Carbon-14 and Iodine-129 Migration in Cement (CIM)

The working programme comprises of the following steps:

Step 1: Finalise experimental design

Step 2: Drill needed boreholes and install initial condition monitoring

Step 3: Develop and construct the experimental equipment (packer system, surface equipment)

Step 4: Emplace circulation interval

Step 5: Start circulation and monitoring

Step 6: Overcore and post-mortem analysis

The project relies largely on experience acquired with previous projects using radionuclides at GTS (LTD, CFM) and will benefit from the lessons learned of these projects with respect to borehole and surface equipment and sampling.

Design

• A large diameter borehole (Ø 388 mm) backfilled with OPC mortar is being used as a proxy for cementitious material used as backfill and/or support material in L/ILW or TRU waste repositories.
  
  
• In the centre of the mortar backfill, a small diameter (Ø 56 mm) borehole was drilled in November 2019 and equipped with a multi-packer system for the circulation of the test cocktail. Figure 1 shows the schematic setup in a sub-vertical borehole. The cocktail will be based on cement water and be prepared with the ions listed in Table 1.
  
  
• The circulation interval will be connected to surface equipment, allowing for continuous monitoring of the fluid composition by online sensors and sampling. The monitoring will focus on activities of the injected radionuclides, chemistry, and microbiology.
  
  
• Three monitoring boreholes drilled at relatively short distance (ca 15 cm) from the circulation borehole will allow to monitor the progress of the weakest sorbing compounds in the rock.
  
  
• Based on the results from the experiment monitoring and discussions between the partners, the in-situ experiment will be overcored to allow for the analysis of diffusion profiles in the mortar and/or in the rock.

Table 1: Isotopes (radioactive and stable) foreseen for the circulation in the CIM experiment.

HPF backfilled borehole

Four vertical or slightly inclined boreholes resulting from the overcoring of the HPF experiment at GTS have been backfilled with OPC mortar in 2004 (Figure 2).

One of these boreholes is being used as circulation borehole for the CIM experiment. The borehole and the depth of the circulation interval was selected in order to position the latter in an area of dense matrix with no distinct water conducting features. The selection was based on available geological logs from the HPF project. Three monitoring boreholes were drilled at short distance (ca 15 cm) from the source borehole.

Figure 1: Setup of the circulation in a sub-vertical borehole

Figure 2:          Schematic 3D view of the HPF and planned CIM borehole relative to the AU-126 shear zone and AU tunnel at the GTS

Carbon-14 and Iodine-129 released from repositories of L/ILW and TRU wastes typically contribute most to dose rates over the long-term (i.e. 10 ka to 100 ka) according to performance assessment (PA) calculations. This is primarily due to the combination of (i) high solubility and (ii) low sorption properties of the chemical forms that the two radionuclides are expected to exist in. In the case of C-14, experimental work on the speciation of carbon during corrosion of activated and un-activated steel under anoxic conditions similar to that of an emplacement tunnel showed the formation of oxygenated and reduced hydrocarbons, including carboxylic acids and CH4.

Methane is not expected to react with cementitious material or with the host rock. Uncertainties remain, however, on the retardation of carboxylic acids in clay and cementitious materials. In the case of formic acid, so far no retardation was expected in near-neutral environments; however laboratory experiments at PSI showed evidence of weak sorption of formic acid on cementitious material.

I-129 originates largely from reprocessed waste and is expected to occur mostly as iodide. Although it is planned that large amounts of cementitious material will be used in L/ILW and TRU waste repositories, there remains much uncertainty on both the release and in-situ retardation of I-129 as well as C-14 species in cementitious materials, especially in naturally aged cement on the field scale.

Aims of the experiment

The following aims were developed based on the wishes from the current partners (NUMO, RWM, SURAO and Nagra):

• Simulate the transport of C-14 and I-129 through aged cementitious backfill of a L/ILW or TRU waste repository and into the saturated host rock
• Provide confirmation on the effect of cementitious on material retarding C-14 and I-129
• Further improve the process understanding of the behaviour of C-14 and I-129 under real in-situ conditions
• Develop a method to upscale the results obtained from extensive laboratory based migration studies to the field/disposal tunnel scale.