A Talc-Based Cement-Poor Concrete for Sealing Boreholes in Rock

Abstract

<p class="MsoNormal"> <span lang="EN-US">Deep investigation boreholes in crystalline rock for site selection of repositories for high-level radioactive waste are proposed to be sealed by installing a series of dense concrete and clay plugs. These should prevent radionuclides from leaking canisters at depth to migrate to the biosphere through the holes. The concrete seals will be installed where the holes intersect water-bearing fracture zones to serve as stable and low-permeable supports for adjacent clay plugs. Low porosity and microstructural stability must be guaranteed for many thousands of years and ordinary Portland cement with organic superplastizer will not fulfill the requirements since the high pH will cause degradation of contacting clay and the organic additive can produce colloids with a capacity to carry radionuclides up to the biosphere. Very cement-poor concrete (<8%) based on low-pH cement and with talc as plasticizer is an option but it matures more slowly, which requires that the construction of seals is made so that sufficient bearing capacity for carrying overlying clay seals is reached.</span><span lang="EN-US"><o:p></o:p></span> </p

Optimal ways of disposal of highly radioactive waste

“…An optimal design principle proposed here is to combine ANDRA's version with the LRDT principle shown in Figure 11, utilizing also newly developed concrete material for filling the space between the large perforated tube, which is preferably made of Navy Bronze with a high content of copper, and the surrounding rock. The concrete has 10% talc as superplasticizer and a content of 5% -10% low-pH Merit 5000 cement, which gives a pH of about 10 and a fluidity that is suitable for pumpingit onsite through pipes placed in a shallow groove in the floor while letting air and water out through a pipe at the crown [13]. The sleeve is also made of Navy Bronze for the sake of chemical stability.…”

Medium-Deep or Very Deep Disposal of Highly Radioactive Waste?

“…The main criteria for the concrete seals in VDH are: (1) sufficient fluidity forcasting; (2) sufficient bearing capacity for carrying the overlying super containers; (3) lower hydraulic conductivity than of the fracture zone to be sealed; (4) insignificant chemical impact on contacting clay seals. They are fulfilled by using talc-based concrete with a density of about 2,000 kg/m 3 and a low content of very fine-grained low-pH cement (< 8 weight percent of the solids) [23].…”

Proportioning of Cement-Based Grout for Sealing Fractured Rock-Use of Packing Models

“…The rheological properties strongly depend on the W/C ratio, and high fluidity is, in principle, not favoured by a high packing density. In practice, superplasticizers therefore have to be added for which the hydrophobic mineral talc was used [12,18]. Palygorskite was used for early strengthening of the grouts in certain tests.…”

“…The hardened grouts should be less permeable than the surrounding rock matrix and to have a granular composition that makes them erosion-resistant and serve as filters for minimizing migration and loss of fines especially where high hydraulic gradients prevail [9]. Figure 1 illustrates a deep hole bored in rock as part of the site investigation for location and construction of an underground repository for disposal of highly radioactive waste (HLW), It must ultimately be effectively sealed for avoiding transport of possibly released radionuclides to the biosphere [9,10,12]. This is achieved by installing dense, expandable clay seals where the rock is poor in fractures, providing axial tightness, and casting concrete where the rock is rich in fractures since clay seals placed here would be eroded and dispersed.…”