Drip Shield Research Articles

Developments in fibre reinforced concrete in tunnelling

AbstractThis paper will provide an overview of the state of the art of the use of fibre reinforced concrete in tunnelling. The major applications for fibres are for the reinforcement of sprayed concrete and pre‐cast concrete segmental linings, but fibre reinforcement is also increasingly used for other applications such as trackbed/trackslab and minor precast concrete elements such as cable ducts, drip shields, and drainage elements. As the technology has matured, a wide variety of fibre types, made from different materials, has emerged. Standards have been developed for the manufacture, testing and application of all these fibre types. Moreover, design guidelines and codes have been published and new design methods are still being developed. The International Tunnelling Association has published several guidance documents in this field. Last but not least, FRC has an important role to play in reducing the carbon footprint of tunnelling projects.

Expected dose and associated uncertainty and sensitivity analysis results for all scenario classes in the 2008 performance assessment for the proposed high-level radioactive waste repository at Yucca Mountain, Nevada

Extensive work has been carried out by the U.S. Department of Energy (DOE) in the development of a proposed geologic repository at Yucca Mountain (YM), Nevada, for the disposal of high-level radioactive waste. In support of this development and an associated license application to the U.S. Nuclear Regulatory Commission (NRC), the DOE completed an extensive performance assessment (PA) for the proposed YM repository in 2008. The conceptual structure and organization of the 2008 YM PA is based on decomposing the analysis into the following scenario classes: nominal, early waste package failure, early drip shield failure, igneous intrusive, igneous eruptive, seismic ground motion, and seismic fault displacement. This presentation describes how results obtained for the individual scenario classes are brought together in the determination of expected dose to the reasonably maximally exposed individual (RMEI) specified by the NRC in the regulatory requirements for the YM repository and presents associated uncertainty and sensitivity analysis results. The following topics are addressed: (i) determination of expected dose to the RMEI from all scenario classes, (ii) expected dose and uncertainty in expected dose to the RMEI for 0 to 20,000yr, (iii) expected dose and uncertainty in expected dose to the RMEI from for 0 to 106yr, (iv) justification for the decomposition procedure used to estimate expected dose to the RMEI from all scenario classes, and (v) effectiveness of individual barrier systems in reducing releases from the repository and thus dose to the RMEI. The present article is part of a special issue of Reliability Engineering and System Safety devoted to the 2008 YM PA; additional articles in the issue describe other aspects of the 2008 YM PA.

Uncertainty and sensitivity analysis for the nominal scenario class in the 2008 performance assessment for the proposed high-level radioactive waste repository at Yucca Mountain, Nevada

Extensive work has been carried out by the U.S. Department of Energy (DOE) in the development of a proposed geologic repository at Yucca Mountain (YM), Nevada, for the disposal of high-level radioactive waste. In support of this development and an associated license application to the U.S. Nuclear Regulatory Commission (NRC), the DOE completed an extensive performance assessment (PA) for the proposed YM repository in 2008. This presentation describes uncertainty and sensitivity analysis results for the nominal scenario class (i.e., for undisturbed conditions) obtained in the 2008 YM PA. The following topics are addressed: (i) uncertainty and sensitivity analysis procedures, (ii) drip shield and waste package failure, (iii) engineered barrier system conditions, (iv) radionuclide release results for the engineered barrier system, unsaturated zone, and saturated zone, and (v) dose to the reasonably maximally exposed individual specified in the NRC regulations for the YM repository. The present article is part of a special issue of Reliability Engineering and System Safety devoted to the 2008 YM PA; additional articles in the issue describe other aspects of the 2008 YM PA.

Evolution of repository and waste package designs for Yucca Mountain disposal system for spent nuclear fuel and high-level radioactive waste

This paper summarizes the evolution of the engineered barrier design for the proposed Yucca Mountain disposal system. Initially, the underground facility used a fairly standard panel and drift layout excavated mostly by drilling and blasting. By 1993, the layout of the underground facility was changed to accommodate construction by a tunnel boring machine. Placement of the repository in unsaturated zone permitted an extended period without backfilling; placement of the waste package in an open drift permitted use of much larger, and thus hotter packages. Hence in 1994, the underground facility design switched from floor emplacement of waste in small, single walled stainless steel or nickel alloy containers to in-drift emplacement of waste in large, double-walled containers. By 2000, the outer layer was a high nickel alloy for corrosion resistance and the inner layer was stainless steel for structural strength. Use of large packages facilitated receipt and disposal of high volumes of spent nuclear fuel. In addition, in-drift package placement saved excavation costs. Options considered for in-drift emplacement included different heat loads and use of backfill. To avoid dripping on the package during the thermal period and the possibility of localized corrosion, titanium drip shields were added for the disposal drifts by 2000. In addition, a handling canister, sealed at the reactor to eliminate further handling of bare fuel assemblies, was evaluated and eventually adopted in 2006. Finally, staged development of the underground layout was adopted to more readily adjust to changes in waste forms and Congressional funding.

Uncertainty and sensitivity analysis for the early failure scenario classes in the 2008 performance assessment for the proposed high-level radioactive waste repository at Yucca Mountain, Nevada

Extensive work has been carried out by the U.S. Department of Energy (DOE) in the development of a proposed geologic repository at Yucca Mountain (YM), Nevada, for the disposal of high-level radioactive waste. In support of this development and an associated license application to the U.S. Nuclear Regulatory Commission (NRC), the DOE completed an extensive performance assessment (PA) for the proposed YM repository in 2008. This presentation describes uncertainty and sensitivity analysis results for the early waste package failure scenario class and the early drip shield failure scenario class obtained in the 2008 YM PA. The following topics are addressed: (i) engineered barrier system conditions, (ii) release results for the engineered barrier system, unsaturated zone, and saturated zone, (iii) dose to the reasonably maximally exposed individual (RMEI) specified in the NRC regulations for the YM repository, and (iv) expected dose to the RMEI. The present article is part of a special issue of Reliability Engineering and System Safety devoted to the 2008 YM PA; additional articles in the issue describe other aspects of the 2008 YM PA.

Waste package degradation from thermal and chemical processes in performance assessments for the Yucca Mountain disposal system for spent nuclear fuel and high-level radioactive waste

This paper summarizes modeling of waste container degradation in performance assessments conducted between 1984 and 2008 to evaluate feasibility, viability, and assess compliance of a repository for spent nuclear fuel and high-level radioactive waste at Yucca Mountain, Nevada. As understanding of the Yucca Mountain disposal system increased, modeling of container degradation evolved from a component of the source term in 1984 to a separate module describing both container and drip shield degradation in 2008. A thermal module for evaluating the influence of higher heat loads from more closely packed, large waste packages was also introduced. In addition, a module for evaluating drift chemistry was added in later PAs to evaluate the potential for localized corrosion of the outer barrier of the waste container composed of Alloy 22, a highly corrosion-resistant nickel–chromium–tungsten–molybdenum alloy. The uncertainty of parameters related to container degradation contributed significantly to the estimated uncertainty of performance measures (cumulative release in assessments prior to 1995 and individual dose, thereafter).

Expected dose for the early failure scenario classes in the 2008 performance assessment for the proposed high-level radioactive waste repository at Yucca Mountain, Nevada

Extensive work has been carried out by the U.S. Department of Energy (DOE) in the development of a proposed geologic repository at Yucca Mountain (YM), Nevada, for the disposal of high-level radioactive waste. In support of this development and an associated license application to the U.S. Nuclear Regulatory Commission (NRC), the DOE completed an extensive performance assessment (PA) for the proposed YM repository in 2008. This presentation describes the determination of expected dose to the reasonably maximally exposed individual (RMEI) specified in the NRC regulations for the YM repository for the early waste package (WP) failure scenario class and the early drip shield (DS) failure scenario class in the 2008 YM PA. The following topics are addressed: (i) properties of the early failure scenario classes and the determination of dose and expected dose the RMEI, (ii) expected dose and uncertainty in expected dose to the RMEI from the early WP failure scenario class, (iii) expected dose and uncertainty in expected dose to the RMEI from the early DS failure scenario class, (iv) expected dose and uncertainty in expected dose to the RMEI from the combined early WP and early DS failure scenario class with and without the inclusion of failures resulting from nominal processes, and (v) uncertainty in the occurrence of early failure scenario classes. The present article is part of a special issue of Reliability Engineering and System Safety devoted to the 2008 YM PA; additional articles in the issue describe other aspects of the 2008 YM PA.

Overview of total system model used for the 2008 performance assessment for the proposed high-level radioactive waste repository at Yucca Mountain, Nevada

A summary is presented for the total system model used to represent physical processes associated with the seven scenario classes (i.e., nominal conditions, early waste package (WP) failure, early drip shield (DS) failure, igneous intrusive events, igneous eruptive events, seismic ground motion events and seismic fault displacement events) considered in the 2008 performance assessment for the proposed repository for high-level radioactive waste at Yucca Mountain, Nevada. The total system model estimates dose to an exposed individual resulting from radionuclide movement through the repository system and biosphere. Components of the total system model described in this presentation include models for (i) climate analysis, (ii) land surface infiltration and associated unsaturated zone flow, (iii) multi-scale thermal hydrology and engineered barrier system (EBS) thermal–hydrologic environment, (iv) EBS physical and chemical environment, (v) WP and DS degradation, (vi) drift seepage and drift wall condensation, (vii) waste form degradation and mobilization, (viii) water and radionuclide movement in the EBS and underlying unsaturated and saturated zones, (ix) radionuclide movement in the biosphere and resultant human exposure, and (x) processes specific to early WP and DS failures, intrusive and eruptive igneous events, and seismic ground motion and fault displacement events.

Lessons Learned from the Yucca Mountain Nuclear Waste Repository Project The Engineered Barrier System

ABSTRACTNuclear waste isolation programs both inside and outside the United States have provided evidence that there are many geologic options for a repository, but virtually all of them rely to some degree on an engineered barrier system (EBS) to isolate and/or retard the migration of radionuclides to the biosphere. At Yucca Mountain, the design of the EBS was unexpectedly challenging because of uncertainties in quantitatively determining the local environment of the EBS particularly during the thermal pulse. The EBS design for the Yucca Mountain site evolved from a thin-walled, limited-lifetime, corrosion-resistant canister through a corrosion-allowance canister, to the present design, which may have a lifetime of more than 106years. The EBS proposed for the Yucca Mountain repository has many individual sub-barriers, beginning with the spent fuel and waste, the cladding of the spent fuel, the geometry of the package, etc. The anticipated modes of degradation of engineering materials, including corrosion of the fuel, of the canister, and of the drip shield proposed specifically for the Yucca Mountain project, and the consequences of the materials degradation on the performance of the repository are presented. The roles of conservative modeling and simplifying assumptions for radionuclide mobilization and transport in the EBS on characterization of the source term are addressed.

Applications in the Nuclear Industry for Thermal Spray Amorphous Metal and Ceramic Coatings

Amorphous metal and ceramic thermal spray coatings have been developed with excellent corrosion resistance and neutron absorption. These coatings, with further development, could be cost-effective options to enhance the corrosion resistance of drip shields and waste packages, and limit nuclear criticality in canisters for the transportation, aging, and disposal of spent nuclear fuel. Iron-based amorphous metal formulations with chromium, molybdenum, and tungsten have shown the corrosion resistance believed to be necessary for such applications. Rare earth additions enable very low critical cooling rates to be achieved. The boron content of these materials and their stability at high neutron doses enable them to serve as high efficiency neutron absorbers for criticality control. Ceramic coatings may provide even greater corrosion resistance for waste package and drip shield applications, although the boron-containing amorphous metals are still favored for criticality control applications. These amorphous metal and ceramic materials have been produced as gas-atomized powders and applied as near full density, nonporous coatings with the high-velocity oxy-fuel process. This article summarizes the performance of these coatings as corrosion-resistant barriers and as neutron absorbers. This article also presents a simple cost model to quantify the economic benefits possible with these new materials.

Implementation of Localized Corrosion in the Performance Assessment Model for Yucca Mountain

A total system performance assessment (TSPA) model has been developed to analyze the ability of the natural and engineered barriers of the Yucca Mountain repository to isolate nuclear waste over the period following repository closure. The principal features of the engineered barrier system are emplacement tunnels (or “drifts”) containing a two-layer waste package (WP) for waste containment and a titanium drip shield to protect the WP from seeping water and falling rock. The 25-mm-thick outer shell of the WP is composed of Alloy 22, a highly corrosion-resistant nickel-based alloy. There are five nominal degradation modes of the Alloy 22: general corrosion, microbially influenced corrosion, stress corrosion cracking, early failure due to manufacturing defects, and localized corrosion (LC). This paper specifically examines the incorporation of the Alloy 22 LC model into the Yucca Mountain TSPA model, particularly the abstraction and modeling methodology, as well as issues dealing with scaling, spatial variability, uncertainty, and coupling to other submodels that are part of the total system model, such as the submodel for seepage water chemistry.

Yucca Mountain engineered barrier system corrosion model (EBSCOM)

A revised engineered barrier system model has been developed by the Electric Power Research Institute to predict the time dependence of the failure of the drip shields and waste packages in the proposed Yucca Mountain repository. The revised model is based on new information on various corrosion processes developed by the US Department of Energy and others and for a 20-mm-thick waste package design with a double closure lid system. As with earlier versions of the corrosion model, the new EBSCOM code produces a best-estimate of the failure times of the various barriers. The model predicts that only 15% of waste packages will fail within a period of 1 million years. The times for the first corrosion failures are 40,000 years, 336,000 years, and 375,000 years for the drip shield, waste package, and combination of drip shield and the associated waste package, respectively.

Failure model and Monte Carlo simulations for titanium (grade-7) drip shields under Yucca Mountain repository conditions

A failure model was developed for the titanium alloy drip shield proposed for the Yucca Mountain nuclear waste repository. The degradation modes considered are hydrogen-induced cracking (HIC) and general aqueous corrosion, processes which are inextricably linked. Failure by HIC is controlled by the environment, the corrosion rate, the material properties, and the hydrogen absorption efficiency which is assumed to decrease parabolically with time. This model includes both oxygen and water reduction coupled to corrosion, and allows for the release of the absorbed hydrogen as the alloy containing the hydrogen converts to oxide. Monte Carlo simulations were employed to predict drip shield lifetimes, and to investigate the effects of the hydrogen absorption efficiency, the critical HIC concentration, the corrosion rate, and the fraction of corrosion supported by water reduction, on the susceptibility of the material to HIC.

Localized corrosion of alloy 22 in the potential Yucca Mountain repository environment

The U.S. Department of Energy (DOE) has indicated that it may use Alloy 22 as the waste package outer container material for the potential high-level waste repository at Yucca Mountain, Nevada. Additionally, a drip shield, made of titanium grade 7 and titanium grade 29, may extend the length of the emplacement drifts to enclose the top and sides of the emplaced waste package. Localized corrosion in the form of crevice corrosion could be one degradation process that may adversely affect the waste package performance. This paper will summarize the work conducted to evaluate the effects of environmental conditions relevant to the potential Yucca Mountain repository, metallurgical states (e.g., mill-annealed and welded plus solution annealed), and similar and dissimilar metal crevices on the crevice corrosion susceptibility of Alloy 22. This work also evaluates crevice corrosion propagation behavior resulting from contact with Alloy 22 or titanium grade 7.

Material corrosion issues for nuclear waste disposition in Yucca Mountain

For more than two decades, an extensive scientific effort has been underway to determine whether Yucca Mountain, Nevada, is a suitable site for a deep underground repository for spent nuclear fuel and high-level radioactive waste. Even though the geologic site is stable, additional engineered barriers are planned, including waste packages, drip shields, and tunnel inverts that will be within the emplacement tunnels. Research is under way into the best materials for corrosion prevention in those engineered barriers to ensure their long-term mechanical integrity.

Integration of Postclosure Safety Analysis with Repository Design for the Yucca Mountain Repository through the Selection of Design Control Parameters

AbstractThe Yucca Mountain repository combines multiple barriers, both natural and engineered, which work both individually and collectively to limit the movement of water and the potential release and movement of radionuclides to the accessible environment. Engineered structures, systems and components (SSCs) are designed to function in the natural environment utilizing materials chosen to perform their intended functions in order to meet the postclosure performance objectives. Similarly, the features of the natural environment are expected to respond to the presence of the repository through geomechanical, hydrogeologic and geochemical changes. At Yucca Mountain, specific features, both engineered and natural have been identified as requiring design control during repository construction and operations. The integration between design and postclosure safety analysis is facilitated using design control parameters. The term “design control parameters” includes functions and performance requirements allocated to SSCs through the design process, as well as the attributes of SSCs that are developed during design (e.g., dimensions; weights; materials; fabrication and quality-control processes; and operating conditions). These control parameters provide an interface between the design and the analyzed postclosure safety bases, which needs to be maintained through the licensing process. Maintenance of the design is controlled through configuration management and procedural safety controls. The design control parameters serve three key purposes. First, they identify key aspects of the design that serve as the design bases for the designers of the SSCs of the facility. Second, they provide a useful input to the analyses of relevant postclosure features, events and processes (FEPs) and are used to either exclude FEPs from the postclosure safety analysis or as an input to models of included FEPs in the safety analysis. Finally, they provide important controlled interface constraints between the design and safety analyses organizations that are amenable to configuration management. Several examples of such design controls will be presented in this briefing. The first type of design controls relates to the location of the underground facility, including standoffs from faults and the ground surface. The second type of design controls relates to the configuration of the engineered features including the spacing of emplacement driftsand drip shield dimensions and characteristics. A third type of design controls relates to constraints on handling, loading and emplacing waste forms in canisters and waste packages in the emplacement drifts.

The potential for the SCC of titanium alloys under repository-relevant environments for U.S. nuclear waste

Stress corrosion cracking (SCC) is one of the most common corrosion-related causes of premature failure of metal structural components. This paper presents an assessment of the performance of titanium alloys (titanium grade 7 for the drip shield and grade 29 for the structural support), which may be subjected to SCC under proposed U.S. nuclear waste repository environments. The threshold stresses for SCC initiation, SCC crack propagation rates, and the threshold stress intensity factors for titanium grades 7, 28, and 29 are discussed.

Second generation waste package design and storage concept for the Yucca Mountain Repository

The reference waste package design and operating mode to be used in the Yucca Mountain Repository is reviewed. An alternate (second generation) operating concept and waste package design is proposed to reduce the risk of localized corrosion of waste packages and to reduce repository costs. The second generation waste package design and storage concept is proposed for implementation after the initial licensing and operation of the reference repository design. Implementation of the second generation concept at Yucca Mountain would follow regulatory processes analogous to those used successfully to extend the design life and uprate the power of commercial light water nuclear reactors in the United States. The second generation concept utilizes the benefits of hot dry storage to minimize the potential for localized corrosion of the waste package by liquid electrolytes. The second generation concept permits major reductions in repository costs by increasing the number of fuel assemblies stored in each waste package, by eliminating the need for titanium drip shields and by fabricating the outer container from corrosion resistant low alloy carbon steel.

Evolution of Chemistry and Its Effects on the Corrosion of Engineered Barrier Materials

ABSTRACTThe evolution of environmental conditions within the emplacement drifts of a potential high-level waste repository at Yucca Mountain, Nevada, may be influenced by several factors, including the temperature and relative humidity within the emplacement drifts and the composition of seepage water. The performance of the waste package and the drip shield may be affected by the evolution of the environmental conditions within the emplacement drifts. In this study, tests evaluated the evolution of environmental conditions on the waste package surfaces and in the surrounding host rock. The tests were designed to (i) simulate the conditions expected within the emplacement drifts; (ii) measure the changes in near-field chemistry; and (iii) determine environmental influence on the performance of the engineered barrier materials. Results of tests conducted in this study indicate the composition of salt deposits was consistent with the initial dilute water chemistry. Salts and possibly concentrated calcium chloride brines may be more aggressive than either neutral or alkaline brines.

Monte Carlo Simulations of the Degradation of the Engineered Barriers System in the Yucca Mountain Repository Using the EBSPA Code

AbstractBased on a probabilistic model previously proposed, a Monte Carlo simulation code (EBSPA) has been developed to predict the lifetime of the engineered barriers system within the Yucca Mountain nuclear waste repository. The degradation modes considered in the EBSPA are general passive corrosion and hydrogen-induced cracking for the drip shield; and general passive corrosion, crevice corrosion and stress corrosion cracking for the waste package. Two scenarios have been simulated using the EBSPA code: (a) a conservative scenario for the conditions thought likely to prevail in the repository, and (b) a worst-case scenario in which the impact of the degradation processes is overstated.