Debating Yucca Mountain29 June 2001
The US Department of Energy has produced and published its Yucca Mountain Science and Engineering Report. This opens the public comment period on the recommendation of Yucca Mountain as a potential repository for spent nuclear fuel and high-level radioactive waste.
The USA has been evaluating methods for the safe storage and disposal of radioactive waste for over 40 years. Many organisations and government agencies have participated in these studies. After analysing various options, disposal in mined geological repositories emerged as the preferred long-term solution for the management of spent nuclear fuel and high-level radioactive waste. This consensus resulted in the Nuclear Waste Policy Act (NWPA) of 1982, establishing US responsibility and policy for radwaste disposal.
Congress established the framework to address issues of nuclear waste disposal in the NWPA, and designated the roles and responsibilities of the federal government and the owners and generators of the waste. Congress assigned responsibility to:
•The DoE to site, construct, operate and close a repository for the disposal of radioactive waste material.
•The US Environmental Protection Agency to set health and safety standards for releases of radioactive material from a repository.
•The US Nuclear Regulatory Commission to promulgate regulations for construction, operation and closure of a repository.
•The generators and owners of high-level radioactive waste to pay the costs of disposal of such radioactive material.
Congress amended the NWPA in 1987, and directed the DoE to investigate Yucca Mountain to determine if it is a suitable site for the first repository. The DoE has studied Yucca Mountain for over 20 years to characterise the site and assess the future performance of a potential repository. Preliminary engineering specifications have been developed for surface and subsurface facilities and the waste package. Analyses that integrate design- and site-specific data and models have been conducted to assess performance of a repository at Yucca Mountain.
The scientific investigations covered by the report include site characterisation studies of the geological, hydrologic and geochemical environment, and evaluation of how conditions might evolve. These analyses considered a range of processes that would operate in and around the potential repository. Since projections for 10,000 years are inherently uncertain, the uncertainties associated with analyses and models of long-term performance are also described, along with the likely impact of these uncertainties on performance assessment.
The NWPA specifies a process for recommendation and approval of a repository site. It requires that the secretary of energy provide a comprehensive statement of the basis for any site recommendation. The NWPA requires that the DoE hold public hearings near the site before the secretary makes a decision whether or not to recommend Yucca Mountain.
As the DoE moves towards a possible site recommendation, it is continuing to evaluate uncertainties in performance assessment models. Such uncertainties result from the long time frames over which performance must be forecast; natural variability in features and processes at the site; inherent limitations on the amount of data that can be collected; and complexities in the processes studied, most notably, the interrelated thermal, hydrologic, chemical and mechanical processes in the underground emplacement drifts and the surrounding rock.
Material for disposal
DoE is responsible for the safe, permanent disposal of spent nuclear fuel from commercial nuclear power plants. The DoE must also dispose of large quantities of DoE-owned high-level radioactive waste from the production of nuclear weapons and smaller quantities of spent nuclear fuel from weapons production reactors, research reactors, and naval reactors.
The NWPA limits the amount of high-level radioactive waste that can be stored in the first US geological repository to 70,000t heavy metal until a second repository is in operation. The materials that may be disposed at Yucca Mountain include 63,000t of commercial spent fuel, 2,333t of DoE spent fuel, and 4,667t of DoE high-level radioactive waste. All the waste forms transported to and received at a repository would be solid materials. No liquid waste forms would be accepted.
As of December 1999, the USA had generated about 40,000t of spent nuclear fuel from commercial nuclear power plants. This amount could more than double by 2035 if all currently operating plants complete their initial 40-year licence period.
Some material to be disposed would come from surplus plutonium from production and decommissioning of nuclear weapons. A nominal 50t of surplus plutonium must be safely disposed of. Current plans call for some surplus plutonium to be combined with uranium to form fuel to be used in commercial reactors. The resulting spent nuclear fuel would be disposed as commercial spent nuclear fuel. Some of the surplus plutonium would be immobilised in ceramic, placed inside stainless steel cans, and placed in canisters. These canisters will be filled with molten high-level radioactive waste glass, which will vitrify into a glass waste form around the stainless steel cans.
The DoE has developed a design for a monitored geological repository at Yucca Mountain that could give future generations the choice of either closing and sealing the repository, or keeping it open and monitoring it for a longer period. The design for the potential repository would not preclude the option for future generations to make societal decisions to monitor the repository for up to 300 years before making decisions to decommission and close the facility.
The operations to be performed include:
•Receiving high-level radioactive waste in NRC-certified shipping casks from rail and truck transporters.
•Unloading, handling and packaging high-level radioactive waste into waste packages suitable for underground emplacement.
•Transporting waste packages from the surface to the underground facility.
•Emplacing waste packages in underground drifts.
•Monitoring operations and repository system performace to ensure the safety of workers and the public.
•Decommissioning and closure.
The design is intended to fulfill the functional requirements defined for the facilities while maintaining the flexibility to adapt to various construction and operational conditions and requirements. Four key aspects of design flexibility are:
•The design’s ability to support different construction methods (change in drift spacing, modular or sequential construction of surface and subsurface facilities).
•The capability to dispose of a wide range of radioactive waste container sizes.
•The ability to support a range of thermal operating modes (defining a larger waste emplacement area or varying ventillation duration and rates).
•The ability to continue to enhance the design to achieve performance-related benefits identified through ongoing analyses.
Thermal management strategy
Radioactive materials in the waste become less radioactive over time. As part of this process, energy is released as heat. In the potential repository, this heat could affect thermal, hydrologic, chemical and mechanical processes. Temperatures above the boiling point of water would dry out the emplacement drifts, limiting the amount of water available to contact waste packages. Lower temperatures might have less effect on rock temperatures and geochemistry, reducing complexities in modelling thermal effects. This may reduce uncertainties in assessing future repository performance.
The Waste Handling Building would receive, prepare and package the waste for emplacement underground in the repository. All waste handling operations would be conducted using remotely operated equipment. Thick concrete walls, air locks, and controlled area access techniques would be used to protect workers from radiation exposure. The Waste Handling Building and equipment would be designed to withstand the effects of earthquakes on repository operations. The Waste Handling Building would house all systems necessary to prepare waste for emplacement. These include:
•The carrier/cask handling system, which would receive and unload transportation casks from rail and truck carriers.
•The assembly transfer system, which would receive casks containing spent nuclear fuel assemblies from commercial reactors.
•The canister transfer system, which would receive transportation casks containing canisters of DoE high-level radoactive waste or spent nuclear fuel, unload the canisters from the casks, and load the canisters into disposal containers.
•The disposal container handling system, which would receive loaded disposal containers from the assembly and canister transfer systems, install and weld closure lids onto the disposal containers.
•The waste package remediation system, which would receive waste packages or disposal containers that are damaged, have failed the inspection process, or have been selected for retrieval from the repository for examination. Waste packages that are damaged or fail inspection would be repaired or repackaged into another container.
The underground facilities would be designed to contribute to the isolation of waste. The total subsurface area required to accommodate 70,000t is about 1150 acres. A larger area, up to about 2500 acres, may be required for lower-temperature operating modes. For the higher-temperature operating mode, the subsurface facilities would be constructed over 23 years.
The barriers important to waste isolation are characterised as natural barriers, associated with the geologic and hydrologic setting, and engineered barriers, discussed in the next section. The engineered barriers are designed specifically to complement the natural system in prolonging radionuclide isolation within the repository and limiting their potential release. The natural barriers at Yucca Mountain include:
•Surface soils and topography.
•Unsaturated rock layers above repository.
•Unsaturated rock layers below repository.
•Volcanic tuff and alluvial deposits below the water table.
Natural barriers would contribute to waste isolation by limiting how much water enters emplacement drifts, and limiting transport of radionuclides through the natural system. The natural system would provide an environment that contributes to the long lives of waste packages and drip shields.
Identification of a subsurface location for the repository was based on several factors to take advantage of natural barriers, including the thickness of overlying rock and soil, the extent and geomechanical characteristics of the host rock, location of faults, and depth to groundwater. The host rock for a potential repository should be able to sustain the excavation of stable openings that can be maintained during repository operations and that would isolate the waste for an extended period after closure. In addition, the rock should be able to absorb any heat generated by the waste without undergoing changes that could threaten the site’s ability to isolate the waste. The host rock should be sufficiently thick to construct a repository large enough to support the design’s intended disposal capacity. Moreover, the amount of suitable host rock should provide adequate flexibility in selecting the depth, configuration, and location of a repository.
The engineered barrier system is designed to complement natural barriers in isolating waste from the environment. The repository design includes the following engineered barriers: the waste package, the drip shield, and the emplacement drift invert.
The engineered barriers would contribute to waste isolation by using long-lived waste packages and drip shields to keep water away from the waste forms, and limiting release of radionuclides from the engineered barriers.
Waste packages would have a dual-metal design containing two concentric cylinders. The inner cylinder would be 5cm thick of stainless steel type 316. The outer cylinder would be 2.0-2.5cm thick corrosion-resistant, nickel-based alloy (alloy 22). The outer cylinder would protect the inner cylinder from corrosion, and the stainless steel would provide structural support for the outer cylinder. Laboratory tests indicate that alloy 22 would last over 10,000 years in the range of expected repository environments at Yucca Mountain. Uncertainties associated with the long-term performance of the waste package are the subject of a peer review. Corrosion tests are continuing in a variety of thermal and chemical environments to provide additional information on the corrosion rate of alloy 22 and to address these uncertainties.
Drip shields would be installed over the waste packages prior to closure. The drip shields would divert moisture that might drip from the drift walls around the water packages to the drift floor. All the drip shields would be the same size, so one design could be used with all the waste packages. The drip shields would be of titanium, to provide corrosion resistance and structural strength, and divert moisture around waste packages for thousands of years. Tests continue on drip shield materials to assess how well current data and models can be extrapolated over long periods of time. The drip shields would maintain their function in the event of rockfalls.
The invert includes structures and materials to support the pallet and waste package, drift rail system, and drip shield. It is composed of two parts: the steel invert structure and the ballast.
Following closure, one function of the granular material in the invert is to provide a layer of material below the waste packages to slow movement of radionuclides into the host rock. Water is not expected to accumulate and flow beneath the drip shields, so the most likely way radionuclides could move is by diffusion through thin films of water on the granular material.
The processes important to the repository’s performance after it is closed include those that control movement of water through the mountain. These processes begin with surface precipitation, a fraction of which infiltrates into the mountain. This infiltration would move through the unsaturated zone to the repository level, then downward to the saturated zone. Within the saturated zone, water would move laterally away, where it could eventually reach a location where a receptor resides. At the repository level, water moving past the engineered barriers would be affected by the physical and chemical processes associated with heat from the waste, and could corrode waste packages and dissolve some waste. These processes could lead to movement of radionuclides out of the repository.
Certain disruptive events that could affect these processes are also considered.
Unsaturated zone flow
Because of the current arid climate at Yucca Mountain and the surface processes of runoff, evaporation and transpiration, the amount of water available to contact and transport radionuclides will be small. However, the availability of water may increase in wetter climates that may occur in the future. The design and operating mode proposed for Yucca Mountain uses heat produced by the waste to limit potential for contact between water and waste packages for thousands of years. It keeps most of the pillar area between the emplacement drifts below the boiling point of water to promote water drainage through the cooler portions of the rock pillars.
In addition, the repository design takes advantage of natural processes that divert water around drift openings. Most water moving in the unsaturated zone flows through fractures. Water flowing in narrow fractures will usually remain in them rather than flow into large openings, because of capillary pressure in the fractures. Thus capillary forces and water flow in unsaturated zone fractures would limit seepage into openings, allowing most water to move past, and not into, emplacement drifts. If water does seep into an emplacement drift, most of it may flow down the drift wall to the floor and drain without contacting either drip shields or waste packages. However, for modelling repository performance, a more conservative approach is used.
Many processes have been studied during the evaluation of Yucca Mountain. Important processes considered in the unsaturated zone flow models include:
•Climate and infiltration.
•Fracture flow or matrix flow within major rock units.
•Flow above the potential repository.
•Flow below the repository, including formation and significance of perched water.
•Effects of major faults.
•Seepage into drifts.
Effects of heat on water movement
In the operating mode, no liquid water can remain in the emplacement drifts, and very little can remain in the nearby rock, as long as the drift wall remains at temperatures above the boiling point of water. The long-term continuing but reduced heat production from waste packages will still cause evaporation in and near emplacement drifts, limiting the amount of water in the rock near the waste packages. The heat from emplaced waste may change the flow properties of the rock, as well as the chemical composition of the water and minerals in the engineered barrier system and surrounding rock.
Physical and chemical environment
The drip shield and waste package lifetimes depend on the conditions to which they are exposed: the in-drift physical and chemical environment. Once water enters a degraded waste package, the transport of radionuclides released from the waste form inside also depends on the drift environment.
The repository environment will be warm, with temperatures at the waste package surface increasing above the boiling point of water. The expected duration of temperatures above the boiling point of water on the surfaces of the waste packages would be thousands of years. The precise time period varies for three main reasons:
•Location within the repository layout.
•Spatial variation in the infiltration of water at the ground surface.
•Variability in the heat output of individual waste packages.
The repository edges would cool first because they would lose heat to the cooler rock outside the perimeter. Water percolating downwards through the host rock in response to infiltration at the ground surface would hasten cooling of the repository.
The chemical environment is expected to be at near-neutral pH and mildly oxidising. Under such conditions, alloy 22 will form a thin, stable oxide layer that is extremely corrosion resistant.
Waste package/drip shield degradation
The drip shield and waste package lifetimes depend on the environment to which they are exposed and the degradation processes that occur. Corrosion is the most important degradation process considered in selecting the materials for the waste package and drip shield.
Because most corrosion occurs only in the presence of water, and because highly corrosive chemical conditions are not expected in the repository, both the titanium drip shield and the alloy 22 outer level of the waste package are expected to have long lifetimes.
Analyses to date indicate that the drip shields and waste packages will be long lived. Current models indicate that significant corrosion is not likely for thousands of years. Nevertheless, the DoE is evaluating whether keeping waste package surface temperatures cooler would improve performance or reduce uncertainty in the models used. Water could contact waste packages sooner in lower-temperature operating modes. However, lower temperature operating modes would likely reduce the potential for corrosion susceptibility of alloy 22.
Water diversion performance
Movement models of water in emplacement drifts focus on seepage. For water to contact a drip shield or waste package, water droplets must form from seepage above the waste package and fall. Other modes of flow, such as film flow on the drift wall, may divert some or all seepage around the drift. Seepage would probably occur at or near faults or fractures that could focus percolation into a drift. Analyses indicate that emplacement drifts have enough drainage capacity to ensure that water would not rise above the level of the invert, even under extreme seepage conditions.
Waste form degradation
The DoE does not expect water to contact waste for over 10,000 years. Even if water were to penetrate a breached waste package before 10,000 years, several characteristics of the waste form and repository would limit radionuclide releases. First, because of the high temperatures, much of the water that penetrates the waste package will evaporate before it can dissolve or transport radionuclides. Both spent nuclear fuel and glass high-level radioactive waste forms dissolve slowly in the waste package environment. Data indicates that most radionuclides in the waste are not very soluble in the warm, near-neutral pH conditions that are expected. To dissolve soluble radionuclides, water must also penetrate the metal cladding of the spent nuclear fuel assemblies.
Release of radionuclides from the waste forms is a three-step process requiring:
•Degradation of the waste forms.
•Mobilisation of radionuclides from the degraded waste forms.
•Transport of radionuclides away from the waste forms.
Radionuclides can only be released after the waste package is breached and air and water begin to enter.
Engineered barrier system transport
The invert below the waste package would contain crushed tuff to limit transport of radionuclides from breached waste packages into the unsaturated zone. Transport could occur either through advection or diffusion.
Unsaturated zone transport
Eventually, components of the repository’s engineered barrier system will degrade and small amounts of water contact the waste. This initial degradation is not likely to occur within the first 10,000 years, but degradation will gradually increase over tens of thousands of years. Even then, features of the site geology and the repository would limit radionuclide migration to the environment and slow that migration by hundreds to thousands of years.
Saturated zone flow and transport
The same basic processes that apply to movement of radionuclides in the unsaturated zone (sorption, matrix diffusion, dispersion and dilution) also apply to transport in the saturated zone. Flowing groundwater transports radionuclides either in solution or in suspension. Any radionuclides released by water contacting breached waste packages would have to migrate through the unsaturated zone down to the water table and then travel through the saturated zone to reach a location where a receptor resides.
Biosphere analyses have been performed to develop conversion factors to estimate doses to a receptor from transport and retention of radionuclides in the biosphere.
Biosphere analyses examine processes and pathways that could disperse or concentrate radionuclides released from the repository.
Disruptive processes and scenarios
Analyses of future repository performance must also consider future events with the potential to compromise the repository’s ability to protect public health and safety. Analysts evaluated various potentially disruptive processes and events that could affect performance. These range from extremely unlikely events to changes in processes that, although low in probability, could affect long-term repository performance.
Attributes of safe disposal
The potential repository can be described with five key attributes important to long-term performance:
•Limited water entering waste emplacement drifts.
•Long-lived waste package and drip shield.
•Limited release of radionuclides from the engineered barriers.
•Delay and dilution of radionuclide concentrations by the natural barriers.
•Low mean annual dose considering potentially disruptive events.
The first four reflect interactions of natural barriers in prolonging containment of radionuclides in the repository and limiting their release. The fifth reflects the likelihood that disruptive events would not affect repository performance over 10,000 years.
Uncertainties in data
Using existing data and validated models, numerous analyses have been and are being performed to help the DoE understand the extent to which the results of performance assessments are robust. The DoE is currently conducting a study to assess the degree of realism in current process models, quantify key uncertainties, and to improve the understanding of conservatism in the models and in performance assessment results.
The quantitative evaluation of postclosure repository performance demonstrates that a repository can be constructed that would protect public health and safety.
The DoE analysed the dose rate in three scenarios: nominal, disruptive, and stylised human intrusion.
The DoE has completed an assessment of the repository’s performance for the nominal case, including individual protection and groundwater protection. The calculated result is that no dose occurs for over 10,000 years after closure.
The primary disruptive event considered in these analyses is volcanic activity. There are two separate models for volcanic disruption; a model for volcanic eruptions that intersect drifts and bring waste to the surface, and a model for underground intrusions that damage waste packages and expose radionuclides for groundwater transport. The probability of igneous disruption is low. The overall probability-weighted mean igneous dose rate reaches a peak during the first 10,000 years of approximately 0.08 millirem per year.
Human intrusion scenario
The DoE assessed the consequences of human intrusion into the repository according to stylised guidelines. The analysis shows that the peak mean total effective dose equivalent to the receptor from human intrusion is approximately 0.008 millirem per year during the proposed regulatory compliance period.
Preclosure safety assessment
A potential repository at Yucca Mountain would be designed and operated to limit worker and public exposures to radiation. A preclosure safety assessment has been conducted to evaluate the performance of the potential repository in the preclosure period.
To begin the safety assessment, the DoE examined a range of potential hazards and the event sequences such hazards could cause, as well as their likelihood and consequences. From this, the DoE identified structures, systems and components important to safety that would be relied upon to protect the public and workers.
The structures, systems and components were classified into grades according to their importance to safety to ensure that appropriate quality assurance controls are implemented during the repository’s lifetime. DoE has developed a preclosure safety test and evaluation programmme to verify that structures, systems and components are designed as specified and perform as required.
With the release of the report, DoE is opening the public comment period on consideration of possible recommendation of the Yucca Mountain site as a repository for high-level radioactive waste. Before any public hearings, DoE will make available for public review additional information and analyses to be part of the basis for any site recommendation decision, including the results of a preliminary evaluation of the site’s performance against the DoE’s proposed site suitability guidelines.