively removing roughly 10 m thick layers of rock for total tracer concentration determination.
With this method, tracer concentration profiles extending less than 200 m into the
rock could be determined with roughly 10 m resolution. Along this same line, CIEMAT demonstrated
that nuclear ion beam Rutherford Backscattering Spectroscopy (RBS) could be successfully
used to quantify Eu(III) depth profiles extending to ~1.5 m (~50 nm resolution) below a polished
clayrock surface, after different in-diffusion times. FZK-INE developed a special cell for carrying
out actinide in-diffusion experiments on clayrock samples maintained at in-situ confining pressures
and an autoradiography technique for quantifying tracer distribution relative to the input surface. A
different approach was taken by the CEA driven by the need to associate the spatial distribution of
tracer mass after an in-diffusion experiment with the corresponding rock mineral-porosity organization
for carrying out diffusion modelling by the TDD method (cf. §3.2). The developed method,
based on hi-resolution Laser Induced Breakdown Spectroscopy (LIBS), allows the simultaneous
determination of tracer and rock mineral element distribution away from the tracer input surface
with a ~3 m spatial resolution. CEA also developed column-based method for determining Da parameters
for sorbing RN diffusion into and out of ~ 2 mm thick clayrock ‘plates’, with the number
of plates and flow rate being varied depending on RN the sorption and diffusion characteristics.
Results of comparison of RN sorption equilibrium in batch and intact rock systems
The question addressed here is quite simple – for a given mass of rock equilibrated with a given activity
of a sorbing RN under otherwise identical conditions, does one measure the same total sorbed
mass of RN if the rock is present as ground particles or as a compact solid? While this question can
be answered fairly readily for weakly and moderately sorbing RN as will be shown below, it turns
out to be quite difficult in the case of strongly sorbing RN. The key objective here is to reach an
equilibrium state in systems containing compact rock samples, i.e. kinetics related to RN mass
transport (diffusion) into the sample have reached insignificant levels. When this is the case, results
can be interpreted using only a chemical equilibrium model, i.e. without diffusion. The results of
measurements carried out by PSI show that sorption equilibrium reached for Na + , Sr ++ and Cs + on
compacted Opalinus clayrock are comparable (within a factor of 2) to corresponding states measured
on crushed samples. Similar results were obtained by SCK-CEN for Sr and Cs on Boom clay.
The Rd values measured on compacted samples tend to be higher than for crushed rock which
might be due to the longer equilibration times used for the compact material studies. Measurements
of Co +2 sorption, while not completely attaining equilibrium within the nearly 700 day experimental
time frame, indicate the same result. Taken together, these results tend to support the following conclusions:
Crushed and whole rock samples have similar sorption site populations per unit mass
(sorption site types and corresponding total concentrations),
Similar mass action laws apply for sorption in intact and crushed materials,
Sorption-induced retardation of RN mass transport (i.e. very low Da value) results in very
long time frames for reaching equilibrium for highly sorbing RN in compact rock systems.
This limits the capacity to directly determine sorption equilibrium for actinides in compact
rock.
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