방사성 폐기물의 유리화

포항공과대학교
Pavel Hrma

페이스북 트위터 카카오톡 네이버밴드 링크복사

주제분류

공학 >정밀ㆍ에너지 >원자력공학
강의학기

2013년 1학기

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3,343

This graduate course teaches students the key aspects of the vitrification of radioactive wastes.

수강안내 및 수강신청
※ 수강확인증 발급을 위해서는 수강신청이 필요합니다

Waste glass requirements

차시별 강의

1.	Waste glass requirements	The purpose of nuclear waste vitrification is explained. The need to immobilize the hazardous and radioactive materials is stressed. The three major issues are identified: 1) glass formulation for easy processability and high product quality, 2) rapid glass making in an electrical melter, 3) understand the long-term release of radionuclides from the waste glass.
	Vitrification technology overview	The history of legacy waste from Pu production is reviewed and historical development of waste vitrification in key countries is outlined. Basic methods of vitrification are elucidated, including the Korean induction heated hot crucible melter.
2.	Glass properties overview	The requirements imposed on waste glass are stated in terms of melt and glass property constraints. The individual properties are reviewed and characterized.
	Waste glass formulation	Selection of glass-forming and modifying additives to make processable and acceptable glass is explained and mathematically formulated. Waste loading maximization is related to waste cleanup lifecycle and repository cost.
3.	Viscosity and electrical conductivity	The key processing properties, viscosity and electrical conductivity, are characterized as functions of temperature and composition. Various ways of these functional representations are described.
	Glass transition temperature	Glass transition is explained and the glass transition temperature (Tg) is defined. The relevance of Tg for waste glasses with respect to radiolytic heating and glass crystallization is clarified.
4.	Glass crystallization	Basic concepts of nucleation and growth of crystals are introduced and the most frequently encountered crystals are reviewed. Nucleation rate, growth rate, and dissolution rate are defined and both graphically and mathematically represented.
	Liquidus temperature	While phase equilibria must not be neglected, melter processing occurs out of equilibrium, and thus kinetics is of the primary importance. Therefore, kinetic equations and TTT diagrams are shown. Liquid-liquid immiscibility is also mentioned.
5.	Glass corrosion	The complexity of glass corrosion requires deep knowledge of diffusion, interfacial heterogeneous reaction, and hydrothermal precipitation of minerals in highly multicomponent media. Basic concepts and relations are introduced.
	Chemical durability	Only short-term resistance of glass against corrosion (durability) can be measured in the laboratory using a series of standardized tests. Long-term (over 105 to 106 years) durability can only be assessed.
6.	crystallization and chemical durability	Waste glass crystallization must not degrade glass durability. Therefore, canister-centerline cooled glasses are tested pro durability. Out of many crystalline forms, nepheline appears the most insidious.
	Waste glass processing	Waste glass technology differs from commercial glass technology in many aspects: the waste is given, only additive raw materials can be selected; feed may be charged in the form of slurry; fining is not essential; etc.
7.	Melter feed makeup	Mineral form of additives and particle size influence cold cap stability and the rate of melting. Drainage of fluxes can destabilize the cold cap, leading to cold cap freezing. Evolved gases produce foam and cavities under the cold cap.
	Melting reactions	Melting reactions release copious gasses and produce intermediate crystalline forms. Eutectic melts of molten salts attack refractory particles. Glass-forming melt tends to trap gases, creating foam. Melter feed reacts while moving down across the cold cap as the heat is conducted upward.
8.	Batch calculation	Glass component can be present in more than one batch material. Matrix calculus is used for designing complex batches. Back calculation is essential. Feed simulants must be carefully designed.
	Thermal analysis	Thermogravimetric analysis (TGA) and differential scanning calorimetry (DCS) measure the kinetics of batch reaction. Batch reactions are multiple and overlapping. Evolved glass analysis (EGA) allows gas-evolving reactions to be identified.
9.	X-ray diffraction and microscopy	Optical and electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), and X-ray diffraction (XRD) are used for the identification of crystalline phases in glasses and in glass batches (melter feeds) during their conversion to molten glass.
	Segregation of molten salts	As nitrates and carbonates decompose, areas occupied by molten salts shrink leaving behind sulfates, chromates, pertechnetates, chlorides). These sparsely-soluble components accumulate in droplets and are transported to the melt surface.
10.	Silica (quartz) dissolution	Quartz, the major feed component, initially reacts with molten salts and then dissolves in melt via diffusion. Diffusion is affected by the formation, growth and motion of gas bubbles. Large grains tend to cluster.
	Volatilization	Volatile species evaporate from the cold cap and escape from the melt to bubbles and the atmosphere. Their Henry’s coefficients are large. The rate of volatilization is controlled by diffusion both in gas and melt. Buoyancy-driven and surface-tension-gradient-driven convection in melt enhance volatilization.
11.	Redox equilibria	Multivalent species are common in waste glasses. Redox reactions generate oxygen bubbles and metal droplets. Redox ratio increases as temperature increases and optical basicity decreases.
	Foaming	Batch gases generate primary foam, redox reaction lead to secondary foam. Foam accumulated under the cold cap slows the melting process by reducing heat transfer. Foam collapses through met drainage and breaking the liquid films separating foam cells.
12.	Settling of solids	Solids of crystalline phases (spinel) can settle on the melter bottom and can block the melt discharge. Crystals settle while growing and dissolving in the melt and form a dense sludge that can be electrically conductive.
	Glass melters	Joule-heated melters use resistance heating via inconel electrodes. Induction-heated melters generate heat electromagnetically either in the crucible wall or within the melt.
13.	Modeling	The basic balance laws with constitutive relations form a system of coupled field equations to be solved for problems defined by the boundary conditions. Mathematical models of melters display the electric, temperature, and velocity fields in molten glass and furnace atmosphere. Models can also include redox equilibria, bubble behavior, and solid particle growth, dissolution, and settling.
	Refractory end electrode corrosion	Molten glass and salts corrode refractory walls of melters by dissolving refractory materials via diffusion under free or forced convection. Intergranular diffusion is also common. Surface tension gradients drive melt-line corrosion and bubble/droplet drilling.
14.	Revision	Topics selected by students are further elucidated.