Long Term Diffusion (LTD)

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Long Term Diffusion project (LTD) Experiment

Circulation of a cocktail of sorbing, weakly sorbing and non-sorbing radionuclides (³H, ²²Na, ¹³¹I, ¹³⁴Cs) in the monopole of WP-1 was started on 7th June, 2007.

Water samples retrieved twice a month and more frequently in the first two months were analysed at the Paul Scherrer Institute (PSI) to estimate the amount of sorption and diffusion with time.

Start of radionuclide injection on 7th June, 2007

November 2009

Overcoring and subsampling of the LTD in-situ experiment

Overcoring and subsampling of the in-situ diffusion experiment of WP-1 after almost 800 days of continuous monitoring and circulation of radionuclides in a packed-off interval were successfully carried out in November 2009

Figure 1: Overcoring of LTD monopole

Figure 2: LTD overcore prior to subsampling

Detailed analyses of the subsamples from WP-1 are currently ongoing at HYRL in Finland and NRI in the Czech Republic. Preliminary results are shown in Figure 3.

Figure 3: Diffusion profiles in Grimsel granite based on preliminary analyses carried out by NRI after almost 800 days of circulation of H-3 (green), Na-22 (pink) and Cs-134 (blue).

The final results from the analyses of the subsamples will be used in post-mortem modelling exercises carried out by four teams later this year and compared with predictive models developed prior to circulation of the radionuclides in order to re-examine current diffusion models used in safety analysis and performance assessments.

Long Term Diffusion project (LTD) Experiment

As part of Phase 2 of the LTD project, a second in-situ experiment has been excavated and is currently undergoing long-term pressure monitoring. During the recent LTD partner meeting in Mizunami, Japan on 27-28th May 2010 the LTD partners selected H-3, Cl-36, Na-22, Ba-133 and Cs-134 as well as stable Se(VI) for use in the second long-term in-situ diffusion test. Several criteria were used in the selection including length of half-lives, safety aspects, feasibility, results from the first in-situ experiment as well as the need to study of more safety relevant species namely Cl-36 and Se. Other work packages in Phase 2 include mock-up lab tests to study Cl-36, H-3 and Se and the application of Positron Emission Tomography (PET) to give insight into heterogeneities of diffusive transport away from a fracture in order to help provide experimental evidence quantifying effective surface area (used for safety assessment).

LTD partner meeting hosted by JAEA in Mizunami, Japan 27th – 28th May 2010. The meeting included a visit to JAEA's underground research laboratory (MIU) in granite. From left to right are Akira Hayano and Yukio Tachi (JAEA), Jussi Ikonen and Marja Siitari-Kauppi (University of Helsinki), Vaclava Havlova (NRI) and Andrew Martin (Nagra).

Long Term Diffusion project (LTD) Experiment

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

Long Term Diffusion project (LTD) Experiment