Bentonite-Kaolinite-sand mixtures as backfill material for nuclear fuel waste disposal vault.

Abstract

The aim of the present investigation is to find a suitable backfill material to be used in a nuclear fuel waste–disposal vault. The requirements for such backfill material with regard to various characteristics-such as permeability, shrinkage, compactibility, erodibility, swelling, swelling pressure and compressibility-are stringent and sometimes contradictory. One must arrive at optimum conditions such that the required specifications for all characteristics of the backfill material are satisfied. While any natural soil or mixtures of soils could be examined for their suitability as backfill material, doing so becomes tedious and time consuming. To overcome this, an attempt has been made in this thesis to study known mixtures of bentonite, kaolinite and sand, in different proportions, to examine their suitability as backfill materials. This makes it easier to compare the properties of any natural soil with those of known standard mixtures and assess suitability. Chapter 1 presents the introduction, discussing aspects such as groundwater contamination and the production of toxic radon gas from nuclear fuel waste. Sites identified in the literature as safe for permanent isolation of various levels of nuclear fuel wastes are also discussed. Chapter 2 provides a detailed literature review regarding background information on nuclear fuel wastes, their sources and the basic principles involved in their disposal. It summarises literature on high level waste, canisters, buffer materials and backfill materials, as well as the complex and diverse geotechnical requirements that backfill material must satisfy. It also reviews mechanisms controlling volume change in soils, swelling potential and swelling pressure, permeability, shrinkage behaviour and erodibility-key geotechnical properties for backfill performance. Chapter 3 details the materials used in this investigation-bentonite, kaolinite and sand-and the test procedures for determining Atterberg limits, free swell indices, compaction characteristics, swelling and compressibility behaviour, and erodibility. Chapter 4 presents discussion of results. Key findings include: • Index properties of bentonite–kaolinite–sand mixtures (liquid limit, plastic limit and plasticity index) computed from individual values were higher than those obtained experimentally. Free swell index exhibited similar behaviour. • Shrinkage limit initially increased with liquid limit/bentonite content up to a certain value, after which it decreased. • Correlations were established between free swell index and liquid limit, and free swell index and bentonite content. • Loose mixtures with <10% bentonite compressed even at a nominal pressure of 6.25 kPa during inundation, while mixtures with 10% bentonite invariably showed swelling. • Time–compression curves were almost linear at low pressures and S shaped (Casagrande type) at higher pressures. • Permeability coefficient decreased by nearly an order of magnitude with decrease in void ratio (0.6–1.6) and increase in bentonite content (15–30%). • Correlations were attempted between compression characteristics and liquid limit/bentonite content. • A strong linear correlation existed between reciprocal of maximum dry density and optimum moisture content, consistent with published literature. • Compaction curves on the wet side of optimum generally lay between 0–4% air voids line. • Pinhole test results showed the mixtures were prone to erosion, with eroded material largely comprising kaolinite (silt sized fraction). • Polyvinyl alcohol (PVA) was found effective in binding particles and preventing erosion. The chapter also evaluates the influence of PVA on index properties, free swell index, compaction, swelling potential, swelling pressure and compressibility. Chapter 5 provides a general discussion of results with specific reference to suitability of bentonite–kaolinite–sand mixtures as backfill materials. Based on results, recommended proportions of bentonite and kaolinite for nuclear waste–disposal backfill are outlined. Major conclusions of the investigation are also summarised.