Symposia

Risk and Performance Analysis of Disposal Options

Robert Budnitz, Lawrence Berkeley National Laboratory

Abstract. The acceptability of any technical approach to disposing of high-level radioactive waste or spent reactor fuel deep underground depends on being able to demonstrate that such disposal can be accomplished with acceptable consequences vis-a-vis public health, safety and security. This demonstration is invariably tailored to support a regulatory judgment by a national nuclear regulatory agency, which in turn will have promulgated legal standards to govern the level of safety and security that must be achieved. Because of the long half-lives of the radioactive constituents, the time period for such a demonstration is inevitably very long (millennia or even hundreds or thousands of millennia), meaning that the demonstration can only be accomplished by analysis rather than by direct experimental evidence. (Of course, many different kinds of experimental evidence are necessary to support the analysis.) In this talk, a review will be presented of the considerations needed to accomplish such an analysis, and of the methods that have been developed, along with a discussion of the great challenges that such a demonstration must address, given the long time periods and major uncertainties involved.

Nanofluids for Enhanced Economics and Safety of Nuclear Reactors

Jacopo Buongiorno, MIT

Abstract. Nanofluids are engineered colloidal suspensions of nanoparticles in water, and exhibit a very significant enhancement (up to 200%) of the boiling Critical Heat Flux (CHF) at modest nanoparticle concentrations (less than or equal to 0.1% by volume). Since CHF is the upper limit of nucleate boiling, such enhancement offers the potential for major performance improvement in many practical applications that use nucleate boiling as their prevalent heat transfer mode. The Massachusetts Institute of Technology (MIT) is exploring the nuclear applications of nanofluids, specifically the following three:

• Main reactor coolant for PWRs
• Coolant for the Emergency Core Cooling System (ECCS) of both PWRs and BWRs
• Coolant for in-vessel retention of the molten core during severe accidents in high-power-density LWRs

The main features and potential issues of these applications are discussed in the paper. The first application could enable significant power uprates in current and future PWRs, thus enhancing their economic performance. Specifically, the use of nanofluids with at least 32% higher CHF could enable a 20% power density uprate in current plants without changing the fuel assembly design and without reducing the margin to CHF. The nanoparticles would not alter the neutronic performance of the system significantly. A RELAP5 analysis of the large-break loss of coolant accident in PWRs has shown that the use of a nanofluid in the ECCS accumulators and safety injection can increase the peak-cladding-temperature margins (in the nominal-power core) or maintain them in uprated cores, if the nanofluid has higher post-CHF heat transfer rate. The third application can increase the margin to vessel breach by 40% during severe accidents in high-power density systems such as Westinghouse AP1000 and the Korean APR1400. In summary, the use of nanofluids in nuclear systems seems promising, however several significant gaps are evident, including, most notably, demonstration of the nanofluid thermal-hydraulic performance at prototypical reactor conditions and the compatibility of the nanofluid chemistry with the reactor materials. These gaps must be closed before any of the above applications can be implemented in a nuclear power plant.

Direct Energy Conversion

Gang Chen, MIT

Abstract. In this talk, I will discuss a few direct-energy conversion technologies that could potentially be useful for directly converting fission energy into electricity, particularly thermoelectric energy conversion. Under a temperature gradient, charges (electrons and holes) in solids will diffuse from the hot-side to the cold-side, creating a measurable voltage. This process, called Seebeck effect, is the basis for thermoelectric power generation. The efficiency of thermoelectric generators depends on the temperature difference between the hot and cold sides of the device and the materials' figure of merit, ZT. This dimensionless quantity is proportional to the electrical conductivity s, the square of the Seebeck coefficient S, the average device temperature T, and inversely proportional to the thermal conductivity k, i.e., ZT=S2sT/k. Recent progress in improving figure-of-merit will be discussed. In addition to thermoelectric energy conversion, I will also briefly discuss some other direct energy conversion technologies.

Innovative Reprocessing Techniques

Emory Collins, Oak Ridge National Laboratory

Abstract. The characteristics and composition of low-enriched-uranium spent nuclear fuel currently produced predominantly in commercial light-water reactors, govern the selection of reprocessing methods needed to separate the desired components, recover those that are re-usable, and manage the remaining wastes. All reprocessing methods must address the functions of

1. disassembly of the solid fuel, cladding, and hardware;
2. fuel-cladding separation;
3. fuel dissolution;
4. component separation;
5. product conversion to recycle fuels;
6. waste treatment; and
7. reagent recovery and re-use.

The PUREX process is currently used for spent fuel reprocessing in commercial plants located in France, Russia, Japan, and the United Kingdom. New safeguards requirements for future commercial reprocessing will include the use of reprocessing methods and equipment that preclude separation of pure plutonium and thereby reduce the possibility of fissile material diversion. In addition, requirements for improved waste repository capacities and reduced amounts of radiotoxic wastes will require the addition of minor actinide recovery and recycle. Moreover, environmental regulations will require improved retention of volatile radionuclides, and waste minimization efforts will require the recovery and re-use of other spent fuel components. All of these new requirements will demand the development of innovative and economic reprocessing techniques.

Avenues in Computational Design and Safety of Nuclear Reactors

Truc-Nam Dinh, Royal Institute of Technology, Stockholm, Sweden

Abstract. Development of nuclear power in the 20th century played an important role in the advancement of the modern computing technology, computational science, multiphase flow and heat transfer, probabilistic risk analysis and other branches of engineering science. For example, fundamentals of computational fluid dynamics, two-phase flow modeling, etc. – first developed for reactor design and plant safety analysis, later paved their ways to remarkable advances and successful applications with other industries. Nuclear power industry also pioneered the use of computational tools, e.g. system codes, in the design, safety evaluation and licensing of complex engineered systems.

In this talk, we discuss the emergence of the next generation of computational tools for nuclear reactor design and safety analysis, their role in securing the nuclear renaissance and their potential contributions to the 21st century's engineering science and environmentally-conscious technology. Due to economic imperative and stringent safety requirements, the plant technology has become increasingly demanding and complex, even when it looks simpler. The key to the plant's economic, reliable and safe operation is in understanding the multi-physics nature and managing the complexity of rare but potentially high-consequence 'events'. On the one hand, advanced methods of multiphysics, multiscale simulation are a must for high-fidelity quantification of various safety threats, as required to manage the narrowing safety margins. On the other hand, novel probabilistic-deterministic engines are needed to enable effective search for system vulnerability. We discuss the intellectual content of these avenues, their complementary relation, and implementation challenges, particularly in ensuring consistence between efforts to advance simulation methods and evolution of state-of-the-art experimental and diagnostic capability, and the risk-oriented fit-for-purpose strategy in resource utilization.

Materials Innovation for the 'Back-End' of the Nuclear Fuel Cycle: Advanced Nuclear Materials & Waste Forms

Rodney Ewing, U. Michigan

Abstract. Increasing global energy demands combined with the need to reduce carbon emissions has lead to a resurgence of interest in nuclear energy. In the United States, the Advanced Fuel Cycle Initiative and the Global Nuclear Energy Partnership consider new nuclear fuel cycle strategies that involve a combination of the advanced processing technologies for nuclear fuels and the reduction of radionuclide inventories by transmutation. These strategies offer new opportunities for the development of innovative materials for the storage and disposal of nuclear waste. In some cases, materials may be 'dual use', such as inert matrix fuels that are also good nuclear waste forms for direct geologic disposal. Principal considerations in the performance requirements for such materials include: the chemistry of the radionuclide, its half-life, its radiotoxicity and, its mobility in the environment. The principal design considerations include:

1. atomic-scale incorporation of the radionuclide and decay products into the solid;
2. corrosion and release mechanisms of incorporated radionuclides;
3. radiation response of the material;
4. response to evolving thermal and radiation fields;
5. long-term performance in different geochemical and hydrologic regimes.

In some cases, composites may be used to optimize different materials properties. One of the major challenges of this type of research is to predict and confirm long-term performance up to periods of hundreds of thousands of years. In order to do this, studies of the corrosion, alteration and radiation response of natural materials is necessary. The development of these new materials, or their composites, plays an important role in safety assessments of geologic repositories – adding yet another barrier between the disposed radionculides and the accessible environment. This paper reviews some of the requirements for the design of corrosion and radiation 'resistant' materials that selectively incorporate radionuclides of high environmental impact.

Fast Reactors: Options and Challenges

Ehud Greenspan, University of California, Berkeley

Abstract. An overview will be given of the technical options for fast reactors including:

• Sodium-cooled reactors
• Lead-alloy cooled reactors
• Steam-cooled fast reactors
• Gas-cooled fast reactors
• Nuclear-battery type reactors

The overview will include a brief description of each of the reactor technologies and a brief discussion of the status of their technical maturity. The ability of the different reactor technologies to contribute to achieving the goals of the GNEP program and of the development of a sustainable nuclear energy system that is friendly to the environment and that could be available to developing countries will be discussed. Open questions related to the commercialization of the most promising reactor options will be defined and research required to address these questions will be suggested.

Advances in Ceramic Materials

Akira Kohyama, Institute of Advanced Energy, Kyoto University

Abstract. Carbon and ceramic materials have been utilized in many areas of fission reactors and the role of those high temperature materials is becoming more important for the advanced nuclear reactor systems. On the other hand, fiber reinforced composite materials have been historical and classical tailored structural materials.

In a few decades, R & D efforts on ceramic composites have been very extensive, especially in the fields of aero-space and energy. Among them, C/C and SiC/SiC R & Ds have been very much emphasized in nuclear energy research.

In the field of fission reactor R & D, gas reactor technology R & D activities have a wide spectrum from near term PMR/PBR to generation IV reactors of very high temperature reactor (VHTR) and gas cooled fast reactor (GFR). Although there have been many progresses in conceptual design study but supporting activities from engineering and materials have been still pre-mature and insufficient.

The materials R & D methodology has been quite unique for the case of ceramic fiber reinforced ceramic matrix composite materials, such as, C/C, C/SiC, SiC/SiC, where ceramic fibers, ceramic matrix and interphase connecting fiber and matrix are the three key components and materials design is done by optimizing these components to meet the material requirement.

In this presentation, starting with the brief introduction of ceramic fiber reinforced ceramic composite materials, material design, process engineering, and performance evaluation methodologies toward the application to advanced nuclear reactors will be provided.

Fusion Breeders to Fuel Fission Burners, a New (Old) Idea for Fusion Development

Wallace Manheimer, US Naval Research Laboratory (Retired)

Abstract. This talk first gives a very brief overview of fusion and its current status.  It then introduces the fusion hybrid, i.e. using fusion neutrons to breed fissile material for fission reactors.  The major part of the talk makes the case that the fusion hybrid could have a significant role to play in sustainable development [1,2].  It focuses principally on one possible configuration, the 'energy park'.  This is an area of a few square miles containing 7 1GWe reactors.  One of these, the key to the energy park, is an ITER sized fusion reactor run as a hybrid.  In addition to the power it produces, it also produces fuel for 5 other reactors of today's design (GEN <~3).  Also in the park is a separation plant which separates the uranium and actinides from other waste, and a cooling pool  to store the short lived, highly radioactive waste for several hundred years.  The seventh reactor is an actinide burner.  The park treats its own waste with a combination of fission, fusion and patience.  Only thorium enters, only electricity (or hydrogen) leaves.  The park has little or no proliferation risk.  In another configuration, the fusion breeder could produce proliferation proof fuel for existing burners or  for export, even to countries we did not fully trust; as long as spent fuel is sent back for treatment.  Another intriguing possibility is fission breeders fueling only themselves, while the much more prolific fusion breeders fuel a large legacy of fission burners.

Finally the talk will discuss the research necessary to make this a reality.  Five of the seven reactors and the separation plant could be built today.  The actinide burner, which might be either a fast or thermal neutron reactor would take some development.  The hybrid fusion reactor would take considerable development, but there is already a large fusion budget in place which might be used a resource for this.

1. W. Manheimer, The Fusion Hybrid as a Key to Sustainable Development, J. Fusion Energy, vol 23, #4, Dec 2004 (cc2005)
2. W. Manheimer, Can Fusion and Fission Breeding Help Civilization Survive?, J. Fusion Energy, vol 25, p 121, 2006

High Burnup Fuels

Mitchell Meyer, Idaho National Laboratory

Geoscience Needs for Subsurface Storage

Mark Peters, Argonne National Laboratory

Abstract. The nuclear fuel cycle is a key concept when discussing a sustainable future for nuclear energy and nuclear waste management. To the first order, there are two approaches to the nuclear fuel cycle. An open (or once-through) fuel cycle, as currently planned by the United States, involves simply disposing of spent nuclear fuel as waste within a geologic repository. New nuclear fuel is only derived from mined uranium. In contrast, a closed (or recycle) fuel cycle, as currently planned by other countries (including France, Russia, and Japan), involves reprocessing spent nuclear fuel by separating useable actinides to put into new fuel rods and disposing of other elements that are inappropriate for nuclear fuel in a geologic repository. It is important to note that regardless of the fuel cycle approach, a geologic repository is required to dispose of long-lived radioactive material. The constraints for a repository design are primarily the type of waste form(s) to be disposed of and the inventory of radionuclides they contain.

There is scientific consensus that the disposal of spent nuclear fuel and high-level radioactive waste in deep geologic formations is potentially safe and feasible, provided that sites are chosen and characterized well, and that the combination of engineered and natural barriers is designed appropriately. Geologic systems are considered suitable for radioactive waste disposal because of their stability over long time periods, their ability to physically and chemically isolate the waste canisters, their property to limit or significantly retard the release of radionuclides, and their relative inaccessibility, preventing unintentional or malevolent interventions. For geologic repositories, the main scenario of possible pathways for radionuclides to reach the biosphere and expose humans to unacceptably high radiation doses is the transport by groundwater that contacts and corrodes waste containers, dissolves the waste form, and leaches the radionuclides into the water, where they migrate in dissolved or colloidal form towards locations where the contaminated groundwater is used as drinking water or for agricultural purposes. Other release and transport mechanisms (such as volcanism, earthquakes, erosion, meteors, and human intrusion) also need to be considered.

The barriers important to waste isolation are broadly characterized as engineered barriers and natural barriers associated with the geologic and hydrologic setting. The engineered barriers are designed specifically to complement the natural system in prolonging radionuclide isolation within the disposal system and limiting their potential release. Natural barriers would contribute to waste isolation by: (1) limiting the amount of water entering emplacement drifts, and (2) limiting the transport of radionuclides through the natural system. In addition, the natural system provides an environment that would contribute to the longevity of the engineered components (disposal canisters and waste forms). The components of the engineered system are designed to complement the natural barriers in isolating waste from the environment.

The grand technical challenge for nuclear waste disposal is the need to understand and predict with sufficient confidence flow and transport processes and performance of materials (engineered and geologic) over geological time scales (at least to a million years), with long-term climate changes and the impact of extreme (disruptive) events (e.g., seismic and volcanic events) taken into account. It is important to enhance the interaction of engineered barriers with natural systems, to maintain retrievability and monitoring, and to prioritize/address the performance in a regulatory framework. The longevity of engineered barrier components depends on the quantity and chemistry of fluids in the surrounding natural system. Finally, there is a need to establish a sound foundation for model abstraction and stochastic approaches used for performance assessment. Specific research opportunities include:

1) Engineered materials performance

A major component of the long-term strategy for safe disposal of nuclear waste is, first, to completely isolate the radionuclides in waste packages for long times, and then, to greatly retard the egress and transport of radionuclides from breached packages. Corrosion is a primary determinant of waste package performance in any storage or disposal environment and will control the delay time for radionuclide release and transport from the waste package. Materials optimization must be realized through a coordinated R&D program. Areas of enhanced understanding include: a) long-term behavior of protective, passive films; b) composition and properties of moisture in contact with metal surfaces; c) rate of penetration and extent of corrosion damage over extremely long times.

2) Radioactive waste form (which is the source for radionuclide releases)

The goal of source term research is to enhance the understanding of the performance and evolution of nuclear waste forms (mainly spent nuclear fuel and nuclear waste glass) and to quantify the release of radionuclides in the evolving near-field environment expected in the repository. A basic understanding of the fundamental mechanisms of radionuclide release and a quantification of the release as repository conditions evolve over time, particularly at longer times (>105 years), must be developed. Radionuclide release will be sensitive to variations in temperature, the radiation field, redox conditions, pH, PCO2, surface area-to-solution volume, and the presence of near-field materials. Among the important processes that can control radionuclide release are:

• the kinetics of waste form corrosion;
• the formation of secondary, alteration phases;
• the reduction and sorption onto the surfaces of near-field materials.

In addition, biogeochemical processes and microbial activity may influence the geochemical environment and promote colloid formation with resultant impacts on waste form stability and radionuclide transport.

3) Natural systems

The term 'geologic disposal' highlights the key role that the natural system plays in isolating the waste from the biosphere for as long as it poses significant risks. Assessment of the barrier capability of the natural system and demonstration of its significance for repository performance need to be based on:

• an understanding of the features, events, and processes that could transport radionuclides from the repository to the accessible environment;
• laboratory experiments and field observations and tests to appropriately characterize the relevant properties at a potential repository site;
• conceptual, mathematical, and numerical models that predict the behavior and performance of the repository system given its site-specific characteristics and properties;
• natural analogs, which help build confidence that the model predictions can be reliably extrapolated to the long time scales required for waste isolation.

Specific features, events, and processes of interest include long-term climate change; groundwater recharge and discharge; percolation processes and definition of groundwater flow fields; near-field effects and thermally coupled processes; radionuclide transport; and low probability disruptive events (such as volcanic and seismic events).

* Based on discussion in M.T. Peters et al., Technical Perspectives Resource Document in Basic Research Needs for Geosciences: Facilitating 21st Century Energy Systems, DOE (2007).

Advanced Fuels for Actinide Recycling

Sylvie Pillon, CEA - France

Abstract. For the management of high level and long life radioactive waste (HLLW), a large and continuous research and development effort is carried out in France in order to provide a wide range of scientific and technical alternatives according to three main options: partition and transmutation, disposal in deep geological formations and long term interim surface or subsurface storage.

For the feasibility demonstration of the HLLW transmutation in nuclear reactor (essentially Np, Am and Cm), an important program dedicated to the development of fuels containing minor actinides is underway. The transmutation of minor actinides has been envisaged to take place preferably in fast critical power reactors of the 4th generation (Sodium-cooled Fast Reactor : SFR and Gas-cooled Fast Reactor : GFR), or in fast sub-critical reactors specifically designed to incinerate minor actinides (Accelerator Driven System: ADS).

Concerning the critical reactors, two recycling methods can be distinguished. Minor actinides can be diluted in all or in part of the fuel, in sufficiently weak concentrations as not to affect fuel and core performance: this is known as homogeneous recycling. Minor actinides can also be concentrated into targets and managed separately from the fuel within the core or in the core periphery: this is known as heterogeneous recycling. As for sub-critical reactor, ADSs employ a fuel composed of minor actinides that significantly contribute to core reactivity.

Contrarily to the homogeneous fuel, the performance of the targets and ADS fuels is very different from that of a standard fuel due to their composition. So a large irradiation program is carrying out to collect data necessary to model their behaviour under irradiation. This program is conducted within a collaboration with industrial partners (EDF) and in the frame of various international collaboration (Europe, USA, Japan, Russia), using the main material test reactors (MTR) in operation (HFR, BOR60 and Phénix).

This talk summarizes the main results of this irradiation program.

Thorium Reactors

Ratan Sinha, Bhabha Atomic Research Centre- India

Abstract. Thorium fuel cycle can be commercially implemented, in a closed fuel cycle mode, provided several technologies in the area of fuel manufacture and fuel reprocessing are developed to provide the necessary economic competitiveness at the required scales of operation. Thorium can be used in conventional reactors, as has been already demonstrated in the past. One has to, however, recognize that the deployment of thorium on a large scale will be needed in a time frame when the volume of nuclear energy related activities globally may increase multi-fold, necessitating enhanced safety, security, waste management and proliferation resistance related concerns to be addressed. It is therefore prudent to conceive of thorium reactors along with a necessity to fulfill enhanced requirements in several key areas.. The presentation will address issues and scientific challenges associated with large scale commercial implementation of thorium reactors and identify some associated R&D directions. The presentation will also provide a broad over view of the Indian activities in these directions.

The Role of Nuclear in the Future Energy Mix

Andrew White, GE Hitachi Nuclear Energy

Abstract. Managing global greenhouse gas emissions from power generation is a daunting challenge and will require a portfolio of technology solutions. As policymakers and utilities move towards 'greener' energy options, the commercial nuclear industry is ramping up to help to meet the demand for emissions-free, base load energy generation. The question for the nuclear industry isn't 'will we build new nuclear power plants,' but instead 'how many can we build and over what time horizon?' The answer to this question will depend on our ability to solve several pressing challenges.

To be sure, there are questions of regulatory certainty. But to a much larger extent, the industry must meet challenges of initial capital costs and construction schedules, human resource availability and expertise, and materials performance and accessibility, while at the same time ensuring that plants operate with optimal safety and reliability. Additionally, we must meet the challenge of managing spent nuclear fuel in the most cost-efficient, sustainable way possible. Industry and academia can partner to find innovative solutions to these challenges to accelerate the positive impacts of a dramatic increase in commercial nuclear power.

Atomistic Simulation for the Development of Advanced Materials

Brian Wirth, University of California, Berkeley

Abstract. Radiation damage, and its consequence to a wide range of material properties, is a central issue in many advanced technologies, from light water reactor lifetime extension to the future development of advanced fission and fusion power plants. Irradiation effects are initiated by the violent displacement of atoms from their lattice sites, and ultimately determined by the subsequent diffusional transport and evolution of the point defects and their clusters, along with the transport of solutes and impurities.

Fortunately, nature has provided hints that it may be possible to scientifically tailor materials to be extremely resistant to irradiation effects by promoting the efficient recombination, or 'self-healing', of the vacancy and self-interstitial point defects that are the principal radiation damage products. This presentation will first introduce the inherently multiscale nature of irradiation effects in materials and then describe a multiscale research paradigm involving a close integration of atomistic-based computational modeling and advanced experimental techniques required to better understand irradiation effects. Select results will be presented from this hierarchical, but atomic-scale based modeling approach that highlight recent improvements in understanding materials degradation in nuclear environments.

In particular, results will be presented that provide a basis for understanding the long-term evolution of displacement cascades and damage accumulation in irradiated Fe-Cu alloys, the observations of Cr enrichment and depletion at grain boundaries observed in various irradiation experiments performed on binary Fe-Cr and ferritic-martensitic alloys, and the dislocation – obstacle interactions in irradiated fcc metals, which control observed mechanical property changes of irradiated materials.

Finally, the scientific questions that determine the potential development of irradiation resistant materials will be discussed.