The catastrophic events that occurred at the Fukushima-Daiichi nuclear power plant in 2011 have led to widespread interest in research of alternative fuels and claddings that are proposed to be accident tolerant. The United States Department of Energy (DOE) through its Nuclear Energy Advanced Modeling and Simulation (NEAMS) program has funded an Accident Tolerant Fuel (ATF) High Impact Problem (HIP). The ATF HIP is a three-year project to perform research on two accident tolerant concepts. The final outcome of the ATF HIP will be an in-depth report to the DOE Advanced Fuels Campaign (AFC) giving a recommendation on whether either of the two concepts should be included in their lead test assembly scheduled for placement into a commercial reactor in 2022. The two ATF concepts under investigation in the HIP are uranium silicide fuel and iron-chromium-aluminum (FeCrAl) alloy cladding. Utilizing the expertise of three national laboratory participants (Idaho National Laboratory, Los Alamos National Laboratory, and Argonne National Laboratory), a comprehensive multiscale approach to modeling is being used that includes atomistic modeling, molecular dynamics, rate theory, phase-field, and fuel performance simulations. Model development and fuel performance analysis are critical since a full suite of experimental studies will not be complete before AFC must prioritize concepts for focused development. In this paper, we present simulations of the two proposed accident tolerance fuel systems: U3Si2 fuel with Zircaloy-4 cladding, and UO2 fuel with FeCrAl cladding. Sensitivity analyses are completed using Sandia National Laboratories' Dakota software to determine which input parameters (e.g., fuel specific heat) have the greatest influence on the output metrics of interest (e.g., fuel centerline temperature). We also outline the multiscale modelling approach being employed. Considerable additional work is required prior to preparing the recommendation report for the Advanced Fuels Campaign.

"To mitigate global warming, we need to develop carbon-free ways to generate power. Nuclear energy currently generates more carbon-free power in the United States than all other sources combined at 55%. To make nuclear as viable a power source as possible, we need to maximize power density and safety. Both of these can be improved with Accident Tolerant Fuel (ATF) materials. Uranium nitride (UN), a candidate ATF material, offers high fuel economy due to its uranium density and improved safety margins from thermal properties. However, its instability in the presence of water, a reactor coolant, must be addressed. This dissertation employs Density Functional Theory-based methods to investigate the atomistic and electronic mechanisms in UN corrosion initiation. To ensure accuracy in future UN models, the effects of magnetic treatments on UN surface stability and corrosion properties are also determined. The performance of advanced nuclear materials must be tested in research reactors before they can be implemented in power reactors. To get real-time temperature data from these tests, sensors are required that can survive the high temperatures and irradiation. To meet these needs, Idaho National Laboratory has been developing High Temperature Irradiation Resistant Thermocouples (HTIR-TCs). Towards increasing temperature resolution and in-pile lifetime, an ab initio method has been developed to predict HTIR-TC performance. The method considers the effects of composition and temperature on performance and has been validated against experiment. To predict the interaction of HTIR-TCs with research reactor coolant, corrosion and oxidation mechanisms have been investigated. By examining the diffusion behaviors of water and oxygen, recommendations are made for which thermoelement materials may be the most resistant to corrosion and/or oxidation."--Boise State University ScholarWorks.

Accident-tolerant fuels (ATFs) are fuels and/or cladding that, in comparison with the standard uranium dioxide Zircaloy system, can tolerate loss of active cooling in the core for a considerably longer time period while maintaining or improving the fuel performance during normal operations [1]. It is important to note that the currently used uranium dioxide Zircaloy fuel system tolerates design basis accidents (and anticipated operational occurrences and normal operation) as prescribed by the US Nuclear Regulatory Commission. Previously, preliminary simulations of the plant response have been performed under a range of accident scenarios using various ATF cladding concepts and fully ceramic microencapsulated fuel. Design basis loss of coolant accidents (LOCAs) and station blackout (SBO) severe accidents were analyzed at Oak Ridge National Laboratory (ORNL) for boiling water reactors (BWRs) [2]. Researchers have investigated the effects of thermal conductivity on design basis accidents [3], investigated silicon carbide (SiC) cladding [4], as well as the effects of ATF concepts on the late stage accident progression [5]. These preliminary analyses were performed to provide initial insight into the possible improvements that ATF concepts could provide and to identify issues with respect to modeling ATF concepts. More recently, preliminary analyses for a range of ATF concepts have been evaluated internationally for LOCA and severe accident scenarios for the Chinese CPR1000 [6] and the South Korean OPR-1000 [7] pressurized water reactors (PWRs). In addition to these scoping studies, a common methodology and set of performance metrics were developed to compare and support prioritizing ATF concepts [8]. A proposed ATF concept is based on iron-chromium-aluminum alloys (FeCrAl) [9]. With respect to enhancing accident tolerance, FeCrAl alloys have substantially slower oxidation kinetics compared to the zirconium alloys typically employed. During a severe accident, FeCrAl would tend to generate heat and hydrogen from oxidation at a slower rate compared to the zirconium-based alloys in use today. The previous study, [2], of the FeCrAl ATF concept during station blackout (SBO) severe accident scenarios in BWRs was based on simulating short term SBO (STSBO), long term SBO (LTSBO), and modified SBO scenarios occurring in a BWR-4 reactor with MARK-I containment. The analysis indicated that FeCrAl had the potential to delay the onset of fuel failure by a few hours depending on the scenario, and it could delay lower head failure by several hours. The analysis demonstrated reduced in-vessel hydrogen production. However, the work was preliminary and was based on limited knowledge of material properties for FeCrAl. Limitations of the MELCOR code were identified for direct use in modeling ATF concepts. This effort used an older version of MELCOR (1.8.5). Since these analyses, the BWR model has been updated for use in MELCOR 1.8.6 [10], and more representative material properties for FeCrAl have been modeled. Sections 2 4 present updated analyses for the FeCrAl ATF concept response during severe accidents in a BWR. The purpose of the study is to estimate the potential gains afforded by the FeCrAl ATF concept during BWR SBO scenarios.

The Accident Tolerant Fuel (ATF) program is focused on extending the time for fuel failure during postulated severe accidents compared to the standard UO2-Zr alloy fuel system. This thesis investigates the feasibility of four different cladding concepts, two of which are zirconium-alloy based and two are SiC-based. The Zirconium-alloy based claddings are 1) Zr4-Chromium coated cladding and 2) Zr4-FeCrAl coated cladding with a molybdenum interlayer (Zr4-Mo/FeCrAl). The SiC-based claddings are 3) composite SiC coated with chromium (SiC/SiC-Cr) and 4) Three layered SiC cladding consisting of inner and outer monolith with a composite layer sandwiched in between (mSiC-SiC/SiC-mSiC). The coated claddings were kept to a 50[mu]m of coating thicknesses, deducted from the base layer thicknesses. The claddings were studied, using the multi-physics fuel performance tool MOOSE/BISON, under steady-state PWR operating conditions as well as two transients: power ramp and loss-of-coolant accident (LOCA). The major finding is that the chromium coated concepts proved to be the most promising in both Zr4 and SiC based claddings. The three layered SiC cladding showed a high probability of failure during normal operation and transient conditions, while the Zr4-Mo/FeCrAl cladding showed high plastic strains in the molybdenum layer making its possibilities of survival questionable. On the other hand, the Zr4-Cr and SiC/SiC-Cr concepts showed acceptable plastic strains for the chromium coatings, with the SiC/SiC-Cr being more advantageous during LOCA scenarios. Both concepts warrant further experimental investigation as well as modelling of beyond design-basis accidents.

As part of a broader spectrum of collaborative activities underpinning nuclear materials research, the Nuclear Energy Agency is supporting worldwide efforts towards the development of advanced materials, including fuels for partitioning and transmutation purposes and accident-tolerant fuels (ATFs). This state-of-the-art report on ATFs results from the collective work of experts from 35 institutions in 14 NEA member countries, alongside invited technical experts from the People's Republic of China. It represents a shared and consensual position, based on expert judgment, concerning the scientific and technological knowledge related to ATFs. The report reviews available information on the most promising fuels and cladding concepts in terms of properties, experimental data and modelling results, as well as ongoing research and development activities. It also includes a description of illustrative accident scenarios that may be adopted to assess the potential performance enhancement of ATFs relative to the current standard fuel systems in accident conditions, a definition of the technology readiness levels applicable to ATFs, a survey of available modelling and simulation tools (fuel performance and severe accident analysis codes), and the experimental facilities available to support the development of ATF concepts. The information included in this report will be useful for national programmes and industrial stakeholders as an input to setting priorities, and helping them to choose the most appropriate technology based on their specific strategy, business case and deployment schedules.

The purpose of Accident Tolerant Fuels (ATF) experiments is to test novel fuel and cladding concepts designed to replace the current zirconium alloy uranium dioxide (UO2) fuel system. The objective of this Research and Development (R & D) is to develop novel ATF concepts that will be able to withstand loss of active cooling in the reactor core for a considerably longer time period than the current fuel system while maintaining or improving the fuel performance during normal operations, operational transients, design basis, and beyond design basis events. It was necessary to design, analyze, and fabricate drop-in capsules to meet the requirements for testing under prototypic LWR temperatures in Idaho National Laboratory's Advanced Test Reactor (ATR). Three industry led teams and one DOE team from Oak Ridge National Laboratory provided fuel rodlet samples for their new concepts for ATR insertion in 2015. As-built projected temperature calculations were performed on the ATF capsules using the BISON fuel performance code. BISON is an application of INL's Multi-physics Object Oriented Simulation Environment (MOOSE), which is a massively parallel finite element based framework used to solve systems of fully coupled nonlinear partial differential equations. Both 2D and 3D models were set up to examine cladding and fuel performance.