Research and Development on Geological Disposal of High-Level Radioactive Waste

3-1

Study of the Groundwater Flow in Coastal Areas=Optical Measurement of the Salinity Distribution on the Saltwater Wedge=

Fig.3-3 Optical measurement of salinity distribution(e.g., saltwater intrusion)
The chamber is 250 mm high, 500 mm wide, and 100 mm deep. Salt
water dyed red is supplied from the left side and nondyed fresh
water is supplied from the right side. A saltwater intrusion
phenomenon appears in the medium.

Fig.3-4 Experimental results for saltwater intrusion(salinity distribution)
The white part at the lower left is the saltwater region and theblack part at the upper right is the freshwater region. The
contour lines are isosalinity lines. Salinity was converted fromreflected light intensity.

Fig.3-5 Simulation results for saltwater intrusion(salinity distribution)
This figure illustrates a result of the numerical simulation for thesame boundary conditions as in the laboratory experiment.
The white mixing zone appears between the saltwater region(red) and the freshwater region (blue).

A safety assessment for the geological disposal ofradioactive waste is based on the groundwater scenario ,which assumes transportation of radioactive nuclides from arepository to the human living environment by groundwater.
For the simulation of radionuclide migration, an appropriate
model is selected on the basis of the characteristics of atarget. For targets in coastal areas characterized by thecoexistence of salt and fresh water, the parameters modeled
include saltwater intrusion caused by the density difference,
upconing of salt water with tunnel excavation, and change inthe position of the salt-fresh water mixing zone as a result ofsea level change. Although efforts have been made tounderstand these phenomena in situ, this understanding islimited owing to extremely complex groundwater flow andsalinity distribution caused by heterogeneity in rock facies
and fracture distribution. Consequently, numerical simulation
coupling multiple phenomena consisting of advection,diffusion, and density flow is essential to complement in situ
assessment.
To verify such groundwater simulation codes including
Dtransu , we constructed experimental equipment

incorporating a chamber filled with glass beads, which
simulates a homogeneous media for saltwater intrusion, andan optical measurement device with a white LED light sourceand high-resolution CCD camera to observe salinity
distribution (Fig.3-3). In the experiments, we measured theintensity of light reflected from the side surface of the salt
water, which is colored with dye of sufficiently low
concentration that acts as a tracer, using the optical device.
We quantified the salinity distribution by applying theKubelka-Munk formula to convert reflected light intensities
into tracer concentrations (Fig.3-4). The optical measurement
device can simultaneously quantify the salinity distribution
and temporal variation for the entire section without
disturbing the flow in the chamber. In addition, the spatial
resolution of the measurement has been greatly improved.
At present, we are comparing the results of ourexperiments with those of numerical simulations (Fig.3-5)
and verifying and upgrading the simulation codes. After
improving the experimental techniques, we intend to study
the phenomena of sea level change and upconing.

Reference
Oda, Y. et al., Optical Measurement of the Salinity Distribution by Saltwater Intrusion Experiment, Doboku Gakkai Ronbunshu Division C: Geotechnics,vol.67, no.2, 2011, p.186-197 (in Japanese).

44 JAEA R&D Review 2012