Stochastic analysis of radioactive waste package performance using first-order reliability method

In order to license an underground radioactive waste repository, it is important to demonstrate regulatory compliance with authoritative regulations. Also it is evidence from NRC's criteria for waste package performance that a stochastic analysis is necessary to provide that these criteria can be met with confidence. The first-order reliability method is an attractive tool to the stochastic analysis and particularly useful when statistical information is incomplete, as is common for problems occurring in the subsurface environment. The method is based on using the first-order Taylor series expansion at a specific linearization point to calculate a measure of reliability. Results from a first-order reliability analysis include an estimate of the probability of exceeding a specified performance criteria and measures of the sensitivity of the stochastic solution to the changes in input random variables and their statistical moments. The method of stochastic analysis is illustrated by analyzing the canister corrosion in a radioactive waste package.     

Stochastic Modeling Concepts in Groundwater and Risk Assessment: Potential Application to Marine Problems

Parameter uncertainty is ubiquitous in marine environmental processes. Failure to account for this uncertainty may lead to erroneous results, and may have significant environmental and economic ramifications. Stochastic modeling of oil spill transport and fate is, therefore, central in the development of an oil spill contingency plan for new oil and gas projects. Over the past twenty years, several stochastic modeling tools have been developed for modeling parameter uncertainty, including the spectral, perturbation, and simulation methods. In this work we explore the application of a new stochastic methodology, the first-order reliability method (FORM), in oil spill modeling. FORM was originally developed in the structural reliability field and has been recently applied to various environmental problems. The method has many appealing features that makes it a powerful tool for modeling complex environmental systems. The theory of FORM is presented, identifying the features that distinguish the method from other stochastic tools. Different formulations to the reliability-based stochastic oil spill modeling are presented in a decision-analytic context.     

Hydrolysis kinetics in anaerobic degradation of particulate organic material: an overview

The applicability of different kinetics to the hydrolysis of particulate organic material in anaerobic digestion is discussed. Hydrolysis has traditionally been modelled according to the first-order kinetics. For complex substrate, the first-order kinetics should be modified in order to take into account hardly degradable material. It has been shown that models in which hydrolysis is coupled to the growth of hydrolytic bacteria work well at high or at fluctuant organic loading. In particular, the surface-related two-phase and the Contois models showed good fits to experimental data from a wide range of organic waste. Both models tend to the first-order kinetics at a high biomass-to-waste ratio and, for this reason, they can be considered as more general models. Examples on different inhibition processes that might affect the degradation of solid waste are reported. Acetogenesis or methanogenesis might be the rate-limiting stages in complex waste. In such cases, stimulation of hydrolysis (mechanically, chemically or biologically) may lead to a further inhibition of these stages, which ultimately affects hydrolysis as well. Since the hydrolysis process is characterized by surface and transport phenomena, new developments in spatially distributed models are considered fundamental to provide new insights in this complex process.     

SLFP: a stochastic linear fractional programming approach for sustainable waste management

A stochastic linear fractional programming (SLFP) approach is developed for supporting sustainable municipal solid waste management under uncertainty. The SLFP method can solve ratio optimization problems associated with random information, where chance-constrained programming is integrated into a linear fractional programming framework. It has advantages in: (1) comparing objectives of two aspects, (2) reflecting system efficiency, (3) dealing with uncertainty expressed as probability distributions, and (4) providing optimal-ratio solutions under different system-reliability conditions. The method is applied to a case study of waste flow allocation within a municipal solid waste (MSW) management system. The obtained solutions are useful for identifying sustainable MSW management schemes with maximized system efficiency under various constraint-violation risks. The results indicate that SLFP can support in-depth analysis of the interrelationships among system efficiency, system cost and system-failure risk.     

Application of a Proposed Generic LLW Waste Package Analysis With Changing Infiltration in a Humid Environment

Corrosion of carbon steel boxes filled with low‐level radioactive waste and buried within a near surface disposal facility in a humid environment was evaluated using an integrated systems approach framework. The time to hydraulic failure from initial burial to development of holes through the wall of a given waste package from pitting corrosion was calculated for four corrosion scenarios under two different corrosion cases. The two corrosion cases chosen were a constant rate of corrosion and a slowing rate of corrosion. Corrosion rates were estimated for carbon steel buried in soil from several historical studies and related to the corrosivity and aeration profile of the soil. The scenarios were chosen to represent a range of possible conditions at current and future U.S. Department of Energy disposal facilities. For each scenario, once the time to hydraulic failure had been estimated, the amount of liquid present in each waste package at the time of failure was calculated as an estimate of leachate available for subsurface transport. The Savannah River E‐Area Engineered Trench was used as a basis for the hypothetical disposal facility. © 2016 Wiley Periodicals, Inc.     

Cement/clay interactions-- a review: experiments, natural analogues, and modeling

The concept of storing radioactive waste in geological formations calls for large quantities of concrete that will be in contact with the clay material of the engineered barriers as well as with the geological formation. France, Switzerland and Belgium are studying the option of clayey geological formations. The clay and cement media have very contrasted chemistries that will interact and lead to a degradation of both types of material. The purpose of this review is to establish an exhaustive list of laboratory experiments so as to identify the reaction sequences in the evolution of both the clay minerals and accessory minerals during their alteration in an alkaline environment. We review the data on clay dissolution kinetics in this environment, and include an invaluable study of natural analogues that allow one to correlate the phenomena in time. The available data and experiments make it possible to construct predictive numerical models. However, as the quality of the data is inhomogeneous, we recommend a continuation of the thermodynamic and kinetic data acquisition. It is obvious that the numerical modeling of the alkaline disturbance will be more relevant if it can combine the advantages of the different detailed models: mineralogical completeness, combined modeling of the clay and cement media, evolution of the porosity, consideration of the pCO2 and all the surface reactions.     

Multi-regime transport model for leaching behavior of heterogeneous porous materials

Utilization of secondary materials in civil engineering applications (e.g. as substitutes for natural aggregates or binder constituents) requires assessment of the physical and environment properties of the product. Environmental assessment often necessitates evaluation of the potential for constituent release through leaching. Currently most leaching models used to estimate long-term field performance assume that the species of concern is uniformly dispersed in a homogeneous porous material. However, waste materials are often comprised of distinct components such as coarse or fine aggregates in a cement concrete or waste encapsulated in a stabilized matrix. The specific objectives of the research presented here were to (1) develop a one-dimensional, multi-regime transport model (i.e. MRT model) to describe the release of species from heterogeneous porous materials and, (2) evaluate simple limit cases using the model for species when release is not dependent on pH. Two different idealized model systems were considered: (1) a porous material contaminated with the species of interest and containing inert aggregates and, (2) a porous material containing the contaminant of interest only in the aggregates. The effect of three factors on constituent release were examined: (1) volume fraction of material occupied by the aggregates compared to a homogeneous porous material, (2) aggregate size and, (3) differences in mass transfer rates between the binder and the aggregates. Simulation results confirmed that assuming homogeneous materials to evaluate the release of contaminants from porous waste materials may result in erroneous long-term field performance assessment.     

Risk assessment for shallow land burial of low level radioactive waste

A composite model for the risk assessment is developed for the shallow-land burial of low-level radioactive waste. The composite model is composed of a source term, repository failure model, geosphere model, biosphere model, and finally, a dose-and-health-effects model. Uncertainty and sensitivity analyses are also carried out. In addition to the time-dependent annual release rate and dose rate for each nuclide, the ultimate risk in terms of cancer death rate is estimated.The results show that the highest value of the annual dose for the repository is less than the individual dose limit to the human body, and the calculated highest cancer death rate is much lower than other various risks. The highest value of the total annual dose appears at about 100 years after disposal, and at this time, dominant nuclides are Ni-63 and Cs-137.From the results of uncertainty and sensitivity analyses, it is found that the uncertainty associated with the public risk is strongly sensitive to the volume flow rate and irrigation rate of surface water and the retardation coefficient of geological structure.It is concluded that the methodology of simplified mathematical modeling for the risk assessment can be an effective tool for the decision making of the radioactive waste repository selection and the management of the repository system.     

Colloids in the mortar backfill of a cementitious repository for radioactive waste

Colloids are present in groundwater aquifers and water-permeable engineered barrier systems and may facilitate the migration of radionuclides. A highly permeable mortar is foreseen to be used as backfill for the engineered barrier of the Swiss repository for low- and intermediate-level waste. The backfill is considered to be a chemical environment with some potential for colloid generation and, due to its high porosity, for colloid mobility. Colloid concentration measurements were carried out using an in-situ liquid particle counting system. The in-house developed counting system with three commercially available sensors allowed the detection of single particles and colloids at low concentrations in the size range 50-5000 nm. The counting system was tested using suspensions prepared from certified size standards. The concentrations of colloids with size range 50-1000 nm were measured in cement pore water, which was collected from a column filled with a highly permeable backfill mortar. The chemical composition of the pore water corresponded to a Ca(OH)2-controlled cement system. Colloid concentrations in the backfill pore water were found to be typically lower than approximately 0.1 ppm. The specific (geometric) surface areas of the colloid populations were in the range 240 m2 g(-1) to 770 m2 g(-1). The low colloid inventories observed in this study can be explained by the high ionic strength and Ca concentrations of the cement pore water. These conditions are favourable for colloid-colloid and colloid-backfill interactions and unfavourable for colloid-enhanced nuclide transport.     

Analysis of a system for remote, dry removal of solidified mixed waste

This paper describes a novel technology for the removal of solidified radioactive waste from underground storage tanks at the DOE Hanford site in southeast Washington. The process involves the use of a unique high pressure system which pulverizes the hardened saltcake with stainless steel pellets to a powder for easy vacuum removal. The steel pellets can be magnetically separated from the waste material for re-use. Specifically this study analyzes the effects of various steel abrasives, pressure of the air stream, and stand-off distance on removal rates and penetration depth on simulated saltcake samples using the high pressure technique. A full-scale test set-up and protocol were implemented to allow for comprehensive testing. To insure reproducibility of the method, tests were then run for the optimum removal parameters. A time-dependent test was also conducted to determine the relationship of removal rates to length of pressure blasts. The results of these tests revealed that stand-off distance and pressure could be positively correlated to removal volumes. Additionally, a statistical analysis confirmed that nozzle angle is independent of removal rate. This study demonstrated that the pellet ‘blaster’ technique is a safe, effective method for removal of radioactive wastes without any increase in either waste mass or volume.     

Assessing recycling versus incineration of key materials in municipal waste: The importance of efficient energy recovery and transport distances

Recycling of materials from municipal solid waste is commonly considered to be superior to any other waste treatment alternative. For the material fractions with a significant energy content this might not be the case if the treatment alternative is a waste-to-energy plant with high energy recovery rates. The environmental impacts from recycling and from incineration of six material fractions in household waste have been compared through life cycle assessment assuming high-performance technologies for material recycling as well as for waste incineration. The results showed that there are environmental benefits when recycling paper, glass, steel and aluminium instead of incinerating it. For cardboard and plastic the results were more unclear, depending on the level of energy recovery at the incineration plant, the system boundaries chosen and which impact category was in focus. Further, the environmental impact potentials from collection, pre-treatment and transport was compared to the environmental benefit from recycling and this showed that with the right means of transport, recyclables can in most cases be transported long distances. However, the results also showed that recycling of some of the material fractions can only contribute marginally in improving the overall waste management system taking into consideration their limited content in average Danish household waste.     

A BIM-based system for demolition and renovation waste estimation and planning

Due to the rising worldwide awareness of green environment, both government and contractors have to consider effective construction and demolition (C&D) waste management practices. The last two decades have witnessed the growing importance of demolition and renovation (D&R) works and the growing amount of D&R waste disposed to landfills every day, especially in developed cities like Hong Kong. Quantitative waste prediction is crucial for waste management. It can enable contractors to pinpoint critical waste generation processes and to plan waste control strategies. In addition, waste estimation could also facilitate some government waste management policies, such as the waste disposal charging scheme in Hong Kong. Currently, tools that can accurately and conveniently estimate the amount of waste from construction, renovation, and demolition projects are lacking.In the light of this research gap, this paper presents a building information modeling (BIM) based system that we have developed for estimation and planning of D&R waste. BIM allows multi-disciplinary information to be superimposed within one digital building model. Our system can extract material and volume information through the BIM model and integrate the information for detailed waste estimation and planning. Waste recycling and reuse are also considered in our system. Extracted material information can be provided to recyclers before demolition or renovation to make recycling stage more cooperative and more efficient. Pick-up truck requirements and waste disposal charging fee for different waste facilities will also be predicted through our system. The results could provide alerts to contractors ahead of time at project planning stage. This paper also presents an example scenario with a 47-floor residential building in Hong Kong to demonstrate our D&R waste estimation and planning system. As the BIM technology has been increasingly adopted in the architectural, engineering and construction industry and digital building information models will likely to be available for most buildings (including historical buildings) in the future, our system can be used in various demolition and renovation projects and be extended to facilitate project control.     

Challenges when performing economic optimization of waste treatment: A review

Strategic and operational decisions in waste management, in particular with respect to investments in new treatment facilities, are needed due to a number of factors, including continuously increasing amounts of waste, political demands for efficient utilization of waste resources, and the decommissioning of existing waste treatment facilities. Optimization models can assist in ensuring that these investment strategies are economically feasible.Various economic optimization models for waste treatment have been developed which focus on different parameters. Models focusing on transport are one example, but models focusing on energy production have also been developed, as well as models which take into account a plant’s economies of scale, environmental impact, material recovery and social costs. Finally, models combining different criteria for the selection of waste treatment methods in multi-criteria analysis have been developed.A thorough updated review of the existing models is presented, and the main challenges and crucial parameters that need to be taken into account when assessing the economic performance of waste treatment alternatives are identified. The review article will assist both policy-makers and model-developers involved in assessing the economic performance of waste treatment alternatives.     

Fate and transport of phenol in a packed bed reactor containing simulated solid waste

An assessment of the risk to human health and the environment associated with the presence of organic contaminants (OCs) in landfills necessitates reliable predictive models. The overall objectives of this study were to (1) conduct column experiments to measure the fate and transport of an OC in a simulated solid waste mixture, (2) compare the results of column experiments to model predictions using HYDRUS-1D (version 4.13), a contaminant fate and transport model that can be parameterized to simulate the laboratory experimental system, and (3) determine model input parameters from independently conducted batch experiments. Experiments were conducted in which sorption only and sorption plus biodegradation influenced OC transport. HYDRUS-1D can reasonably simulate the fate and transport of phenol in an anaerobic and fully saturated waste column in which biodegradation and sorption are the prevailing fate processes. The agreement between model predictions and column data was imperfect (i.e., within a factor of two) for the sorption plus biodegradation test and the error almost certainly lies in the difficulty of measuring a biodegradation rate that is applicable to the column conditions. Nevertheless, a biodegradation rate estimate that is within a factor of two or even five may be adequate in the context of a landfill, given the extended retention time and the fact that leachate release will be controlled by the infiltration rate which can be minimized by engineering controls.     

Influence of composition variations on the initial alteration rate of vitrified domestic waste incineration fly-ash

Vitrification as a waste stabilization technology has often been considered applicable only to high-level radioactive waste for which, with the use of suitable additives, it yields a vitreous material with excellent chemical durability. It has become apparent in recent years that some waste forms-notably domestic waste incineration fly-ash purification residues--contain most of the ingredients of a vitrified material, although their composition variations are difficult to control. It is thus important to ensure not only that the materials are suitable for vitrification, but also that the resulting product exhibits acceptable long-term behavior under all circumstances. An initial study showed that, allowing for the compensation changes inherent in the melting process builtby EDF**, the residue collected by a single fly-ash dust separation defines a composition range within which the suitability of the vitrified material can be verified. "Vitrified material" refers to a melted material that contains no unmelted inclusions after cooling, but that may contain a variable fraction of crystallized phases. Five composition parameters were identified for the long-term behavior assessment: the concentrations of the three major elements (silicon, aluminum and calcium), the total alkali metal (sodium and potassium) concentration, and the sum of the concentrations of two toxic elements (zinc and lead). The other elements were assumed constant at molar ratios representative of industrial wastes. The experimentation plan methodology applied to the composition range identified fourteen materials suilable for developing and validating first-order models of the material components. The fly-ash composition had a very significant effect on the degree and kinetics and crystallization in the vitrified material within the experimental composition range; the cooling rate was the determining factor for some of the fourteen materials studied. Two crystailine phases predominated: spinels rich in chromium, zinc, aluminum, magnesium and iron formed quickly on cooling, and accounted for about 2 vol% of the final material. Gehelenite (Ca2Al2SiO7) crystallized massively in some vitrified materials, accounting for more than half the final product and giving it a rock-like appearance. The effect of composition alone must therefore be distinguished from the effect of crystallization on the leaching behavior. Soxhlet tests were conducted for 14 days according to a protocol based on that of the French AFNOR draft standard NF-M 60313 to determine the maximum alteration rate in pure water at 100 degrees C. The measured rate ranged from 4 to 40 gm(-2) day(-1), illustrating the crucial role of the silicon concentration: within the test composition range, a low silicon content (< 30 wt%) tended to result in a significantly higher initial rate. However, the initial rate alone is not sufficient to assess the chemical durability of the material. Further tests will be carried out at 25 degrees C under conditions approximating those of a proposed disposal site to highlight the role of the alteration layer and the effect of rising concentrations in solution on the decreasing alteration rate.     

An Updated Cost Study for Enhanced Sludge Washing of Radioactive Waste

This article updates previous cost savings studies conducted to evaluate the use of enhanced sludge washing (ESW) of high‐level radioactive waste at the United States Department of Energy Hanford Site. The cost savings estimate was updated using stochastic analysis based on new information from the Independent Review of Hanford High Level Waste Volume and the more recent Tank Waste Remediation System Operation and Utilization Plan. It is estimated that implementation of ESW in the tank waste remediation system (TWRS) at the Hanford Site can save approximately $4.8 billion compared to the use of an alternative, simpler water wash. The simpler water wash dissolution was found to be 85 percent as effective as the ESW dissolution. Further, the updated remediation cost estimate of $4.8 billion savings is uncertain only within ±$1.6 billion at the 95 percent confidence interval. © 2002 Wiley Periodicals, Inc.     

Sustainable disposal of municipal solid waste: Post bioreactor landfill polishing

Sustainable disposal of municipal solid waste (MSW) requires assurance that contaminant release will be minimized or prevented within a reasonable time frame before the landfill is abandoned so that the risk of contamination release is not passed to future generations. This could be accomplished through waste acceptance criteria such as those established by the European Union (EU) that prohibit land disposal of untreated organic matter. In the EU, mechanical, biological and/or thermal pretreatment of MSW is therefore necessary prior to landfilling which is complicated and costly. In other parts of the world, treatment within highly engineered landfills is under development, known as bioreactor landfills. However, the completed bioreactor landfill still contains material, largely nonbiodegradable carbon and ammonia that may be released to the environment over the long-term. This paper provides a conceptual analysis of an approach to ensure landfill sustainability by the rapid removal of these remaining materials, leachate treatment and recirculation combined with aeration. The analysis in this paper includes a preliminary experimental evaluation using real mature leachate and waste samples, a modeling effort using a simplified mass balance approach and input parameters from real typical bioreactor cases, and a cost estimate for the suggested treatment method.     

Comparison of leaching tests to determine and quantify the release of inorganic contaminants in demolition waste

The changes in waste management policy caused by the massive generation of waste materials (e.g. construction and demolition waste material, municipal waste incineration products) has led to an increase in the reuse and recycling of waste materials. For environmental risk assessment, test procedures are necessary to examine waste materials before they can be reused. In this article, results of column and lysimeter leaching tests having been applied to inorganic compounds in a reference demolition waste material are presented. The results show a good agreement between the leaching behaviour determined with the lysimeter unit and the column units used in the laboratory. In view of less time and system requirements compared to lysimeter systems, laboratory column units can be considered as a practicable instrument to assess the time-dependent release of inorganic compounds under conditions similar to those encountered in a natural environment. The high concentrations of elements in the seepage water at the initial stage of elution are reflected by the laboratory column leaching tests. In particular, authorities or laboratories might benefit and have an easy-to-use, but nevertheless reliable, method to serve as a basis for decision-making.     

A Review Of The Management Of Radioactive Wastes In Medical Institutes

Amongst the waste from health care institutions, radioactive waste represents a special category since it cannot be modified or neutralized by any available conventional means. Accordingly, disposal of radioactive waste can mean only its transfer from a place where it represents some hazard to somewhere else where it can be retained without undue risk. Radioactive waste arises in health care institutes as a result of diagnostic, therapeutical or research uses of unsealed radioactive substances. Sometimes, sealed sources withdrawn from further use might also be subject to disposal. Most radionuclides used in medicine are short-lived beta-, or beta-gamma emitters and represent a low risk, if properly handled, that is if due care is taken to prevent significant contamination of the workplace and personnel. Low-activity gaseous and liquid waste can usually be discharged to the environment directly; medium-activity or high-activity waste should be stored for variable periods to allow natural decay before specialized disposal.This paper presents a review of the different types of radioactive wastes produced in hospitals, and introduces many of the sources of generation and subsequent disposal options. An example is given of the wide range of guidance available, both by national bodies in Hungary and international agencies, such as the International Atomic Energy Agency (IAEA), World Health Organisation (WHO) and International Committee on Radiological Protection (ICRP).     

Analysis of high-level waste glass performance by the physical and geochemical simulation code STRAG4

A source term release analysis code for high-level waste glass has been developed for simulation of long-term dissolution behavior under repository conditions. The STRAG4 code consists of models for (a) element diffusion in both the bulk glass and surface layer, (b) glass dissolution kinetics at the interface between glass and water, and (c) geochemical reactions of dissolved elements in underground water. The simulations for various conditions of glass dissolution, including a static and dynamic system, show accordance with the experimental observations even in the relatively complicated case where bentonite is present. Long-term dissolution analyses of a borosilicate waste glass were carried out as preliminary study. The calculations were achieved by considering detrimental effects due to interactions between the glass and surrounded materials which are presumed to be in a repository environment (i.e., compacted bentonite, corrosion products from the iron overpack, and underground water). The environmental conditions such as temperature and geochemical reactions are also taken into account in the calculations. The results suggest the life of the waste glass would be more than 50,000 years even if the glass surface area increased by a factor of 10 due to crack formation.