Long Term Diffusion (LTD) Work Planning - Diffusion Processes Study

This task involves the analysis of 19 archived core sections for evidence of matrix diffusion of the radionuclide ¹³⁷Cs from the earlier Radionuclide Migration Programme (RMP).

The specific aims of this work are:

• Determine the presence of ¹³⁷Cs in the matrix
  
• Observe any mineralogical controls on the route of Cs migration
  
• Define any structural controls on Cs migration
  
• Development of methodologies for determining formation factor
  
• Increase the understanding of the behaviour of radionuclides at the transition of the advectively dominated part of the flow field towards the rock matrix
  

Background
During earlier work at the Grimsel Test Site (GTS), the radioactive tracer ¹³⁷Cs was injected into a water-conducting fracture to examine its in situ retardation properties. The behaviour of ¹³⁷Cs in the flow field was then derived from examination of the radionuclide breakthrough curves. However a very small amount of the radionuclide tracer remained within the flow field.

Around five years later, a different dipole in the same water-conducting fracture was used in another experiment with sorbing tracers ( Radionuclide Migration Programme - RMP ). This new flow field was then injected with resin and subsequently overcored for detailed analysis of the sites of radionuclide retardation. The initial analysis showed the ¹³⁷Cs from the earlier experiment within the granodiorite matrix surrounding the fracture.

Initial Studies
The presence of ¹³⁷Cs in the samples was determined via gamma spectrometry measurements of the remaining 19 core slices. In addition to the gamma spectrometry, the slices have also been photographed in normal and UV light and the spatial distribution of the radionuclides (in the fracture and in the matrix) has been determined using a state-of-the-art beta autoradiography scanner (Fuji film BAS 1800i).

Res Möri (Geology and Geochemistry AG) and Thomas Huegel (GI) perform high resolution photography of the rock samples with the 8 mega pixel Fujifilm CAMILLA system.

Max Rüthi, Paul Scherrer Institute (PSI) checks the gamma spectrometer.

Initial Results
The images produced from the beta autoradiography have suggested radionuclides may be found away from the main shear zone.

Rock slab with resin filled advective flow paths (grey) and beta-autoradiograph of the same flow path. The beta-autoradiograph of a thin section shows activity along grain-boundary pores outwith the flow path.

Samples were then very carefully sub-sampled and a series of samples at increasing depth from the shear zone were analysed with high precision gamma spectrometry at the Paul Scherrer Institute. These results suggest that ¹³⁷Cs may have diffused up to 5 cm into the rock matrix.

Activity profiles of 137Cs and 60Co in the rock matrix bordering a flow channel (green). The autoradiograph in the background indicates activity in the flow paths and grain-boundary pores in the adjacent matrix

Another sample was send to Dr Hu at Lawrence Livermore National Laboratory (LLNL). The samples was analysed using a very high precision instrument called Laser - Ablation Inductively Coupled Plasma - Mass Spectrometry (LA-ICP-MS). This allowed the activitiy of ²³⁷Np to be determined at increasing depth from the shear zone. This technique uses a laser to sample very precisely and allows high resolution profiles to be produced.

LA-ICP-MS produced profile at increasing distance from shear zone suggesting diffusion of ²³⁷Np up to 2 cm from the shear zone