Our research work encompasses a number of elements of this domain, from the impacts of new technology and policy decisions on resource utilization and waste generation to addressing current challenges pertaining to the responsible long-term disposition of used nuclear fuel. Current areas of interest include fuel cycle assessments of novel reactor concepts (including molten salt reactors and other Gen-IV reactor types) used to inform policymakers as to long-term R&D priorities, analysis in support of continued safe storage of used nuclear fuel assemblies stored at reactor sites across the U.S. and preparations for eventual disposition, as well as modeling and simulation to support the development and deployment of material accountancy and safeguards techniques for the next generation of fuel cycle facilities.
Nuclear Fuel Cycle Safeguards
With the promise of transformative capabilities in advanced nuclear fuel cycles also comes tremendous challenges, especially in terms of developing viable strategies for material accountancy and safeguards. Material recovery techniques such as electrochemical separations (“pyroprocessing”), advanced reactor concepts such as liquid-fueled molten salt reactors, and fuel cycle concepts based upon continuous recovery and reuse of long-lived actinide materials all present challenges to current safeguards methodologies oriented chiefly around light water reactor designs. Our research leverages understanding of fuel cycle facilities and operations to devise new strategies for measurements and analyses (including how most effectively to employ radiation detection techniques) in order to meet these challenges, ensuring that the next generation of nuclear technologies can realize the potential of enhanced economics, resource utilization, and sustainability without compromising security.
Some recent examples of our work include:
- Assessments of material accountancy strategies for electrochemical separations facilities
- New techniques for measuring and quantifying trace fissile material samples
- Developing analysis techniques for material accountancy in molten salt reactors
Advanced Nuclear Fuel Cycles & Radioactive Waste Management
One of the key long-term sustainability concerns with the nuclear fuel cycle is the issue of high-level radioactive waste management. Numerous options for advanced fuel cycles have been proposed premised on the recovery and reuse of some or all of the fissionable, long-lived actinide materials. Key to making decisions about the transition to alternative fuel cycles is an understanding of the relative tradeoffs, including assessing the viability of different strategies, further research needs required to realize such systems, and the long-term costs and benefits of different fuel cycle options.
Meanwhile, regardless of whether the United States chooses to move beyond the present once-through fuel cycle, the federal government must fulfill its obligations to see through the responsible final disposition of used fuel assemblies stored at reactors throughout the country. Owing to the present absence of an operational geologic repository, this means that the present prognosis for used fuel management will entail storage times on the order of decades or more. As such, a particularly relevant concern is in understanding the long-term behavior of fuel materials held in storage, including how these considerations will influence planning for the eventual removal and transportation of these assemblies from reactor sites to a permanent repository.
Used nuclear fuel storage and disposition analysis
Our research includes a broad spectrum of activities within the realm of used nuclear fuel management, including assessments of material evolution of the fuel cladding material over time, source term assessments of fuel held in storage (including heat and radioactivity), instrumentation strategies for monitoring fuel held in storage, along with policy-relevant research into areas such as optimal removal and transportation strategies.
One of the most important considerations for long-term storage of used nuclear fuel in dry storage canisters pertains to the micro-structural changes to the cladding material over time. Here, the concerns primarily revolve around redistribution of hydrogen compounds within the zirconium cladding (“zirconium hydrides”). The elevated temperatures within dry storage canisters may allow for the migration of these hydride materials at sufficiently high temperatures. This realignment of hydrogen at a microstructural level within the clad may introduce changes to the macroscopic properties, including possible embrittlement of the fuel cladding material, which in turn has substantial implications for future transportation of storage casks. Some of our work thus focuses on different aspects of this problem, including evaluation of the time-dependent temperature evolution of the cladding materials within the U.S. inventory of dry storage canisters (working with ORNL and the UNF ST&NDARDS toolkit). Other work includes the development of materials performance models designed to predict the behavior of hydrogen within zirconium cladding such to better understand the macroscopic changes (such as changes to ductility and susceptibility to cracking and corrosion) to the cladding material over time.
Other work in this area includes collaborations with industry to evaluate novel instrumentation for monitoring of in-cask conditions, including indicators of corrosion or cladding failure.
Fuel cycle options assessment & decision-making tools
Essential to understanding both the benefits and challenges to realizing novel fuel cycle concepts are the tools to assess the requirements and impacts of different strategies. To this end, our work includes contributions to fuel cycle modeling and assessment tools, such as the open-source Cyclus nuclear fuel cycle simulator code (http://www.fuelcycle.org) and industry-supported tools such as eVinci and ORION. In each of these cases, the goal is to develop sufficiently robust tools and analysis methods such to enable assessments of the relevant tradeoffs for both novel reactor concepts as well as fuel cycle strategies (i.e., material recovery options) based on dimensions such as resource impacts, cost, gaps in technical knowledge, repository impacts, and security considerations, with the goal of being able to inform policymakers concerning R&D priorities. Examples of these types of options include the relevant impacts of new reactor concepts such as small modular reactors, molten salt reactors, fast-spectrum reactors designed to consume actinides, and microreactors for remote deployments, as well as novel use cases such as cogeneration of process heat and electricity from high-temperature reactors.
Molten Reactor Systems
Molten salt reactors are unique in their use of high-temperature alkali-halide salts as a fuel form. This feature makes their fuel cycles different from other reactor types and presents unique opportunities for safety and fuel cycle performance. Gaseous fission products, most importantly xenon-135, can be bubbled out of the fuel salt. Meanwhile, the fuel continuously homogenizes as it circulates about the primary loop, avoiding hotspots and depleting uniformly as the reactor generates power. Refueling can be done in continuous manner, reducing the need for excess reactivity compensation. The radical difference of liquid fuel also presents challenges for spent fuel handling, reprocessing, and safeguards. Both advantages and challenges of liquid fuel systems present compelling research directions.
Much of the work in the nuclear fuel cycles area is computational in nature, with our faculty leveraging strong connections with Oak Ridge National Laboratory located nearby. These collaborations include contributing to the development of state-of-the-art computational tools, including the SCALE code package used for reactor licensing and safety analysis, the VERA toolkit developed for the Consortium for Advanced Light Water Reactors (CASL), along with open-source tools such as the Cyclus nuclear fuel cycle simulator.
Nuclear Engineering Linux Cluster
Serving as the backbone for much of our computational research work is the growing Nuclear Engineering Linux Cluster (https://necluster.ne.utk.edu) which consists of over 60 computing nodes, most of them based on 8-core Intel i7 processors, presently totaling over 700 cores. The computing nodes are rack-mounted boxes, connected to the head node via Gigabit Ethernet. Engineering software available on the cluster includes the latest versions of standard simulation tools such as MCNP, SCALE, VERA, Serpent, ROOT, Geant4, and MATLAB/SIMULINK. The cluster’s scheduler uses Torque/Maui to closely resemble clusters available at ORNL, allowing users to run both small, single-threaded jobs as well as massively parallel jobs over multiple physical cores.