High-Level Waste Dissolution and Secondary Mineral(S) Formation and Dissolution

1987 ◽  
Vol 112 ◽  
Author(s):  
S. J. Zavoshy

AbstractA mathematical model of waste dissolution that includes the formation of secondary solid precipitate(s) explicitly is formulated. For a static type of experiment, the model indicates that the concentration of the highly soluble species compared to the concentration of the main component of the matrix is equal to their corresponding density in the matrix (i.e., congruent release). For low solubility nuclides, the solubility limit of precipitate(s) may be exceeded (depending on the waste composition) and a supersaturated solution is temporarily achieved. From the model, the concentration of radionuclides and mass flux at the waste form surface in a geologic repository is obtained. Numerical results (Sr-90) indicate that the liquid-surface concentration rises very rapidly after dissolution starts. In the first week, the solubility limit of Sr precipitate(s) is exceeded and Sr solid precipitate(s) will be formed. Subsequently the concentration decreases and becomes almost equal to the solubility limit and remains so for up to 1 year. It decreases later due to radioactive decay. After the first week, the surface concentration and surface mass flux are independent of the retardation coefficient throughout the leaching process. The mass transfer rate is determined by the matrix dissolution rate and not by the solubility of the precipitate.

1991 ◽  
Vol 235 ◽  
Author(s):  
E. Johnson ◽  
K. Hjemsted ◽  
B. Schmidt ◽  
K. K. Bourdelle ◽  
A. Johansen ◽  
...  

ABSTRACTIon implantation of lead or indium into aluminium results in spontaneous phase separation and formation of lead or indium precipitates. The precipitates grow in topotactical alignment with the matrix, giving TEM images characterized by moiré fringes. The size and density of the precipitates increase with increasing fluence until coalescence begins to occur. Implantations at elevated temperatures lead to formation of larger precipitates with well developed facets. This is particularly significant for implantations above the bulk melting point of the implanted species. Melting and solidification have been followed by in-situ TEM heating and cooling experiments. Superheating up to ∼ 50 K above the bulk melting point has been observed, and the largest inclusions melt first. Melting is associated with only partial loss of facetting of the largest inclusions. Initial growth of the inclusions occurs by trapping of atoms retained in supersaturated solution. Further growth occurs by coalescence of neighbouring inclusions in the liquid phase. Solidification is accompanied by a strong undercooling ∼ 30 K below the bulk melting point, where the smallest inclusions solidify first. Solidification is characterized by spontaneous restoration of the facets and the topotactical alignment.


Author(s):  
Amina Manel Bouaziz ◽  
M.N. Bouaziz ◽  
A. Aziz

Free convective of nanofluid inside dispersive porous medium adjacent to a vertical plate under the effects of the zero mass nanoparticles flux condition and the thermal and solutal dispersions is studied. Buongiorno's model revised is used considering Darcy and non Darcy laminar flows, and isothermal or convective flux outer the wall. Dimensionless governing equations formulated using velocity, temperature, concentration and nanoparticle volume fraction have been solved by finite difference method that implements the 3-stage Lobatto collocation formula. The numerical data obtained with semi or full dispersions cases are compared to predictions made using the non dispersive porous medium. Taking into account the dispersions, the influence of the zero mass nanoparticles flux condition is examined to test the validity of the control active nanoparticle assumption. It is found mainly that the thermal transfers can reach more than 100% in connection with the case where of a semi-dispersion of the porous medium is applied. Realistic condition, i.e. zero mass flux should be addressed for the heat transfer rate rather than the mass transfer rate, discovered markedly different to the active condition. This signifies the importance of considering the zero nanoparticles mass flux and dispersions in the performance characterization of nanofluid flow in porous media.


1997 ◽  
Vol 506 ◽  
Author(s):  
S.V. Stefanovsky ◽  
S.V. Ioudintsev ◽  
B.S. Nikonov ◽  
B.I. Omelianenko ◽  
T.N. Lashtchenova

ABSTRACTSince the early of the 1990s the method of inductive melting in a cold crucible (IMCC) has been applied at SIA “Radon” for production of various wasteforms, including glasses and Synroc-type ceramics. Sphene-based glass-ceramics composed of glass and crystalline phases were considered as appropriate wasteform for High Level Waste immobilisation. Investigation of two glass-ceramic specimens prepared with the IMCC has been performed using optical microscopy, XRD, SEM/EDS, and TEM methods. The samples produced consist of vitreous and crystalline phases. The vitreous phase consists of two varieties of glass formed by the immiscibility of the initial melt onto two separate liquids. One of the glasses is observed as spherical microinclusions in the matrix glass. The glass of the microspheres are differed from the matrix glass composition by higher contents of Ca, Ti, Ce, Sr, Zr (or Cr), while the matrix glass contains higher amounts of Si, Al, and alkalies. The crystalline phases with sphene- and perrierite-like structures have been also occurred. Their total quantity reaches up to 50 vol.%. The synthetic perrierite has similar unit-cell parameters with its natural mineral analogs with the only exception in two-fold value of c dimension. Zr, Ce, and Sr are incorporated into synthetic sphene and perrierite, while Cs is hosted by the glass phases.


1979 ◽  
Vol 101 (2) ◽  
pp. 201-206 ◽  
Author(s):  
Y. Taga ◽  
K. Nakajima

The effects of phosphorus on the friction and wear characteristics of Cu-5 at. percent Sn-P alloys containing 1–5 at. percent P were studied using a pin on disc apparatus. The results showed that the decrease in both the coefficient of friction and the rate of wear became conspicuous with the increase in quantity of Cu3P coexisting in the matrix; its amount increases with the content of phosphorus. The structural changes in the surface of the specimen due to heating in a vacuum were observed by using Auger electron spectroscopy and X-ray photoelectron spectroscopy. It was seen that the surface concentration of phosphorus strongly increased after heating at 573K, whereas the diffusion of tin atoms was markedly retarded. It was concluded from these results that the behavior of phosphorus atoms in the surface during sliding played an important role in the friction and wear characteristics of Cu-Sn-P alloys.


1996 ◽  
Vol 465 ◽  
Author(s):  
T. H. Pigford ◽  
E. D. Zwahlen

ABSTRACTRecent proposals for a new U.S. standard for high-level waste disposal would limit the average dose to individuals in the vicinity surrounding a geologic repository. This would be a new approach to protecting the public from environmental releases of radioactivity. Heretofore, criteria adopted for geologic disposal have limited the reasonable maximum exposure to a future hypothetical individual. Here we present quantitative analyses of the relation between maximum exposure and vicinity-average exposure, resulting from future human use of ground water contaminated by radioactive releases from a repository.Estimating the vicinity-average exposure would require postulates and guesses of location and habits of future people. Exposure probabilities postulated by others show that proposed dose limit to the vicinity-average individual would be a far more lenient standard than the traditional dose limit to reasonably maximally exposed individuals. The proposed vicinity-average dose limit would allow far greater concentrations of contaminants in ground water than would be allowed by normal standards of ground water protection. A safety standard that limits vicinity-average exposure should also include limits on maximum exposure.


1988 ◽  
Vol 3 (2) ◽  
pp. 257-266 ◽  
Author(s):  
V. J. Laraia ◽  
William C. Johnson ◽  
P. W. Voorhees

A treatment of diffusion limited growth of a coherent spherical precipitate into supersaturated solution is presented. It is found that the growth kinetics are affected by dilatational coherency strains and by compositionally induced strains in the matrix phase. Numerical solutions to the time-dependent problem are obtained and are compared to the quasistationary solution. The parabolic growth coefficient is a function of the transformation strain, partial molar volumes of the components, elastic constants in each phase, interfacial compositions and far-field composition while, in contrast, the growth coefficient in the absence of stress is a function only of the reduced supersaturation. Elastic effects shift the interfacial concentration of the matrix in the direction of the far-field concentration, reducing the effective driving force for growth. At the same time, compositionally induced strains increase the diffusive flux, increasing the growth rate.


Author(s):  
Si Y. Lee

The engineering viability of disposal of aluminum-clad, aluminum-based spent nuclear fuel (Al-SNF) in a geologic repository requires a thermal analysis to provide the temperature history of the waste form. Calculated temperatures are used to demonstrate compliance with criteria for waste acceptance into the geologic disposal system and as input to assess the chemical and physical behavior of the waste form within the Waste Package (WP). The leading codisposal WP design proposes that a central DOE Al-SNF canister be surrounded by five Defense Waste Process Facility (DWPF) glass log canisters, that is, High-level Waste Glass Logs (HWGL’s), and placed into a WP in a geologic disposal system. A DOE SNF canister having about 0.4318m diameter is placed along the central horizontal axis of the WP. The five HWGL’s will be located around the peripheral region of the DOE SNF canister within the cylindrical WP container. The codisposal WP will be laid down horizontally in a drift repository. In this situation, two waste form options for Al-SNF disposition are considered using the codisposal WP design configurations. They are the direct Al-SNF form and the melt-dilute ingot. In the present work, the reference geologic and design conditions are assumed for the analysis even though the detailed package design is continuously evolved. This paper primarily dealt with the thermal performance internal to the codisposal WP for the qualification study of the WP containing Al-SNF. Thermal analysis methodology and decay heat source terms have been developed to calculate peak temperatures and temperature profiles of Al-SNF package in the DOE spent nuclear fuel canister within the geologic codisposal WP.


Author(s):  
Robin Nazzaro ◽  
William Swick ◽  
Nancy Kintner-Meyer ◽  
Thomas Perry ◽  
Carole Blackwell ◽  
...  

The U.S. Department of Energy (DOE) oversees one of the largest cleanup programs in history—the treatment and disposal of 356,260 cubic meters of highly radioactive nuclear waste created as a result of the nation’s nuclear weapons program. This waste is currently stored at DOE sites in the states of Washington, Idaho, and South Carolina. In 2002, DOE began an accelerated cleanup initiative to reduce the estimated $105-billion cost and 70-year time frame required for the program. The U.S. General Accounting Office (GAO), an agency of the U.S. Congress, evaluated DOE’s high-level waste program to determine the status of the accelerated cleanup initiative, the legal and technical challenges DOE faces in implementing it, and any further opportunities to improve program management. GAO found that DOE’s initiative for reducing the cost and time required for cleaning up high-level waste is evolving. DOE’s main strategy continues to include concentrating much of the radioactivity into a smaller volume for disposal in a geologic repository. Under the accelerated initiative, DOE sites are evaluating other approaches, such as disposing of more of the waste on site or at other designated locations. DOE’s current savings estimate for these approaches is $29 billion, but the estimate is not based on a complete assessment of costs and benefits and has other computational limitations. For example, the savings estimate does not adequately reflect the timing of when savings will be realized, which distorts the actual amount of savings DOE may realize. DOE faces significant legal and technical challenges to realize these savings. A key legal challenge involves DOE’s authority to decide that some waste with relatively low concentrations of radioactivity can be disposed of on site. A recent court ruling against DOE is a major threat to DOE’s ability to meet its accelerated schedules. A key technical challenge is DOE’s approach for separating waste into high-level and low-activity portions. At the Hanford Site in Washington State, DOE is planning to implement such a method that will not be fully tested until the separations facility is constructed. This approach increases the risk and cost of schedule delays compared to fully testing an integrated pilot-scale facility. However, DOE believes the risks are manageable and that a pilot facility would unnecessarily delay waste treatment and disposal. DOE has opportunities to improve management of the high-level waste program. When it began the initiative to reduce costs and accelerate the high-level waste cleanup schedule, DOE acknowledged it had systematic problems with the way the program was managed. Although DOE has taken steps to improve program management, GAO has continuing concerns about management weaknesses in several areas. These include making key decisions without a sufficiently rigorous supporting analysis, incorporating technology before it is sufficiently tested, and pursuing a “fast-track” approach of simultaneous design and construction of complex nuclear facilities. DOE’s management actions have not fully addressed these weaknesses.


2007 ◽  
Vol 261-262 ◽  
pp. 47-54 ◽  
Author(s):  
Ya. Matychak ◽  
V. Fedirko ◽  
A. Prytula ◽  
I. Pohreljuk

This paper deals with the development of a theoretical model for reactive diffusion during new-phase formation when its volume fraction is negligible. A mathematical analysis of this physical model is carried out and an equation for mass balance at the interface is derived. The latter includes a correlation between the transport flux of nitrogen to the surface (J1) followed by both diffusion dissolution (Jdiff) and segregation at the defects due to chemical interaction with metal atoms (JR.). The analytical-experimental procedure for the determination of the coefficient of mass transfer which controls the density of the outer flux (J1) is proposed. The effect of temperature and time upon the kinetics of nitriding is estimated. The additive dependences of both the nitrogen concentration in a distant band and the depth of the diffusion zone upon time, temperature and pressure during isothermal exposure are shown. Peculiarities of the nitrogen concentration distribution at the interface, as a function of temperature of isothermal exposure, are found. It is established that, for a given duration of exposure, an increase in temperature does not always tend to increase the concentration of the impurity at the interface. This is caused by a more intense diffusion flow into the matrix. In accordance with the concept of time-independence of the surface concentration during saturation, the diffusion fluxes Jdiff → ∞ with decreasing time (τ→ 0). This artefact is eliminated by an equation, derived here, for a diffusion flux defined using new boundary conditions. The equations for calculating mass increase include the effect of segregation to the surface, which causes a deviation from a parabolic dependence of the mass change.


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