Basic Research Needs for Synthesis
The artwork on the cover is a cross-section scanning electron microscope image showing the corroded 

surface of Inconel 600 alloy after exposure to a mixture of molten salts at 750°C. Scanning electron 

microscope image courtesy of Donovan Leonard (Oak Ridge National Laboratory).
Basic Research Needs for Future Nuclear Energy

BASIC RESEARCH NEEDS WORKSHOP FOR

Future Nuclear Energy 

REPORT OF THE BASIC ENERGY SCIENCES WORKSHOP FOR FUTURE 

NUCLEAR ENERGY

Chair:

Kelly Beierschmitt (Idaho National Laboratory)

Co-chairs:

Michelle Buchanan (Oak Ridge National 

Laboratory)

Aurora Clark (Washington State University)

Ian Robertson (University of Wisconsin–Madison)

Panel Leads: 

Design and Discovery of Coolants and Liquid 

Fuels

Phillip Britt (Oak Ridge National Laboratory)

Alexandra Navrotsky (University of California–

Davis)

Physics and Chemistry of Interfaces

Amit Misra (University of Michigan)

Jim Wishart (Brookhaven National Laboratory)

Understanding Behavior at Coupled Extremes

Bruce Mincher (Idaho National Laboratory) 

Izabela Szlufarska (University of Wisconsin–

Madison) 

Design and Discovery of Structural Materials 

and Solid Fuels

Peter Burns (University of Notre Dame) 

Pete Tortorelli (Oak Ridge National Laboratory, 

retired)

Crosscutting Themes

Paul Fenter (Argonne National Laboratory)

Andy Gewith (University of Illinois, UrbanaChampaign)

Brian Wirth (University of Tennessee–Knoxville

Technical Coordinator:

Jeremy Busby (Oak Ridge National Laboratory)

Basic Energy Sciences Team:

Linda Horton

Bruce Garrett

John Vetrano

Philip Wilk

Katie Runkles, BES Administrative Lead

Office of Nuclear Energy Team:

Stephen Kung

Sue Lesica

Oak Ridge National Laboratory Publications Team: Brenda Wyatt, Deborah Counce, and Kathy 

Jones

https://science.energy.gov/bes/community-resources/reports/

Basic Research Needs for Future Nuclear Energy

Table of Contents iii

TABLE OF CONTENTS

Abbreviations, Acronyms, and Initialisms ............................................................................................... v

Executive Summary .................................................................................................................................. vii

1. Introduction ......................................................................................................................................... 1

2. Priority Research Directions .............................................................................................................. 3

PRD 1: Enable Design of Revolutionary Molten Salt Coolants and Liquid Fuels ............................... 3

PRD 2: Master the Hierarchy of Materials Design and Synthesis for Complex, Reactor 

Environments ......................................................................................................................... 11

PRD 3: Tailor Interfaces to Control the Impact of Nuclear Environments ......................................... 17

PRD 4: Reveal Multiscale Evolution of Spatial and Temporal Processes for Coupled 

Extreme Environments .......................................................................................................... 27

PRD 5: Identify and Control Unexpected Behaviors from Rare Events and Cascading 

Processes ................................................................................................................................ 35

3. Panel Reports .................................................................................................................................... 43

Panel A: Design and Discovery of Coolants and Liquid Fuels ........................................................... 45

Panel B: Physics and Chemistry of Interfaces .................................................................................... 57

Panel C: Understanding Behavior at Coupled Extremes .................................................................... 65

Panel D: Design and Discovery of Structural Materials and Solid Fuels ........................................... 75

Panel: Crosscutting Themes ................................................................................................................ 85

4. References .......................................................................................................................................... 95

Appendix A: Workshop Participants .................................................................................................... A-1

Appendix B: Workshop Agenda ............................................................................................................ B-1

Basic Research Needs for Future Nuclear Energy

Abbreviations, Acronyms, and Initialisms v

ABBREVIATIONS, ACRONYMS, AND INITIALISMS

0D zero-dimensional

1D 1-dimensional

2D 2-dimensional

3D 3-dimensional

AHTR Advanced High Temperature Reactor 

AIMD ab initio molecular dynamics

AM advanced manufacturing

APT atom probe tomography

ARE Aircraft Reactor Experiment

bcc body-centered cubic

BES Office of Basic Energy Sciences

BESAC Basic Energy Sciences Advisory Committee

BRN Basic Research Needs

CVD chemical vapor deposition

DFT density functional theory

DOE Department of Energy

EAC environment-assisted cracking

fcc face-centered cubic

GIF Generation IV International Forum

HBS high-burnup structure

HF hydrogen fluoride

IASCC irradiation-assisted stress corrosion cracking

LWR light water reactor

MD molecular dynamics

MGI Materials Genome Initiative

MSR molten salt reactor

MSRE Molten Salt Reactor Experiment

NE DOE Office of Nuclear Energy
Basic Research Needs for Future Nuclear Energy

vi Abbreviations, Acronyms, and Initialisms

NMR nuclear magnetic resonance

ORNL Oak Ridge National Laboratory

PDF pair distribution function

PRD Priority Research Direction

SANS small-angle neutron scattering

SEM scanning electron microscopy/microscope

STEM scanning transmission electron microscopy/microscope

TEM transmission electron microscopy/microscope

UV ultraviolet

VIS ultraviolet-visible

VHTR very-high-temperature reactor

XRD x-ray diffraction

XRIM x-ray reflection interface microscopy
Basic Research Needs for Future Nuclear Energy

Executive Summary vii

EXECUTIVE SUMMARY 

Nuclear power today accounts for approximately 20% of the electrical supply for the US. It provides 

reliable energy that helps to stabilize the national grid. In contrast to current nuclear power facilities, 

future concepts operate at higher temperatures, are operationally more efficient, more fully utilize the 

energy stored within the fuel, and reduce the amount of waste produced. Recent advances in chemistry 

and materials sciences provide an unprecedented opportunity to develop the critical systems—the fuel, 

coolant, and structural materials—needed for deployment of advanced nuclear reactor designs. To address 

the challenge of stringent demands on material systems as their properties and performance continuously 

evolve over the operational lifetime, the frameworks to uncover the underlying processes that cause 

reduced performance have been developed over the last decade. There is now the prospect of designing 

materials that report upon their own damage, are self-healing, and leverage what would be considered 

deleterious chemical processes to increase their performance within advanced reactor designs. Success 

will provide an energy source needed to secure a stable and safe energy supply for the nation. 

To identify the key research directions required to discover the chemical processes and material 

degradation mechanisms that result in loss of performance of the fuel, coolant, and structural materials, 

nearly 150 theoreticians, computational experts, and experimentalists from industry, national laboratories, 

academia, and federal agencies participated in a workshop held in August 2017. Based on the discussions 

held at the workshop, five Priority Research Directions (PRDs) were identified. These PRDs defined the 

key scientific challenges that must be met as well as the opportunities that can be exploited to achieve a 

multi-scale spatial and temporal understanding of fundamental processes that govern the properties and 

performance of the different material systems required for advanced reactors.

This report recognizes that the requirements needed to realize future nuclear energy systems present a 

daunting challenge for the chemical and materials sciences. Recent advances in modeling and simulation, 

data analytics, synthesis and processing science, and in situ characterization capabilities are discussed as 

an unparalleled opportunity: it is now possible to uncover the fundamental science governing the 

chemical processes and material responses that result in the degradation in properties and loss of 

performance of the fuel, coolant, and structural materials. The report concludes that the knowledge 

resulting from the PRDs defined during the workshop will enable the design, discovery, and production of 

revolutionary materials, coolants, and fuels required for operation in complex, evolving, harsh 

environments. The challenges and opportunities outlined will build the scientific basis required to further 

the development of a new generation of nuclear energy sources to meet the future energy needs of the 

nation for the next century.

Priority Research Directions for Future Nuclear Energy

Enable design of revolutionary molten salt coolants and liquid fuels. Key question: How can we 

characterize and predict the structure, dynamics, and energetics of molten salts— including an evolving 

chemical composition across length and time scales? For many future reactor concepts, molten salt 

coolants and liquid fuels are key elements. During reactor operation, these materials exhibit a 

heterogeneous and evolving chemical composition, leading to large variations in structure, dynamics, and 

thermodynamics over broad time and length scales. Elucidating the changes in chemical composition, as 

well as the impact of these changes on physical and chemical properties, is necessary to optimize their 

designs. Attaining understanding will require new approaches to understand phase evolution and 

thermodynamics, including the use of model systems to benchmark computational models, theories, and 

instrumentation. 

Master the hierarchy of materials design and synthesis for complex, reactor environments. Key 

question: How do we design, synthesize, and process superior materials able to function and perform 
Basic Research Needs for Future Nuclear Energy

viii Executive Summary

over decades in the extreme environments of advanced nuclear reactors? The infrastructure for nuclear 

energy consists of solid materials. The application of computational methods coupled with innovative 

synthesis and processing tools will permit tailoring both the composition and structure of solid fuels and 

structural materials for targeted functionality. This capability will foster precise creation of specific 

materials for experiments to probe the underlying physical and chemical processes and mechanisms 

responsible for the degradation of materials exposed to extreme radiation, heat, stress, and chemical 

reactivity. 

Tailor interfaces to control the impact of nuclear environments. Key question: How can the multitude 

of inextricably linked chemical and physical processes that occur at interfaces be controlled? Interfaces 

between materials and phases dominate the infrastructure of energy production—from structural 

components exposed to liquid coolants, to the interior microstructures of constituent materials. Interfaces 

in future nuclear systems—both fluid/solid and solid/solid—will be exposed to a wide variety of extreme 

and evolving conditions that can lead to degradation of materials and eventual failures. The complex 

networks of processes that occur at these interfaces need to be understood and controlled to ensure longterm performance and reliability.

Reveal multiscale evolution of spatial and temporal processes for coupled extreme environments. 

Key question: How can computational and experimental techniques be integrated to bridge spatial, 

temporal, and energy scales that underpin materials’ behavior and chemical transformations in coupled 

extreme environments? Many mechanical and chemical processes can occur separately or in combination 

over a wide range of time and length scales, acting to degrade materials subjected to the extremes 

envisioned for future reactor systems. Integration of experimental and computational capabilities is 

required to reveal the complex networks of processes and mechanisms that evolve over such broad 

temporal and spatial scales. This critical knowledge is required to inform the design of a new generation 

of materials and predict their long-term behavior and performance. 

Identify and control unexpected behaviors from rare events and cascading processes. Key question: 

How do we identify, anticipate, and control rare events that initiate cascading processes and cause 

aberrant properties and materials’ responses? In the extreme conditions found in nuclear energy 

environments, high-impact, low-frequency events can change the properties of materials in ways that 

could ultimately lead to unexpected behaviors and potentially catastrophic failures. Variability in 

materials, impurity-controlled reactivity, sensitivity to radiation/radiolysis, localized transient behavior, 

and chemical evolution can act separately or synergistically to trigger a critical event and launch a 

cascade of unforeseen events. Scientific understanding of these conditions is central to the design of 

future nuclear energy systems.
Basic Research Needs for Future Nuclear Energy

Introduction 1

1. INTRODUCTION

Nuclear energy is a critical component of the US energy portfolio, reliably producing energy around-theclock and complementing electricity generation from combustion of fossil fuels and from renewable 

sources such as solar and wind. Today, the United States has 99 operating nuclear power plants, largely 

based on a water-cooled reactor design, that supply approximately 20% of the nation’s electricity 

(https://www.eia.gov/energyexplained). The future of nuclear energy requires new concepts to move 

beyond the water-cooled designs of the previous century to achieve improved efficiency, greater fuel 

utilization, reduced high-level waste generation, and enhanced safety. 

Meeting these requirements necessitates developing entirely different concepts for reactor systems 

comprising coolants, fuels, and structural materials that can last through decades- to centuries-long 

reactor lifetimes and withstand some of the most extreme environments known—operational temperatures 

approaching 1000°C, intense radiation fields, and corrosive environments, among others. These 

conditions far exceed those of the current operating reactor fleet, requiring revolutionary advances in 

chemistry and materials science to realize the full potential of future nuclear energy systems. These 

advances will be achieved only by addressing gaps in our understanding of fundamental chemical and 

physical processes projected to occur under the extreme conditions encountered in future nuclear systems,

and by using this knowledge to design a new generation of materials and chemical systems. 

The Office of Basic Energy Sciences (BES) within the Department of Energy (DOE) Office of Science 

convened a workshop August 9–11, 2017, charged with identifying fundamental research priorities in 

chemistry and materials science needed to address these knowledge gaps. With this focus, the workshop 

was intended to go well beyond the topics covered by the 2006 Basic Research Needs workshop report on 

Advanced Nuclear Energy Systems and the 2007 Basic Research Needs workshop report on Materials 

under Extreme Environments, taking full advantage of the array of advances that have been realized in 

materials and chemistry—including characterization and computational techniques and tools—over the 

intervening decade. Before the workshop, BES worked closely with the DOE Office of Nuclear Energy 

(NE) to define future requirements for nuclear energy from the perspective of fuels, materials, and other 

areas relevant to future reactor concepts. In addition, leaders in nuclear energy from universities, industry, 

and national laboratories partnered to prepare a factual status document (posted separately with this report 

at https://science.energy.gov/bes/community-resources/reports/), which provided a reference for the 

workshop attendees and served as a foundation for the discussions.

The workshop included approximately 80 participants and 60 invited observers, representing universities, 

national laboratories, industry, and DOE offices (Appendices A and B). A plenary session opened the 

workshop with five presentations focused on key challenges and opportunities for future reactor concepts. 

The presentations emphasized the enormity of the challenges—understanding myriad physical and 

chemical processes across scales ranging from femtoseconds to decades and from angstroms to meters. 

Workshop participants were divided into four technical panels for in-depth discussions: 

1. Design and Discovery of Coolants and Liquid Fuels

2. Physics and Chemistry of Interfaces

3. Understanding Behavior at Coupled Extremes

4. Design and Discovery of Structural Materials and Solid Fuels

A fifth panel considered research opportunities that cut across the four panel areas. These panels were 

charged with defining Priority Research Directions (PRDs) that are critical to address the challenges for 

future nuclear energy technologies. 
Basic Research Needs for Future Nuclear Energy

2 Introduction

From these panel discussions, five PRDs were identified: 

1. Enable design of revolutionary molten salt coolants and liquid fuels

2. Master the hierarchy of materials design and synthesis for complex, reactor environments

3. Tailor interfaces to control the impact of nuclear environments

4. Reveal multiscale evolution of spatial and temporal processes for coupled extreme environments

5. Identify and control unexpected behaviors from rare events and cascading processes

These PRDs, discussed in depth in the next chapter of the report, define the key scientific challenges that 

must be met, as well as the opportunities that can be exploited, to achieve a multiscale spatial and 

temporal understanding of fundamental processes that govern the properties and performance of the 

different material systems required to function in advanced reactors. The remainder of the report is 

devoted to summaries of the broader panel discussions. These summaries provide additional context and 

background relevant to the community’s endeavors to advance this critical research. The challenges and 

opportunities outlined in this report provide the scientific base for innovation in materials systems and 

chemical processes required for nuclear energy sources that will safely and economically meet future 

energy needs.

Designing Materials for the Extreme Reactor Environment

Images courtesy of Oak 

Ridge National 

Laboratory, Pacific 

Northwest National 

Laboratory, and 

University of Michigan

Physical and chemical processes in extreme environments—including corrosion, oxidation, cracking, and 

fatigue—span broad length and time scales from atoms (left) to components (right), which can result in 

degradation of desired properties and, ultimately, failures.

Materials in nuclear reactors are exposed to some of the highest extremes in temperature, pressure, 

radiation, chemical reactivity, stress, and strain; and they must be designed to withstand these extremes for 

decades. Understanding and mitigating the mechanisms responsible for degradation are critical to meet 

the challenges for future nuclear reactors. Achieving this understanding requires information gleaned from 

experiments and modeling—from the atomic scale, to explain how atoms and molecules are arranged and 

may evolve, all the way to the mesoscale, to elucidate effects that result in macroscopic changes.
Basic Research Needs for Future Nuclear Energy

Priority Research Direction 1 3

2. PRIORITY RESEARCH DIRECTIONS

PRD 1: Enable Design of Revolutionary Molten Salt Coolants and Liquid Fuels

Key question: How can we characterize and predict the structure, dynamics, and energetics of molten 

salts, including an evolving chemical composition across length and time scales?

Over the past decade, efficiency, cost, and safety have motivated increased interest in the use of molten 

salts as coolants and fuels in next-generation nuclear reactors used for electricity generation and to supply 

high-temperature, low-pressure heat. The selection of the salt composition depends on the specific 

applications, fuel, and coolant and has numerous criteria and constraints. Consider solid-fuel salt-cooled 

reactors, wherein the molten salt flows over the solid fuel and removes heat to the primary heat exchanger 

and/or serves as the secondary coolant. In this case, the molten salt must have good thermal stability, 

good heat transfer and hydrodynamic properties, low vapor pressure and chemical reactivity, and 

chemical compatibility with the reactor materials. For a liquid salt–fueled reactor—in which fissile, 

fertile, and fission products are dissolved in the molten salt—the molten salt must further have good 

neutronics (i.e., a low neutron capture cross-section), radiochemical stability, and solubility of the fuel 

and fission products. To optimize the numerous (and in some cases conflicting) requirements of the 

physical, chemical, and radiochemical properties of molten salts for desired applications, new methods 

and tools are needed to characterize and predict their structure, dynamics, and energetics across length 

and time scales. These will lead to the development of new molten salt compositions with superior 

properties, which will directly impact the efficiency, safety, and operational lifetimes of next-generation 

nuclear reactors.

Scientific Challenges

The extreme operational conditions of molten salt coolants and liquid fuels challenge even state-of-the art 

experimental measurement and theoretical prediction. This difficulty limits understanding of the 

fundamental chemical and nuclear reactions and physical properties within these systems, and 

compromises the ability to optimize and leverage the full potential of advanced reactor systems. At any 

instant, the fluid may be heterogeneous and exist in localized equilibria that evolve in time as a result of 

dynamic chemical and nuclear processes. Complex speciation imparted by fission products and trace 

impurities has an unknown effect upon important properties, including heat and mass transfer, which in 

turn perturb the net thermodynamics, kinetics, and transport in both a chemical and physical context. One 

must account for the temporal evolution due to reactor events that include radiation absorption and 

scattering, nuclear processes (in which the fission and activation product chemistry is not completely 

known), and the presence of corrosion products that develop over time as a result of reactions of the 

liquid with the surrounding materials. Multiple time scales are relevant, from exceedingly fast nuclear 

events to the long-term behavior over the lifetime of the reactor. Concomitantly, many spatial scales must 

be considered—from the atomic-scale organization of molten salts about ions, to the mesoscale nucleation 

and growth of insoluble materials, to the macroscopic-scale impact on physical properties of materials 

and components within the reactor. Understanding of the relevant chemistry, physics, and materials 

science that must be transferred across coupled spatial and temporal scales is a critical factor. It underpins

the needs for technological and scientific innovation that must be broadly reliable not only for sciencebased decisions but also for regulatory approval. The following are the essential scientific challenges:

• Understand the structure, dynamics, and thermodynamics of molten salts across the extreme 

operational conditions of advanced reactors.

• Understand and reliably predict the equilibrium phase diagrams and thermodynamic properties of 

multicomponent systems containing fission products and trace constituents.
Basic Research Needs for Future Nuclear Energy

4 Priority Research Direction 1

• Characterize and describe the evolution of nonequilibrium systems. 

• Develop high-throughput experimental methods to create thermodynamic data and incorporate these 

in interactive databases to guide molten salt selection. 

• Understand the speciation and oxidation states of fission products and develop methods of monitoring 

the fission-driven changes to the local chemical potential to aid the development of predictive 

capabilities for long-term reactor behavior, including the role of transients.

• Elucidate the impact of fission and activation product chemistry upon salt properties, corrosion, and 

neutronics. This effort includes understanding corrosion and materials compatibility issues to predict 

and avoid system failures. 

• Develop in situ and operando methods to characterize salt quality, impurities, and fission and 

corrosion products in real time to optimize reactor chemistry and operations.

Research Thrusts

To address the scientific challenges, this PRD identifies the following research areas that need to be 

pursued to provide the fundamental understanding necessary for the predictive design of new coolants and 

liquid fuels for future nuclear energy.

The structure and dynamics of molten salts in extreme environments and the 

connection between local behavior and macroscale properties

It is beginning to be possible to use average atomic-scale structure to understand and predict macroscopic 

properties for simple salts; however these approaches need further development to address 

multicomponent systems and added complexity involving minor and trace constituents, such as fission 

products and impurities. To understand the atomic-scale structure and dynamics of molten salts and their 

impact on chemical and physical properties—including viscosity, thermal conductivity, solubility, and 

vapor pressure—modeling and simulation must be coupled with advanced spectroscopic and scattering 

methods. Optical, Raman, and x-ray adsorption spectroscopy; nuclear magnetic resonance (NMR); and 

neutron scattering techniques are among those that can provide insights into the structure, coordination 

environment, and oxidation state of species in solution. For example, Figure 1 illustrates ab initio 

molecular dynamics (AIMD) simulations of a ZnCl2 melt at 327°C alongside neutron diffraction and 

Raman measurements, which collectively indicate three major species present in the melt. [Alsayoud 

2016] 

An additional challenge is the in situ and operando measurement of molten salt properties under extreme 

environments (1000°C and higher in the presence of radioactivity). Recently, a high-temperature (up to 

1000°C) airtight cell that can handle radioactive samples was developed for x-ray adsorption 

measurements of solid and molten actinide fluorides (Figure 2). This cell was used in high-temperature 

extended x-ray absorption fine structure measurements of LiF-ZrF4 mixtures, which revealed the 

quantitative distribution of different anionic zirconium complexes. [Bessada 2017] Sample cells have also 

been designed for in situ spectroscopy and spectroelectrochemistry of actinides in high-temperature 

molten salts. [Polovov 2008] In addition, methods are needed for real-time monitoring of chemical and 

physical processes.
Basic Research Needs for Future Nuclear Energy

Priority Research Direction 1 5

Figure 1. Coupled ab initio molecular dynamics with neutron diffraction and Raman 

vibrational spectroscopy to identify distinct polymorphs in ZnCl2 molten solutions.

| Reprinted with permission from A. Alsayoud et al. 2016. Structure of ZnCl2 melt. Part I: Raman 

spectroscopy analysis driven by ab initio methods, J. Phys. Chem. B 120, 4174–4181. Copyright 2016 

American Chemical Society

High-throughput measurements of 

phase diagrams and 

thermodynamic parameters of 

complex molten salt systems

The phase diagram is the primary tool for 

understanding the phase behavior of 

molten salts, as it defines the temperaturecomposition space for a single-phase 

liquid (see the sidebar “Phase Diagrams”). 

Thermodynamic properties can be 

extracted from the phase diagrams (and 

conversely, phase diagrams can be 

constructed from thermodynamic data). 

Although phase diagrams can be 

determined by a variety of experimental 

methods, they tend to require a large 

Figure 2. Hightemperature cell for xray adsorption 

measurements of 

molten salts. | Reprinted 

from J. Nucl. Mater. 494, 

October, C. Bessada et al., 

High temperature EXAFS 

experiments in molten 

actinide fluorides: The 

challenge of a triple 

containment cell for 

radioactive and aggressive 

liquids, 192, copyright 2017, 

with permission from 

Elsevier
Basic Research Needs for Future Nuclear Energy

6 Priority Research Direction 1

Phase Diagrams

Phase and predominance diagrams are graphical representations of free energy minimization, constrained 

by heat capacities, enthalpies, and entropies of formation/reaction. They provide details on speciation, 

phase transitions including melting, and dissolution and mixing of major and trace components in both the 

molten and solid phases. Such data can be obtained by experiment and by calculation from first principles 

or using more empirical potentials. An example of phase diagrams for the Li/Na/K nitrate ternary mixture is 

shown in the diagram below at left, with the calculated (blue) and experimental (black) eutectics. The blue 

lines represent binary eutectics in the presence of the third component and are not meant to depict the 

ternary phase diagram. The experimental and predicted phase diagrams are different; improvements are 

needed in the ability to accurately predict thermodynamics properties of molten salts. 

Obtaining accurate thermodynamic data experimentally is labor-intensive and is done only by a small 

number of research groups. For complicated multicomponent molten salt coolants, the available data are 

grossly insufficient; for molten salt fuels, the situation is further complicated by the combination of high 

temperatures; radiation effects; and compositional change over time resulting from fission, transmutation, 

and dissolution of corrosion products. As a molten salt reactor operates, trace constituents that collect in the 

fluid phase can influence the chemistry of the salt matrix and the speciation of ions dissolved into the 

medium. One trace constituent is oxygen, which can significantly shift the speciation of both major and 

minor constituents. The generalized predominance diagram below at right shows the impact of oxygen on 

the redox chemistry for uranium in LiCl-KCl eutectic at 500°C. Note the complexity of the diagram.

Experimental eutectics in black and computed 

eutectics in blue. The black triangle, black curves, and 

black pentagram are ternary data, and the filled black 

circles are the binary eutectics. Crosses indicate 

temperature contours (K). Insets show the free energy of 

mixing for the liquid phase and the pure solid–liquid free 

energy difference connecting plane at three 

temperatures: T(a)<T(b)<T(c). | Reprinted from 

S. Jayaraman et al., Molten salt eutectics from atomistic 

simulations, Phys. Rev. E, 2011, 84, 030201. Image courtesy of 

American Physical Society under the terms and conditions of 

the Creative Commons license.

Predominance diagrams of U, in NaCl-KCl at 

750°C. | Reprinted from R. Abdulaziz et al. 2016. 

Predominance diagrams of spent nuclear fuel 

materials in LiCl-KCl and NaCl-KCl molten salt 

eutectics, | Int. J. Electrochem. Sci. 11, 10417-10435.

Image courtesy of Int. J. Electrochem. Sci. under the 

terms and conditions of the Creative Commons license

number of high-purity samples; and there needs to be rigorous control (and measurement) of impurities, 

including dissolved metals, water, and oxygen-containing impurities (e.g., oxides, hydroxides, sulfates, 

carbonates, and oxyhalides). For liquid salt–fueled reactors, the impact of fission products (which include 
Basic Research Needs for Future Nuclear Energy

Priority Research Direction 1 7

more than 40 elements) on the phase diagram must be determined. To address this challenge, we can 

build upon advances from the materials, high-temperature chemistry, and pharmaceutical industries to 

develop high-throughput approaches for screening candidate molten salts—with respect to properties like 

electrochemical potential, solubility, or heat transfer—and accelerate the development of new salt 

compositions. Knowledge of the phase transition, formation, and fusion enthalpies and entropies of the 

components and of the thermodynamic mixing properties of the melts is essential for accurate constraint 

of phase equilibria. In particular, thermodynamic parameters that enter databases and computational 

models must be grounded in experimental reality. Indeed, robust databases of a variety of thermodynamic 

properties could be revolutionary in this context. They should include 

• Heat capacities from near absolute zero to operational temperatures

• Heats of mixing in appropriate molten salt systems

• Heats of fusion of common systems, including binary and ternary fluorides and chlorides

• Heats of dissolution of fission products and other impurities in molten salts, measured as a function of 

both molten salt composition and solute concentration

High-throughput methods that use calorimetry combined with spectroscopy or electrochemical methods 

need to be developed to handle radionuclides in air- and water-free environments. Experiments at 

moderate pressure may provide advantages in terms of suppressing volatility and overcoming 

containment problems, for example, by the use of a noble gas pressure medium in a diamond anvil cell. 

Levitation experiments, circumventing containment problems, may also be useful in determining phase 

diagrams and fusion enthalpies and in studying structure via neutron and synchrotron x-ray techniques. 

These and other new methodologies need considerable development, especially for application to extreme 

and corrosive environments that combine high temperatures and radiation fields.

Speciation and oxidation states of minor and trace constituents and their impact on 

molten salt properties

One of the challenges of a liquid salt–fueled reactor is that the chemical constituents are constantly 

changing because of the generation and decay of fission, activation, and corrosion products and the 

presence of impurities (such as dissolved metals, water, and oxygen-containing impurities). It is essential 

to understand which constituents are volatile, soluble, insoluble, or sometimes soluble in the molten salt 

and how they impact the properties of the salt. The thermophysical properties that directly impact heat 

transfer are strongly connected to the composition and impurity concentrations of the molten salt. Thus, 

optical spectroscopy, NMR, electrochemical methods, and x-ray and neutron scattering and spectroscopy 

are needed to understand the speciation and solvation of minor constituents in the molten salt and to 

understand their impact on properties and reactivity. For example, it was discovered in the Molten Salt 

Reactor Experiment (MSRE) that the fission product tellurium promoted cracking in the Hastelloy N 

reactor vessel by leaching chromium from the alloy. [Keiser 1977] Cracking may be reduced by 

controlling the redox potential of the salt through variation of the U3+/U4+ ratio. Thus, methods are needed 

to provide real-time data on the changing composition (and properties) of the molten salt operando to 

optimize reactor performance. Since many of the nuclei are NMR-active, NMR methods could possibly 

be developed to monitor lanthanide ingrowth, as well as flow fields, viscosity, and diffusion coefficients. 

Given that many fission products are radioactive, initial studies could also focus on nonradioactive 

surrogates, i.e., stable isotopes of the elements.

One of the advantages of molten salt reactors (MSRs), and a reason for the increased interest in them, is 

their ability to burn actinides (for the production of electricity), which reduces the need for geologic 

storage of spent nuclear fuel. This opportunity also creates additional technical challenges in 

understanding the solution-phase chemistry and reactivity of actinides. For example, the Molten Salt 
Basic Research Needs for Future Nuclear Energy

8 Priority Research Direction 1

Advanced Reactor Transmuter (MOSART) reactor, one of the designs actively being considered that 

would encompass complete actinide transmutation, uses transuranic trifluorides (Np, Pu, Am, and Cm) 

from light water reactor spent nuclear fuels as the fuel in a mixture of LiF (15 mol %), NaF (58 mol %), 

and BeF2 (27 mol %). The fraction of actinides transmuted in 60 years is 91–92%, but heavier actinides 

(including Cf and Bk) are also produced during the lifetime of the reactor. [Forsberg 2003] The actinide 

component can be divided into two regimes—the light actinides actinium through plutonium, and the 

heavy actinides americium through fermium. 

The light actinides have electronic properties that parallel those of the transition metals owing to the 

radial extension of the 5f electrons beyond the valence shell. As the actinide series is traversed, relativistic 

effects alter the spatial availability and energetics of 5f, 6d, and 7s orbitals, which drive actinide reactivity 

under conditions relevant to MSRs. The challenges for the lighter actinides center on the directed 

management of oxidation states to enable in situ separations. To minimize corrosion and enable online 

actinide recovery via pyroprocessing, these actinides are trivalent while in the molten salt solution. 

Although the trivalent state is common for the heavier actinides, uranium through plutonium generally 

persist in higher oxidation states, and knowledge of the behavior of their trivalent state is limited. This 

lack of understanding impedes the ability to predict actinide speciation and redox chemistry in molten salt 

systems. As an example, the chemistry of trivalent protactinium is highly relevant to the thorium fuel 

cycle (in which 232Th is transmuted into 233Th, which beta decays to 233Pa and then to the fissile 233U), yet 

facilities and isotopes available for work with protactinium are both limited. Only recently has the 

speciation of protactinium fluorides been probed, wherein the PaF8

– anion is proposed to be the 

predominant Pa(V) complex in concentrated solutions of aqueous hydrogen fluoride (Figure 3). The 

challenges for the heavier actinides stem from limited knowledge of their chemistry, especially in the 

extreme conditions of MSRs. The chemistry of transcurium actinides, including tetravalent berkelium, 

and of divalent elements heavier than berkelium, is relevant to predicting the chemistry controlling both 

equilibrium and nonequilibrium phenomena in these systems.

Figure 3. Select protactinium(V) fluoride structures synthesized in De Sio and Wilson 2014. | Reprinted with 

permission from S. M. De Sio and R. E. Wilson. 2014. Structural and spectroscopic studies of fluoroprotactinates, Inorg. 

Chem. 53(3), 1750–1755. Copyright 2014 American Chemical Society.

Data-driven advances in computational models and methods for predictive 

understanding of the structure, dynamics, and thermodynamics of molten salts

The development of predictive methods and algorithms for studying salt chemistry is a major scientific 

challenge. This is the case because of (1) the complex nature of the salt mixtures and (2) the dynamic salt 

composition due to the production and decay of fission products, and its impact on chemical reactivity 

and physical properties across length and time scales. Quantum mechanics and classical dynamics—

combined with coarse-grained, continuum, and thermodynamic models and validated with experimental 
Basic Research Needs for Future Nuclear Energy

Priority Research Direction 1 9

data—will achieve predictive insights into solvation, speciation, and chemical reactivity from the atomic 

scale to bulk thermophysical and transport properties at longer length and time scales.

Few studies are reported on molten salts that are based on first principles electronic structure calculations 

and are capable of explicitly treating polarization effects and chemical reactivity. [Bengston 2014; Song 

2017] Classical interatomic potentials have been applied with some success to predict molten salt 

properties. [Jabes 2012; Dewan 2013; Luo 2016; Nam 2014; Song 2017] Here, the MSR salt fuel can be 

treated as a set of cations that are changing their identity (through nuclear transmutation and fission) and 

their oxidation states embedded in a sea of fluoride or chloride anions. The current approach is to fit a 

classical potential model to a set of results from first principles electronic structure calculations, with 

improved parameterization from available liquid state data. This suffices only if configurations can be 

sampled and there are no changes in the oxidation state or the valency; however, for more complex 

systems, many different force field interactions will be needed, with a concomitant increase in 

complexity. Thus, fundamentally different approaches are required to address the issue of dynamic timeand space-dependent chemistry.

For application to heavy elements, all electronic structure methods must include relativistic effects; yet 

the accuracy of these approaches is not uniformly robust. Actinides in lower oxidation states have many 

close-lying electronic states in which the electrons mix within the 7f orbitals, resulting in a rich multiplet 

structure with implications for the electronic properties of molecules. The modeling of redox reactions 

(critical for speciation) requires the ability to treat different numbers of f electrons at the same level of 

accuracy, but currently this cannot be done. These fundamental limitations—which are ultimately all 

related to the inability of current electronic structure methods to capture the multiple ways f electrons can 

interact with one another, with electrons in other shells, and with their surrounding environment—need to 

be addressed to advance understanding of the structure, solvation of ions, and thermodynamics of molten 

salts.

Molecular dynamics (MD) simulations are needed to predict the structure and dynamics of salts in the 

bulk and at the salt/reactor and salt/vapor interfaces. They can be used to predict and explain transport 

properties, as well as explain the origins of features found in x-ray and neutron structure factors and 

radical distribution functions. First principles MD simulations of pure molten salts at multiple 

temperatures are needed to obtain temperature-dependent thermodynamics, kinetics, and transport 

properties; and speciation is needed for the prediction of phase diagrams and other physical properties. 

Innovative approaches to long-time MD sampling are needed to predict nucleation, [Kathmann 2009; 

Kathmann 2006] releases of volatiles, and rates of reactions, including redox reactions. Knowledge of the 

constituents is needed to understand interfacial behavior leading to MSR material degradation, i.e., 

corrosion. At longer length and time scales, integration of thermochemical information into higher-order 

(multi-element) systems is critical to predict complex liquid and solid solution behavior. Thermophysical 

and transport properties, such as thermal conductivity and viscosity, are needed for the modeling of 

material flow that is required to model fissile reactions and MSR performance. Such computational 

studies will require large simulation sizes, on the orders of tens of millions of atoms, to cover the required 

spatial scale. This will require access to DOE’s highest-performance computers and optimized exascale 

software.

With the development of new methods for high-throughput data collection and the availability of 

databases, new computational methods and approaches are needed to query and visualize phase diagrams 

and thermodynamic data. Thermodynamic models are needed to predict properties beyond conditions that 

can be experimentally measured, such as in highly radioactive samples. These advances would accelerate 

the development of new molten salt systems and provide the data needed for the design of an MSR. 
Basic Research Needs for Future Nuclear Energy

10 Priority Research Direction 1

Uncertainty quantification is a unifying feature for all the development described above. It is critical to 

provide error bars from the computations (as well as from experiments), either through careful analysis of 

each term in the modeling or through proper sampling. New applied mathematical approaches are needed 

for computationally more efficient uncertainty analysis. 

Potential Scientific Impact

MSRs use molten salts as the primary coolant at atmospheric pressure. Either the molten salts can flow 

past the solid fuel to remove heat to the primary heat exchanger, or the fuel can be dissolved in the molten 

salt that flows through the primary heat exchanger. For molten salts to be used in future nuclear energy 

applications, significant fundamental scientific challenges must be addressed to advance the 

understanding of the structure, properties, and reactivity of molten salts and the solvation of actinides and 

fission products. Addressing the proposed research directions will enable the following scientific impacts: 

• The design and synthesis of new salt-based coolants and fuels based on fundamental knowledge of 

the physical and chemical properties of the individual components

• Predictive understanding of the impacts of minor and trace solutes on physical and chemical 

properties of molten salt fuels

• Development of high-throughput characterization methods that may be applicable to other fields of 

study

• Improved understanding of actinide chemistry and identification of new emergent behavior for heavy 

elements in extreme environments

• Advances in experimental methods to measure properties in situ and operando under extreme 

environments 

• Advances in computational methods to address f element chemistry, and validated predictive models 

of structure, dynamics, and properties that bridge length and time scales, which are applicable to 

many problems across DOE’s research portfolio

Potential Impacts on Energy Technology

There is significant worldwide interest in the development of MSRs because they provide new 

opportunities to rationalize the nuclear fuel cycle by using a variety of fissile (233U, 235U, or 239Pu) and 

fertile (232Th or 238U) materials to supply cost-competitive, safe, nuclear electricity generation and hightemperature, low-pressure industrial heat. However, a fundamental lack of understanding of how the 

changing chemical composition of a molten salt changes the salt’s chemical and physical properties—and 

the subsequent impact on the reactor design parameters, operational parameters, and lifetime—is one of 

the many factors that has hindered the development of liquid salt–fueled reactors. Addressing the 

scientific challenges discussed in this PRD will provide the scientific foundations necessary to allow the 

development of compact high-temperature reactors enabling safe, cost-effective electricity generation and 

excess heat for other uses such as desalination and hydrogen production. New molten salts will be 

developed for fluid-fueled reactors compatible with a variety or fissile or fertile fuels, allowing a 

reduction in the amount of spent nuclear fuel. The fundamental knowledge and advances gained for in 

situ and operando characterization will also provide new insights into reprocessing methods, corrosion, 

and optimal reactor operation.

Molten salts are also envisioned as coolants, in the primary or secondary loop, for high-temperature (600–

1000°C and beyond) nuclear reactors that will be more efficient. This research will lead to improved 

high-temperature fluids with superior heat transfer properties for other applications, e.g., solar thermal 

energy.
Basic Research Needs for Future Nuclear Energy

Priority Research Direction 2 11

PRD 2: Master the Hierarchy of Materials Design and Synthesis for Complex, 

Reactor Environments

Key question: How do we design, synthesize and process superior materials able to function and perform 

over decades in the extreme environments of advanced nuclear reactors?

Future nuclear reactor systems will require a new generation of materials designed to withstand extreme 

environments, including materials designed with hierarchical architectures incorporating 

multidimensional phases with complex alloy or compound chemistries. Developing such materials calls 

for an approach to accelerate the discovery and design of materials for performance under coupled 

extremes, leveraging advanced experimental techniques and modern data science approaches. The 

synthesis of materials with targeted microstructures is also a critical component for scaling from atomistic 

to ensemble responses and developing predictable microstructural pathways and performance. This PRD 

will enable high-throughput science for the discovery and design of materials to meet the needs of future 

nuclear energy systems. A key challenge is to bridge from atomistic mechanisms to the collective 

responses of hierarchical architectures under coupled extremes.

Scientific Challenges

The scientific challenge is to elucidate the underpinning science enabling the design of radiation-resilient 

materials that possess emergent phenomena that exploit multiple length scales, including interfacial 

metastability, hybrid architectures, and/or complex chemistries. The predictive design of materials 

requires fundamental understanding of thermodynamic and kinetic pathways in chemically and 

structurally diverse material architectures that are far from equilibrium. The key elements of the scientific 

challenges embodied in this PRD include 

• Bridge from atomic-scale defect and interface mechanisms to collective response of microstructures 

under coupled extremes

• Predict kinetic and thermodynamic pathways to enable microstructural design for extreme environments

• Design new synthetic processes to produce tailored microstructures 

• Develop accelerated testing approaches with tightly integrated feedback between modeling and 

experiments

Research Thrusts 

To address the scientific challenges outlined, the following research thrusts are provided to inspire 

fundamental research required for the design of next-generation radiation-resilient materials required for 

future nuclear energy concepts.

Bridge from atomic-scale defect and interface mechanisms to collective response of 

microstructures under coupled extremes 

In structural materials and fuels, internal interfaces, such as grain and interphase boundaries, are 

heterogeneities that serve as sites for a large number of synergistic phenomena that can either anneal and 

reduce, or nucleate and grow damage. Impeding our understanding of the complex interfacial changes taking 

place under extreme environments are the vastly different time and length scales over which these mechanisms 

operate and the resulting manifestations of damage occur. Therefore, it is highly desirable to design interfaces 

that are adaptive to these extreme environments and that serve to heal, as opposed to accumulate, damage. 

However, to reach this goal will require a fundamental understanding of atomistic mechanisms taking place at 
Basic Research Needs for Future Nuclear Energy

12 Priority Research Direction 2

the single interface level, as illustrated in Figure 4a [Vattré 2014] and by recent progress in understanding 

fundamental unit mechanisms of defect-interface interactions in irradiated materials. [Beyerlein 2013] 

(a)

(b)

Figure 4. (a) Computational design of patterned interfaces using reduced order models. |Image 

courtesy of Massachusetts Institute of Technology. Reprinted from Vattré et al. 2014. Computational design 

of patterned interfaces using reduced order models, Sci. Rep. 4, 6231 under the terms and conditions of the 

Creative Commons Attribution 4 license. 2014 (b) Analysis of defect accumulation at the 

microstructure level to establish correlations between damage state, grain orientation, and 

grain boundary properties. | Reprinted from El-Atwani et al. 2017. Direct observation of sink-dependent 

defect evolution in nanocrystalline iron under irradiation, Sci. Rep. 7, 12 under the terms and conditions of 

the Creative Commons license

Bridging to the complexity introduced by the different elements that compose a microstructure ensemble—as 

captured in Figure 4b, which shows the defect accumulation relevant to the grain boundary network [El-
Basic Research Needs for Future Nuclear Energy

Priority Research Direction 2 13

Atwani 2017]—represents a synergistic understanding needed to design tailored microstructures for 

predictable properties. Therefore, the following are the scientific questions that must be addressed:

• How do we understand the full range of defect and interface mechanisms induced by coupled 

irradiation extremes across broad length and time scales? 

• How do we harness emergent phenomena at mesoscales in far-from-equilibrium microstructures to 

reduce or repair damage?

To address these questions, advances are required in the ability to design and synthesize specifically 

tailored microstructures, which will enable the examination of the ensemble response of these novel 

hierarchical materials. New computational tools also are necessary to bridge atomistic phenomena at 

adaptive interfaces to phase-field or other mesoscale models of microstructure evolution. Finally, new 

frontiers in 3D and in situ quantitative characterization techniques in coupled environments are critical to 

understanding mechanisms across nano- to meso-dimensional and temporal scales.

Predict kinetic and thermodynamic pathways to enable microstructural design for 

extreme environments

A fundamental understanding of the thermodynamic and kinetic pathways that determine microstructure 

evolution is needed to develop design principles for material design for extreme environments. The key 

scientific questions include

• How does microstructure influence transport and physical properties?

• How does microstructure influence radiation resistance?

• How do properties evolve in a dynamically complex microstructure under highly transient 

conditions?

• How does the structure-property relationship change under coupled extremes with far-fromequilibrium conditions?

A critical need thus exists to establish microstructure-property relations that will enable materials design 

and result in predicable performance that targets specific properties and/or attributes (e.g., thermal 

conductivity, radiation stability, and fission product retention). The ability to fabricate designed 

microstructures represents an essential first step required to fully evaluate these attributes in the context of 

kinetic and thermodynamic pathways. Further, capturing and interpreting the full range of data from these 

experiments will require the integration of data science approaches and machine learning techniques to 

accelerate the fundamental understanding of designer microstructures and their implications for long-term 

performance during reactor lifetimes.

Design new synthetic processes to produce tailored microstructures 

The realization of next-generation structural materials and nuclear fuels will require novel synthetic 

methods. Critical to this goal is the understanding of the fundamental mechanisms of phase metastability 

that are governed by kinetic pathways—rather than thermodynamically favored pathways—specifically to 

enable the formulation and synthesis of new phases. Building on current efforts using kinetically driven 

microstructure modifications—such as irradiation, [Bellon 2015] additive manufacturing, [Gu 2012] and 

extreme deformation [Arshad 2015]—novel microstructures can be fabricated to answer specific 

fundamental questions regarding material resistance to degradation under coupled extremes. Similarly, 

using gamma irradiation in aqueous solutions has enabled chemistry to be driven by solvated electrons 

formed in solution, as depicted in Figure 5; the strength of the irradiation dose correlates with the 
Basic Research Needs for Future Nuclear Energy

14 Priority Research Direction 2

concentration of solvated electrons formed, 

thereby dictating the reduction routes of the 

synthesis. [Belloni 2006] Uniform nonthermodynamically predicted AgNi alloy 

nanoparticles of predesigned stoichiometries 

have been successfully synthesized and 

characterized, [Zhang 2010] and this method 

has been extended to nuclear fuel 

compositions. [Nenoff 2013]

To develop advanced synthesis methods, 

however, will require an understanding of 

reaction mechanisms for a number of future 

materials needs, including targeted selfhealing properties based on defect sink 

strength and defect reaction kinetics. 

Examples of successful materials 

development would include the ability to 

immobilize fission gases into the fuel 

(thereby minimizing environmental 

contamination) or design defect sinks into 

structural materials. This need lends itself 

directly to the use of high-performance 

computing to model and predict events such 

as metal/gas interactions, defect growth, 

and nanoscale structures. Not only will 

fundamental research be enabled by this 

predictive capability, but also the 

simultaneous development of computational 

toolboxes will enable the continuous and 

ever-more-complex design of novel 

advanced materials combining material 

neutronics, thermodynamics, kinetics, reversible chemistry, and quantitative microstructure description. It 

is thought that the exploitation of combinatorial chemistry and big data can propel the exploration of 

novel materials’ compositions for intrinsically safe fuels.

Develop accelerated testing approaches with tightly integrated feedback between 

modeling and experiments

Under the steady state operation of a reactor, core materials may exhibit dramatic changes in behavior. 

The transitions are characterized by a sharp departure in the rate of evolution of the microstructure. As 

this change is dependent on many operational and material variables, understanding the mechanisms 

controlling these events and predicting them remain major challenges. For example, void swelling 

undergoes an extended incubation period that is interrupted by a rapid acceleration of the swelling rate. 

Such observations have been made in both metallic alloys [Zinkle 2013] and fuels [Iltis 2017] as 

demonstrated in Figure 6. As the timing of this rate change is dependent on many operational and material 

variables, it is very challenging to predict it. Other examples include precipitate formation in reactor 

pressure vessel steels [Ke 2017] and breakaway irradiation growth/oxidation in Zircaloys. [Garner 2007]

(a) (b)

Figure 5. Radiation pathway for nanoparticle and alloy 

synthesis. | Reprinted from Catal. Today 113(3–4), J. Belloni, 

Nucleation, growth, and properties of nanoclusters studied by 

radiation chemistry: Application to catalysis, 141–156, copyright 

2006, with permission from Elsevier
Basic Research Needs for Future Nuclear Energy

Priority Research Direction 2 15

Conversely, changes in external stimuli can induce changes in the rate of microstructure evolution or 

property behavior (hot spots). These may be rare or frequent events in which the temperature, neutron 

flux, and coolant chemistry undergo a transient or excursion that induces changes to the microstructure. 

An example is a temperature excursion 

that induces the nucleation of defects or 

anneals out existing defects, thus altering 

the subsequent evolution of the 

microstructure. This effect was evident 

during shutdown of a fast-flux test 

facility, where a slow temperature change 

resulted in the nucleation of a fine array of 

dislocation loops that significantly 

changed both the sink strength and sink 

nature and hardened the alloy. [Garner 

1993] Other examples are changes in 

coolant chemistry, such as an excursion in 

the dissolved oxygen content, or an 

increase in water conductivity due to the 

unintentional injection of ionic species 

into the coolant. Such changes induce an 

alteration of the growing oxide and can 

dramatically alter processes such as 

localized corrosion or stress corrosion 

crack growth.

Regardless of the driver, these events will 

dictate the course of microstructure and 

property evolution after their occurrence, 

so that microstructure evolution will follow a different path from what it would have taken in the absence 

of the event. Thus, there is a great need to develop the capabilities to predict the occurrence of these 

microstructure discontinuities and the ensuing evolution of the microstructure. 

A fundamental challenge is to understand these threshold/hot-spot/slope-discontinuity processes, whether 

they are intrinsic to the process or stimulated by an external event. The question then is how to design 

experiments and simulations to discover these physical processes. Specifically, can accelerated testing 

(computational/experimental) techniques be developed to predict rate changes in the microstructure of a 

material under steady external conditions (temperature, stress, chemistry, flux)? Further, can 

experimental/simulation techniques be developed to assess how the rate of microstructure evolution 

changes with external stimuli (temperature, stress, chemistry, flux)? In both cases, new experimental 

techniques are needed to allow validation of the developed computer simulations. Researchers must also 

be able to identify regimes in which rates of microstructure evolution change dramatically with 

microstructure or stimuli. A major challenge is to develop these techniques and the underpinning models 

to span large time scales and determine microstructure evolutions efficiently over entire lifetimes. Such 

advancements (validated tools as well as locations of thresholds and hot spots) will enable improved alloy 

design (e.g., composition, microstructure, processing).

Potential Impacts

Mastering hierarchical architectures has the potential to greatly impact the design, development, 

operation, and safety of advanced nuclear energy systems. The impact will manifest itself in two forms:

Figure 6. Cavity swelling in iron base cladding alloys. 

| Reprinted from Acta Mater, 61(3), S. J. Zinkle and G. S. Was, 

Materials challenges in nuclear energy, 735–758, copyright 2013, with 

permission from Elsevier.
Basic Research Needs for Future Nuclear Energy

16 Priority Research Direction 2

1. Advanced multiscale models to assist with the design and development models

2. New fuel, structural, and functional materials that permit reactors to operate at higher flux and 

temperature, allowing longer lifetimes and smaller evacuation zones around advanced nuclear reactor 

systems 

Every advancement in experimentally validated models and development of new materials containing 

hierarchical architectures provides an additional tool to increase the confidence in the regulatory process 

of any advanced nuclear reactor systems put into operation.
Basic Research Needs for Future Nuclear Energy

Priority Research Direction 3 17

PRD 3: Tailor Interfaces to Control the Impact of Nuclear Environments

Key question: How can the multitude of inextricably linked chemical and physical processes that occur at 

interfaces be controlled?

The requirements for nuclear power plants with long lifetimes and high reliability demand understanding 

of the chemistry and materials science of dynamic interfaces under extreme conditions far beyond our 

current knowledge. These interfaces, including solid/fluid and solid/solid ones, play critical roles in 

enhancing the performance of coolants (gas and liquid) and solid materials in reactor systems. Interfaces 

provide pathways for efficient heat transfer, are preferential sites for chemical reactions, and oftentimes 

are places where materials failure is initiated. The confluence of the development of modern computer 

algorithms, new computer architectures, and advanced nanoscale characterization tools provides exciting 

opportunities to discover the fundamental science of how complex interfaces evolve under radiation, 

high-temperature, high-stress, and corrosive environments, and to lay the foundation for the design of 

next-generation materials and coolants to support future nuclear energy technologies.

Understanding, controlling, and even tailoring interfacial processes—whether at the fluid coolant/solid 

interface or the solid/solid interface resulting from propagation of corrosion fronts—is critical to future 

nuclear energy technologies because these processes ultimately determine material reliability. Clarifying 

the collective effects of radiation-induced reactions at interfaces and of thermal interactions under the 

anticipated conditions of new reactor technologies requires knowledge well beyond our current 

understanding established on water-based reactors. Mastering these processes is particularly urgent 

because future nuclear reactor concepts involve the development of new materials and extreme operating 

conditions about which little of the fundamental science is currently known. An added complication 

beyond the extreme conditions is the presence of corrosion products at the interface and of suspended 

nanoparticles (generated by radiolysis and corrosion processes), which can enhance radiolytic damage 

rates and yields in a manner that is not currently predictable.

Radiation effects represent an important part of this PRD. Radiation damage in materials, at both surfaces 

and interfaces, arises both from energy losses associated with electronic excitations and ionizations, and 

from nuclear displacements. Electronic excitations and ionizations mainly result from the inelastic 

scattering of low-energy electrons produced by higher-energy particles, such as gamma rays, x-rays, 

(recoil) protons, and alpha particles. Nuclear displacements arise from momentum transfer and recoil 

events in ballistic nuclear collisions. Radiation effects induce chemical changes in the coolant and driving 

processes at the coolant/solid interface, leading to corrosion that extends from nanometers to millimeters 

into the material. Although coolant chemistry and bulk corrosion have typically been considered 

separately in the past, they are intimately connected; future research must address them as a system. For 

example, there is a systematic mechanistic path connecting radiation-driven chemistry at the coolant/solid 

interface to the moving oxidation fronts typical of corrosion. The same processes that cause radiationinduced loss of atoms from the solid surface into the coolant often correlate with enhanced motion of 

point defects in protective layers and penetration of corrosion into the material. As shown in Figure 7, the 

effect of radiation on corrosion can depend considerably on the coolant composition and the presence or 

absence of air. Radiation also introduces a flux of point defects to solid/solid internal interfaces, e.g., 

grain boundaries and precipitate/matrix interfaces, which cause structural and compositional 

modifications. Furthermore, the accumulation of radiation-produced damage within the interior of a grain 

modifies the deformation response. The consequences of these modifications can have a profound and 

negative impact on the structural integrity of the component. For example, dislocation interactions with 

grain boundaries are important in rupturing the oxide and exposing the bare metal to the coolant—a 

critical component in irradiation-assisted stress corrosion cracking. Current understanding of deformation 

processes in irradiated metals and the resultant decrease in mechanical properties is limited to the 

operational conditions of water-cooled reactors.
Basic Research Needs for Future Nuclear Energy

18 Priority Research Direction 3

Scientific Challenges and Opportunities

Understanding energy, charge, and heat 

transport processes at solid/fluid (gas and liquid) 

interfaces in nuclear reactor environments

Absorption and dissipation of radiation energy arising 

from the exchange of energy, charge, and reactive species 

between the phases is significantly more complicated in 

heterogeneous multi-phase systems than in homogeneous 

systems. The basic mechanisms of these processes are 

poorly understood; yet they can lead to dramatic 

transformations of surface and subsurface layers of the 

material, as well as the contacting fluids. Special attention 

must be paid to the reactive intermediates at the interface, 

including surface defects, active groups, and specific 

adsorption complexes. These intermediates can be created 

directly and locally at the interface, or from the mobile 

electronic excitations or reactive species generated 

radiolytically in the subsurface layers of the material and 

trapped at the interface. These sites can be major focal 

points for interfacial charge transfer, initiating a broad 

range of processes at interfaces, such as corrosion.

Important processes that occur at the solid–fluid interface 

under the radiation conditions typical of reactors include

• Enhanced radiolysis, due to energy and polaron

(hole/electron) transfer from the solid phase, of the 

fluid coolant at or near the surface of the structural 

materials (and nanoparticles dispersed in the fluid). It 

leads to enhanced evolution of products (e.g., O2, H2, 

Cl2) that may alter the chemistry of the coolant.

• Dissolution of the solid material (metal, oxide, or ceramic) into the fluid, which contaminates the 

coolant and alters properties of the structural materials.

• Erosion and defect formation on the surface of the material leading to uniform or local degradation 

after long operation cycles.

• Material restructuring and/or elemental segregation that changes the surface reactivity with respect to 

the coolant.

• Damage to protective layers/coatings that may lead to complete loss of their protective properties.

Comprehending the mechanisms of these interrelated processes will require in situ and operando 

experimental techniques that are closely coupled to the insights provided by theory and multiscale 

computational methods.

Mastering corrosion processes under extreme thermal and radiolytic conditions

The ultimate vision for controlling corrosion would be to develop an understanding of the kinetics of 

corrosion for any microstructural unit and establish a set of design rules and synthesis processes for 

Figure 7. Example of coupling between 

radiation effects in coolant and corrosion of 

the base solid. Above, γ-irradiation 

suppresses the intrinsic corrosion of carbon 

steel by the ionic liquid [P14,666][Br]. Below, 

radiation in the presence of air accelerates 

corrosion of the same steel in normally 

unreactive [P14,666][NTf2]. | Reprinted from 

Corros. Sci. 102, R. P. Morco et al., Corrosion of 

carbon steel in the [P14666] [Br] ionic liquid: The 

effects of gamma-radiation and cover gas, 1–15, 

copyright 2016, with permission from Elsevier
Basic Research Needs for Future Nuclear Energy

Priority Research Direction 3 19

combining microstructures to produce a thin, impervious, protective, and stable protective film for any 

specific environment. 

Important phenomena that need to be understood to reach this vision include

• Competition between thermodynamics and kinetics in driving the formation and movement of the 

corrosion front

• The effects of physical heterogeneities on the competition between local thermodynamic and kinetic 

drivers

• Changes to local electronic effects caused by dilute impurity atoms

• Differences caused by external factors, such as radiation, that create nonequilibrium structures, or 

coolants that change kinetics or thermodynamics at surfaces

Chemistry of solid–fluid interfaces in extreme environments

Understanding and controlling physical and chemical effects occurring at interfaces are extremely 

important for the development and deployment of the next generation of nuclear reactors. However, 

intense radiation fields, high temperatures, and above-atmospheric pressures lead to highly 

nonequilibrium environments that have not been fully considered. Much of our knowledge of the 

processes that occur at interfaces is empirical. A detailed fundamental mechanistic understanding of the 

myriad of processes that occur in these complex environments is necessary to obtaining a predictive 

understanding that can be applied to the design and development of new materials and coolants for 

advanced nuclear reactors. Emerging technologies in both the experimental and theoretical fields offer 

huge potential for achieving the goals of understanding and controlling the fundamental chemical and 

physical processes occurring at interfaces.

Radiolytic processes occurring at solid/fluid interfaces may be initiated either by direct energy deposition 

in the local region or following the transport of energy, mass, or charge through either phase to the 

interface. The efficiency of energy and charge transfer events at solid/fluid interfaces depends upon the 

chemical compositions of the fluid, as well as the interface terminations, microscopic structure, 

nanoparticle phases, and dimensions of the material. Processes initiated on a short time scale can also 

manifest themselves at much longer times and at distances far removed from the initial energy deposition 

site. This multiscale nature poses great challenges in obtaining realistic mechanistic information, as well 

as in predicting future events.

The elucidation of radiolytic processes at heterogeneous solid/fluid interfaces presents even more 

challenges. Energy and polarons induced in either phase by incident radiation can migrate to and across 

the interfaces, inducing damaging radical and redox chemistry in the coolant and corrosion in materials. 

The transport mechanisms of energy, mass, or charge through the interface are often unknown and 

difficult to quantify. For example, the energy transport mechanism through the interface depends upon the 

degree of electron-phonon coupling. Strong coupling leads to heating and thermal processes. Weak 

coupling enables localization of electronic energy, leading to nonthermal damage and desorption 

processes. 

A wide variety of interfaces (solid/liquid, solid/gas, and liquid/gas) will exist in advanced nuclear energy 

systems. The anticipated high operational temperatures will require the design and discovery of new 

materials and coatings for both fuels and structural components. There are many classes of materials that 

may be considered, including ceramics, graphite and carbides, MAX phase materials (layered carbides 

and nutrides), and metallic glasses. With respect to coolants, helium is the most common gas proposed for 

solid/gas reactors because of its chemical inertness at high operating temperatures. Although helium itself 
Basic Research Needs for Future Nuclear Energy

20 Priority Research Direction 3

is chemically inert, reactive impurities such as O2, N2, H2O, CO, and CO2 (as well as possibly a variety of 

small hydrocarbons) will be present in small quantities and will pose challenges in interfacial chemistry. 

In molten salt and liquid metal reactors, solid/liquid interfacial chemical processes will dominate; 

however, vapor phases—as either head space, an interfacial layer, or bubbles (produced by fission gas, 

cavitation, chemistry, or temperature excursions)—would be expected in high-temperature environments. 

The wide range of chemical and physical processes that occur at solid/fluid interfaces must be understood 

to meet the demands of advanced nuclear reactors. 

Understanding how irradiation-induced defects impact solid/solid interface response to 

stress

Internal solid/solid interfaces such as grain and phase boundaries play a determining role in the response 

of materials to stress and radiation-induced changes in microstructure and composition. These interfaces 

act as sinks for point defects, solutes, and impurities; and the ability of the interfaces to effectively absorb 

these strongly influences the radiation resistance of the material. For example, the development of 

nanostructured ferritic alloys—which contain a very high density of nanostructured oxide features, small 

grains, and a high dislocation density—results in an increase in the resistance to radiation damage and an 

improvement in the creep properties. [Odette 2018] Similarly, introducing a large number of interfaces, as 

in a multilayered composite of dissimilar metals, has been shown to enhance the radiation resistance. 

[Misra et al. 2007] Interfaces also dictate the mechanical response, serving as both sources of and sinks 

for dislocations, as sites of stress concentrations that ultimately become failure initiation sites, and as 

failure pathways for some instances of environmental assisted cracking. The responses of these interfaces 

are dependent on compositional and structural modifications that are driven by point defect fluxes and 

dislocation interactions, by temperature, by stress, and by the local environment. For example, grain 

boundaries serve as the sources of dislocations that produce deformation channels in a deformed 

irradiated metal. The interaction of these channels with grain boundaries and the transfer of strain across them, 

at least at low levels of strains and below 350°C, is governed by the same rules as in unirradiated metal, except 

that the magnitude of the local stress must be sufficient for the dislocations emitted from the grain boundary to 

overcome the radiation-produced obstacle field and propagate away from it. [Cui et al. 2014] 

There is a paucity of information on how these processes are modified at large strains and stresses, high 

damage levels, and high temperature, and on the continuous evolution of the composition and structure of 

the interfaces. Similarly, there is a need to understand how the evolution of precipitates under extreme 

conditions impacts the structural integrity of the metal. The fundamental processes by which these 

interfaces react to the external stimuli that will occur in advanced reactor designs must be revealed so that 

processing strategies can be developed to optimize not the initial but the evolved structure, composition,

and properties of the interfaces to ensure long-term structural integrity. This will entail novel 

combinations of modeling, simulation, advanced microstructural characterization, and smart processing to 

determine how to control and manipulate interfaces to achieve the desired properties.

Advances in experimental techniques

Experimental techniques for probing interfacial phenomena, especially in situ and operando, are required to 

understand interfacial processes that occur in the extreme environments anticipated for advanced nuclear 

reactors. Recent advances in characterization tools—especially spectroscopic, imaging and scattering 

techniques—have made it possible to examine mechanisms responsible for radiation damage at interfaces 

across the spatial and temporal scales relevant to energy transfer, strain transfer, and chemical reactivity. For 

example, new experimental techniques that span ultrafast (attosecond) time resolution and ultrahigh spatial 

resolution (a few angstrom) are rapidly evolving. These include, but are not limited to, the following tools:
Basic Research Needs for Future Nuclear Energy

Priority Research Direction 3 21

• Time-resolved ultraviolet, VIS, mid-infrared, and Raman spectroscopy

• High–spatial-resolution x-ray and neutron probes and 3D x-ray microscopy

• Time-resolved and ultrafast x-ray probes

• Time-resolved electron microscopy and 3D electron tomography

• Atomic force microscopy and other imaging techniques in situ under irradiation

• 3D compositional analysis via techniques such as atom probe tomography

As an example, novel x-ray reflection interface microscopy (XRIM) techniques have been used to directly 

probe the radiation-stimulated dissolution of calcite as shown in Figure 8. The development of corrosionresistant experimental chambers that can also work at high temperatures and pressures is necessary, and 

some have been developed (Figure 9). These corrosion-resistant in situ and operando sample 

environments allow measurement using many spectroscopic and scattering probes under realistic 

operating conditions.

Figure 8. X-ray imaging of radiation-stimulated calcite dissolution. | From N. Laanaitet al. 2015. X-ray–driven 

reaction front dynamics at calcite-water interfaces, Science 349(6254), 1330–1334. Reprinted with permission from AAAS

A wide variety of experimental microscopic, surface science, and radiolysis techniques have been 

developed in recent years and have been applied to elucidate the mechanistic detail of interactions and 

processes at solid/liquid and solid/solid interfaces pertinent to water-based reactors. This huge array of 

sophisticated methodologies can now be applied to the solid/fluid and solid/solid interfaces under stimuli 

relevant to next-generation nuclear reactors. 

Experimental advances must be accompanied by theoretical developments to establish the comprehensive 

understanding needed for implementing future reactor systems. Developing multiscale, integrated models 

for complex interfacial processes is required. New computational tools (electronic structure, embedded 

methods, multiscale simulations, and machine learning) can lead to mechanism-based design of materials 

and interfaces. Development of an integrated mechanistic model is required for the design of 

complementary and compatible reactor materials and coolants.
Basic Research Needs for Future Nuclear Energy

22 Priority Research Direction 3

Figure 9. Corrosion-resistant apparatus for high-pressure and high-temperature x-ray studies. |Reproducted 

from M. S. Elbakhshwan et al. 2016. “Sample environment for in situ synchrotron corrosion studies of materials in extreme 

environments,” Rev. Sci. Instrum. 87, 105122, with permission of AIP Publishing

Understanding corrosion under radiation, stress, and high temperature

Whereas corrosion has been studied for decades, much of the reported work tends toward empirical 

observations, is very system specific, or focuses on a limited microstructural component of the corrosion 

process. Thus, while much has been learned, there is not a unifying scientific foundation that ties together 

the nonequilibrium kinetics and thermodynamics of moving interfaces and atomic motion—the local 

microstructural, morphological, and physical features, as well as the environmental effects that influence 

the collective corrosion response from the earliest interactions through a decades-long processes. The 

priority for future research thus needs to tie together an understanding of the following:

• The free energy or kinetic rates that lead to structural changes associated with oxidation or 

dissolution. We call these the drivers of corrosion.

• The local microstructural features and the local compositional changes or fluctuations that establish 

the local free energy or kinetic rates. Since any system undergoing corrosion quickly moves away 

from homogeneity, this local variability can trigger very different responses than would be expected 

from a homogeneous system. We thus call these triggers for corrosion.

• Changes to the environment of the material being corroded, or to the fluid causing the corrosion, can 

dramatically change the corrosion response. These changes could include radiation that changes 

material microstructures or rates of atomic diffusion. They could also include radiation or impurities 

in the coolant that change the coolant–metal interaction rates. In every case, the pathways for 

corrosion are strongly dependent on these environments. We thus call these corrosion changes 

dependencies.

• The chemical reactions and/or the collective motion of atoms that takes place, given the drivers, 

triggers, and dependencies. Since this collective motion of atoms defines the progression of the 

overall corrosion response, we call these collective motions the mechanisms of corrosion.
Basic Research Needs for Future Nuclear Energy

Priority Research Direction 3 23

The ultimate goal would be to develop a set of scientific principles, valid across multiple material 

systems, that allow prediction of corrosion paths and therefore mechanisms. These principles would 

include the knowledge necessary to design specific chemistries and microstructures for control of 

mechanisms such that the material forms thin, stable, protective surface layers. To reach the grand vision 

of controlling corrosion in any specific environment (dependencies), it is critical to make systematic 

advances in understanding corrosion-controlling processes (the drivers and triggers and their interplay) at 

multiple scales. The discovery process would work from understanding corrosion in idealized systems; to 

understanding how the presence and/or addition of defects changes corrosion pathways; to understanding 

how external stressors like radiation, mechanical stress, or environmental impurities alter the corrosion 

response. Although the emphasis here is on corrosion-related failure, it is necessary to understand how 

stress, temperature, and irradiation impact the deformation processes and the mechanical response, as 

these may trigger alternate failure mechanisms.

Idealized systems

Idealized systems should be 

understood first before moving on to 

complex systems with defects and 

external effects. As discussed in 

Figure 10, an idealized system refers 

to the simplified model of a coolant, 

a protective film, and the solid, in 

which each of these three 

components is chemically and 

structurally homogeneous. In these 

systems, it is important to understand 

the nonequilibrium thermodynamic 

and kinetics of the moving 

interfaces, including the balance of 

dissolution and regrowth. Key issues 

include the drivers that control 

whether the interfaces are flat, are 

complex in three dimensions or 

perhaps oscillate with time, and how 

these couple to the dependencies, such as the coolant in a reactor. The fundamental processes that lead to 

delamination and failure of protective oxide films are still controversial in the current scientific literature, 

an issue that needs to be resolved. For instance, what are the relative roles of destabilizing drivers—such 

as stresses in a protective oxide film due to electromechanical issues such as high electric fields, misfit at 

the interface to the metal, and deformation processes—in relation to the roles of triggers for delamination, 

such as voids in the oxide, the metal, or the interface? While these phenomena matter for the substrate 

material, they also will lead to the presence of impurities and nanoparticles in the coolant liquid, which 

leads to other concerns. These are issues for which the revolutionary advances in characterization and 

modeling tools made over the last decade offer the opportunity for key scientific breakthroughs that will 

facilitate significant improvement in future nuclear materials.

Defected systems

Although much fundamental understanding of oxidation and corrosion can be achieved by investigating 

pure metal systems, it is crucially important to extend this understanding to multicomponent alloys and to 

determine the roles of structural and chemical heterogeneities (see Figure 10). These multicomponent 

alloys are typically thermomechanically processed with the goal of achieving optimized microstructures 

that improve the initial properties of the metal (e.g., yield strength) and its corrosion resistance. However, 

Figure 10. A schematic 

representation of a corroding 

system, including a solid 

material, a protective layer, 

and a coolant. An idealized 

system consists simply of a 

coolant, a solid, and potentially 

a protective layer. Realistic 

(defected) systems include 

heterogeneities, such as 

alloying elements, secondphase precipitates, surface 

roughness, and grain 

boundaries. In extreme 

environments of nuclear 

reactors, one also needs to 

account for the external 

stressors, such as radiation and 

environmental impurities.

| Image courtesy of I. Szlufarska, 

University of Wisconsin–Madison
Basic Research Needs for Future Nuclear Energy

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the irradiation continuously modifies the composition and structure of the metal, requiring an 

understanding of how these changes modify the mechanical integrity and the corrosion response.

Many details of how alloying elements change corrosion are scientifically ambiguous, despite enormous 

advances in the empirical development of alloys that show improved resistance in selected cases. At the 

level of simple solid solutions of trace elements, it remains unclear what are the relative roles of changes 

in the lattice parameters and hence interfacial stresses, surface segregation, or electronic phenomena such 

as dopant-induced states in protective oxide films. There is clear evidence that even small levels of metals 

such as molybdenum drastically change the corrosion of systems involving chromium oxide protective 

layers, [Morco et al 2016; Morco et al. 2017] but the underlying science is not fully understood.

Extending understanding to real materials with grain boundaries, dislocations, and second-phase 

precipitates involves further complications and poses further scientific questions. Microstructural features 

of the alloys, such as grain size, texture, and grain boundary character distribution, can dramatically affect 

the mechanical properties and the corrosion mechanism; and these must be understood on a fundamental 

level. For example, the early stages of oxidation are often associated with internal oxidation, via which 

oxygen diffuses along the grain boundaries of the alloys, oxidizing these internal surfaces. Although the 

thickness of the internally oxidized layer represents wasted thickness, the phenomenon of internal 

oxidation can anchor the overlying protective oxide layer, rendering it mechanically more stable. Grain 

boundaries represent high-energy sites in the alloy, and improper heat treatment can lead to sensitization,

[NRC 2011] which can cause phases that more readily corrode to form at grain boundaries, or protective 

elements such as chromium to precipitate out. More recent studies have shown that grain boundary type 

or character can profoundly affect corrosion; in steels, low-energy grain boundaries are typically less 

prone to corrosion than their high-energy counterparts, providing a powerful tool to control corrosion in 

an alloy. All of these phenomena need to be coupled to the underlying science of the thermodynamics and 

kinetics of corrosion, connecting the complex behavior of engineered alloys back to the idealized systems 

and to the relevant drivers, triggers, mechanisms, and dependencies.

External effects in corrosion 

In nuclear reactors, materials are subjected to external stressors (dependencies) that include radiation, 

mechanical stress, and environmental impurities—all of which can affect the degradation of the coolant 

and the metal. In particular, radiation effects are particular to the environments of nuclear reactors, and

they lead to phenomena that do not occur otherwise. These phenomena range from radiation-enhanced 

diffusion in the solid material to radiolytic desorption of oxygen from protective oxides into the coolant, 

and radiolytic enhanced diffusion (Figure 11). Although we already understand some of the consequences 

of radiation in the solid, there are still too many unknowns. The effects of external stressors can also 

couple to one another, as in the case of irradiation-assisted stress corrosion cracking, [Hoffman et al.] and 

are almost certainly important for other failure mechanisms such as fretting-assisted corrosion. 

Radiation effects in the coolant can also be significant, and they can couple to the corrosion mechanisms 

in the base solid. Specifically, the solvation properties and redox properties of the solution affect the 

kinetics of the individual processes involved in corrosion, and they impact corrosion pathways. Radiolysis 

produces redox-active species, which in turn can alter corrosion mechanisms. The effects of radiolysis on 

corrosion varies depending on the physical/chemical properties of the solution, and the coupling of all 

these phenomena needs to be understood. 

In summary, there is already empirical evidence that the development of protective oxide films can be 

drastically different with and without radiation, but our understanding of the science underpinning these 

processes is incomplete. 
Basic Research Needs for Future Nuclear Energy

Priority Research Direction 3 25

Opportunities

To address the aforementioned scientific challenges, it will be necessary to develop and adopt state-ofthe-art theoretical, modeling, and experimental tools in an integrated fashion. On the theory and modeling 

side, the goal is to develop predictive models that are based on an understanding of thermodynamic and 

kinetic drivers, as well as of mechanisms and triggers, that control materials degradation in the coupled 

extreme environments (dependencies) of nuclear reactors. In addition, modern techniques, such as data 

mining and machine learning algorithms, provide a new exciting opportunity to learn from historical as 

well as new data to predict systems (e.g., alloy or coolant compositions) that have superior corrosion 

resistance. A significant opportunity to unravel the synergistic effects of extreme environments is the 

development of in situ techniques that allow for coupling of the effects of radiation, environments (i.e., 

controlled chemistry and temperature) and nanoscale characterization in a single experiment.

Potential Impacts on Energy Technologies

Enhancing the properties of surfaces under harsh conditions will lead to increased and improved 

interfacial performance that extends across DOE, from reactors to waste storage. Radiation and interfaces 

are ubiquitous in the nuclear power industry, and processes initiated at interfaces are responsible for 

subsequent corrosion and failure of components. The need for understanding and controlling processes 

occurring at interfaces is wide ranging and of vital importance. 

Success in advancing the fundamental knowledge of the processes occurring at interfaces exposed to high 

radiation fields, temperature, and pressure will lead to improved safety and stability of the current reactor 

fleet by increasing safety margins in, for instance, loss-of-coolant accidents. Reduced corrosion and 

erosion of materials will be obtained, as well as the decreased transport and deposition of activity within 

gas, liquid metal, and molten salt reactor systems. Expected advances from this research will lead to fuels 

and cladding materials better able to withstand higher burnup, higher temperature gradients, and other 

hazards associated with the next fleet of nuclear reactors, as well as increased lifetimes of advanced 

reactor systems. 

Absent the burden of radiation, similar performance challenges exist in the use of molten salts, ionic 

liquids, and other fluids employed in heat transfer systems. For example, the understanding of hightemperature interfacial chemistry and corrosion processes obtained in the context of nuclear applications 

will be equally valuable for the deployment of durable and reliable systems for thermal conversion of 

solar energy.

Basic Research Needs for Future Nuclear Energy

Priority Research Direction 4 27

PRD 4: Reveal Multiscale Evolution of Spatial and Temporal Processes for 

Coupled Extreme Environments 

Key question: How can computational and experimental techniques be integrated to bridge spatial, 

temporal, and energy scales that underpin materials’ behavior and chemical transformations in coupled 

extreme environments?

The materials used as fuels, coolants, and structural components in advanced nuclear reactors are exposed to 

multiple extreme external stimuli, e.g., temperature, stress, pressure, environment, and radiation. Each of 

these external stimuli independently has the potential to degrade the properties and performance of the 

material; when they are coupled synergistically, the degradation processes may be modified or accelerated. 

The continuous evolution of the intrinsic properties of the fuel, coolant, and structural materials induced by 

these external stimuli introduces additional variables that add to the complexity of identifying, 

understanding, and designing against the degradation processes. For example, the presence of the fuel 

dissolved in a molten salt complicates the corrosion processes. In the fluid at the interface with the structural 

material, radiation-induced ballistic collisions with the salt can change near-surface salt chemistry, which 

changes the local driving forces for dissolution. Additionally, the salt chemistry constantly changes with 

irradiation, adding new elements to the salt and thus changing corrosion potentials.

The time scale on which reactions can occur is as short as 0.1 femtosecond, but these initial events can 

trigger degradation mechanisms on time scales of months and years, with ultimate failure occurring in 

decades. Understanding the reactions also requires a multiscale approach that extends from angstroms to 

meters; see the sidebar “Corrosion from the Atomistic to the Continuum Level” for an example. To 

address the scientific challenges associated with determining performance degradation issues under 

changing external and internal variables will require advances in computational and experimental 

methodologies and forging of a new and stronger integration between them. Each will have to become 

multiscale within itself and integration between them attained. The experiments will need to be conducted 

in real time while the material is exposed concurrently to high temperatures, pressures/stresses, and/or 

radiation fluxes. This PRD recognizes the need for multiscale, multi-method approaches to identify the 

degradation processes that will operate in the coupled extreme environments envisioned for advanced 

nuclear systems.

Scientific Challenges and Opportunities

This PRD seeks to discover the fundamental changes in the properties of fuels, coolants, and structural 

materials that occur over spatial and temporal scales spanning multiple decades. They occur as a result of 

the coupled extreme environments that can act independently or in concert to degrade the properties and 

performance of the materials. A key scientific challenge is that the systems under investigation and the 

external variables themselves are not static but continually evolving. That is, determining the degradation 

of a material requires that the degradation in properties and responses be determined not only in pristine 

materials but also in various degraded states. Furthermore, the degradation processes themselves occur 

over the operational lifetime, and they are likely to be modified as the fuel, coolant, and structural 

material evolve. Since these are time-dependent processes, there is also a need to assess processes and 

mechanisms in environments that mimic the operational conditions, and to be able to do so over a wide 

range of spatial and temporal scales. 

Thus the following are key scientific challenges to be addressed:

• How are the intrinsic properties of the fuel, coolant, and structural materials modified by the extreme 

environments found in advanced nuclear reactors? For example, can we determine how exposure to 
Basic Research Needs for Future Nuclear Energy

28 Priority Research Direction 4

the irradiation environment over the operational lifetime affects the physical and chemical properties 

of the coolant? 

• What is the impact of exposure to coupled extreme environments on the spatial and temporal 

dependencies of the key degradation processes and mechanisms? For example, what are the 

relationships between the physical and chemical properties of the coolant and the attendant corrosion 

processes and mechanisms?

Corrosion from the Atomistic to the Continuum Level 

Corrosion is a multi-spatial and multi-temporal problem. The spatial range varies from initiation events at 

the atomic level through nanometer-scale defects in materials that then migrate and cluster to create 

larger defects, eventually leading to failure over a long time scale under the influence of stress and 

environment. Processes at multiple length scales dictate both corrosion susceptibilities and modes of 

attack, as well as how a surface layer regulates these processes to provide protection. To understand 

these processes requires study at the resolution limits of existing experimental methods in order to 

understand the mechanisms necessary to guide improvements in materials design.

From a computational viewpoint, the atomic structure and surface morphology of a homogenous 

region can be modeled by density functional theory and Monte Carlo methods (lower left). Although 

this information provides some insights into mechanisms, major scientific challenges are to advance the 

ability to investigate the complexity of materials that will be deployed as structural materials in 

advanced reactors, and to introduce and explore the relevant complex structures. Furthermore, it is 

essential that computational methods evolve to explore the roles of microstructural heterogeneities, 

grain boundary nano-chemistry and structure at the 1–1000 nm scale, and grain texture that often 

governs corrosion. | Images courtesy of Oak Ridge National Laboratory, Pacific Northwest National

Laboratory, and University of Michigan

To address these scientific challenges, the following research thrusts were identified. These seek to reveal 

the multiscale evolution of spatial and temporal processes that operate in coupled extreme environments 

and lead to degradation in the desired property of a material. The research thrusts recognize the need to 

conduct in situ, operando experiments with multi-modal diagnostic capabilities to assess the fundamental 

mechanisms and processes. These experiments cannot be conducted in isolation but must be fully 

integrated with advanced computational methods. 
Basic Research Needs for Future Nuclear Energy

Priority Research Direction 4 29

Discovery of multiscale processes that lead to degradation of coolants and structural 

materials

Experimental framework

Through advances in scattering, spectroscopy, and electrochemical techniques, it is now possible to 

employ simultaneously a variety of diagnostic tools to uncover the physical and chemical processes that 

govern the degradation in performance of material 

systems over different spatial and temporal scales. The 

information gained at one spatial and temporal scale 

informs the interpretation of the data at successive 

spatial and temporal scales, until ultimately a 

comprehensive understanding is achieved of the 

processes and mechanisms at the atomic and molecular 

scales, though the mesoscale, to the continuum scale. 

Three-dimensional data sets can now be garnered to 

provide a snapshot in time of the reaction kinetics, 

bringing closer the concept of coupling 3D spatial data 

with temporal information. Moreover, and important to 

answering the challenges relevant to advanced nuclear 

systems, experimental capabilities have been or are 

being developed to enable the experiments to be 

conducted in real time and under environmental 

conditions that duplicate or replicate the application 

environments. If these approaches include multiple in 

situ diagnostic capabilities—coupled modality 

capabilities—they can provide information on the 

reaction kinetics over a range of temporal and spatial 

scales under actual operational environments. 

For example, it is now feasible to monitor in situ the 

corrosion of a metal exposed to a high-temperature 

liquid metal or aqueous environment under simultaneous 

irradiation. Figure 11 provides an example of such an 

experimental station. [Raiman and Was 2017] Using this approach, it was shown that the irradiation of 

316L austenitic stainless steel in water with an added hydrogen environment resulted in the formation of a 

thinner and more porous oxide with a lower chromium content than in the unirradiated material. [Raiman 

and Was 2017] This difference was attributed, following post-corrosion characterization, to the 

dissolution of chromium-rich spinel oxides in irradiated water, leading to an accelerated rate of inner 

oxide dissolution. The results further suggested that a pre-formed protective oxide may be effective in 

preventing chromium loss in irradiated water. Similarly, lead bismuth eutectic produced a less dense and 

more porous oxide on HT9 in the presence of irradiation. [Frazer et al. 2016] These results show the 

potential of building experimental instruments and developing techniques to enable real-time monitoring 

and in situ characterization in which the material of interest is subjected to multiple external stimuli 

simultaneously. In this case, in situ diagnostic capabilities need to be developed to provide insight into the 

reaction kinetics not achievable from post-reaction characterization. 

Figure 11. Proton beam irradiation of 316L 

stainless steel while exposed to hightemperature water with added hydrogen to 

study the effect of radiation on corrosion. 

| Reprinted from J. Nucl. Mater. 493, S. S. Raiman 

and G. S. Was, Accelerated corrosion and oxide 

dissolution in 316L stainless steel irradiated in situ in 

high temperature water, 207–218, copyright 2017, 

with permission from Elsevier
Basic Research Needs for Future Nuclear Energy

30 Priority Research Direction 4

The production of crystalline oxide phases 

during corrosion may or may not produce 

protective layers, and failures in the oxide phase 

may result in localized corrosion. Understanding 

the nucleation of oxide phases requires 

knowledge of precursor states and the growth of 

these nuclei over a wide temporal and spatial 

range. For example, Ingham et al. used in situ 

synchrotron small- and wide-angle x-ray 

scattering to determine the oxidation of steel 

exposed to a CO2-saturated brine solution. They 

observed the initial formation of amorphous 

nanoparticle iron carbonates and their evolution 

over time into nucleation sites and crystal 

formation of FeCO3 (sederite). [Ingham et al. 

2015] Current density was observed with the 

onset of nucleation, and it dropped again as 

protective oxide formation occurred (Figure 12). 

Coupled modality characterization techniques 

such as this have the capability to elucidate 

reaction mechanisms that occur over a range of 

siderite particle sizes on a solution interface. However, the combination of methods required likely differs 

depending on whether the coolant allows passivation and the subsequent mode of corrosive attack. 

Adding the ability to add simultaneous temperature, radiation, or loading would extend the capabilities of 

such an experimental method to address degradation mechanisms applicable to advanced nuclear reactors.

Computational framework

In addition to adopting a multiscale experimental 

approach, there is a pressing need to bridge the 

spatial and temporal scales computationally. 

Figure 13 shows the time and length scales 

appropriate to the different calculations. [Stan 

2009] Bridging these scales will require combining 

the accuracy and flexibility of electronic structure 

calculations with the speed of classical potentials 

by merging and exploiting the best insights from 

the fields of machine learning, advanced 

optimization, and atomistic simulations. Model 

machine learning algorithms, such as deep learning 

and genetic algorithms, are 

needed for the enhanced sampling of training data 

with experimental inputs. There is a need to 

leverage state-of-the-art advances in modelbuilding algorithms, such as genetic algorithms, 

compressive sensing, and deep neural networks, to 

describe scenarios that deviate far from 

equilibrium. Yet to leverage these advanced 

intelligent learning approaches requires 

experimental and theoretical data that reveal the essential phenomenological patterns that then enable 

predictive capability. At present, a single framework encompassing such information across multiple 

Figure 12. Plot of current density (light line) during 

siderite growth on iron as monitored by in situ x ray 

diffraction (heavy lines). | Reproduced from B. Ingham et 

al., 2015, First stages of siderite crystallisation during CO2

corrosion of steel evaluated using in situ synchrotron smalland wide-angle x-ray scattering, Faraday Discuss. 180(0), 171–

190, with permission of The Royal Society of Chemistry

Figure 13. The time and length scales appropriate 

to different calculation modalities. | Reprinted from 

Mater. Today 12(11), M. Stan, Discovery and design of 

nuclear fuels, 20–28, copyright 2009, with permission from 

Elsevier
Basic Research Needs for Future Nuclear Energy

Priority Research Direction 4 31

length and time scales is lacking; this remains a scientific challenge. A second key scientific challenge for 

the computational community is to advance from idealized surfaces and systems to address ones with 

heterogeneities on the surface and in the bulk, including those that develop as the materials evolve under 

irradiation; and to develop the computational capabilities that explore the spatial scale of relevance to the 

degradation mechanism. This challenge includes developing potentials that account appropriately for 

magnetic effects, impurities, precipitates, and so on.

Development of an integrated framework

While the multiscale experimental and computational approaches present significant challenges in their 

own domain, another major challenge is the synergistic coupling of experimental and computational 

methods across all relevant spatial and temporal scales. This must be met to address the loss in 

performance of materials used in complex and multi-component harsh environments. 

An example of the insight into fundamental processes that can be obtained by coupling theory and 

experiment is provided by the recent studies of radiation damage evolution in equimolar metallic alloys, 

including those that contain five or more elements—often referred to as “high-entropy” alloys. The 

enhanced properties of these concentrated solid-solution alloys are attributed to a combination of 

thermodynamic and kinetic effects, the local lattice distortions, and to the combined effect of the intrinsic 

properties of the elements. The ab initio Korringa–Kohn–Rostoker coherent-potential-approximation 

method—which was implemented within density functional theory (DFT)—provided insight into the 

variation in the density of states as the number of elements increased in the nickel-based alloy series 

formed with Cr, Co, Mn, Fe, and Pd additions. [Zhang et al. 2015; Zhang et al. 2017] The results showed 

a substantial decrease in the electron mean free path, which impacts how heat is transported away from 

the displacement cascade. The electrical and thermal conductivity were predicted and experimentally 

shown to be significantly lower in the more complex alloys than in pure nickel. Molecular dynamics 

(MD) computer simulations showed that the defect migration and formation barriers were higher in the 

binary alloys than in nickel, and that the production of extended defects was related to the collision and 

not the ballistic phase of the displacement cascade. [Beland et al. 2016] Together, these results suggest 

that the radiation resistance of these alloys should increase with increasing alloy complexity. In general 

terms, this prediction has been substantiated by several different experiments, although it has emerged 

that a specific element can play a more significant role than the number of elements. [ Zhang et al. 2015; 

Lu et al. 2016] The theoretical and computer simulations clearly provided an interpretation of the 

experimental results in terms of intrinsic properties not accessible in the irradiation experiments. It now 

remains to create enough information about the patterns of behavior associated with alloy series to train 

learning algorithms and enable the capability to predict materials properties in more-complex, highercomponent materials.

A second example demonstrates that new experimental approaches are needed to understand the 

predictions of calculations of material degradation processes. Much is known from first-principles 

calculations about how electroactive ions diffuse through a lattice, [Van der Ven et al. 2013] but little is 

known experimentally about the atomic-scale processes involved in ion diffusion. The ability to 

characterize such phenomena over the length and time scales required for comparison with theory has 

until now been limited by the available experimental tools. This has in turn restricted the ability to 

develop validated design guidelines to manage ion diffusion in materials. The capabilities of free electron 

x-ray lasers may enable much more detailed insight into ion and atom migration in complex materials 

under operating conditions. For example, x-ray photon correlation spectroscopy has already been shown 

to be a powerful probe of atomic diffusion, [Leitner et al. 2009] but it is currently limited to relatively 

slow time scales (>ms). Advanced high-flux x-ray photon correlation spectroscopy with high-repetitionrate free electron lasers has the potential to enable studies of ion diffusion at high spatial resolution. 

[Carnis et al. 2014]
Basic Research Needs for Future Nuclear Energy

32 Priority Research Direction 4

Development of an integrated computational and experimental framework to 

understand environment-assisted cracking

Understanding uniform modes of corrosion requires understanding transport rates, interface reaction 

mechanisms, and rate-controlling processes. These phenomena depend on the coolant and coolant/fuel 

hybrids being considered. The challenge will be to address the modes of corrosive damage that combine 

several external stimuli with material transformations driven by related factors. The combination of 

factors gives concern regarding sophisticated modes of corrosion such as local microstructural attack, dealloying, and environment-assisted cracking (EAC).

Dissolution of structural materials is a key factor in heavy liquid metals based on Pb, Bi, and Sn and light

liquid metals such as Na or Li. Although the dissolution can be mitigated by purposely doping the liquid 

metal with passivating elements such as oxygen, the detailed understanding of the transport of elements 

through these interfaces is not fully understood. The complexity of the challenge is magnified because 

these phenomena are accompanied by microstructural effects such as de-alloying and associated phase 

transformations, multiple layers of different oxide types, and porosity, making a temporally and spatially 

accurate description of the phenomena difficult. For sodium and lithium, the degradation is attributed to 

the impurities in the liquids. Again, it is the transport of specific metal species to the surface that governs 

the progress of the phenomena and controls the impurities; therefore, the chemical reactions are 

challenging and need to be addressed in a fundamental fashion.

Salt-cooled systems will operate at temperatures above 600°C. These systems exemplify an environment 

in which materials must perform in the coupled extreme environments of high temperature and radiation. 

The corrosion itself is a competition of thermodynamic drivers at the corrosion front and kinetic processes 

as atoms diffuse in response to reactions at the fluid/metal interface. Additionally, the radiation-induced 

changes in the microstructure and, more important, in the composition of the salt, influence the 

competition between thermodynamics and kinetics of the corrosion reactions. The high temperature leads 

to corrosion, specifically dissolution of alloying elements such as chromium from structural materials into 

the salts, which reduces the free energy of the system 

(Figure 14). [Lister 2012] The dissolution is faster at 

grain boundaries, which act as fast pathways for 

chromium to reach the surface. Under radiation, the 

grain boundary composition changes as a result of 

radiation-induced depletion of chromium at the 

boundary. [Bruemmer et al. 1999] Nominally, this 

reduction of chromium at the grain boundaries will 

slow down the loss of chromium to the salt, but the 

complexity of these competing nonequilibrium 

processes remains to be understood.

Identifying how the coupling of multiple 

external stimuli expands or contracts the 

susceptibility window for EAC

EAC of a particular metallic system occurs when 

there is a critical combination of chemical, 

electrochemical, and mechanical factors that, when 

coupled, render a material susceptible. These factors 

produce crack initiation and growth that likely would 

not occur in the absence of any one of these factors. 

The operating temperature, the coolant and fuel, the 

pressure, and the radiation constantly modify the 

Figure 14. Correlation between chromium 

content in the alloys and weight loss due to 

corrosion. Samples from nickel-based alloys were 

exposed to FLiNaK salt at 850°C for 500 hours. |

Reprinted from Molten Salt Chemistry: From Lab to 

Applications, eds. F. Lantelme and H. Groult, K. 

Sridharan and T. R. Allen, “Corrosion in Molten Salts, 

241–267, Elsevier, Oxford, copyright 2013, with 

permission from Elsevier
Basic Research Needs for Future Nuclear Energy

Priority Research Direction 4 33

coolant, fuel, and the intrinsic properties of the structural material and the surface state, which will enable 

or accelerate interfacial reactions. The key scientific challenge will be to identify—through the synergistic

coupling of multiscale experimental and computational approaches—the chemical, electrochemical, and 

mechanical driving forces; the dependencies on the evolving material condition; and the mechanisms 

behind crack initiation and propagation in the bulk and at specific sites. That is, the key challenge is not 

only to define how the coupling of extreme environments modifies the susceptibility window for EAC but 

also to uncover the fundamental mechanisms that govern the change. 

Liquid metal embrittlement, for example, is a specific form of EAC in which bare metal-to-metal contact 

occurs. Although liquid metal embrittlement is a commonly observed phenomenon in systems such as 

Ga/Al, Hg/Al, Pb/ferritic steel, and many others, many aspects remain unknown—such as the intrinsic 

and extrinsic drivers and dependencies of this phenomenon. For example, if the introduction of impurity 

species into the grain boundary is insufficient to cause it to fail, as some DFT calculations have 

suggested, what are the other forces driving embrittlement? Is the change in the grain boundary structure 

due to the introduction of the impurity the additional driving force [Luo et al. 2011]? Is the plastic 

deformation and its attendant impact on the grain boundary an additional driving force [Gorse et al. 

2011]? The scientific challenge is to uncover these driving forces and determine how they are modified 

by the continuously evolving structural material and external variables. 

Potential Impacts 

Successfully meeting the challenges outlined will enable a deeper fundamental scientific understanding of 

the evolution of material properties and performance under exposure to stress, temperature, corrosive 

environment, and radiation. This understanding will be achieved by coupling new, multi-modal 

measurement techniques integrated with theory and modeling. The impact will be an improvement of 

system operating lifetimes that leverage initial capital investments in plant construction; improved energy 

generation efficiency through improved heat transfer resulting from decreased degradation of fuel 

cladding/coolant interfaces; and the resultant improved safety that arises from more durable, dependable 

materials in both primary and support plant systems. It can be reasonably imagined that new tools and 

deeper understanding will have important impacts on other energy systems as well, including new 

materials for renewable energy technologies, such as concentrated solar power; for energy storage 

technology, such as advanced batteries; and for the transport of energy from source to load locations. 

Finally, strategies for designing new materials tailored for specific applications will emerge based upon 

this new knowledge, perhaps even taking advantage of synthesis under the extreme conditions under 

which these reactors will operate. 

Basic Research Needs for Future Nuclear Energy

Priority Research Direction 5 35

PRD 5: Identify and Control Unexpected Behaviors from Rare Events and 

Cascading Processes 

Key question: How do we identify, anticipate, and control rare events that initiate cascading processes 

and cause aberrant properties and materials’ responses? 

The extreme conditions associated with nuclear energy systems emphasize the importance of 

understanding processes that can be controlled by multiple factors. One such factor is so-called rare 

events that are characterized by vastly different time scales of the event, the accumulation of those 

processes, and the eventual outcome in the context of system reactivity or behavior. Other important 

factors include unpredictable stochastic processes, and minority species (e.g., defects, impurities) whose 

reactivity or physical properties impact the system inordinately. Many phenomena depend in a nonlinear 

fashion on differences in structure; the distributions of defects and impurities; or even the fluctuations in 

energy, properties, or organization, rather than the average. Fluctuations can adopt dramatically different 

behavior depending upon the extreme conditions within nuclear energy systems, leading to unanticipated 

reaction pathways. A robust, validated mechanistic knowledge of material and chemical processes at the 

time and length scales of the controlling events and interactions is needed to attain a predictive 

understanding of the role of high-impact/low-frequency processes that can lead to unexpected reactivity 

or materials degradation, including catastrophic failures. The need to understand the controlling events 

associated with singular processes is a major challenge for experimental, computational, and theoretical 

approaches. 

Scientific Challenges and Opportunities

The exposure of materials to extreme chemical and physical 

perturbations can lead to unexpected behaviors that are 

challenging to anticipate, predict, and control. (See the sidebar 

“Unexpected Sensitivity of bcc Alloys to Radiation Damage.”) 

Under such circumstances, an understanding of the average 

properties of a system responding to such perturbations—for 

example, through the ensemble integration of stochastic 

processes—is not sufficient. Rather, it is necessary to obtain a 

more robust understanding of the wide array of materials processes 

that occur, including 

• Incorporating the role of rare (low-probability) events and the associated variability in behavior, such 

as including deviations from Gaussian statistics or an ensemble average that is not well formed

• Understanding how the manifestation of processes can change across length and time scales, such as 

connecting atomic-, nano-, meso- and eventually macro-scale processes

• Assessing the role of minority, often trace, components, such as defects and impurities

In some cases, seemingly identical structures can react in distinctly different fashions. Detecting and 

probing these phenomena at the appropriate length and time scales—through experimental, 

computational, and theoretical approaches—is a major scientific barrier that has been identified as a key 

element in a transformative research agenda (e.g., in the DOE BES report Challenges at the Frontier of 

Matter and Energy [Hemminger 2015]).

“A new type of energy science 

can be developed to understand 

real materials under real-world 

conditions, including even the 

slow and statistically rare events 

that can dictate success or 

failure of materials.”

From Hemminger 2015, p. 20
Basic Research Needs for Future Nuclear Energy

36 Priority Research Direction 5

Unexpected Sensitivity of bcc Alloys 

to Radiation Damage

Metallic alloys based on the body-centered-cubic 

(bcc) crystal structure are generally observed to 

exhibit superior radiation tolerance compared with 

face-centered-cubic (fcc) alloys. For example, the 

radiation-induced dimensional change (volumetric 

void swelling) observed in austenitic (fcc) stainless 

steels is about an order of magnitude higher than 

in ferritic/martensitic (bcc) steels (upper image). 

Consistent with this observation, both vanadium 

and iron, in their elemental forms, have a bcc 

structure and exhibit very low void swelling rates 

during fast neutron irradiation. Therefore, it was 

quite surprising that a bcc V-5%Fe alloy showed 

the greatest swelling of any known metallic 

material, and that damage in dilute V-Fe bcc 

alloys showed extreme sensitivity to the 

concentration of iron. In these alloys, a sharp 

increase in the swelling rate was observed upon 

addition of just a small additional percentage of 

iron (lower image). 

The swelling behavior of bcc V-Fe solid solutions 

directly challenges our understanding of the 

underlying relationships among crystal structure, 

composition, and radiation damage. At the same 

time, it presents a concrete example of how an 

unexpected result presents a unique window for 

breakthroughs in developing improved radiationtolerant materials and greater flexibility in system 

design.

Swelling of fcc and bcc steels as a function of 

heavy ion dose in units of displacements per atom. 

| Reprinted from Acta Mater., 61/3, S. J. Zinkle and G. S. 

Was, Materials challenges in nuclear energy, 735–758, 

copyright 2013, with permission from Elsevier.

Sensitivity of radiation damage to V-Fe BCC alloy 

composition. | Reprinted from J. Nucl. Mater., 258–263, K. 

Fukumotoet al. Swelling behavior of V-Fe binary and V-Fe-Ti 

ternary alloys, 1431–1436, copyright 1998, with permission 

from Elsevier.

Therefore, this PRD is focused on the need to isolate, study, model, and predict the singular processes that 

ultimately may control materials behavior. This is a daunting challenge, as there can be multiple layers of 

events masking the underlying controlling phenomena. To address these key issues, the following 

research challenges have been defined: 

• Predicting unexpected responses to radiation exposure

• Elucidating fluctuations and defect evolution

• Revealing the origin of irradiation-assisted stress corrosion cracking

• Understanding impurity-controlled materials behavior in molten salts
Basic Research Needs for Future Nuclear Energy

Priority Research Direction 5 37

An illustrative, but far from complete, set of research directions is provided herein that delineate 

unexpected behaviors controlled by rare events (usually unknown), minority species, and/or cascading 

(nonlinear) processes that are relevant to future nuclear energy systems. Mechanistic studies of such 

phenomena at the appropriate length and time scales will provide the conceptual underpinnings of the 

relevant chemical processes and materials systems that are needed to overcome many of the technological 

barriers to advanced nuclear reactor operations.

Predicting unexpected responses to radiation exposure 

Damage to materials by neutrons (or heavy 

ions) begins as an atomic displacement 

cascade (Figure 15). It is normally assumed 

that this process is additive, so that the 

damage can be characterized within the 

context of the average absorbed energy 

(e.g., quantified by net displacements per 

atom). Yet, the dramatic differences in 

cascade profiles for two nominally 

equivalent initial conditions (Figure 15) 

illustrate the highly variable response of a 

material under heavy-ion exposure and the 

intrinsically stochastic nature of these 

processes. 

A dramatic breakdown of the conventional 

concept of linear response is demonstrated 

in heavy-ion damage to solids. Specifically, 

a significant increase in the damage cross 

section can occur from the interaction between the electronic energy dissipation by ions and preexisting 

atomic defects, as observed in single-crystal SrTiO3. [Weber et al. 2015] For instance, amorphous tracks 

can preferentially occur in regions with preexisting defects as a result of increased thermal spikes in the 

damaged regions (Figure 16). That is, electronic energy dissipation through electron-lattice coupling at 

defects is a controlling factor in this 

unexpected sensitivity to ionizing radiation. 

Another example of deviation from the 

expected linear responses of materials and 

the critical role of preexisting defects is 

illustrated by controlled studies of ion 

irradiation of SiC, a potential fuel cladding 

material in future nuclear energy systems. In 

this case, an unexpected result of radiation 

exposure is the substantial annealing of 

preexisting defects and restoration of 

structural order in SiC upon ion–electron 

collisions. This structural healing is 

illustrated in a cartoon (Figure 17) showing 

two projected electron microscopy images 

of SiC. The top image shows structural 

disorder within a damaged crystal (smeared 

Figure 15. Comparison of defect production in two 

nominally equivalent high-energy displacement cascades 

in iron (shown separately in green and purple). | Image 

courtesy of R. E. Stoller, Oak Ridge National Laboratory

yg p g 

non-damaged 

region

pre-damaged 

region

Ionization radiation

Figure 16. The synergistic effect of preexisting defects is 

illustrated: ionization leads to rapid amorphization in a 

defective region but no observable irradiation effects in a 

damage-free region. | Adapted from W. J. Weber et al. 2015. 

Synergy of elastic and inelastic energy loss on ion track formation 

in SrTiO3, Sci. Rep. 5, 7726. Reprinted under terms and conditions 

of the Creative Commons license
Basic Research Needs for Future Nuclear Energy

38 Priority Research Direction 5

spots in top slab), and the bottom image shows 

removal of damage upon exposure of the material to 

high-energy ions. This behavior is understood to arise

from local heating of the material by energy 

transferred from the energetic ions to SiC electrons 

via inelastic ionization processes. [Zhang et al. 2015] 

These examples highlight a few of the unexpected 

responses of materials to irradiation, especially when 

ionization and defect production occur 

simultaneously. Although self-healing processes 

could lead to longer-lasting components in extreme 

radiation environments, the distribution of the 

elementary damage events and the sensitivity of 

materials to preexisting damage represent significant 

challenges to the ability to predict material 

performance in extreme radiation environments. 

Elucidating fluctuations and defect evolution

Understanding stochastic processes is critical to the 

design and selection of materials, particularly for 

ensuring the longevity of materials under extreme 

conditions. Materials evolution is sensitive to the kinetics of multiple competing mechanisms. A critical 

challenge is to go beyond a simple “mechanistic” understanding of a single event and to instead gain 

understanding of the entire range of events—including the outlying events that can ultimately control 

behavior, particularly in intense radiation environments that can result in far-from-equilibrium conditions. To 

achieve this understanding, new experimental and theoretical techniques are needed to probe the complex 

energetic landscape that the system experiences and allow the identification of the full range of outcomes that 

may occur, their relative probabilities, and their 

ultimate impact on material behavior. 

An example of the challenge addressed here is 

manifested in the defect evolution of materials that 

can cascade and eventually lead to nucleation of 

precipitates, cracks, and voids and the coarsening of 

interstitial clusters. Understanding all of these is 

fundamental to the design of radiation-resistant 

materials (see the sidebar “Unexpected Sensitivity of 

bcc Alloys to Radiation Damage”). The stochastic 

variability in the evolution pathway in bodycentered-cubic (bcc) iron was explored explicitly 

through simulations that tracked the system energy 

(Figure 18). These studies enabled the direct 

observation of a novel dislocation structure that was 

distinct from any previously proposed mechanisms, 

and they provided a direct link between experiment 

and simulation to aid the understanding of radiation 

resistance in the systems. This finding highlights the 

need to develop approaches that can explore the 

roles of controlling events within fluctuating systems.

Figure 17. Bombarding a material with energetic 

charged particles can heal, rather than 

damage, the crystalline structure of SiC through 

local heating. | Reprinted from Y. Zhang et al. 2015. 

Ionization-induced annealing of pre-existing defects in 

silicon carbide. Nat. Commun. 6, 8049, under terms 

and conditions of the Creative Commons license.

Figure 18. Multiple pathways of interstitial cluster 

evolution in bcc iron with comparable large 

transition states can exhibit substantial differences in 

the evolutionary path of the system. | Reprinted with 

permission from H. Xu et al. Phys. Rev. Lett., 110, 2013. 

Solving the puzzle of ⟨100⟩ interstitial loop formation in bcc 

iron, 265503. Copyright 2013 by the American Physical 

Union.
Basic Research Needs for Future Nuclear Energy

Priority Research Direction 5 39

Revealing the origin of irradiation-assisted stress corrosion cracking 

Corrosion is the result of a complex suite of processes controlled by charge transfer, diffusion, 

temperature, and other influences (see PRD 3). Yet, in many cases, the controlling behavior is the result 

of the localized breakdown of a passive layer or, at high temperatures, a protective, thermally grown 

oxide. [Shibata and Takeyama 1976; Wu et al. 1997] Irradiation-assisted stress corrosion cracking 

(IASCC) is a particularly important material degradation mode for nuclear systems. For certain alloys, the 

susceptibility to failure by fracture due to the synergistic effects of stress in a corrosive environment is 

significantly enhanced in the presence of radiation damage. [Was et al. 2011] Painstaking, multifaceted 

experimental research and atomistic modeling have now revealed that IASCC of austenitic stainless steel 

in a prototypic water reactor environment is controlled by the interaction of dislocation channels 

(localized regions of plasticity caused by concentrated dislocation movement) with grain boundaries. 

[McMurtry et al. 2015] Under specific crystallographic/microstructural conditions, a dislocation channel 

at a grain boundary can lead to stress intensification, which ultimately can lead to crack formation. 

Surprisingly, when these conditions are met (identical channels intersecting the same grain boundary), 

corrosion cracks occur in only 18% of the cases. [McMurtry et al. 2015] This is a clear demonstration of 

how—despite the unraveling of complex phenomena to reveal the mechanistic underpinnings of 

IASCC—the underlying structural, mechanical, and/or chemical characteristics that cause dislocation 

channels to trigger cracking under extremes of radiation and reactivity continue to challenge our 

understanding of initiating events. 

Understanding impurity-controlled materials behavior in molten salt

The composition of and chemistry in operating nuclear reactor fluids (e.g., a molten salt) are modified by 

fission product evolution, by impurities, and by the reactivity of component materials—such as the 

chemical dissolution of fuel, moderator, and structural materials (see PRD 1). Results from the MSRE 

provide several compelling examples of the sensitivity of material behavior to transient/fluctuating 

chemistry. 

The principal constituents of the MSRE fluoride salt were 

LiF, BeF2, and ThF4, together with the UF3 and UF4 fuel 

[Ting et al. 1977] and associated fission products. Of these 

many components, the presence of a few minor elements 

(e.g., Te, Nb, 3

H) was particularly problematic with respect 

to key operational factors (such as corrosion). [Williams and 

Toth 2005; Ting et al. 1977] The fate of tellurium in the salt, 

in particular, provides an excellent example of the synergies 

between fuel/coolant chemistry and material compatibility. 

Tellurium was found to embrittle the Hastelloy N in contact 

with the salt, but only at higher salt oxidation potentials. 

Post-exposure examination showed considerably more 

tellurium within the grain boundaries from the oxidizing 

domain than from the reducing domain, with a sharp 

transition in embrittlement susceptibility over a narrow 

potential range (Figure 19). This finding demonstrates how 

the presence of minor constituents can exert an outsized

influence over materials integrity with just small changes in 

a key operating parameter of the reactor fuel/coolant (i.e., 

the redox potential)—especially at specific defect sites 

where impurities and geometry can dominate local 

chemistry.

Figure 19. Influence of oxidation potential

on the embrittlement of Hastelloy N in the 

presence of dissolved tellurium. The red 

circle suggests a suitable operating point 

at which the tellurium concentration in 

Li2BeF4 is below the threshold for severe 

embrittlement. | Reprinted from D. E. Williams

and L. E. Toth. 2005. Chemical Considerations 

for the Selection of the Coolant for the 

Advanced High-Temperature Reactor, Oak 

Ridge National Laboratory Letter Report 

ORNL/GEN4/LTR-05-011
Basic Research Needs for Future Nuclear Energy

40 Priority Research Direction 5

Most of the phenomena discussed have complex drivers related to physical and chemical processes and 

may depend on minor elements and/or highly localized structures. Material behaviors are often directed 

by coupling between chemistry and local structure on a small scale. Examples include bilayer grain 

boundary complexions in bismuth-doped copper and their role in embrittlement [Kundu et al. 2013] and

IASCC, in which the effects of deformation and radiation on grain boundary structure are as important as 

those of reactions at those interfaces. By their nature, the linking of these underlying structural and 

chemical factors challenges our capabilities to resolve them at the appropriate length and time scales 

needed to identify controlling events and processes. 

New characterization methods that probe how, where, and how often and/or how fast events occur are 

critically needed to understand the controlling aspects of stochastic processes. For example, as discussed 

earlier, to understand the full range of processes that occur under these extreme conditions, a suite of 

experimental techniques such as in situ microscopy, scattering, and spectroscopic approaches are needed 

that operate at different length or time scales. Ideally, these events need to be studied with simultaneous 

high spatial resolution and large fields of view, and in situ/operando, so rare events or very dilute species 

and isolated structures can be observed (as described in the Crosscutting Themes panel report). These 

challenges can be addressed, at least in part, by observing events that are externally triggered to analyze 

both the subsequent initiation and temporal evolution of changes in the system. For example, highresolution experimental probes, such as scanning transmission electron microscopy, x-rays, and neutron 

diffraction, that provide atomic-scale sensitivity require the use of high-energy-density probe beams. As a 

result, studies conducted with these high-energy, highly focused probes may provide essential information 

about the role of radiation fields that will be encountered within a reactor environment, such as radiolysis 

of the molten salt coolant. Such studies can provide both insights into the mechanisms, and quantitative 

information—such as the mobility of defects, chemical fronts, and transformations—that is critical for 

validation of atomic-level simulations of materials and as input for longer-length-scale modeling. 

The availability of new characterization tools to provide high-resolution and even dynamic information 

on materials systems must be coupled with new computational approaches. Data analytics tools, such as 

uncertainty quantification, are needed to identify key variables that affect the various chemical and 

physical processes. Traditional and ab initio 

statistical mechanics methods must be 

coupled to techniques that can probe the full 

potential energy surface, wherein the 

appropriate collective coordinates are used to 

elucidate the distributions of outcomes so 

that the effects of internal and external 

variables upon distributions can be 

ascertained. Nudged elastic band 

calculations, metadynamics, accelerated 

dynamics, and other approaches [Barducci et 

al. 2011; Henkelman et al. 2000] are critical 

to exploring the transition states that govern 

temperature-dependent mechanisms and 

rates. Kinetic Monte Carlo approaches must 

be generalized to allow the discovery of new 

mechanisms, as opposed to reliance upon a 

priori assumptions of mechanisms. Highly 

accurate quantum mechanical techniques 

must be scaled to the appropriate system to determine local minima in energy landscapes and the 

transformation barriers that exponentially control reaction rates (Figure 20). 

Figure 20. Schematic complex energy landscape 

highlighting local minima and barriers between them. 

| Reprinted from J. C. Schön. 2006. Wanderungen in der 

energie-landschaft, Physik Journal 9, 20–22, Wiley permission 

granted
Basic Research Needs for Future Nuclear Energy

Priority Research Direction 5 41

Potential Impacts

The extreme energy densities associated with nuclear fission reactions, along with the significant impacts 

on society associated with uncontrolled releases of radiation, necessitate a truly robust understanding of 

the material and chemical processes that underlie future nuclear energy systems. Understanding the roles 

of controlling rare events or stochastic processes will facilitate a validated, predictive understanding of 

materials and chemical responses in these harsh and reactive environments and the extrapolation of 

laboratory results to conditions outside of the specific conditions (e.g., longer length and time scales). 

This will lead to the ability to improve descriptions of material behaviors under real-world conditions and 

of their relationships to potential failures. Ultimately, this perspective will enable the rapid development 

and deployment of new, long-lived, radiation-tolerant, environmentally resistant materials with enhanced 

safety, efficiency, and lifetimes. This understanding will be enabled by compositional and microstructural 

design that is informed by the rare events that affect the long-time behavior of materials and control their 

lifetimes. It is also expected that such a deep understanding of the material response will lead to the 

development of non-perturbing sensing capabilities that can be used for detecting deleterious rare events, 

and perhaps even their precursors, so that the impacts of these events can be mitigated in real time.

Basic Research Needs for Future Nuclear Energy

Panel Reports 43

3. PANEL REPORTS

As part of the Future Nuclear Energy workshop, panelists were divided into four technical panels to 

discuss the current status and recent advances in each topic and then outline scientific challenges and 

opportunities. Their deliberations are captured in panel reports, which are presented in this section of the 

workshop report. The following are summaries of the panel reports. 

The panel on Design and Discovery of Coolants and Liquid Fuels addressed the need to understand and 

design around the numerous, and in some cases conflicting, requirements of the physical, chemical, and 

radiochemical properties of molten salts for deployment as the coolant or fuel in a reactor. At any instant, 

the fluid may be heterogeneous and exist in localized equilibria that evolve in time as a result of dynamic 

chemical and nuclear processes. For example, complex speciation imparted by fission products and trace 

impurities (including corrosion products) has an unknown effect upon important properties, including 

heat and mass transfer, which in turn perturb the net thermodynamics, kinetics, and transport in both a 

chemical and physical context. Understanding these will lead to the development of new molten salt 

compositions with superior properties which will directly impact the efficiency, safety, and operational 

lifetimes of next-generation nuclear reactors.

The panel on Physics and Chemistry of Interfaces discussed research needs required for understanding 

the wide range of physical and chemical processes that occur at interfaces, both surface (solid/fluid) and 

buried (solid/solid), that are proposed for the variety of advanced reactor concepts. These interfaces will 

have critical impacts on the performance, safety margins, and operational lifetimes of future reactors. To 

predict and control materials evolution and degradation, this panel concluded that a fundamental 

understanding of the reaction mechanisms active at the surfaces of and within materials in a complex 

combination of environments is critically needed. For example, one area identified was the interest in

understanding how radiolytic energy, charge, and heat are transported at a variety of solid/liquid and 

solid/solid interfaces. The impact of defects on the reactivity at interfaces was also viewed as an 

important issue. Once the fundamental mechanisms of the myriad chemical and physical processes that 

occur at interfaces are illuminated, this knowledge can catalyze the development of a new generation of 

materials and coatings designed to control and even repair interfaces exposed to extreme environments. 

The panel on Understanding Behavior at Coupled Extremes focused on understanding the dynamic 

and highly coupled processes that occur under extreme conditions. Simple studies of materials and 

systems at a single given extreme condition were not deemed to be sufficient for these challenges, 

because of the combination of radiation fields, high temperature, pressure, and stress. These extreme 

conditions can be synergistic and yield processes and mechanisms that are active only when they are 

studied concurrently. Only investigations under coupled conditions can provide insight into the full 

complexity of interfacial reactions, transport phenomena, microstructural evolution, and material defect 

formation and propagation. As these inextricably linked processes are unraveled, it will be possible to 

develop new materials, interfaces, and multicomponent systems that survive in harsh, nonequilibrium 

environments where high-energy, transient species are present.

Discussions in the Design and Discovery of Structural Materials and Solid Fuels panel summarized a 

need for greater fundamental knowledge of nuclear material behaviors based on a mechanistic 

understanding of the interplay of compositional modifications, defect formation, deformation, and 

dynamically changing microstructure and nanostructure/chemistry as they are exacerbated by extreme 

environments. This information will be critical for the development of materials with stable structures at 

the nano/micro scales that can withstand harsh environments, and even of revolutionary materials that can 

self-repair. This panel noted that development of robust materials design rules must incorporate holistic 

understanding of materials and fuels behavior in nuclear energy environments to enable the specification 
Basic Research Needs for Future Nuclear Energy

44 Panel Reports

of robust materials and fuels for radiation environments in a way that substantially shortens the current 

decades-long development cycle.

A major challenge that emerged in the workshop is addressing the multitude of chemical and physical 

processes that occur in structural materials, coolants, and fuels which must be understood across broad 

spatial and temporal scales—from atomic to macroscopic length scales and from femtoseconds to years. It 

was concluded that these requirements, which were common to all four panels, cannot be met with 

today’s analytical and computational tools. Therefore, the Crosscutting Themes panel focused on two 

areas. The first was the need for new characterization approaches that take advantage of in situ 

measurements, including multiple simultaneous measurements that can probe both chemical reactions and 

structural changes, to provide a more complete view of the multitude of impacts that occur in the extreme 

conditions to which these materials are exposed. The second was the development of multiscale modeling 

capabilities to understand and predict the full range of chemical and physical phenomena that occur under 

extreme environments. Importantly, it was underscored that experimental methods need to be closely 

integrated with computational tools to predict and simulate degradation processes, and that deep data 

analysis will be needed to interpret and assimilate data from multiple experiments. With these approaches, 

the goal of understanding and predicting chemical and physical changes in materials, coolants, and fuels 

can be achieved, and will expedite the development of future nuclear energy systems.
Basic Research Needs for Future Nuclear Energy

Design and Discovery of Coolants and Liquid Fuels 45

Panel A: Design and Discovery of Coolants and Liquid Fuels

Nuclear energy currently provides about 20% of US electricity and approximately 60% of its zero-carbon 

electricity. In the future, nuclear energy can play a significant role in meeting mid-century energy 

demands in an environmentally friendly manner; but advanced technologies are needed to replace the 

current fleet of aging nuclear reactors and move past the limitations of light water reactor technologies. 

Advanced reactor concepts offer the potential benefits of enhanced safety and reliability, greater resource 

utilization, improved proliferation resistance, co-production of process heat for industrial operations, and 

economic competitiveness. In 2002, the Generation IV International Forum (GIF) created a Technology 

Roadmap that defined the necessary research and development to allow for the deployment of nextgeneration energy systems after 2030. The GIF selected 6 reactor systems from more than 100 concepts, 

which were confirmed in the 2014 Technology Roadmap Update for Generation IV Nuclear Energy 

Systems. [OECD 2014] They were the gas-cooled fast reactor, lead-cooled fast reactor, MSR, sodiumcooled fast reactor, supercritical–water-cooled reactor, and very-high-temperature reactor (VHTR). As

discussed in the factual status document for this workshop, each reactor technology has its own set of 

advantages, challenges, and fuel and coolant requirements. To meet the DOE–NE goal “that by the early 

2030s, at least two non-light water advanced reactor concepts will have reached technical maturity, 

demonstrated safety and economic benefits, and completed licensing reviews sufficient to allow 

construction to go forward,” [DOE 2017] significant research and development is needed to overcome the 

research challenges and determine the most promising reactor technology.

Recently, there has been growing national and international interest in MSRs because of the potential 

benefits in higher efficiencies and resource utilization, enhanced safety, and economics. [WNA 2017] In 

response to this interest, DOE–NE organized a Molten Salt Chemistry Workshop in April 2017 to identify 

science-based, technology-driven approaches to accelerate MSR development and deployment. [ORNL 

2017] High-temperature molten fluoride and chloride salts have many advantages. 

1. They can serve as coolants with solid fuels and/or have the fissile or fertile fuel dissolved in the liquid 

salt.

2. They have excellent heat transfer properties and a high operating temperature that allow for compact 

design and high operating efficiencies. 

3. They are safer (i.e., operate at atmospheric pressure, have a large negative temperature coefficient of 

reactivity [i.e., when the fuel heats up, it expands and becomes less reactive], and do not react with 

water or air). 

4. The fuel is highly soluble in the coolant. 

5. They can be considered for other applications (e.g., solar thermal and metal extraction). 

An MSR has the flexibility to operate with a variety of fissile materials, including 235U, 233U, and 239Pu, 

and can either burn actinides from spent nuclear fuel or use fertile material to generate (breed) fissile 

material, e.g., 233U from 232Th. However, there are many unique technical challenges that must be 

addressed because of the chemical nature of the MSR. The chemistry manifested during reactor operation 

evolves over time because of the generation and decay of fission products and is complicated by the 

ubiquitous problem of corrosion and reactivity of trace impurities, such as water and oxygen. Reducing 

conditions and nonaqueous media force the presence of lower-valent actinides, e.g., trivalent and divalent 

oxidation states; and, depending on the fuel cycle deployed, actinide chemistry from thorium to 

californium is relevant in these reactors. The increased presence of noble metals and radiation could also 

result in unexpected reactions during reactor operation. These factors are all compounded by the limited 

characterization data on the chemical species in a MSR and their impact on the chemical and physical 
Basic Research Needs for Future Nuclear Energy

46 Design and Discovery of Coolants and Liquid Fuels

properties. The limited number of studies of complex molten salts means limited data are available on 

predicting and controlling their behavior. Although there are significant chemical challenges facing the 

application of molten salts as coolants and liquid fuels in advanced reactors, scientific advancement in 

these areas offers the greatest opportunities for future reactor concepts and thus was the focus of this 

panel. 

High-temperature, liquid-fueled MSRs have been investigated since 1948, initially as part of a program to 

develop a high-energy-density nuclear reactor to power an airplane. In 1954, scientists at Oak Ridge 

National Laboratory (ORNL) demonstrated the concept of a high-temperature fluoride salt–fueled reactor 

in the Aircraft Reactor Experiment (ARE). The reactor was 2.5 MW(t), fueled with UF4 (6 mol %) 

dissolved in NaF-ZrF4 (53–41 mol %) and moderated with BeO. The reactor operated for 9 days with an 

outlet temperature of 860°C and was found to be stable and self-regulating. At the end of this program in 

the late 1950s, MSR technology research was focused on developing a thermal breeder reactor based on 

the thorium fuel cycle (i.e., 232Th to 233U). From 1965 to 1969, ORNL operated the 8 MW(t) MSRE, 

which demonstrated many of the key features of a liquid salt–fueled reactor. The fuel for the reactor was 

initially UF4 (0.9 mol %, composed of 33% 235U and 67% 238U) dissolved in 7

LiF-BeF2-ZrF4 (65-29.1-5 

mol %), but in subsequent experiments, it was changed to 233U. The salt flowed through a graphite 

moderator at atmospheric pressure at 650°C. Lithium-7 was used to prevent unwanted formation of 

tritium from 6

Li by neutron-induced fission. The secondary coolant salt was 7LiF-BeF2 (2:1, FLiBe). 

This research showed that the MSR concept was feasible but also highlighted the challenges with salt 

chemistry, fission product chemistry, analytical chemistry, materials and corrosion chemistry, radiation 

chemistry, and separations chemistry. [Grimes 1970; Rosenthal et al. 1970] From 1970 through 1976, 

research at ORNL focused on designing the molten salt breeder reactor, which would use LiF-BeF2-ThF4-

UF4 (71.7-16-12-0.3) as the fuel and operate at approximately 700°C. The reactor was to be moderated by 

graphite, with a 4 year replacement schedule, and the secondary coolant was NaF-NaBF4. Although the 

MSR program was closed in 1976 in favor of other reactor technologies, this research laid the foundation 

for a new direction in MSR research. MSRs are now envisioned as a potential way to manage the fuel 

cycle without aqueous reprocessing of spent fuel and solid fuel fabrication from recycled actinides, 

provide novel fueling and refueling schemes, and offer a pathway to safer, economical nuclear electricity 

generation and the co-production of high-temperature, low-pressure process heat for industrial operations.

This vision was also recognized by the GIF and has generated significant national and international 

interest in MSR research. [WNA 2017]

Current Status and Recent Advances

Currently, a wide variety of MSR concepts are being considered using fluoride or chloride salts. [ORNL 

2017] MSRs under consideration today are either solid-fueled salt-cooled or liquid salt–fueled designs. In 

solid-fueled salt-cooled reactors, such as the VHTR or Advanced High Temperature Reactor (AHTR) 

(also known as the fluoride salt–cooled high-temperature reactor), the molten salt (i.e., the primary fluid) 

flows over a solid ceramic fuel in the form of prisms, plates, or pebbles. A secondary fluid, which can 

also be a molten salt, removes the heat from the primary fluid in a heat exchanger to generate power via 

gas or steam turbine. Molten salts have excellent heat transfer properties, similar to those of water, and 

have the advantage of operating at low (atmospheric) pressure. In liquid salt–fueled reactors—such as the 

ARE and MSRE—the fissile or fertile material and the fission products are dissolved in the molten salt 

that circulates through the primary heat exchanger. Heat is removed by a secondary (molten salt) coolant 

to generate electricity. The reactor can operate as a burner or converter (i.e., consuming more fissile 

material than it makes) or a breeder, which makes more fissile material than it consumes (e.g., 233U 

from 232Th). 
Basic Research Needs for Future Nuclear Energy

Design and Discovery of Coolants and Liquid Fuels 47

The choice of molten salts depends on whether the salts are used as the solvent for the fissile or fertile 

fuel, as the primary coolant (as in the AHTR), or as the secondary (heat transfer) coolant. As discussed by 

Grimes [1970] for the MSRE, for applications as the fuel or primary coolant, the elements must have a 

low neutron capture cross section and good moderating properties at the neutron energies of the reactor 

(i.e., fast or thermal) (Figure 21). The salt must dissolve more than a critical concentration of fissile 

material (235U, 233U or 239Pu) and must dissolve the fission products without loss of physical properties 

(Figure 22). One of the challenges is to determine which fission products remain dissolved in the salt, 

which are volatile, and which ones precipitate on surfaces (and where). The salt must be stable against 

radiation and high temperatures, and it must possess low vapor pressure over a wide operating range 

(500–750°C for MSR and up to 900°C for AHTR). The salt must possess good heat transfer and 

hydrodynamic properties to serve as a heat transfer fluid. [Williams et al. 2006] It also must be 

compatible with the reactor and moderator materials.

Figure 21. Approach to screening candidate molten salts for the MSRE (where σt(abs) is total neutron 

absorption cross-section). | Image courtesy of Oak Ridge National Laboratory

Finally, the salt should be low in cost, be economical to purify, and allow separation of fission products 

and recovery of bred and unburned fissile materials. Examples of fluoride fuel salts primarily for thermal 

spectrum applications include (1) 7

LiF and/or NaF and BeF2 combined with low concentrations 

(<5 mol %) of UF4/PuF3 and (2) 7

LiF and/or NaF and high concentrations of UF4/ThF4 (near eutectic: 25–

30 mol % actinide). For fast neutron energies, chloride salts have some attractive features compared with 

the fluoride salts; specifically, the chloride salts have higher solubility for the actinides, and the actinide 

trichlorides form lower melting solutions. However, because of the high melting point of NaCl, other salts 

need to be mixed in, such as MgCl2, KCl, or LiCl, to lower the melting point. Also, Na37Cl is preferred 

because 35Cl produces 36Cl, a long-lived energetic beta emitter, as an activation product.
Basic Research Needs for Future Nuclear Energy

48 Design and Discovery of Coolants and Liquid Fuels

The secondary coolant does not need 

to meet the neutronics and radiation 

resistance requirements of the 

primary coolant. However, it does 

have the same heat transfer, 

hydrodynamic, and materials 

compatibility requirements. These 

properties include melting point, 

vapor pressure, viscosity, density, 

heat capacity and volumetric heat 

capacity, and thermal conductivity. 

The salt also should be chemically 

stable and not react with water or air 

in the event a leak develops. 

Numerous reports have reviewed and 

summarized the literature for molten 

salt coolants as the primary coolant 

[Williams et al. 2006; Samuel 2009; 

IEA 2013] or secondary coolants.

[Williams 2006] These reports have 

also highlighted the gaps in the 

thermodynamic data for candidate 

molten fluoride salts and the even greater lack of data available for chloride salts (Figure 23). 

LiF NaF KF RbF CsF BeF2 CaF2 LaF3 ZrF4 ThF4 UF4 PuF3 UF3

x x x x x x x x x x LiF

x x x x x x x x x NaF

x x x x x KF

x x x RbF

x x CsF

x x x x BeF2

x x CaF2

x LaF3

ZrF4

x x ThF4

x UF4

PuF3

UF3

Binary

LiF NaF KF RbF CsF BeF2 CaF2 LaF3 ThF4 UF4 PuF3

x x x x x x x LiF-NaF

x x x LiF-KF

LiF-RbF

x x LiF-CsF

x x x LiF-BeF2

x LiF-CaF2

LiF-LaF3

x x LiF-ThF4

x LiF-UF4

x x x NaF-BeF2

x NaF-ThF4

x BeF2-ThF4

Ternary

Figure 23. Available thermodynamic data (marked by “x”) for binary and ternary fluoride salt mixtures for 

MSRs, developed at the Institute for Transuranium Elements of the Joint Research Centre of the European 

Commission, Karlsruhe, Germany. | Reprroduced from E. Capelli, Thermodynamic Characterization of Salt 

Components for Molten Salt Reactor Fuel, Figure 1.2, PhD Thesis, University of Delft, 2016.

To optimize a molten salt for a particular application, many criteria must be met and many experimental 

parameters measured. In most cases, mixtures of salts are used to obtain the desired properties because the 

melting point of an individual salt is too high for coolant applications (e.g., the melting point of LiF is 

845°C). Since each salt mixture and salt composition will have different physical and chemical properties, 

and these properties will impact the solubility of fission products and impurities, the amount of 

experimental data that is needed to discover or optimize a molten salt mixture is extremely large. Phase 

diagrams are the most important tool for understanding fuel and coolant salt properties; but other physical 

properties need to be measured for individual salts and mixtures—including vapor pressure, viscosity, 

density, heat capacity and volumetric heat capacity, and thermal conductivity, and surface tension. Many 

60 80 100 120 140 160

10-8

10-6

10-4

10-2

100

235U (14.1MeV)

238U (14.1MeV)

235U (Fast)

238U (Fast)

235U (Th)

Cumulative Yield (%)

Mass Number

Figure 22. Fission products (atomic mass on the x-axis) arising from 

various neutron environments. Note the increased production of 

noble metals in the presence of a fast (2 MeV) spectrum relevant 

to certain molten salt reactor designs. | Image courtesy of Michael 

A. Koehl, Colorado School of Mines
Basic Research Needs for Future Nuclear Energy

Design and Discovery of Coolants and Liquid Fuels 49

of these properties are sensitive to impurities, including dissolved metals and oxygen-containing 

impurities such as oxides, hydroxides, sulfates, carbonates, and oxyhalides. Therefore, rigorous methods 

must be developed to prepare high-purity samples, and rigorous analytical practices are needed to make 

reproducible measurements. Given the number of measurements required and the large variety of 

experimental variables, high-throughput methods need to be developed to accelerate precise property 

measurement and database generation. Computational methods and thermodynamic models are also 

needed to rapidly advance the exploration of molten salt properties to enable materials-by-design 

approaches. 

Since the original ARE and MSRE, there have been many advances in experimental, computational, and 

analytical methods to explore the structures, properties, and reactivities of molten salts. Since 

thermodynamic properties are determined by atomic structure and inter-atomic interactions, modern 

spectroscopy methods can provide new insights into the structure and dynamics of molten salts. For 

example, modern x-ray and neutron scattering—along with optical spectroscopy, nuclear magnetic 

resonance (NMR), and spectroelectrochemistry methods—can provide quantitative insights into the 

oxidation states of transition metals, lanthanides, and actinides, and the local structure and bonding.

[ORNL 2017] One of the challenges is providing sample environments in which these measurements can 

be made at high temperatures (up to 1000°C) in the presence of radioactive species (although surrogates 

can be used for many studies). These data can then be used to validate computational calculations to 

predict thermodynamic properties, especially for difficult-to-measure sample environments (i.e., high 

temperature and high radiation). 

The development of computational methods to predict optimal molten salt compositions, properties, and 

reactivities would dramatically increase the rate of scientific discovery. However, a multiscale approach 

must be employed, and there are serious computational challenges associated with these types of 

computational methods. At the molecular level, static ab initio methods must be able to accurately 

account for relativistic effects and determine properties like oxidation and reduction potentials with a high 

level of fidelity. Time-dependent chemical reactions necessitate ab initio molecular dynamics, for which 

increased compositional complexity mandates the inclusion of extended polarization effects and longer 

simulation time scales that can probe the role of fluctuations in external conditions. [Bengtson et al. 2014; 

Song et al. 2017] At longer length and time scales, classical molecular dynamics (MD) simulations are 

needed to predict the physical and transport properties of molten salts, and they can be applied to 

understanding interfacial behavior that can lead to materials degradation. Such approaches will require 

benchmarked numerical descriptions of the interatomic interactions (force fields). Ultimately, to achieve 

predictive insight across scales—from the atomic and molecular chemical reactivity to bulk 

thermophysical and transport properties—a combination of quantum mechanics, classical dynamics, 

course-gained models, continuum models, and thermodynamics models must be integrated. Doing so will 

require the use of DOE’s high-performance computing resources. 

Scientific Challenges and Opportunities

To push forward the development of advanced reactor concepts and realize the potential benefits of 

enhanced safety, reliability, resource utilization, and proliferation resistance, many fundamental 

challenges need to be overcome. The application of molten salts as coolants and liquid fuels in advanced 

reactors offer the greatest opportunities for future advanced reactor concepts, but there are significant 

chemical and materials challenges that need to be addressed. The scientific challenges and opportunities 

for molten salts are organized into three key areas: (1) time-dependent radiochemical processes, (2) trace 

constituents in extreme environments, and (3) tailored interfaces for extreme environments, which are 

discussed in detail in the following sections.
Basic Research Needs for Future Nuclear Energy

50 Design and Discovery of Coolants and Liquid Fuels

Time-dependent radiochemical processes 

One of the challenges in a liquid salt–fueled reactor is time-dependent radiochemical processes. 

Understanding the fate and transport of fission products is key to controlling the salt chemistry/properties 

and reactor performance. Although the decay pathways of fission products are well understood and 

independent of the chemical environment (see Figure 22), the chemical and physical properties of these 

elements and their interactions with the environment are different at each step of the decay chain. A key 

challenge is to understand the solubility of the fission products as a function of system composition 

(chemical potentials) and external variables. [Compere et al. 1975] In early studies of the MSR, noble gas 

behavior was of particular interest because those gases are neutron poisons, i.e., neutron absorbers, that 

can affect the performance of the reactor. For example, 137I (t1/2 = 24 s) is soluble in the fluoride molten 

salt but decays to become an entrained noble gas, 137Xe (t1/2 = 3.8 min), which will further decay to 137Cs 

(t1/2 = 30 years). If the gas is removed from the salt, radioactive cesium will plate in the outgas system; but 

if the xenon gas stays entrained in the salt, the resulting cesium will be soluble, based on its free energy as 

the fluoride salt. Thus, there is a need to understand the binding, coordination environment, and Lewis 

acid-base properties of the fission products. Although individual elements can be experimentally studied 

with (nonradioactive) surrogates, it is difficult to study the dynamics and changing chemistries of shortlived fission products. Thus, computational methods are needed to complement experimental studies to 

understand the chemical and physical properties of short- and long-lived fission products.

Another key challenge is to understand how the fission products change the chemical and physical 

properties of the salt mixtures as a function of time. For example, the redox potential of the melt may 

change with fission; and free halide ions could be released, which change the Lewis acid-base properties 

of the melt and the corrosivity (i.e., fission of 235UF4 can produce 141Ba2+ and 92Kr and free fluoride ions). 

Understanding how fission products interact with the reactor materials is also important. In the MSRE, it 

was found that the fission product tellurium contributed to surface cracking of the walls of the reactor by 

leaching chromium from the Hastelloy N alloy. [Keiser 1977] Understanding whether the fission products 

change thermophysical properties of the salt—such as thermal conductivity, melting point, viscosity, and 

heat capacity—is also important for efficient reactor operation. 

Fission liberates large amounts of energy, as both electromagnetic radiation and kinetic energy of the 

fragments, into the local environment. This process is repeated with many various outcomes for each 

fission product, and the impacts on the physical and chemical properties of the molten salts need to be 

understood. During the MSRE project, the fluoride fuel and coolant salts appeared to be relatively 

immune to irradiation damage, and any primary radiolytic event was rapidly reversed by recombination. 

However, the long-term (>5 years) impact of high radiation fields on a molten salt is not known. If the 

fluoride molten salt was allowed to solidify and cool below 200°C, significant radiolysis and chemical 

changes occurred. [Williams et al. 1996] Chloride salts have not been studied under the high radiation 

fields found in MSRs. However, molten alkali chloride salts have shown good chemical and radiation 

stability in pyroprocessing of spent nuclear fuel. Nevertheless, experiments need to be conducted with 

chloride salts under more intense irradiation conditions, as would be found in a MSR, to confirm their 

stability and identify any reaction products. 

To study the time-dependent processes relevant to an MSR, real time in situ characterization tools are 

needed that operate in extreme environments (high temperatures and radiation fields) to monitor the 

species in solution. Optical (ultraviolet-visible-infrared and Raman) spectroscopy, electrochemical, and 

spectroelectochemical methods in conjunction with chemometric methods can be used to provide 

quantitative information on the identity, concentration, and oxidation states of dissolved species in 

complex, multicomponent solution. To determine the structure and bonding of species in molten salts, 

neutron diffraction and x-ray absorption spectroscopy can be applied at high temperatures. Hightemperature solid state and liquid NMR can also provide insights into the local structures of species in 
Basic Research Needs for Future Nuclear Energy

Design and Discovery of Coolants and Liquid Fuels 51

molten salts, since many of the species are NMR-active, including 7Li+, 19F−, 23Na+, and selected 

lanthanide and actinides (including 235U3+,4+) cations. 

Computational methods are needed to predict the solubility of fission products, the impact on 

thermophysical properties, and the fate and transport of fission products, especially for short-lived species 

and those that are hard to measure experimentally. There is also a need to fill data gaps for the properties 

of the molten salts by using modeling and simulation. Computational predictions for molten salts are a 

major scientific challenge because of the complex nature of the salts and the molecular, mesoscale, and 

macroscale processes that occur over a wide range of time scales (from nanoseconds to years). As a start, 

experimentally validated predictive computational methods that employ first-principles molecular 

dynamic simulations are needed to describe the structure and dynamics of molten salt systems. 

Understanding and predictions of the compositional and redox changes that arise as a result of fission, as 

well as the solubility and transport of the fission products, are needed. Understanding the fates of 

insoluble fission products (i.e., where they deposit and why) is important for safe operation of the reactor. 

Ultimately, quantum mechanics, classical dynamics, coarse-grained models, continuum models, and 

thermodynamic models need to be integrated and validated with experimental data to enable predictive 

insights on chemical reactivity at the atomic level to thermophysical and transport properties. 

Computational studies will require large-scale simulations with tens of millions of atoms to cover the 

spatial scale and will require the use of DOE’s high-performance computers with optimized exascale 

software codes.

Addressing the scientific challenges described will have significant scientific impacts on advanced 

nuclear reactors, as well as other areas of energy technology. Characterization and computational tools 

will be available for time-dependent phenomena to monitor the health of systems in real time and enable 

predictive system response to perturbations. These tools could be used in other areas beyond MSRs, 

including catalysis, batteries, corrosion, and pyroprocessing. New insights will also be gained into 

complex chemistry in extreme environments (i.e., high temperature and high radiation fields). These 

studies will also enable improved heat transfer, a better understanding of actinide chemistry, and insights 

into processing of molten salt fuels and coolants.

Trace constituents in extreme environments

One of the challenges in molten salt chemistry is understanding the evolution and reaction chemistry of 

the trace constituents (ppm or less) and their impact on the thermophysical properties of a molten salt

mixture. Trace constituents include impurities, activation products such as tritium, corrosion products, 

and low concentrations of fission products. New characterization techniques need to be developed to 

quantitatively determine the concentration levels of constituents at low concentrations (ppm or less), as 

well as follow their reactions in real time under extreme environments. As discussed earlier, optical 

spectroscopy and electrochemical methods coupled to chemometrics hold great promise, but the levels of 

detection need to be pushed below ppm levels. New approaches to low-level detection of elements in a 

complex matrix could exploit the resolving power and large dynamic range of mass spectrometry. 

However, a sampling method is needed for real-time analysis of molten salts at high temperatures. These 

analytical tools could then be coupled to high-throughput methods to collect data on the large number of 

species present under a variety of experimental conditions while measuring the properties of the solutions. 

A major advance would be to extend these studies to include radiation fields and radioactive materials, 

since there are a limited number of experimental facilities that can perform these kinds of experiments. 

High-throughout methods would greatly accelerate the generation of databases of a variety of 

thermodynamic properties of molten salts and accelerate the discovery of new molten salts with superior 

stability, low reactivity, and excellent heat transfer properties. 
Basic Research Needs for Future Nuclear Energy

52 Design and Discovery of Coolants and Liquid Fuels

Corrosion can result from reactions of trace impurities or fission products with the reactor materials.

[Delpech et al. 2010] For example, corrosion rates for salts that were used as received and remained open 

to the atmosphere were four orders of magnitude higher than those obtained for purified salts in a closed 

loop. [Mishra and Olson 2005] Oxidizing impurities typically found in molten salts include water and 

hydrogen fluoride, but they may also include metallic impurities. Other corrosive impurities that must be 

removed include oxides, hydroxides, sulfates, sulfides, and carbonates. In molten salts, corrosion depends 

directly on the thermodynamic driving force of the corrosion reactions, since the products of metal 

corrosion by fluoride melts are soluble (e.g., Cr + 2HF → CrF2 + H2) in the molten salt and passivation is 

precluded. [Grimes 1970] Thus, thermodynamic data are needed for the metals, trace constituents, and 

potential halide-containing reaction products to determine if a reaction will occur over the operating 

temperature range. This information will facilitate the design of chemically stable reactor materials.

The ability to use computational methods to understand the impacts of trace constituents on the chemical 

and physical properties of molten salts will be valuable in accelerating the development of stable reactor 

materials and improving the operational efficiency and lifetimes of reactors. Static and dynamic ab initio 

methods are needed to model redox states and chemical reactions in solution, and classical MD 

simulations are needed to model transport properties. One goal is to develop computational methods that 

can predict the properties of systems with ppm levels of impurities and obtain accurate results. 

Computational methods can also be used to extrapolate limited experimental data into extreme 

environments of high temperature or high radiation fields. 

Addressing these scientific challenges will have significant scientific impacts. These advances will lead to 

an improved understanding of how trace constituents impact chemical properties and system efficiencies 

in MSRs and other systems. They will enable improved understanding of other complex electrolyte 

systems and develop new computational and experimental techniques. These studies will lead to an 

improved understanding of fundamental structure, binding, and reactivity of actinides; and the advances 

in characterization tools will lead to improved methods to deal with low-level impurities in a variety of 

energy applications. Overall, these advances will lead to an improved understanding of the roles of trace 

elements in corrosion and in improved reactor performance and longer reactor lifetimes.

Tailored interfaces for extreme environments

Materials compatibility of molten salts under extreme environments has been a key challenge ever since 

the ARE and MSRE and a potential bottleneck to their future use as coolants and fuels. There is a need to 

understand, at the molecular level, the structure and chemical reactivity at the fluid/solid interface to 

provide important insights into corrosion by fission products and their adsorption onto surfaces—such as 

metal alloys and graphite—and to aid in the design of new materials with interfaces tailored for improved 

material lifetimes and performance under extreme environments. There are still unresolved questions 

about chemical compatibility and embrittlement by fission products, and some recent MSR designs are 

proposed to operate at higher temperatures and use chloride salts. [WNA 2017; ORNL 2017] Much less is 

known about the compatibility of chloride salts with structural materials for MSR applications, and some 

studies appear to show greater corrosion resulting from chloride than from fluoride salts. In addition to 

the effects of corrosion, radioactive fission products may deposit at interfaces, providing a fixed source of 

radiation and heat. Their presence would impact reactor operation and maintenance, and deposition of 

neutron poisons on the (graphite) moderator in the core would impact reactor performance (see Table 1). 
Basic Research Needs for Future Nuclear Energy

Design and Discovery of Coolants and Liquid Fuels 53

Table 1. Fission product distribution in the fuel, on the graphite moderator, on the Hastelloy reactor walls, 

and in the purge gas of the MSRE. | Source: W. R. Grimes. 1970. Molten-salt reactor chemistry, Nucl. Appl. Technol.

8(2), 137–155.

Nuclide Quantity found (% of calculated inventory)

In fuel On graphitea On Hastelloy-Na In purge gasb Total

99Mo 0.17 9.0 19 50 78

132Te 0.47 5.1 9.5 74 89

129Te 0.40 5.6 11.5 31 48

103Ru 0.033 3.5 2.5 49 55

106Ru 0.10 4.3 3.2 130 138

95Nb 0.001 to 2.2 41 12 11 61

95Zr 94 0.14 0.06 0.43 95

89Sr 83 8.5 0.08 17 109

140Ba 96 1.9 0.05 0.48 98

141Ce 0.33 0.03 0.88

144Ce 0.92 0.03 2.7

131I 60 0.11 0.3 19 80

a Calculated on assumption that average values for surveillance specimens are representative 

of all graphite and metal surface in MSRE.

b These values are the percentage of production lost assuming mean values found in gas 

samples apply to the 4-liter/min purge.

To investigate the reaction, transport, and deposition of fission products at the fluid/solid interface will 

require the development of characterization methods and sample environments to study surface reactions 

in situ under extreme environments. Computational methods need to be combined with experimental data 

to help describe the reactions at the interface and gain predictive insights into designing stable fluid/solid 

interfaces over a wide range of time scales. Methods also need to be developed to aid in the design of new 

materials, with knowledge of the molten salt and fission product chemistry and radiation effects so 

interfaces can be tailored to resist corrosion and keep their structural integrity.

Most corrosion studies to date with molten salts have relied on macroscopic ex situ measurements under 

static conditions and postmortem analysis, typically by transmission electron microscopy with energydispersive spectroscopy. This approach leaves a gap in the understanding of the molecular structure, 

species, and dynamics at the liquid/solid interface and the kinetic parameters that determine their 

evolution. For example, studies of the compatibility of the NaBF4/NaF coolant with Hastelloy N (in a 

convection loop) found the loop partially plugged after 8765 h of operation at 607°C. Postmortem 

analysis found deposition of Na3CrF6; and large quantities of Cr, Fe, Ni and Mo—which are major 

elements in Hastelloy N—were found in the salt. [McCoy et al. 1970] New in situ characterization 

methods and sample environments are needed for extreme environments to characterize the molecularlevel chemistry in real time to understand interfacial reactions in molten salts, including redox reactions, 

precipitation, and diffusion of fission products into metal (or graphite) or leaching of metal into solution. 

A number of surface-sensitive optical spectroscopy techniques (e.g., reflection-adsorption infrared 

spectroscopy, sum frequency generation, and surface-enhanced Raman) and x-ray and neutron scattering 

techniques (e.g., reflectometry and grazing incidence angle), in combination with electrochemical 

methods can be used to probe the reactions, the metal species, and the oxidation state of the salt phase at 

the interface. To aid in the understanding of interfacial reactions under more realistic conditions, 

experiments should be run under flowing salts, in a flow loop, to capture the impact of thermal gradients, 

fretting, and erosion on the mechanistic studies. The combined effects of chemistry and radiation on 
Basic Research Needs for Future Nuclear Energy

54 Design and Discovery of Coolants and Liquid Fuels

corrosion should also be studied because of the potential for synergist effects. Characterization of deposits 

that form at the interface, as well as of any elements leached from the surface, will provide a greater 

understanding of the chemical reactions and the reaction kinetics among the molten salt, fission products, 

and reactor material. Understanding the interfacial chemistry in situ and under realistic condition will help 

in predicting materials behaviors and lifetimes and accelerate the development and deployment of new 

materials suitable for MSRs and for other applications under extreme environments.

Another important interface that needs to be controlled is the gas/liquid interface. In the MSRE, the salt 

mixture was sprayed into the gas space of the pump bowl, above the sump pump, to allow noble gases 

such as krypton and xenon (a neutron poison)—which are only sparingly soluble in the fluoride salts—to 

be removed by a helium sweep gas. Tritium, a moderate-lifetime beta emitter, is also produced by both 

ternary fission and a number of light-element neutron reactions. Because of its high mobility through 

alloys at high temperatures, it is important to understand its reactions at the gas/solid and gas/liquid 

interfaces. Tailoring of interfaces to prevent the diffusion of tritium would be a major advance. Another 

interfacial process that it is important to understand is mist formation in the pump bowl, which can carry 

nonvolatile fission products into the gas stream (see Table 1). It is critically important to understand the 

nucleation of particles and transport of fission products across the gas/liquid interface and gas-phase 

entrainment to account for the radioactive inventory and for the safe operation of the reactor. It is difficult 

to experimentally study the gas/liquid interface. Therefore, this topic is an ideal challenge for 

computational modeling, in which first-principles calculations can be combined with classical MD 

simulation using validated force fields, and with mesoscale simulation, to provide predictive insights into 

behavior at the gas/liquid interface. 

One of the challenges in using molten salts as coolants and fuels is finding a material that is corrosionresistant, radiation-resistant, and structurally strong. A classic approach to corrosion protection is to apply 

a coating onto a structural material, but oxide coatings dissolve in molten salts. The free energy driving 

force for corrosion in molten salts is controlled by the redox potential of the salt matrix. It has been shown 

that reducing conditions (resulting from varying the U3+/U4+ ratio, for example) suppress corrosion; but 

they have been found to make other reactions between the salt and the solid, such as alloying, more likely.

[Williams et al. 2006] Thus, a delicate balance is needed between oxidative and reductive degradation, 

and real-time monitoring is needed to control the redox properties of the salt. If a coating could be 

developed that controls the redox potential of the interface relative to the molten salt and fission products, 

and a method could be found to bond the coating, a corrosion-resistant structural material could be 

developed. To advance the design of new materials, computational studies are needed to guide the choice 

of corrosion-resistant materials. Novel processing methods, such as additive manufacturing, are needed to 

tailor the properties of the interface by adjusting the chemical composition, by causing microstructural 

morphology of the structural component, or by developing novel methods to bind the coating to the 

structural material. These approaches could also have the advantage of reducing the time required to 

qualify a material for an application. 

There is a need to develop computational models to describe molecular, mesoscale, and macroscale 

interfacial processes over a wide range of time scales (from nanoseconds to years) to gain a predictive 

understanding of interfacial processes and to gain insights into the design of new tailored interfaces. 

Doing so requires multiscale simulations that couple information from first principles studies to classical 

MD to mesoscale simulations to address several key challenges—including predicting slow processes, 

such as corrosion, transport processes at the gas/liquid and liquid/solid interfaces, nucleation and growth 

of deposits formed at interfaces, and coupling of radiation and chemical reactions. These studies would 

feed into the development of new alloys and tailored interfaces for extreme environments.

Addressing these scientific challenges will improve the understanding of corrosion mechanisms in alloys 

and compositionally graded surfaces. This will enable the design of new engineered materials with 
Basic Research Needs for Future Nuclear Energy

Design and Discovery of Coolants and Liquid Fuels 55

tailored interfaces for use under extreme environments with predicted lifetimes. Understanding the 

transport of fission products in the gas phase and at gas/solid and gas/liquid interfaces would also 

improve the safety and operation of the reactor. New in situ surface-sensitive tools need to be developed 

to monitor the health of the reactor materials in real time; these could be used for other applications under 

extreme environments. 

Conclusion

Molten salt coolants and fuels offer the potential benefits of enhanced safety, reliability, resource 

utilization, and proliferation resistance; but there are many fundamental chemical and materials 

challenges that need to be addressed. Since the original work with the first MSR in the 1960s and 70s, 

there have been significant advances in analytical tools, especially x-ray and neutron scattering, that can 

be applied to in situ measurement of the structure and dynamics of molten salts in extreme environments. 

There also have been advances in computational methods for predictive understanding of the structure, 

dynamics, reactions, and thermodynamics of molten salts. These tools will help provide a deeper 

fundamental understanding of chemical and physical properties of molten salt, which will aid in the 

design of new salt compositions with superior properties. Understanding the solubility, reaction, and 

transport of fission products and trace constituents in molten salts will aid reactor design, operation, waste 

management, and safety. New corrosion-resistant materials can be designed from a knowledge of the 

interfacial reactions of the molten salts, fission products, and trace constituents to improve reactor 

lifetimes. To realize the potential for molten salts as coolants and fuels, new fundamental understanding is 

needed of chemical and physical properties of molten salts and their interactions and reactions with 

fission products, reactor materials, and trace constituents under extreme environments. 

Basic Research Needs for Future Nuclear Energy

Physics and Chemistry of Interfaces 57

Panel B: Physics and Chemistry of Interfaces

Future reactor designs under consideration that employ gas, liquid metal, and molten salt coolants will 

operate under increased temperatures (600 to 1100°C) along with higher radiation fluxes (order of 

magnitude increases in the displacement damage). Further, these new reactor designs will need to be 

operated efficiently and reliably over decades of performance with minimal upkeep. These challenging 

conditions will require materials for use in those reactors with performance characteristics far beyond 

those of current materials. In particular, surfaces and interfaces in structural materials and fuels must 

withstand reactor conditions under irradiation, corrosion, and other types of extreme conditions without 

increased downtime or operational risk. Much of the damage incurred in these materials occurs at 

interfaces. Tailoring of surfaces exposed to fluids (gases/liquids) and buried interfaces (solid/solid 

interfaces) will be essential to the development of material systems to reach the elevated damage, 

temperature, and corrosive environments that are proposed for next-generation reactors. 

A greater understanding of the myriad chemical and physical processes that occur at these various types 

of interfaces is critically needed to control and even mitigate these processes. Controlling interfaces under 

reactor operation, used fuel storage and disposition, and accident conditions for reactor operation and 

used fuel storage will contribute to increased stability and safety margins. Increasing the interfacial 

performance of materials by enhancing the stability of surfaces for advanced reactor environments (high 

temperature, radiation, and pressure, among others) will lead to reduced corrosion and erosion of 

materials and minimize the transport and deposition of radioactivity within future gas, liquid metal, and 

molten salt reactor concepts. Improvements in fuel behavior in advanced reactors, enabled by increased 

understanding of the evolution of internal interfaces within fuels and structural materials, will produce 

fuels and claddings better able to withstand higher burnup, higher temperature gradients, and harsher 

corrosive environments. Equally, the nature of the coolant and its chemistry at high temperatures under 

radiolytic conditions is a crucial determinant of interfacial material degradation. Likewise, the 

development of a capability to predict defect interactions at solid/solid internal interfaces, such as 

interphase and grain boundaries, precipitates, and dispersoids, will provide the scientific underpinning for 

the microstructural design of advanced materials with tailored responses in future nuclear technologies.

Current Status and Recent Advances

Solid/fluid interfaces

Radiation-induced processes at and near interfaces contribute to corrosion, dissolution, hydriding, 

catalysis, and many other phenomena that impact nuclear energy technologies. In fact, radiolysis effects 

at interfaces may be quite different from those that occur in the bulk material. Further, radiation energy 

absorbed in one phase may contribute to the chemistry in the other one, significantly increasing the 

effective yields of the radiolytic products (Figure 24). Interfacial energy transfer involves interactions of 

active species, charges, and excitations between the contacting phases. For example, radiation-chemical 

yields of H2 with respect to the radiation energy deposited to water adsorbed on an oxide surface may 

increase 10- to 100-fold relative to bulk water [Petrik 2001; Poston 2014]; and on the surfaces of alkali 

halide salts, they increase 10,000- to 100,000-fold. [Alexandrov 1987] 

Detailed mechanisms of such efficient energy transfer are not well understood, but it is believed that lowenergy electronic excitations generated in solids (e.g., electrons, holes, excitons, or mobile defects) may 

play a decisive role. [Petrik 2001; Poston 2014] Typically, mobile excitations generated by radiation in a 

certain subsurface layer (from nm to μm thick) may reach the interface during their lifetimes and react with 

the adsorbed coolant molecules. The flux of the excitations to the interface depends on a variety of 

parameters, including the trapping defect density in the bulk, and can be controlled by doping. For example,
Basic Research Needs for Future Nuclear Energy

58 Physics and Chemistry of Interfaces

~0.3% of Nb5+ in ZrO2 completely suppresses 

energy transfer to adsorbed water and thus H2

production. [Petrik 2001] Interfacial energy 

transfer is most efficient for low-dimensional 

systems—thin films, nanoparticles, and porous 

materials—in which a larger fraction of the 

mobile excitations can reach the surface. These 

excitations can induce chemistry in molecules 

adsorbed at or near the interface. The various 

mechanisms are complex and insufficiently 

understood. They depend on the electronic 

structures of the adsorbate and the adsorbent at 

the surface, where non-idealities (such as 

structural and compositional defects and 

impurities) could play important roles.

Our understanding of processes at liquid/solid 

interfaces (e.g., corrosion, oxidation, 

deposition) is mainly limited to water-based 

reactor conditions. While this body of 

knowledge informs our approaches, interfaces 

with molten salts, liquid metals, and gases under high temperature (and/or pressure) conditions pose many 

new challenges. Very little is known about the generation, migration, and reactivity of electronic 

excitations and polarons (holes and electrons) and their subsequent chemical effects near the interfaces of 

new-generation nuclear materials with reactive media.

Filling this knowledge gap will require the coordination of experimental and theoretical investigations 

involving multidisciplinary teams of chemists, material scientists, physicists, engineers, and computer 

scientists. Experimental techniques with ultrafast time resolution and ultrahigh spatial resolution can 

reveal initial charge and energy transfer events that lead to interface degradation and ultimately corrosion. 

These include pulsed irradiation (electrons, photons, and ions) in combination with time-resolved 

detection methods (e.g., optical and vibrational spectroscopy, luminescence, mass spectrometry) to apply 

reaction dynamics and surface science methods for the mechanistic characterization of interfacial 

reactivity and its impact on surface transformation (e.g., oxidation, corrosion, new phase formation). New 

in situ and operando methods for investigating interfacial catalysis have enabled many scientific 

advances, spurred along by the tremendously increasing performance of synchrotron and neutron 

facilities, advanced imaging, and other analytical techniques. These methods need to be adapted to the 

demanding environmental conditions expected in future nuclear technologies.

Computation plays an extremely important role in understanding the chemical and structural evolution of 

heterogeneous systems, in which continuum models and empirical relations do not apply. A prime 

example of this is the field of low-temperature molten salts (ionic liquids). True comprehension of the 

molecular origins of their complex physical behavior on multiple length and time scales has been enabled 

only by molecular dynamics (MD) simulations of increasing sophistication. [Freitas 2014; Shimizu 2016] 

Quantum MD simulations can follow the evolution of excess charges produced by radiolysis, which is the 

first step in understanding the reactivity of ionic liquids. [Xu 2015] Events on the molecular scale impact 

the reliability of nuclear systems only if their effects cascade to ever-larger dimensions; thus, it is 

important to connect to simulations on larger scales, up to macroscopic systems, in a multiscale 

computational framework. MOOSE-BISON-MARMOT is an example of a computational framework that 

has been developed to model the evolution of nuclear fuel in tight integration with experiment. Similar 

Figure 24. Radiolytic energy delivered near interfaces or 

into nanoparticles can result in higher yields of charge 

separation and reaction products in the surrounding fluid 

phase as a result of migration of radiolytic species to the

interfacial region. | Image courtesy of James Wishart, 

Brookhaven National Laboratory
Basic Research Needs for Future Nuclear Energy

Physics and Chemistry of Interfaces 59

integration of multiscale computational tools is essential to establish a mechanistic understanding that will 

permit informed tailoring of materials and interfaces for durable and reliable nuclear systems. 

Solid/solid interfaces

Most damage processes in materials initiate at solid/solid interfaces, including the formation of voids and 

fission gas bubbles, solute and impurity segregation, depletion, clustering, and phase transformation. 

These atomic-level processes eventually lead to material failure: swelling, hardening, intergranular 

cracking, fast corrosion along grain boundaries, degradation of thermal conductivity, and other types of 

deterioration. [Brinkman 1954; Sickafus 2007] The current understanding of these processes is mainly 

qualitative and limited to observations and phenomenological modeling. Thus, our knowledge of these 

processes falls far short of providing the scientific underpinning necessary to design materials for the 

sorts of tailored responses required for the development of future nuclear systems. New understanding of 

radiation-tolerance mechanisms has recently been gained through the systematic design of materials with 

tailored interfaces and the integration of experiments with theory and computation [Han 2013; Beyerlein 

2013; Demkowicz 2010] (Figure 25). However, insights from altering single-dimensional variables (e.g., 

grain size, layer thickness, particle size or spacing) are not sufficient for designing materials for radiation 

tolerance and damage inhibition.

Figure 25. (a) Cu-Nb composites with 20 nm individual layer thickness are nearly void-free after helium ion 

irradiation at 450°C to 3 dpa. Arrows indicate small, isolated voids in the thicker copper layers. (b) 

Irradiation damage and grain growth were observed in nanocrystalline copper under identical conditions. 

| Images courtesy of Massachusetts Institute of Technology, W. Han, Design of radiation tolerant materials via interface 

engineering, Adv. Mater., 2013, 25, 6975–6979

To gain understanding of damage occurring at solid/solid interfaces requires the ability to characterize 

materials across broad time and length scales. The recent development of new experimental 

characterization techniques has enabled ultrahigh–spatial-resolution imaging and spectroscopy and 

provides new avenues to investigate the fundamentals of solid/solid interfaces. Beyond 2D 

characterization of model interfaces, 3D imaging and chemical mapping using transmission electron 

microscopy (TEM) and atom probe tomography (APT) provide unique information regarding the 

inherently complex structure and chemistry of solid/solid interfaces and radiation-induced solute 

redistribution and precipitation. [Dong 2013; Marquis 2010; Ardell and Bellon 2016] APT and fast data 

processing permit detailed insight into the structure of these materials. The temporal dimension is also 

available through in situ experiments in coupled extremes—such as irradiation, controlled environment, 

and nano-mechanics—via TEM or synchrotron x-ray characterization. In situ environmental TEM 

provides a tool to reveal the effects of temperature and pressure on oxidation and corrosion mechanisms 

and rates. However, it should be noted there is a trade-off in that high-resolution techniques are generally 
Basic Research Needs for Future Nuclear Energy

60 Physics and Chemistry of Interfaces

also low volume; techniques are needed to bridge the spatial gap so the knowledge gained at high 

resolution can be extended to understand bulk behavior. 

Parallel to the experimental advances, new computational approaches have also become available to 

quantify interface structures, discover new mechanisms, and elucidate defects and interactions across 

multiple length and time scales. [Vattre 2014; Bai 2010; Choudhury 2015; Dholabhai et al. 2014]

Scientific Challenges and Opportunities

Solid/fluid interfaces

Understanding the mechanisms at solid/fluid interfaces presents enormous challenges because of the 

higher temperatures, radiation doses, and pressures expected in future nuclear reactor concepts and 

systems. Radiolysis and charge transfer at interfaces under extreme conditions generate reactive 

intermediates that can lead to changes in the coolant chemistry, dissolution of the solid into the fluid, 

corrosion of the buried interfaces (metal-oxide and intergranular interfaces), erosion and defect formation 

on the surface, material restructuring and/or elemental segregation that changes the surface reactivity, and 

damage to protective layers or coatings [Tupin 2015] (Figure 26). 

Figure 26. Schematic illustrating the mechanism of corrosion of zirconium hydride and impact of 

precipitated hydrides on Zircaloy-4 corrosion behavior. | Reprinted from Corros. Sci., 98, M. Tupin et al., 

Mechanism of corrosion of zirconium hydride and impact of precipitated hydrides on the Zircaloy-4 corrosion behavior, 

478–493, copyright 2015, with permission from Elsevier

The scientific challenges include 

• Understanding and quantifying how radiolytic energy, charge, and heat are transported at solid/fluid 

and fluid/fluid interfaces (e.g., metals, oxides, salts, bubbles, and particles of many kinds)

• The roles defects play in solid/fluid interface reactivity and how they evolve in high-radiation and 

high-temperature environments

• How to design coatings and meta-materials that can control interfacial reactivity under extreme 

conditions
Basic Research Needs for Future Nuclear Energy

Physics and Chemistry of Interfaces 61

• The development of integrated mechanistic models leading to coherent design of complementary and 

compatible materials and fluids (e.g., equipment and coolant)

• The use of interfacial effects to increase the heat capacity and fuel carrying capacity of molten salts.

Interfacial phenomena must be probed across broad spatial and temporal scales relevant to energy transfer 

and chemical reactivity, using advanced and emerging experimental spectroscopic, imaging, and 

scattering capabilities. New experimental techniques that achieve ultrafast time resolution and ultrahigh 

spatial resolution are rapidly progressing. For example, interface-specific detection methods such as 

grazing-incidence scattering, and nonlinear optical techniques that exploit the asymmetry of interfaces 

have been successful in aiding the understanding of interfacial structure and reactivity.

However, new in situ characterization methods are needed to characterize these materials and chemical 

processes in real time under the extreme conditions expected in future nuclear reactors. These new 

analytical tools are needed to examine chemical and physical processes on the surfaces of nanoparticles, 

at buried interfaces, selectively within phase domains (solid, liquid, gas, and by composition), and in 3D 

on the meso and macro scales—all with high spatial and time resolution and chemical specificity. These 

advanced experimental techniques will require close cooperation with theory and data analytics to 

interpret the observed phenomena, predict spectroscopic and structural fingerprints for detection and 

assignment, and suggest specific targets for experimental investigation. 

Integrated mechanistic models for complex interfacial processes across the atomic-, nano-, and 

mesoscales are required for the design of complementary and compatible reactor materials and coolants.

On the atomic scale, quantum calculations and MD simulations can be used to predict energetics and 

model nonequilibrium phenomena, multi-state dynamics, and reactivity. An important challenge will be 

to accurately describe radiation-driven reactions at interfaces. On the scale of nanometers, modeling is 

needed to understand the assembly and evolution of structural features such as the formation of particles, 

layers, and segregated phases, and the forces and dynamics that drive their organization into larger-scale 

structures. Results from quantum calculations and modeling must be reliably embedded into models to 

predict aging behavior and the evolution of material properties over longer time and length scales. The 

development and integration of more powerful computational tools (electronic structure, embedded 

methods, multiscale simulations, and machine learning) should lead to the mechanism-based design of 

materials and interfaces that provide control over interfacial reactivity under the demanding conditions of 

next-generation reactors.

Solid/solid interfaces

A key challenge in mastering the radiation tolerance of fuels and structural materials is to obtain a 

fundamental understanding of the unit mechanisms of defect interactions at internal interfaces, such as 

grain and interphase boundaries, as a function of interface chemistry and structure. Once these unit 

mechanisms are elucidated, two significant new areas of science emerge:

1. Understanding and predicting the behavior of ensembles of randomly distributed interfaces within a 

hierarchical microstructure

2. Designing adaptive internal interfaces that heal rather than nucleate and propagate damage 

Multiscale simulations, informed by physical mechanisms from unit to collective response, are required to 

develop predictive models of material behavior for increased efficiency, decreased performance margins, 

and reduced safety concerns in future nuclear systems.
Basic Research Needs for Future Nuclear Energy

62 Physics and Chemistry of Interfaces

The fundamental insights gained from the unit mechanism and ensemble studies can be used to creatively 

design new material microstructures that provide enhanced properties satisfying the operational 

parameters of the proposed future nuclear system technologies. As the field advances to mechanism-based 

design, increasingly complex aspects of the reactor environment can be modeled in combinations of 

controlled environments.

Many concurrent extreme conditions will be present in any future reactor, including displacement 

damage, introduction of transmutants, elevated temperatures, large temperature gradients and temperature 

changes, mechanical loads, and/or other types of harsh environments. Combinations of these stressors 

may result in synergistic effects that may be catastrophic to the essential properties of the advanced 

materials chosen. Some of these responses have been highlighted in conventional materials systems, such 

as radiation-induced segregation, stress corrosion cracking, shadow corrosion, and crud. However, little 

work has been done to investigate the roles of various grain boundary types, interface natures, or surface 

structures in the responses to the extreme environments that will be present in advanced reactors. This 

deficiency is further compounded as material systems with nanocrystalline, nanolayer, or nano-dispersion 

structures are considered, as even less is known about the responses of ensembles of interfaces to these 

complex overlapping environments. [Han 2013; Demkowicz 2010; Beyerlein 2013; Wu 2016; Parish 

2017; Stan 2017; Tupin 2015] Combining state-of-the-art multiscale modeling with in situ experimental 

testing will greatly enhance understanding of the physical and chemical processes that occur at these 

intertwined interfaces, boundaries, and surfaces in extreme reactor environments. Given the complexity of 

the alloy compositions and chemical heterogeneities (e.g., metal-oxide) at interfaces, advanced 

computational approaches such as machine learning are needed to elucidate the information contained in 

these data [Xue 2016].

From the fundamental knowledge of kinetics at interfaces, defect interactions, stress development, 

structure, and chemistry, and considering the elusive role of impurities, two major scientific questions 

emerge: 

1. How do we design materials with hierarchical structures of interfaces to produce tailored responses at 

irradiation extremes? 

2. How do we accelerate the development of novel interface-enabled radiation-resilient materials using 

advanced, experimentally validated, physics-based computational models?

Conclusion

Understanding the wide range of physical and chemical processes that occur at surface (solid/fluid) and 

buried (solid/solid) interfaces that are proposed for the variety of advanced reactor concepts is a daunting 

challenge. All of the proposed future nuclear energy technologies involve harsh environments (high 

radiation dose, high temperature, mechanical strain, and corrosive environments) beyond present 

experience that will require a new generation of materials designed for tailored responses. Interfaces will 

have critical impacts on the performance, safety margins, and operational lifetimes of next-generation 

reactors. To predict materials evolution and degradation, a fundamental understanding of the reaction 

mechanisms active at the surfaces of and within materials in a complex combination of environments is 

critically needed. This knowledge will be enabled through advancements in characterization and 

computation tools that will provide detailed insights into material and coolant degradation at interfaces 

across broad time and length scales. For example, advancements in combined in situ electron microscopy, 

APT, and synchrotron and neutron scattering techniques, coupled with multiscale modeling, could 

provide ways to map out the 3D evolution of interfaces under the harsh environments proposed for 

advanced nuclear reactors. 
Basic Research Needs for Future Nuclear Energy

Physics and Chemistry of Interfaces 63

Once a fundamental understanding of the mechanisms of the myriad chemical and physical processes that 

occur at interfaces is achieved, this knowledge will catalyze the development of a new generation of 

materials designed to perform in the extreme environments anticipated in future nuclear energy systems. 

Recent advancements in experimental tools and computational methods provide confidence that 

fundamental research will succeed in understanding the evolution of surfaces and buried interfaces; 

develop science-based, experimentally validated computational models; and subsequently design tailored 

materials for the advancement of future nuclear energy technologies.

Basic Research Needs for Future Nuclear Energy

Understanding Behavior at Coupled Extremes 65

Panel C: Understanding Behavior at Coupled Extremes

Understanding the behavior of materials in the 

complex environment of an operating reactor is a 

daunting scientific challenge. Even for mature 

technologies, such as present-day light water 

reactors that generate approximately 20% of US 

electricity, [IAEA 2017] there remains a huge 

gap in knowledge that inhibits our ability to 

understand and mitigate the phenomena that 

degrade these systems. The extreme 

environments that will be encountered in future 

nuclear energy systems, which will use 

nonaqueous coolants and operate at extreme 

conditions, will present an entirely new set of 

challenges beyond those encountered in existing 

reactors. It is critical to understand the full range 

of effects that operating conditions can have on 

materials used in future nuclear systems. [OECD 

2011] To achieve this understanding requires 

fundamental research that examines the behavior 

of materials systems—from the atomic to the 

macroscopic scale—under the full range of 

extreme conditions relevant to future nuclear

systems. Figure 27 illustrates the 

multidimensional nature of materials degradation 

under such extreme conditions as a quadruple phase diagram.

Current Status and Recent Advances

There are many known routes that can degrade materials and eventually lead to failure—extremes in 

temperature, stress, and corrosion, among others—and each can involve complex systems of physical and 

chemical processes. The combination of extreme environments encountered in a nuclear reactor can alter 

these processes, leading to an even more complex system of degradation pathways. Radiation-induced 

defects in the solid can lead to the formation of chemical and structural inhomogeneities (e.g., 

precipitates, defect clusters), as well as to segregation of elements to sinks (e.g., grain boundaries and 

interfaces). These microchemical changes can in turn have a significant impact on corrosion, e.g., by 

depleting grain boundaries of corrosion-suppressing elements, through the effect of stress on chemical 

reactions, by changing the local electrochemical potential. For example, at interfaces, corrosion is a major 

contributor to the degradation of materials. Corrosion is a collection of complex processes that may 

involve liquid/solid interfacial charge transfer, dissolution of metal surfaces, and/or adsorption-induced 

processes. However, these processes may vary depending upon the specific environment encountered. For 

example, corrosion in localized spaces, such as cracks and pits, with high surface-to-volume ratios may 

exhibit different chemistries from those of planar surfaces. This situation becomes more complicated 

when protective surfaces are used, as in the case of a pressurized water reactor. The buildup of surface 

oxide films protects the underlying metal against corrosion, and characterization studies have provided 

insight into the nature of the passive film and its growth (Figure 28). At the atomic level, this involves 

the interfacial transfer of metal cations, which induce significant changes in the physical and (electro-) 

Figure 27. Illustration of the multidimensional aspects 

of materials degradation in nuclear environments. 

| Image courtesy of Peter Hosemann, University of 

California–Berkeley
Basic Research Needs for Future Nuclear Energy

66 Understanding Behavior at Coupled Extremes

chemical nature of the interfacial 

region with time. Concentration 

gradients of dissolved metal 

cations may develop as corrosion 

progresses. The dissolution of 

metal cations can lead to the 

formation of new dissolved 

metal species (e.g., with varying 

valence and coordination) or 

precipitated structures, such as 

dispersed colloid particles, 

gelatinous solids, crystalline 

grains, or thin films. Further 

complications arise if the 

chemical and physical properties 

of the solution and the oxide 

phase are not uniform or 

stationary. These changes in the 

interfacial region, in turn, can 

strongly affect the rate of redox 

reactions and alter corrosion 

pathways. 

These complex corrosion 

processes may become even more complicated in the presence of an intense neutron radiation field. 

Currently, a conventional 3 GWth reactor at full power experiences a neutron radiation dose rate of ~400–

500 MGy/h in the vicinity of the core. This constant source of energy results not only in direct changes in 

the solid material but also in the radiolysis of the coolant, causing the formation of reactive species that 

further contribute to material degradation. Currently, there is an appreciable understanding of the various 

species that are formed in irradiated water [Buxton et al. 1988]; and the extrapolation of this chemistry 

from neutral, low-temperature water to reactor conditions has also been achieved. [Elliot and Bartels 

2009] However, our understanding of similar processes in nonaqueous coolants, including gases, liquid 

metals, and molten salts, required for the next generation of advanced nuclear energy systems that will 

operate under more extreme environmental conditions is still in its infancy. 

Radiation can decompose coolant molecules into redox-active species that may strongly influence metal 

oxidation and particle growth. Corrosion kinetics and radiolysis kinetics are highly interdependent. 

Existing models based on water-cooled reactors are 2D (time, oxide thickness); but in practice, the 

process is inhomogeneous in 3 dimensions, with multiple phases and structures at the nanoscale. The 

quantitative effect of radiolysis on corrosion varies depending on the physical and chemical properties of 

the solution. Examples include the effect of γ-radiation on carbon steel, known to vary considerably 

depending on pH. [Daub et al. 2011] Further, enhanced corrosion under radiation has also been observed 

on zirconium alloys. Zirconium samples under irradiation grow significantly thicker oxide layers than do 

unirradiated samples. Similar synergistic effects include hydrogen, helium, and displacement damage 

leading to enhanced swelling (Figure 29); radiation-enhanced creep; or fretting issues. These are 

examples of coupled phenomena that lead to accelerated materials degradation, compared with a 1D

problem.

Figure 28. Oxide layer growth kinetics as a function of passive film 

thickness. 24 Caberra and Mott theory applies for films thinner than X1 

and Wagner theory applies for films thicker than the Debye length 

and X1. | Reprinted with permission from A. Atkinson, Transport processes 

during the growth of oxide films at elevated temperature, Rev. Mod. Phys. 57, 

437, April 1, 1985. Copyright 1985 by the American Physical Society
Basic Research Needs for Future Nuclear Energy

Understanding Behavior at Coupled Extremes 67

Individually, each extreme—radiation, corrosion, 

stress, temperature, among others—poses its own 

challenge to an understanding of the reactor 

environment. However, it is the combined effects 

of coupled extremes that are particularly 

challenging to understand and that pose a major 

scientific challenge (see the sidebar “Coupling 

Three Extremes: High Pressure, Energetic Ion 

Irradiation, and High Temperature”). One 

example of coupled environments includes the 

simultaneous effects of chemical environment 

and stress. Flow-induced vibrations and 

dimensional changes of materials under 

irradiation can lead to stresses, which may result 

in undesirable wear of materials. This coupling of 

mechanical wear and chemical corrosion is

known as tribocorrosion, [Beaudouin et al. 2006]

fretting-corrosion, [Chitty et al. 2011] or erosioncorrosion. [Davis and Frawley 2009; Jana and 

Stack 2005; Srinivasa et al. 2008; Stack et al. 

2010; Long et al. 2017; Xue et al. 2014]

Tribocorrosion, fretting fatigue, and wear 

problems have major economic and safety 

impacts for the nuclear industry. Flow-induced 

vibration, component-to-component contact, and 

corrosion processes will also occur in advanced 

nuclear energy systems that employ alternative 

coolants such as sodium, lead, helium, or molten salts, making tribocorrosion and wear an important issue 

to understand. Another example of important phenomena that result from the coupling of stress to the 

chemical environment is environmentally assisted cracking. Harsh environments may produce vacancies 

by a number of mechanisms, which can eventually lead to cracking. This issue can be expected to become 

worse under the extreme operating conditions that are anticipated in future nuclear energy systems. 

[Arioka 2014; Arioka et al. 2016] There is only a limited fundamental understanding of the behavior of 

materials under such conditions. 

Proposed advanced nuclear energy concepts will use nonaqueous coolants and may operate at much 

higher temperatures, pressures, and radiation dose rates than the current generation of light water reactors. 

Thus, the multidimensional challenges discussed present an even more formidable challenge in these 

future reactor systems. These new coolants will interact with structural materials, presenting new types of 

corrosion processes. Thus, it is critical to develop a fundamental knowledge of the full range of 

phenomena governing coolant/material interactions to enable the design of next-generation nuclear 

reactor systems. 

Corrosion of materials exposed to molten salts can be exacerbated by irradiation, just as in the case of 

aqueous coolants; accelerated corrosion has been reported even if the irradiation of the solid material took 

place before its exposure to molten salts. [Li et al. 2017; Zhu et al. 2017] The effect of irradiation on the 

molten salts themselves is poorly understood; however, molten salt ions can be expected to be further 

transformed by radiation. For example, the fluoride ion present in a FLiBe salt can produce an F-radical 

and an electron. Because the molten salt is a conductive medium, these free electrons can react with a 

cation or a surface acting as an electron sink. The reactive F-radicals may combine to form F2 (gas), 

interact with other ions and impurities, or interact with the wall material. In general, pure molten salts do 

Figure 29. Synergistic effects of helium and hydrogen 

ion beam irradiation leading to swelling. | Reprinted 

from J. Nucl. Mater., 283–287, Part 1, N. Sekimura et al., 

Synergistic effects of hydrogen and helium on 

microstructural evolution in vanadium alloys by triple ion 

beam irradiation, 224–228, copyright 2000, with permission 

from Elsevier
Basic Research Needs for Future Nuclear Energy

68 Understanding Behavior at Coupled Extremes

not react with the wall materials; however, corrosive reactions can be catalyzed by the presence of 

impurities in the coolant, such as oxides and water. [IAEA 2009] Because the molten salt medium is often 

sparged with a reducing gas mixture that may contain H2 and/or HF, the HF may accelerate corrosion if 

there is any oxygen or water still in the system. Adding radiation to this chemically complex environment 

compounds the issues and, at present, the combined effects of impurities and irradiation upon corrosion 

processes in these molten salt systems are unknown.

Coupling Three Extremes: High Pressure, Energetic Ion Irradiation, and High 

Temperature

Multiple extreme conditions have been 

coupled in the past to mimic the formation 

of fission tracks within Earth’s interior by 

exposing natural zircon, at a pressure of 7.5 

kbar and a temperature of 250°C, to a 

beam of relativistic heavy ions. [Lang et al. 

2008] The approach is based on a recently 

developed method using swift heavy ions

and a heatable diamond anvil cell (see 

photo at left). Pressure is created by 

squeezing two opposing diamond anvils 

against a sample chamber with an 

aperture (diameter ~0.5 mm) drilled in a 

steel gasket and filled with liquid as the 

pressure-transmitting medium. Heavy ions 

of relativistic kinetic energies (35 GeV), 

produced by one of the world's largest ion 

accelerators (GSI Helmholtz Center), are 

injected through one of the two anvils of a 

pressure cell into the pressurized sample. 

The high kinetic energy of the ion beam is

required because it first must traverse the 

millimeter-thick diamond anvil of the highpressure cell before reaching the 

pressurized sample. Temperature is 

maintained during irradiation by means of 

molybdenum heating wires wrapped 

around each diamond. The response of 

materials to coupled extremes can be 

dramatically different from the behavior 

under separately applied extremes. For 

example, the application of high pressure simultaneously with the deposition of large energy densities 

provides a new strategy for the formation and recovery of unique, high-pressure structures that are 

otherwise inaccessible. [Lang et al. 2009]

Photograph (top) and schematic illustration (bottom) of the 

irradiation experiment for exposing pressurized heated 

samples to a beam of relativistic heavy ions. The very small 

sample (300 μm in diameter and 40 μm thick) is enclosed in 

the diamond anvil cell. The inset displays the optical 

micrograph of a zircon crystal in the cell. For the irradiation, 

the cell is placed in air, 45 cm behind the beam line window. 

The schematic (not to scale) shows the details of the anvil 

cell, including the resistance heating coil (shown for only one 

anvil). | Reprinted from Earth Planet. Sci. Lett. 274, M. Lang et al., 

Fission tracks simulated by swift heavy ions at crustal pressures, 355–

358, copyright 2008, with permission from Elsevier

Finally, how the reactor is operated (such as load following), can also lead to high stresses, adding another 

dimension of complexity in these extreme environments. These stresses are accommodated by dislocation 

slip, leading to formation of dislocation channels within the irradiated matrix. The intersection of these 

channels with grain boundaries can result in their being blocked, which causes an out-of-plane displacement 

that ruptures the protective oxide. [McMurtrey et al. 2015] This exposure of the bare metal to the coolant 

can initiate an irradiation-assisted stress corrosion-cracking event. What remains to be resolved is why 

cracking initiates at one blocked dislocation channel rather than at an adjacent one. Is it simply a stochastic 

event, or are there intrinsic local differences in the structure and composition of the grain boundary? Liquid 

metal embrittlement in heavy metal–cooled reactors is also known to exist; and the combined effects of 
Basic Research Needs for Future Nuclear Energy

Understanding Behavior at Coupled Extremes 69

stress, temperature, radiation, and salt environments are virtually unexplored. Based on studies in both 

irradiation-assisted stress corrosion cracking and liquid metal embrittlement, the composition as well as the 

structure of the grain boundaries has emerged as an area requiring atomistic-level understanding. 

It can be seen from this discussion that there are substantial gaps in our knowledge that it is critical to 

resolve as we strive to meet the full potential of future nuclear energy systems. These challenges can be 

met only by developing a fundamental understanding of the myriad processes that occur under multiple 

extremes of radiation, stress, and heat in novel, nonaqueous systems.

Scientific Challenges and Opportunities

Understanding the dynamic and highly coupled processes that occur at material interfaces present under 

extreme conditions is crucial to the development of future nuclear energy systems. However, simple studies 

of materials and systems at a single given extreme will not be sufficient. For instance, in molten salt–cooled 

systems, corrosion is driven by dissolution of metallic elements into the salt, as no protective oxides can be 

formed. Chromium dissolution is a primary concern. Given that dissolution is faster along grain boundary 

pathways, because radiation 

changes grain boundary 

compositions as a result of 

radiation-induced segregation, the 

corrosion response is directly 

coupled to radiation as well as 

surface chemistry. Studies under 

coupled conditions can provide 

insight into the full complexity of 

interfacial reactions, transport 

phenomena, microstructural 

evolution, and material defect 

formation and propagation. This is 

because the effects resulting from 

the combination of radiation fields, 

high temperature, pressure, and 

stress may compound one another 

and yield processes and 

mechanisms that are active only 

when multiple extremes are 

concurrently studied (Figure 30).

As these inextricably linked 

processes are unraveled, 

researchers will be challenged to 

develop new materials, interfaces, and multicomponent systems that survive in harsh, nonequilibrium 

environments where high-energy, transient species are present. Although most chemical analyses are 

currently performed by characterizing the molecules and materials before and after experiments in 

extreme conditions, it is a challenge to understand the changes in the sample during the experiment. 

These experiments will be facilitated by the development of computational models that can provided 

additional insight into the complexity of the coupled environments to accurately predict material 

characteristics across different length and time scales (Figure 31).

Figure 30. Illustration of a chemical system that may initially appear 

simple but is complicated by radiation and environmental effects. 

| Image courtesy of J. M. Haschke and W. J. Siekhaus, Lawrence Livermore 

National Laboratory
Basic Research Needs for Future Nuclear Energy

70 Understanding Behavior at Coupled Extremes

Figure 31. (Top) Example of the experimental multi–length scale observation of localized failure of irradiated 

materials. | Reprinted by permission from Macmillan Publishers Ltd: J. Nucl. Mater., A. Reichardt et al., Nanoindentation 

and in situ microcompression in different dose regimes of proton beam irradiated 303 SS, 486, 323–331, copyright 2017; 

Reprinted from Science Direct 43(11), J. Kacher et al. In situ and tomographic observations of defect free channel 

formation in ion irradiated stainless steels, 1099, copyright 2012, with permission from Elsevier (Bottom) Modeling efforts 

attempt to reproduce this behavior. T. Diaz de la Rubia et al. 2000. “Multiscale modelling of plastic flow localization 

in irradiated materials,” Nature, 406, 871–874

3 mm 3 μm
Basic Research Needs for Future Nuclear Energy

Understanding Behavior at Coupled Extremes 71

Given the extreme conditions anticipated for future nuclear energy systems, a new generation of materials 

are needed that are capable of operating reliably for long periods of time. To facilitate the design of these 

materials, there is an urgent need to understand the coupled effects of these extreme environments. There 

are complementary types of research that will help to fulfill this need, including new experimental 

techniques to couple these extremes, new characterization tools to collect data in situ, and accurate 

models to simulate and ultimately predict materials properties and characteristics

New characterization methods are needed to provide an improved understanding of the complex and 

linked chemical and physical processes that ultimately lead to degradation of materials, both structural 

materials and coolants, under realistic conditions. This understanding will ultimately provide a knowledge 

base to allow for the targeted development of materials that retain their properties and resist degradation 

under the harsh environments of future nuclear systems. In particular, there is a need to develop multimodal techniques that can provide multiple dimensions of information on a single sample in situ. There is 

also a need to integrate time-resolved spectroscopic and imaging techniques (e.g., microscopy, x-ray, 

neutrons) with multiscale/integrated-scale modeling and in situ data analysis and visualization. Doing so

will allow for the design and implementation of experiments with higher spatial and temporal resolutions

than are available to each individual technique. Advances in data analytics and machine learning 

techniques can allow for efficient reconstructions from imaging and x-ray or neutron scattering data,

which can be imported to create atomistic or continuum models. Such approaches will enable the 

development of “experimentally informed models,” thus bridging the gap between modeling and 

experimentally probed structures and conditions.

To model materials phenomena and properties at coupled extremes, there is an urgent need to combine 

the accuracy and flexibility of electronic structure calculations with the speed of classical potentials by 

merging and exploiting the best insights from the fields of machine learning, advanced optimization, and 

atomistic simulations. There is a need to develop flexible atomistic potential models that can describe 

many real important cases, such as nonstoichiometric defect formation, distributions of defects that 

include possible clustering or accumulation at surfaces or interfaces, charge and defect migration across 

interfaces, the effects of doping, and finite-size effects in thin films. In particular, there is a need to move 

beyond the fitting of specific data sets to reproduce specific properties that are valid only at equilibrium 

conditions, to fitting vast and diverse data sets obtained from first principles and experiments. Model 

machine learning algorithms, such as deep learning and genetic algorithms, are needed for enhanced 

sampling of training data with density functional theory and experimental inputs. There is a need to 

leverage state-of-the-art advances in model-building algorithms—such as genetic algorithms, compressive 

sensing, and deep neural networks—to describe scenarios that deviate far from equilibrium.

Multiscale modeling and multidisciplinary research have been the conventional paradigms for some 

years. But for a problem as complex as corrosion under irradiation, stress, and other nonlinear fields,

researchers need to step beyond the conventional paradigms. There is a compelling interest in an 

integrated approach that crosses the barriers of specialization and in which theory and experiment are 

equal partners. Theory needs to move beyond mere replication of experiments to computation that can 

inform the direction of targeted experiments; and, in parallel, the experiments need to inform what classes 

of models and theories need to be explored. For example, phase field models of kinetics at interfaces need 

to be coupled with in situ measurements such as environmental electron microscopy. These need to be 

cross-referenced to ab initio calculations of key thermodynamic drivers, such as the activation energy of 

point defect migration in radiation and stress fields and the dependence of surface free energies on 

chemisorbed species from the external environment. This approach will allow elucidation of the 

fundamental mechanisms of materials transformation under extreme conditions. Further, to achieve 

mechanistic understanding, molecular dynamics computational experiments using potentials generated 

from higher-level first principles methods of nonequilibrium structures need to be combined with 

structural probes—such as x-ray scattering, and chemical probes—such as x-ray photoelectron 
Basic Research Needs for Future Nuclear Energy

72 Understanding Behavior at Coupled Extremes

spectroscopy and electron energy loss spectroscopy (see the sidebar “Coupling Three Extremes with 

Analytical Spectroscopy”).

The integration of new characterization techniques during experiments at coupled extremes, and more 

accurate modeling, will improve our understanding of the processes in nonequilibrium 

kinetics/thermodynamics relevant to interface chemistry. This should be realized through a quantitative 

understanding of the effects of solution conditions coupled with radiation fields to ultimately 

revolutionize methods of assessing reactor structural materials, waste storage, disposal containers, and 

emergency preparedness.

Coupling Three Extremes with Analytical Spectroscopy

In situ and operando analytical techniques are necessary to fully understand chemical reactions that occur 

under coupled extreme conditions. Pre- and post-sample analysis will not always provide the same 

information as an examination conducted directly in the extreme environment. Yet the technique itself must 

survive the extreme conditions as well. An example is provided by the use of laser spectroscopy for the 

examination of reactions in supercritical water. In this work, a short-pulse pump 266 nm YAG laser and a 

continuous low-power probe argon ion laser beam were both focused onto a supercritical water cell.

[Rodriguez et al. 2007] They produced a thermal lensing signal with a strength that could be maximized by 

changing the overlap of the two beams. Despite a very significant mirage effect due to thermal gradients in 

the cell and absorption by the water itself, the thermal lensing signal strength for aqueous benzoic acid in 

supercritical water was found to be linear, with a concentration in the sub-millimolar range. The supercritical 

water cell was machined from a solid block of 316 stainless steel (photo on left) with an internal volume of 12 

mL.

Although only two windows were necessary to perform the analytical work described, [Rodriguez et al. 2007] 

an arrangement of the five sapphire windows allowed for additional experiments using laser excitation 

coupled with fluorescence measurements, or e-beam irradiation of the sample coupled with optical 

measurements. The windows were sealed with gold gaskets. The cell was heated using five separate 

cartridge heaters, and the temperature was measured by a thermocouple and controlled to within 1° by 

adjusting the current to the heaters. The cell pressure was maintained using a solution that was pumped into 

the cell through a stainless steel line preheated to the desired temperature using an Isco syringe pump. Thus, 

analytical measurements under supercritical water pressures and temperatures were possible, and the setup 

provided an additional capability to simultaneously irradiate the sample with an electron beam. The photo 

at right shows the laser transiting the cell while under temperature and pressure, as viewed through an 

orthogonal window.

(Left) Supercritical water cell equipped with five sapphire windows allowing simultaneous irradiation and 

spectroscopy. (Right) Laser light transiting the cell while it is under temperature and pressure. | Images 

courtesy of Bruce Mincher, Idaho National Laboratory

Conclusion

A major goal in research in understanding behavior at coupled extremes is to develop a deep 

understanding of the synergistic effects among the radiation fields, temperatures, static and dynamic 
Basic Research Needs for Future Nuclear Energy

Understanding Behavior at Coupled Extremes 73

stresses, and environments to which a material can be exposed in conventional and advanced reactor 

systems. This effort will be the basis for underlying theory-based models that can be benchmarked against 

well-defined experiments unifying those conditions. The coupled nature of these harsh environments will 

necessitate the development of new experimental techniques that can examine a range of scales in time 

and space, and characterization methods to perform in situ measurements of the system. Furthermore, an 

integrated approach to modeling and experiments is necessary, as it will facilitate a better understanding 

both of the extensive data currently available from operating reactors and of future advanced energy 

systems, enabled by modern data mining and data extraction by modern algorithms. These models need to 

progress toward a continuous integrated scale approach to predict macroscale effects based on atomicnano-, or micro-scale processes. 

Among the important research questions that such techniques will answer are these:

• What is the role of radiation in corrosion? 

• What is the role of stress in the fracture of irradiated alloys? 

• What is the influence of aqueous and/or nonaqueous coolants in stress corrosion cracking?

Such an integrated approach promises exciting opportunities to further a fundamental understanding of 

the chemistry and materials science that occurs in the extreme environments that will be created future 

nuclear energy systems.

Basic Research Needs for Future Nuclear Energy

Design and Discovery of Structural Materials and Solid Fuels 75

Panel D: Design and Discovery of Structural Materials and Solid Fuels

The realization of future nuclear energy systems will require accelerated and robust means to master the 

discovery and design of structural materials and solid fuels stable for use in extreme conditions. Such 

capabilities are not yet available but will emerge from enabling fundamental research on key materials 

and chemical processes that takes advantage of advances in theory, computation, experimental 

techniques, synthesis, and processing.

At least six types of Generation IV nuclear reactor concepts are at various stages of development. [Busby 

et al. 2017] All of the designs aim for much higher operating temperatures and higher burnup of fuels 

than current reactors. Materials will need to withstand extreme conditions of radiation flux, temperature, 

corrosion, and strain over unprecedented timescales. Insights into and approaches to the discovery of new 

materials and related phenomena must be pursued at a level sufficiently fundamental to establish realistic 

physical mechanisms as the basis for accurate modeling and simulation frameworks. Robust materials 

design rules and enabling synthesis and processing routes can then be established.

The past decade has seen impressive advances in the theory and modeling of structures and processes; 

profound progress in the computational and experimental sciences; the emergence of the Materials Genome 

Initiative (MGI; NSTC 2014; https://www.mgi.gov/) and other, related accelerated development approaches 

[Jain et al. 2013; Kuehmann and Olson 2009; Curtarolo et al. 2013]; and the genesis and improvement of 

synthesis and processing methods. [DOE 2007; DOE 2016] Holistically, these advances provide many 

opportunities to seek and perfect design strategies for the materials/fuels needed for future reactors. The 

capabilities to discover and design materials specifically tailored for advanced nuclear energy applications 

require further development of fundamental knowledge of 

structural, chemical and materials processes that underlie

synthesis, processing, and design. Powerful methods to probe 

matter at sufficiently resolved time and length scales will be 

enabling. Progress will rely on verified predictive mechanistic 

models of materials to ensure tight integration among 

experimentation, theory, and simulation and the use of 

advanced methods of data synthesis and machine learning.

Current Status and Recent Advances

The complex evolution of nuclear fuel (typically UO2 in the 

current fleet of reactors) arises from its changing chemical 

composition, radiation-induced microstructural changes, and 

high thermal gradients. [Olander 1976; Olander 2009; Zinkle 

and Was 2013] During reactor operation, the generation of 

fission and activation products and the formation of 

transuranium elements result in the incorporation of onethird of the elements of the periodic table in the fuel. These 

are distributed over gas bubbles, in metallic and oxide 

precipitates, in solid solution in UO2, and in other forms. 

Because of this incorporation and the other processes, fuel 

undergoes vast changes in grain size, grain boundaries, and 

the introduction of new interfaces (Figure 32). Furthermore, 

phonon transport is altered, leading to changes in the fuel 

temperature profiles.

Figure 32. Illustration of microstructural 

changes in traditional fuel (UO2) that defy 

current predictive capability. | Reprinted 

from Mater. Today, V. Rondinella et al., The high 

burn-up structure in nuclear fuel, 24–32, 

copyright 2010, with permission from Elsevier
Basic Research Needs for Future Nuclear Energy

76 Design and Discovery of Structural Materials and Solid Fuels

Proposed solid fuels include oxides, nitrides, and carbides of the actinides Th, U, or Pu. [Olander 2009] 

The distribution of fission products and transuranic elements strongly depends upon the initial form of the 

fuel, the chemical properties of the fuel matrix, and cladding design and composition. Some fuels retain 

gaseous fission products at high temperature, whereas others release them. Although UO2 fuel has been 

used for decades, the detailed understanding of and rigorous modeling approaches for its in-reactor 

evolution lack many of the mechanistic underpinnings, insights, and approaches needed for efficient 

design of new fuels. For example, at the atomic-scale level of modeling, extensive work has focused on 

individual species of fission products, their energetics, and how the fission product moves and interacts 

with extended defects such as grain boundaries. However, the computational framework for modeling the 

full multicomponent system, including all of the fission products and microstructure evolution, remains to 

be developed. Furthermore, the combination of high irradiation dose, fission product buildup, and low 

temperatures that limit defect mobility results in the formation of a high-burnup structure (HBS) on the 

periphery of light water reactor fuel pellets. [Zinkle and Was 2013; Bleiberg et al. 1963; Rodinella and 

Wiss 2010; Matze and Kinoshita 1007] An understanding is needed of some individual processes (e.g., 

resolution of bubbles [Olander and Wongsawaeng 2006) and the overall process of HBS formation. 

Insight into the microstructural evolution of HBS will further enable the development of predictive design 

strategies for new fuels.

The interaction of neutrons with structural components in the fission core results in radiation damage 

ranging from tens to hundreds of dpa. [Zinkle and Was 2013] High temperatures, thermal gradients, and 

radiation damage synergistically combine into a particularly extreme environment with respect to 

materials degradation. Radiation creates high concentrations of defects in materials, far beyond those 

under thermal equilibria, and these migrate and drive microstructural evolution processes. Microstructural 

evolution and thermal and mechanical loads result in high stresses that alter the materials and often 

compromise their integrity. These harsh conditions also are influenced by the chemistry of the heat 

transfer medium (coolant) and, through interfacial processes, cause undesirable effects on solid fuels and 

structural materials in fission power systems. 

Obviously, robust design of nuclear materials requires the ability to rigorously predict the effects of 

radiation on the structure and hence the properties of materials. However, despite considerable advances 

made both theoretically and in computer simulation, there remains a need to incorporate, in a physically 

realistic way, all of the processes and mechanisms that occur during irradiation. For example, void 

swelling occurs in metals at doses >10 dpa, at temperatures within 0.3–0.6 of the typical alloy melting 

point, [Zinkle and Was 2013; Zinkle and Snead 2014] and is associated with an incubation period or a 

threshold. Although theories describing processes such as point defect generation, point defect transport, 

absorption of defects at sinks, and void growth due to vacancy or gas absorption at voids are separately 

available, an overall framework capable of predicting void swelling (and other defect processes) in 

different metals and microstructures is yet to be developed. Addressing this challenge presents an 

opportunity to accelerate the design and certification of new material systems. Applications of and 

advances in phase field modeling will be of great help in this regard, as will further progress in linking 

different time and length scales (multiscale modeling). The formulation of quantitative structure-propertymechanism relationships for the bulk and interfaces must be further developed and verified. This type of 

quantitative understanding will require modeling methods with improved accuracy to access the large 

time and length scales required for unraveling the collective phenomena necessary to the computational 

design of radiation-tolerant materials. [Demkowicz et al. 2010] 

Presently, the ability to discover and design new structural materials and fuels is limited by the amount of 

accumulated knowledge based on prior experimental, analytical, and reactor operational experience. 

There have been materials advances and insights regarding qualitative design approaches, particularly in 

tailoring nanostructured features. [Zinkle and Snead 2014; Demkowicz et al. 2010; Odette et al. 2008] 

Furthermore, recent advances toward stabilizing nanostructures at high temperatures suggest that 
Basic Research Needs for Future Nuclear Energy

Design and Discovery of Structural Materials and Solid Fuels 77

materials with finely tailored microstructures can be designed to withstand thermal and radiation 

extremes. [Chookajorn et al 2014; Miller et al. 2005] However, there are opportunities to improve the 

efficacy of discovery and design through investment in basic research that targets improvements in 

predictive capabilities and accelerated materials development. Improved fundamental understanding is 

needed in a number of areas, including high-temperature reaction pathways; phase formation; decay 

processes during absorption and scattering of neutrons; material interactions with fission products, 

gamma radiation, and self-decay, including HBS; and underlying structure-properties-processingperformance relationships (all discussed elsewhere in this report).

Scientific Challenges and Opportunities

It is widely agreed that the historical and current timeline for developing most new materials (on the order 

of 10–20 years) must be shortened to meet the continually increasing demands set by the performance, 

economic, environmental, and energy expectations for advanced technologies. Accelerated discovery and 

design of materials is therefore critical and was the major driving force for the MGI 

(https://www.mgi.gov/). For nuclear materials, including fuels, the development timeline based on the 

current state of the art is even longer. Therefore, contracting the time between discovery and materials 

deployment for future nuclear systems is essential. This effort logically starts with developing accelerated 

methods of discovery; of materials design, synthesis, and processing; and of validating the properties and 

performance of new compositions and structures.

The barriers to accelerating the materials development process for nuclear systems are scientific, 

technological, and regulatory in nature. Efforts to meet the scientific challenge will immensely aid in 

overcoming the other two barriers. Therefore, there is a need for greater fundamental knowledge of 

nuclear material behaviors based on a mechanistic understanding of the interplay of compositional 

modifications, defect formation, deformation, and dynamically changing microstructure and 

nanostructure/chemistry as they are exacerbated by extreme environments. In this context, the 

overarching scientific goal related to robust materials design rules must be a holistic understanding of 

materials and fuels behavior in nuclear energy environments to enable the specification of robust 

materials and fuels for radiation environments in a way that substantially shortens the traditional 

development cycle.

The goal is challenging, but it presents compelling opportunities to unravel basic material and chemical 

processes that will enable transformative advances in designing structural materials and solid fuels for 

future nuclear energy systems. This effort will naturally benefit from recent worldwide efforts to develop 

effective frameworks for accelerating materials discovery and design via advances in theoretical and 

simulation and in handling and analyzing large data sets. These approaches—often combining theory, 

modeling, and experiment—have been successful in developing functional materials for targeted 

applications (e.g., electrical energy storage devices [Ceder 2010] and a structural alloy for military 

aircraft landing gears [Olson et al. 2013]) much more quickly than is traditional for the respective 

material classes. These positive outcomes provide a clear motivation for conducting basic research to 

enable accelerated discovery and design of nuclear materials and fuels.

Current (and future) specific successes of materials discovery and design frameworks are rooted in the 

ability to calculate energetics and to fundamentally relate atomic and microscopic processes to 

desired/targeted macroscopic engineering properties. [Ceder 2010; Olson 2013] Typically, these 

atomic/micro-to-macro relationships are better established for functional materials (e.g., batteries, 

thermoelectrics, photovoltaics) than for structural materials and nuclear fuels. Computationally guided 

design of new structural alloys has been successful when the targeted properties could be directly related 

to specific microstructural characteristics. This was the case when in-depth scientific knowledge about 

hardening and strengthening mechanisms in steels was used to develop ferrous alloys for landing gear and 
Basic Research Needs for Future Nuclear Energy

78 Design and Discovery of Structural Materials and Solid Fuels

armor applications. [Olson 2013; Saha and Olson 2007] Similarly, processing-structure-properties 

relationships are at the core of computationally based synthesis and processing of materials. [Brough et al. 

2017] In all cases, these fundamental relationships form the foundation for effective high-throughput 

screening (computationally and experimentally) and predictive design approaches. Accordingly, such 

knowledge bases frame the present discussion of basic research needs to substantially reduce the time 

required to discover new compositions, structures, and processing routes and to design, develop, and 

produce materials for future nuclear energy systems.

Illustrating by examples, the following are possible research areas that are responsive to the basic research 

needs emanating from the scientific goal and opportunities.

Self-healing materials through control of sink strength and chemical complexity

Point defects and defect clusters are created homogeneously in a solid under irradiation and are 

eliminated or trapped at chemical or structural inhomogeneities. The buildup of sustained fluxes of excess 

point defects in irradiated alloys often leads to detrimental evolution of the microstructure and properties, 

including swelling, creep, and radiation-induced segregation and precipitation. Therefore, a promising 

design strategy for radiation-resistant materials is the introduction of a high density of sinks and traps for 

point defects and gas atoms. 

Sinks can be efficient at absorbing and eliminating 

point defects. Atomistic simulations show their 

efficiency can be strongly affected by the interface 

structure of a sink, particularly the degree of 

coherency between the sink and the matrix. 

[Demkowicz et al. 2008] It is therefore necessary to 

measure sink efficiencies experimentally to validate 

atomistic models and assess the coherency for the 

interfaces present in irradiated alloys, which often 

depart from ideal. Past measurements were indirect 

and focused on voids or precipitate-denuded zones 

near extended sinks such as grain boundaries. [Han et 

al. 2012] New approaches are required; an example is 

the use of thin films with controlled microstructure 

[Mao et al. 2015] (Figure 33), particularly in 

combination with high-throughput computational 

screening. [Curtarolo et al. 2013] Direct 

measurements of 3D sink structures are particularly 

needed to elucidate the role of coherency, interfacial 

segregation, and elastic modulus in sink behavior. 

This schema will facilitate accelerated, computationally enabled design of nanoprecipitates or other 

nanoscale structural features with optimized sink strength.

Looking to future nuclear energy systems, it is desirable that materials be designed with 

microstructures/nanostructures that are stable under the thousands of dpa and extensive chemical 

evolution (up to 40% change in composition) that are experienced in solid fission fuels. Fission gas 

bubble mobility in a fuel may be reduced by incorporating sinks with species-specific chemical affinity. 

Understanding how to tailor the surface energy of the fuel to enhance sinks’ gas retention inside smaller, 

higher-pressure bubbles is seen as a key to advanced design and synthesis. Incorporation of trapping sites 

(microstructural or chemical) for mobile radionuclides through advanced synthesis will be needed.

Figure 33. Sink efficiency η measured during 1.8 

MeV Kr irradiations at 300°C for Cu/Nb, Cu/V, and 

Cu/Ni interfaces. The Cu/Nb interface is close to 

a perfect sink (η ≈ 1), Cu/Ni is close to a very poor 

sink (η ≈ 0). | Reprinted from Acta Mater. 82, S. Mao et 

al. Quantitative comparison of sink efficiency of Cu-Nb, 

Cu-V and Cu-Ni interfaces for point defects, 328–335, 

copyright 2015, with permission from Elsevier
Basic Research Needs for Future Nuclear Energy

Design and Discovery of Structural Materials and Solid Fuels 79

Recent progress in understanding energy dissipation and the initial stages of defect evolution as controlled 

by the degree of chemical complexity [Zhang et al. 2017] is promising for future alloy design. Extreme 

chemical complexity modifies both energy dissipation and defect energy landscapes. Therefore, 

maximally disordered alloys—as represented by concentrated equiatomic single-phase solid solutions—

provide possibilities for alloy design in general, and radiation resistance in particular, that are 

fundamentally different from conventional approaches for single-element–based alloys. The use of 

concentrated single-phase alloys has allowed unambiguous interpretation of results based on the 

energetics of the radiation damage and alloying processes, and the formulation of simple alloying rules to 

modulate dynamics and control defect production. [Gramberg et al. 2016; Troparevsky et al. 2015; Zhao 

et al. 2016; Lu et al. 2016] This approach lends itself to the type of predicative capability necessary for 

design via compositional tools and models and to high-throughput screening of many compositions. 

[Troparevsky et al. 2015] A basis for designing new radiation-tolerant materials and structures by 

manipulating compositional complexity (Figure 34) would complement and extend the high-sink-strength 

approaches discussed earlier. However, a key barrier to implementing such a strategy is that classical 

potentials are not adequate to deal rigorously with this level of alloy complexity. Consequently, new ways 

of producing highly accurate potentials for N-component systems (where N>3) are needed, as is the 

development of scalable ab initio simulation methods that can deal with moderate numbers of atoms (103

–

104

) to explore defect production and evolution.

Figure 34. Aspirational approach to a design basis for radiation damage tolerance by understanding and 

controlling early-stage damage through compositional complexity. | Left and center images courtesy of Oak 

Ridge National Laboratory. Right image reprinted with permission from Y. Zhang et al. 2017. Atomic-level heterogeneity 

and defect dynamics in concentrated solid-solution alloys, Curr. Opin. Solid State Mater. Sci. 21(5), 221–237. Copyright 

2017 American Chemical Society

Optimized microstructures may be created through a combination of micro/nano chemical and 

microstructural/nanostructural elements. Doing so requires the ability to understand and control the 

relationship between chemistry and structure. Both theory [Si-Ahmed et al. 1982; Sniegowski et al. 1983; 

Wolfer and Mansur 1980] and experiment [Allen et al. 2005] have shown that the growth of 

microstructural features is strongly influenced by changes in chemistry at the surface of the defect. But 

experiment, theory, and computation have not sufficiently advanced to adequately capture the chemistry–

structure interplay at these interfaces (see PRD 3). Thus, the understanding of radiation effects often is 

limited to small subsets of the microstructure, in isolation from the whole.

Synthesis and processing to create stable, tailored microstructures

The synthesis and processing of advanced reactor materials requires basic research focused on developing 

reaction pathways and thermomechanical processes via fundamental understanding of new chemistries 

and associated phase thermodynamics, interfacial dynamics, and the role of deformation. These efforts 

may benefit from recent and future advances in nanoscale and processing science. [DOE 2007, 2016] 

Design targets include new final compositions; an understanding of the thermodynamics of new phases, 

the energetics governing pathways to them, and their long-term stability; the inherent neutronics of these 

phases; and the use of thermal and deformation processing to achieve the targeted microstructures and 

associated properties for extreme nuclear energy environments.
Basic Research Needs for Future Nuclear Energy

80 Design and Discovery of Structural Materials and Solid Fuels

Understanding the fundamental chemistry and physics of phase 

formation, which is often governed by kinetics rather than 

thermodynamics, will enable the formulation and synthesis of nonpredicted phases and new discoveries. For example, gamma 

irradiation of water enables phase formation driven by solvated 

electrons in solution; the irradiation dose correlates with the 

concentration of solvated electrons formed, thereby dictating the 

reduction routes of the synthesis. [Belloni 2006] Targeted outcomes 

include (1) controlled reduction of metal ions, (2) well-known rates 

of reaction, (3) radiation-stable products, and (4) temperaturestabilized phases. Uniform nanoparticles of metals, alloys, or coreshell assemblies are typically desired. Despite not being predicted by 

equilibrium thermodynamics, AgNi alloy nanoparticles of designed 

stoichiometries have been synthesized and characterized [Zhang et 

al. 2009]; and the method has been extended to nuclear fuel 

compositions (see Figure 35). [Zhang et al. 2010; Nenoff et al. 2011; 

Nenoff et al. 2013] 

Methods targeting metastable phases may hasten discovery. These 

include the initial synthesis of reactive novel actinide organometallic phases, oxohydroxides, and 

nanomaterials that are subsequently used as highly reactive reactants. One example is actinide-containing 

nanoporous metal-organic frameworks. These are a means of synthesizing metastable phases as 

precursors/reactants to novel condensed phases. [Li et al. 2017] Examples of other pathway targets 

include a direct pathway for converting UF6 to fuel and uranyl nanostructures [Qui et al. 2012] that are 

fully formed as nanoclusters and therefore stable, possibly incorporating self-healing properties.

Once the basic understanding of chemical complexity and of sink design for radiation tolerance is 

available, predictable and efficient processing routes are needed to incorporate such features into 

materials at the necessary concentrations. Developing such routes will require an understanding of 

reaction mechanisms during thermomechanical treatments, including targeted self-healing properties and 

defect sinks/defect reaction kinetics. It is possible to incorporate desirable features during processing, e.g., 

via powder metallurgy and heat treatment in nano-oxide dispersion-strengthened steels, [Odette et al. 

2008; Miller et al. 2005; Ukai et al. 2002; Klueh et al. 2005] or via severe plastic deformation, as in CuNb nanolaminates produced by accumulative roll bonding. [Chee et al. 2010] Alternatively, one could 

rely on the ability of materials to self-organize during irradiation, such as in a high density of 

nanoprecipitates. [Demkowicz et al. 2010; Chee et al. 2010] The resulting nanofeatures may be stable

under irradiation, in contrast to the metastablility created in them during fabrication. For solid fuels, basic 

research supporting the design and development of advanced chemical techniques and methods is needed 

for processing materials with tailored sink strength and porosity.

Additive manufacturing (AM) (also known as layer-by-layer deposition, direct digital manufacturing, and 

3D printing) has grown explosively as an industrial materials processing route to more efficient use of 

starting materials (versus subtractive manufacturing processes) and to building tailored, sometimes 

intricate, structures from 3D models using compositions controlled by the feedstocks. [Frazier 2014; 

Pietryzk 2015] As in other processing schemes, an understanding of thermodynamics; reaction kinetics; 

matter and heat transport; thermal gradients; and associated deformation, chemical reaction, or 

solidification phenomena are keys to AM process development and control to achieve the desired 

structure and properties. Conventional materials produced by AM can often be considered new materials 

with unique properties. The ability to spatially vary and control microstructural features or composition 

using AM techniques allows for all-encompassing material, component, and system design, avoiding the 

limitations of conventional manufacturing. Application of AM to nuclear energy is in its infancy, but an 

Figure 35. d-UO2 NPs at pH=3, with 

7 days irradiation. | Image courtesy 

of T. M. Nenoff et al. Synthesis and low 

temperature in situ sintering of uranium 

oxide nanoparticles, Chem. Mater. 

2011, 23(3), 5185–5190, copyright 2011 

American Chemical Society
Basic Research Needs for Future Nuclear Energy

Design and Discovery of Structural Materials and Solid Fuels 81

example of progress in this area is using metal AM to produced integrated fuel plates for the High Flux 

Isotope Reactor at Oak Ridge National Laboratory. [Hehr et al. 2017] From a basic science viewpoint, 

however, the greatest value of AM may be (1) for fabricating innovative specimens and experimental 

platforms to efficiently explore and discover phenomena related to extreme environments, and 

specifically (2) for exploring fundamental issues related to enabling materials design, processing, and/or 

high-throughput screening of materials (as discussed later in this section) predicted to be stable under 

extremes.

Development of accurate predictive methods and models

The reactor environment presents the challenge of predicting key materials properties such as stability, 

durability, aging, fission gas production, decay species interactions, and corrosion. Developing accurate 

predictive methods to build “surety” into all facets of advanced nuclear reactors (including structural 

materials and nuclear fuels) is essential, but the difficulty in predicting materials behavior in extreme 

environments is a major challenge to developing better materials design approaches. However, the 

problem lends itself to high-performance computing, which allows the modeling and prediction of new 

structural materials and fuels and/or of events such as metal-gas interactions, defect growth, and the 

formation and evolution of nanoscale structures that serve as bases for design rules. Current advances in 

ab initio molecular dynamics simulations—combining traditional molecular dynamics with electronic 

structure methods that use density functional theory (DFT) electronic optimizations at each time step 

[Parkes et al. 2016]—will allow the inclusion of temperature effects. This will improve the ability to 

design novel fuel and structural materials (and their surface chemistry interactions) based on predicted 

behavior at realistic temperatures. 

Advanced efforts in f-electron element electronic structure modeling, combined with dynamic thermal 

and neutronic effects, are essential for the breakthrough design of new solid fuels. Simultaneously, model 

development and testing on high-performance computing resources is necessary. These tools will allow 

for fundamental code development that addresses the multitude of permutations of the radiological 

environment relatively rapidly and will accelerate the discovery and design of new fuels. 

Despite many years of effort, in many cases, accurate atomic potentials are still not available for 

compositions associated with nuclear materials, even in the unirradiated condition. Improved potentials 

must be validated by DFT and potentially more accurate treatments and more rigorous approaches such as 

quantum Monte Carlo. [Martin et al. 2016] (These potentials will include far-from-equilibrium conditions 

(e.g., when atoms are very close, as occurs in radiation damage or near high-energy transition states) and 

potentials for chemically complex, multicomponent systems.) The validations must fully consider 

relativistic effects to lower this barrier to materials design. New approaches focused on an automated 

evaluation of a landscape of parameters to generate optimal potentials would help accelerate discovery 

and design. Additionally, testing of developed potentials for application in extreme environments will 

require a substantial body of critical experimental data for properties at those conditions—much of which 

is missing, challenging to obtain at the atomistic level, and typically not sought.

The ability to model materials behavior over many length scales is a current challenge for many systems 

that are less complex than those that operate at the extremes of the nuclear reactor environment (see PRD 

4). However, progress in this area is essential for advanced design approaches because property targets 

and material/component lifetimes serve as the desired outcomes. The degradation front moves though 

materials, creating both time and spatial distributions of damage. The relationships among nanoscale 

atomic interactions of atoms, bonds, and neutronics need to be correlated with the mesoscale damage 

front movement through structural and actinide-bearing materials to accelerate the design process. The 

ability to relate phase field modeling to known compositions and structures that accurately reflect 
Basic Research Needs for Future Nuclear Energy

82 Design and Discovery of Structural Materials and Solid Fuels

radiation damage and other degradation modes, and then apply that model to advanced compositions and 

phases, is a necessary predictive capability for materials design.

High-throughput screening

High-throughput computational screening is an important part of accelerated computational design and 

synthesis/processing of both functional and structural materials. [Curtarolo 2013] Therefore, there is a 

need for basic scientific research and complementary tools and techniques that validate high-throughput 

screening of new theories, atomic potentials, models, and analysis frameworks for the structural materials 

and solid fuels needed for future nuclear energy systems—including those necessary for materials design. 

In addition, effective relevant schemes are needed for high-throughput experimental data generation for 

nuclear materials. One particularly promising approach for radiation-tolerant materials design, as well as 

discovery, is the use of thin films to screen for the inherent sink strength of various interface 

configurations and compositions. With design considerations and targets in mind, high-throughput 

experimental methods for screening thermal stability, cracking, corrosion susceptibility, and so on are ripe 

for development. These will include possible uses of AM. Rigorous ways of generating appropriate data 

computationally for structural materials and nuclear fuels also should be explored. Such advances can 

significantly reduce the time required to develop next-generation nuclear materials. 

As high-throughput experimental and computational data are generated, advanced data analysis tools and 

machine learning techniques must be applied to fully use and digest the information in these data sets. 

Such tools could also be applied to the extensive existing databases for current materials to unearth trends 

heretofore unrecognized. Use of these advanced analytical and learning tools may not only validate 

existing models used in computational materials design; they may also lead to unanticipated results and 

the discovery of new materials and improved modeling and simulation approaches resulting in even better 

design rules for structural materials and solid fuels.

Physically based models to guide and accelerate materials design and discovery 

through experiments and integration with theory and modeling

Advances in understanding of the basic physics and chemistry of structural and nuclear fuel materials 

(surrogate and actinide-containing)—from atomic behavior through molecular to bulk material 

properties—is necessary for their eventual use in environments containing high radiation fields, high 

temperatures, reactive species, and the production and decay of fission products. For materials design, the 

experimental and theoretical work to foster this understanding needs to inform predictive models that 

uniquely relate composition and associated microstructures to the properties and performance relevant to 

the characteristic extreme conditions of nuclear systems. Developing the underlying science for these 

models will enable the efficient design of structural materials and solid fuels based on the absorption and 

scattering of neutrons, the presence of other fission products, gamma radiation, self-decay, and the effects 

of deformation and high-temperature environments.

Further experimentation with ion irradiation is needed to understand and model damage and synergistic 

effects in reactor materials (that is, in a neutron environment) and to accelerate dose accumulation to 

examine long-term behavior. However, rigorous methods to convert analytical results from ion beam 

experiments to equivalent damage under neutron irradiation are generally lacking. Better approaches to 

translating results from ion beam irradiation into equivalent neutron doses and dose rates will result in a 

much larger database. It can be used to develop better models for predicting the extent of damage and its 

effects on material properties, as functions of composition and structure, to aid the design of new 

materials. When this challenge is met, such approaches can also be used for accelerated testing and 

validation of design rules. 
Basic Research Needs for Future Nuclear Energy

Design and Discovery of Structural Materials and Solid Fuels 83

Experimental research is key for (1) validating and informing future models for materials design and (2) 

observing processes that are not currently captured by any models. The latter will include the discovery of 

rare or unexpected events and stochastic processes (see PRD 5) that may lead to entirely new approaches 

to the design of structural materials and solid fuels. Although doing so is highly challenging, 

measurements must be made at the right precision and scale to validate these models for design. The 

physics that drive experimentally accessible intrinsic length and time scales are increasingly accessible 

via atomistic-level first-principles electronic structure and molecular dynamics models. A set of models 

exists at even smaller length scales that experimental techniques still cannot probe. Enhanced diffraction 

techniques in high momentum transfer ranges are necessary to enable accurate pair distribution function 

analysis, or new revolutionary techniques that are able to probe accurately at extremely small length 

scales. Such progress can lead to transformational atomic-scale discovery and design rules for new 

nuclear materials.

Material design approaches would also benefit from feedback from refined or re-imagined experimental 

methods at longer length scales to better characterize materials in real applicable radiation and high-heatflux conditions. New in situ and correlative techniques are needed that access extreme environments and 

explore synergistic effects and their implications for materials design. Of particular interest in this area is 

the potentially immense value of in situ characterization to research on predictable synthesis routes and 

processing for modified and new structural materials and fuel forms. One example is the formation of 

lanthanide nanostructures with simultaneous imaging using a microfluidic stage in a transmission electron 

microscope. [Nenoff et al. 2012] The nanostructures that formed were directly correlated with the electron 

dose of the beam (see Figure 36). Basic nuclear materials research that can benefit from this capability 

includes studies of fuel materials, such as uranium carbides or nitrides that are sensitive to oxidation, and 

structural alloys with novel nanoscale additives processed in various ways. Another example is the use of 

neutron scattering to capture stress development during friction stir weld processing of an alloy. [Woo et 

al. 2017]

Figure 36. Y(NO3)3•4H2O solution. (1) initial nanostructure nucleated by focusing the electron beam, (2) the 

electron beam focused to a point on the nanoparticle in solution, (3) microstructure formation, 

(4) continued microstructure growth, (5) and (6) final microstructure resulting from the focused electron 

beam, with possible cavity growth. All images are at the same magnification. (Background honeycomb 

pattern is saturation of camera due to focusing the electron beam.) | Images courtesy of Sandia National 

Laboratories

Basic Research Needs for Future Nuclear Energy

Crosscutting Themes 85

Panel: Crosscutting Themes

During the workshop on Basic Research Needs for Future Nuclear Energy, all four panels pointed to the 

need to understand the fundamental chemical and physical processes that occur under extreme conditions, 

in order to enable the development of future reactor concepts. The processes that occur in structural 

materials, coolants, and fuels must be understood across broad spatial and temporal scales—from atomic 

to macroscopic length scales and from femtoseconds to years. These requirements, which were common 

to all four panels, cannot be met with today’s analytical and computational tools. New approaches are 

needed that take advantage of in situ measurements—including multiple simultaneous measurements that 

can probe both chemical reactions and structural changes—to provide a more complete view of the 

multitude of impacts that occur in the extreme coupled conditions to which these materials are exposed. 

Importantly, it was underscored that experimental methods need to be closely integrated with 

computational tools to not only predict and simulate degradation processes but also enhance and 

accelerate the interpretation of experimental data. This integration requires advancing the scalability of 

computational approaches so as to model realistic systems, as well as effecting new methods that leverage 

increasing experimental knowledge about the energy surfaces that dictate system behavior at different 

length scales. New data analysis approaches are needed to interpret and assimilate heterogeneous data 

from vastly different experimental and computational methods, and to accelerate initial data interpretation 

to optimize experiments—especially those conducted at major facilities where access time is at a 

premium. The development and integration of these experimental and computational methods will 

expedite the rapid advancement of future nuclear energy systems by supporting the essential need to 

understand and predict chemical and physical changes to materials, coolants, and fuels.

Current Status and Recent Advances

New approaches are being developed to rapidly assess the response of relevant materials through in situ 

and operando studies. Integration of multiple characterization tools will allow simultaneous monitoring of 

various processes. It is critically important to closely couple theory with experiment, to predict and 

confirm experiments and to bridge gaps across time and length scales to fully understand future nuclear 

energy systems. Next-generation algorithms will be required that span from ab initio density functional 

theory calculations to continuum models that capture the complexity of liquid/solid and solid/solid 

interface interactions, and that are applicable to the complex alloy compositions envisioned for 

application. New methods assimilating information from multiple experimental and computational studies 

that represent different length and time scales or dimensionality are being developed, as are advances in 

data analytic methods. 

Recently developed techniques and expanded capabilities at national neutron and x-ray synchrotron 

facilities provide unprecedented opportunities to probe interactions and reactions in real time, at

temperature, and in realistic chemical environments. These capabilities—including spectroscopy, 

scattering, and imaging techniques—have been employed to study materials in situ and operando. These 

explorations include imaging of structural materials and even systems of components, local structure 

determination, examination of interfaces, and investigations of chemical dynamics. Liquid and molten salt 

structures can be determined at high temperatures in both irradiated and unirradiated conditions. These 

capabilities are just now starting to provide new insights into the structure-process-property relationship 

in fuel, coolant, and structural materials. 

X-ray, neutron, and electron microscopy imaging technologies have made tremendous advances in recent 

years, including improvements in resolution and sensitivity and the capability to investigate in situ and 

operando conditions. For example, recent advances in the resolution of spatially resolved neutron 

detectors allow the use of neutron resonance absorption imaging to characterize fuel elements in the
Basic Research Needs for Future Nuclear Energy

86 Crosscutting Themes

cladding. Figure 37 shows neutron contrast tomographic 

imaging of UO2 pellet assemblies containing tungsten 

rods and particles of different dimensions. [Tremsin et 

al. 2013] The densities of the two materials are high and 

similar, and conventional absorption contrast imaging is 

insensitive to the difference. However, in the resonancebased image, tungsten rods and particles are readily 

identified as the dark regions. The same approach can be 

used to image isotopic distributions, such as 235U 

particles in a 238U matrix, to reveal heterogeneity in the 

distribution.

Advances in aberration–corrected transmission electron 

microscopes hold the promise of incorporating in situ 

tools with the objective lens while retaining high 

resolution capabilities. In Figure 38, an Ni3Al sample is 

exposed to oxygen in situ at high resolution so the 

atomic mechanisms of ledge growth of the oxide can be 

directly observed. The two images show the same 

sample initially and 40.5 seconds after introducing 

oxygen into the system. They demonstrate the value of 

evolutionary measurements of a single sample, enabled 

by nondestructive techniques that make growth of oxide 

layers apparent.

Scanning transmission electron microscopy (STEM)

instrumentation now provides high-atomic-scale 

imaging with equally high-spatial-resolution spectroscopy, providing column-by-column chemically 

sensitive atomic-scale imaging. For example, using Kα emission lines, element-specific distributions of 

zirconium and yttrium at a grain boundary in yttria-stabilized zirconia were obtained with aberrationcorrected STEM (Figure 39). [Feng et al. 

2016] The bottom panels show elementspecific line profiles that clearly reveal the 

heterogeneous distribution of zirconium 

across and along the grain boundary. 

Similar advances in electron microscopy 

enable researchers to decipher local 

valence, oxidation state, and varied aspects 

of the electronic structure of buried 

internal interfaces. Importantly, new 

capabilities have been developed for 

studying samples in more realistic 

environments using electron microscopy, 

although further advances are needed to 

observe the full range of materials required 

for applications to nuclear systems.

Figure 37. Nuclear resonance contrast 

tomographic imaging of UO2 pellet assemblies 

containing tungsten particles (dark regions). 

Assembly 1: densities of Urania varying from 48 

to 96% of theoretical density. Assembly 2: 

tungsten inclusions introduced with 75–200 μm 

wires and particles. Assembly 3: PMMA 

(plastic) inclusions used to introduce voids 

during pellet sintering, as well as tungsten 

inclusions similar to those in assembly 2. 

| Reprinted from Nuc. Mater. 440(1–3), A. S. Tremsin 

et al., Non-destructive studies of fuel pellets by 

neutron resonance absorption radiography and 

thermal neutron radiography, 633–646, copyright 

2013, with permission from Elsevier

Figure 38. In-situ observation of oxide growth on Ni3Al 

imaged at high resolution in an aberration-corrected 

transmission electron microscope. | Image courtesy of 

Guangwen Zhou, SUNY Binghamton
Basic Research Needs for Future Nuclear Energy

Crosscutting Themes 87

Complementary simulation of 

these heavy-element–bearing 

nuclear materials remains a 

significant challenge, as 

electronic structure methods 

must include relativistic effects, 

both scalar and spin-orbit 

splitting. [Wilson 1988; Dolg 

2015] Rigorous four-component 

solutions are available [Visscher 

et al. 1994] but are typically 

intractable except for 

benchmarking of quite simple 

systems. To enable codes to take 

advantage of modern 

supercomputer architectures, 

such as the next-generation 

supercomputer Summit, the Oak 

Ridge Leadership Computing 

Facility (a DOE Office of 

Science user facility) has chosen 

the DIRAC code as 1 of the 13 

partnership projects in its Center 

for Accelerated Application 

Readiness program. [Visscher et 

al. 2017] DIRAC builds on earlier work on the parallelization of MOLFDIR. [Pernpointer et al. 2000]

The limitations in the reliability of computed energetic and materials properties become particularly 

important in light of the importance of atomic-level defects and their role in the cascade of events that 

eventually result in material failure. For example, localized inhomogeneities, such as ultra-small 

quantities of a deleterious solute at grain boundaries, may have an outsize influence on the local 

brittleness. Innovative approaches are needed to isolate that “needle in the haystack” through spatiospectroscopic approaches combined with computation that can isolate and identify chemical perturbations 

and their unique signatures. There is a need for new potentials to extend simulation capability beyond 

metals that contain more than two or three elements, and for protocols that allow input from one 

simulation method to be passed to the next-higher length scale method.

Perhaps the greatest opportunity—and the most significant challenge—is to combine characterization 

techniques, computation, and data analytics to fully interrogate the data and to use insights from one

approach to inform the interpretation of results from the others. There are opportunities to rapidly explore 

materials space to evaluate, in silico, new compositions, properties, and performance. Such approaches 

have seen success in areas outside the nuclear field and have resulted in reduced times for alloy 

development and deployment and for assessing the performance of components in service. Furthermore—

with enhancements in computer architecture, the advent of exascale computing, the application of 

machine learning methods to materials discovery, and developments in instrumentation that narrow the 

spatial and temporal differences between simulation and experiment—the time is opportune to work at the 

intersection of experimental capabilities, simulation, and data analytics. 

As an example of the advantages of combining different methods consider how the structure of molten 

ZnCl2 was elucidated using a combination of neutron diffraction, Raman spectroscopy, and simulation. 

Ab initio molecular dynamics (AIMD) was used to model the structure of ZnCl2 at 327°C. The structure 

Figure 39. Abberation-corrected STEM energy-dispersive x-ray 

spectroscopy elemental maps for (a) zirconium and (b) yttria at a 

grain boundary in yttria-stabilized zirconia. Kα emission intensity profiles 

for (c) zirconium and (d) yttria. Different scales are used for the two 

intensity profiles. Scale bar is 1 nm. | Image courtesy of B. Feng et al. 2016.

Atomically ordered solute segregation behaviour in an oxide grain boundary, 

Nat. Commun. 7, 11079.
Basic Research Needs for Future Nuclear Energy

88 Crosscutting Themes

factors and pair distribution functions derived from AIMD matched well with those measured previously 

by neutron diffraction. Raman spectroscopy was also used to investigate the structure of liquid ZnCl2.

This information was directly compared with the results of MD simulations, enabling direct testing of the 

simulation and refinement of training sets. [Alsayoud 2016]. Although the experimental studies were 

done separately, it is possible that techniques could be combined within a single experiment to allow 

changes in structure at a surface to be compared with chemical changes in the interfacial fluid (see the 

sidebar “Advanced Materials Characterization: Neutron Total Scattering Experiments”). This study also 

demonstrated the power of closely coupling experimental measurement with simulation to obtain more 

detailed understanding of the resulting structures. In this context, advances in scaling to enable longer 

AIMD simulation time scales and on-the fly analysis of a simulation could ensure either that simulated 

systems are at equilibrium, or that enough statistics have been gathered to yield predictive insight.

Advanced Materials Characterization: Neutron Total Scattering Experiments

Neutron scattering techniques like those available at the Spallation Neutron Source DOE user facility provide 

unique capabilities for studying nuclear materials. For example, neutron pair distribution function (PDF) can 

provide diffuse scattering contributions that are omitted in traditional diffraction experiments. PDF can 

provide unique structural insight into local aperiodic defect configurations, as well as disordered and 

amorphous domains. Each peak in a PDF represents a specific atom-atom correlation; the position 

corresponds to the pair spacing distance, and the area under the curve is related to the coordination 

number. Further, neutrons scatter strongly from low-Z elements, permitting detailed analysis of both cation 

and anion defect behavior. The image at left shows an example of how neutron PDF was applied to 

characterize local structural evolution and irradiation-induced defects as a function of irradiation fluence in 

irradiated thoria. [Palomares et al. 2017] The defects in ThO2 result in a change in the mean O–Th–O bond 

angle and a distortion of the local ThO8 polyhedra. Neutron PDF can also be used to assess the local order in 

disordered complex oxides, as shown in the image at right. Disordered Ho2Zr2O7 pyrochlore with a longrange defect-fluorite structure consists of short-range orthorhombic distortions, which result in a higher 

degree of local order than previously was thought. [Shamblin et al. 2016]

Neutron scattering results for irradiated thoria and disordered complex oxides. | Top left: image courtesy of R. 

Palomares, University of Tennessee–Knoxville. Bottom left: R. I. Palomares et al., Defect accumulation in swift heavy ionirradiated CeO2 and ThO2, J. Mater. Chem. A 5, 12193–12201 (2017). Right: Reprinted by permission from Nature Publishing 

Group. Nat. Mater., Probing disorder in isometric pyrochlore and related complex oxides, J. Shamblin et al., copyright 

2016.
Basic Research Needs for Future Nuclear Energy

Crosscutting Themes 89

High-fidelity, high-performance computing is 

becoming more predictive for certain physical 

properties and processes that are relevant to nuclear 

fuels as well. For example, recent studies have 

extensively investigated the diffusion mechanisms 

of xenon through density functional theory and 

atomistic modeling. [Andersson et al. 2015] 

Figure 40 provides a demonstration of the good 

agreement of MD simulations of displacement 

cascade–induced diffusion of U, O, Kr, and Xe 

with experimental data indicating a regime of 

temperature-independent diffusion in nuclear fuel. 

These simulations [Cooper et al. 2016] used 

recently developed interatomic potentials and 

modeled the diffusion of both the host U and O on 

the fluorite UO2 lattice, along with the fission gases 

Kr or Xe at a local concentration of 1%. Figure 40a 

shows snapshots of the evolution of a 75 keV 

displacement cascade at 327°C in which the U and 

O atoms are represented by semi-transparent 

spheres. Their color indicates the local temperature 

(red = hot and blue = cold). Displaced Xe atoms are 

represented by opaque spheres; their color indicates 

the displacement relative to the initial configuration 

(red indicates displacement greater than 0.5 nm).

These examples illustrate the significant 

opportunities that exist for atomistic modeling of 

actinide-bearing materials (both solids and liquids) 

at realistic simulation conditions. Such simulations 

are also vital in their capacity to inform higherscale coarse-grained finite element and even phase 

field simulation methodologies. Recent work has 

illustrated the ability of modeling to advance the 

fundamental understanding of radiation-induced 

microstructural evolution. An impressive ability to 

capture the multiscale nature of radiation effects on 

body-centered-cubic structural materials has 

recently been articulated, at least in terms of the 

evolution of the interstitial defect clusters and subvisible vacancy cluster distribution in neutronirradiated iron. [Wirth et al. 2015] Radiationinduced precipitation in both steels and binary alloys has also been successfully studied using 

atomistically informed phase field models. [He et al. 2017] 

Opportunities exist to combine in situ irradiation experiments with modeling, to examine the fine details 

of defect evolution under continuous irradiation, and to verify fundamental assumptions/uncertainties 

with respect to damage production and the kinetics/energetics of irradiation defects. For example, there is 

a need for additional, high-fidelity experimental characterization of the defect cluster microstructure in 

irradiated materials at irradiation temperatures covering the range over which interstitial and vacancy 

point defects migrate, annihilate, and cluster. This will help to validate the defect cluster diffusion kinetics 

Figure 40. Atomistic modeling of radiation effects in 

materials are rapidly becoming more predictive. (a) 

Visualization of defects created by 75 keV 

displacement cascade evolution in UO2 containing 

1% xenon (top panel); the darker colored spheres 

indicate xenon atoms displaced from their original 

positions. (b) Resulting diffusion coefficient of U, O, 

Xe, and Kr calculated by molecular dynamics (solid 

lines) and compared with available experimental 

data (dashed lines). | Reprinted from J. Nucl. Mater. 

481, M.W.D. Cooper et al., Simulation of radiation driven 

fission gas diffusion in UO2, ThO2 and PuO2, 125–133, 

copyright 2016, with permission from Elsevier
Basic Research Needs for Future Nuclear Energy

90 Crosscutting Themes

and reaction rate constants that are inherent to the reaction-diffusion cluster dynamics. Kinetic Monte 

Carlo modeling approaches also are needed. Future modeling and experimental studies are also needed to 

further understanding of the interactions among defect clusters and their interactions with extended 

microstructural sinks. For example, there remain numerous unresolved questions about defect cluster 

evolution as the result of cascade overlap. And questions remain regarding the quantitative details of how 

to treat the reaction rates between defect clusters and various microstructural sinks—including 

dislocations, grain boundaries, nanometer-size precipitate interfaces—as well as reactions among the 

defect clusters themselves. Finally, the radiation defect evolution modeling results have focused almost 

entirely on the evolution of vacancy- and interstitial-type defect clusters in irradiated materials, without 

any consideration of the coupling of the defect and solute/impurity fluxes. There is longstanding literature 

on radiation-induced segregation in austenitic alloys, as well as more recent work on radiation-induced 

segregation in ferritic-martensitic materials. The coupling of the defect and solute fluxes that occur under 

irradiation to thermodynamic predictions of radiation-induced phase formation is receiving increasing 

attention. Seamless integration of evolving defect physics with radiation-induced segregation and changes 

in second-phase precipitates represents a true grand challenge within the radiation effects community.

Rapid materials discovery for use in future nuclear energy systems could be expedited by coupling highthroughput experiments and computations. Generation of large datasets coupled with advanced data 

analytics could help define design rules for a new generation of materials. To date, novel high-throughput 

methods have been developed for parallel structure-property. One example is a testing rig that can 

accommodate multiple specimens (in this case, produced by additive manufacturing) subjected to load, 

environment, and radiation fluxes together with in situ diagnostics [Bruemmer et al. 2014]. Other 

combinatorial approaches for synthesis and rapid screening for properties such as phase equilibria, 

radiation tolerance, and susceptibility to specific degradation methods are also still nascent. 

Analogous computational screening of materials aligned in the vein of the Materials Genome Initiative 

has advanced tremendously in other energy conversion sectors that benefit from calculations of 

thermodynamic quantities. [Curtarolo et al. 2013] Nevertheless, the evolving and nonequilibrium nature 

of materials in reactor conditions demands progression beyond the current state of the art. New

approaches need to incorporate 

1. Multiple stimuli (radiation, stress, corrosion, temperature)

2. New methods for high-throughput computation of kinetic parameters (e.g., diffusion) that enable the 

inference of likely, and potentially unconventional, kinetic pathways from knowledge of energy 

landscapes and transition rates

3. Finite temperature phase equilibria, including the possible effects of anharmonicity

A truly integrated and closed loop with high-throughput experiments will advance materials 

understanding and design and is on the horizon.

Science Challenges and Opportunities: 

The extreme conditions in a nuclear reactor represent significant challenges to the structure and chemical 

stability of the component materials—the fuels, coolants, and structural materials. The following 

representative challenges identified in the workshop panels highlight the hierarchical and multiple length 

and time scales at which damage can occur, and how the presence of a multitude of interfaces and phase 

boundaries can lead to unanticipated behavior that is difficult to predict or even characterize at the current 

state of the art. The motivation and opportunities for advanced characterization, computational, and 

analysis methods is demonstrated.
Basic Research Needs for Future Nuclear Energy

Crosscutting Themes 91

Microstructural control of oxidation response

The performance of a metallic component exposed to high temperatures in a water-, sodium-, lead-, or 

gas-cooled reactor is susceptible to degradation and associated metal loss from oxidation. An ideal 

material would quickly form a thin, impenetrable, stable oxide. Real materials form complex oxides that 

may lead to spallation and reinitiation of fresh oxidation, or they may form local pitting or cracking sites.

As an example, three different oxides can form on the single-metal surface of the nickel-based Alloy 625 

after exposure to supercritical water (Figure 41). [Xu et al. 2012] This complexity is believed to be 

controlled by the different types of crystallographic organization on the surface, whereon a “flat” oxide 

forms on a surface having a <111> grain, while a “rough” oxide forms on the non–<111> surfaces. The 

boundary shown is a high-angle boundary intersecting the surface, leading to a flaky oxide that differs 

from that formed on bulk grains. Thus, the nature of the oxide formed is dependent on the grain 

orientation and may be sensitive to the boundary type too. This richness in oxidation behavior (i.e., 

different oxides form simultaneously, and the macroscopically formed oxide is a composite of distinct 

oxides) leads to the possibility that the overall oxide can be designed through thoughtful combinations of 

individual grain and grain boundary building blocks. Making use of a design platform based upon 

microstructural organization, and enhancing corrosion resistance, require 

• a far better understanding of the atomic structure and composition of the native oxide as a function of 

grain orientation

• a molecular-level understanding of the interaction of the coolant with the native oxide and of how 

these processes are modified as impurities are introduced into the coolant

• an understanding of the combined kinetic and thermodynamic pathways that lead to oxide formation 

in individual microstructural building blocks and at interfaces

• the ability to control the synthesis of the desired of microstructural building blocks at interfaces 

Figure 41. The corrosion morphology changes depending on the surface orientation to the environment or 

the grain boundary orientation. Systematic design of the overall material structure could better control the 

stability of the overall oxide, thus extending component lifetime. | Image courtesy of Todd Allen, University of 

Wisconsin–Madison

Toward those ends, one approach is to develop novel methods to rapidly capture large data sets from 

oxidation experiments and computations and use machine learning approaches to generate knowledge and 

predict and optimize microstructural design strategies.

Local compositional changes induced by irradiation and temperature

Irradiation at elevated temperatures induces numerous changes in the microstructure and local 

composition. For example, irradiation produces 2D and 3D defects, e.g., dislocation loops, stacking fault 

tetrahedra, and voids. It also induces compositional modifications associated with the diffusion of point 

defects, which result in changes in the composition of the grain boundaries as well as segregation of 
Basic Research Needs for Future Nuclear Energy

92 Crosscutting Themes

elements to and from the irradiation-induced defects. For example, it has been shown that in facecentered-cubic equimolar alloys such as NiCoFeCrMn and NiCoFeCrMnPd, elemental segregation occurs 

at loops produced by ion irradiation at 500oC and by high-energy electron irradiation at 400°C 

(Figure 42). The segregation of elements to an interface may also have a beneficial impact. For example, 

stabilization of nanograined metallic structures is possible through control of the composition of the grain 

boundary. This may indicate possible strategies to design new alloy systems for radiation environments. 

The fundamental mechanisms governing 

elemental segregation need to be discovered in 

the emerging alloy systems that will be used in 

advanced reactor systems. The influence of 

compositional changes at defect sinks on damage 

evolution under prolonged irradiation at high 

temperature needs to be understood. Similarly, 

the role of irradiation-induced compositional and 

structural changes at interfaces, along with the 

effects of stresses and temperature on corrosion 

and liquid metal embrittlement, need to be 

uncovered.

Degradation driven by coupled 

phenomena

Materials in reactor systems are subjected 

simultaneously to external variables that can act 

independently or synergistically to degrade the 

properties of the material systems used in nuclear 

reactors. The external variables include radiation, 

applied or residual stress, temperature, and 

corrosive environments due to the coolant and 

fuel. Some degradation mechanisms—e.g. 

irradiation-assisted stress corrosion cracking—appear to occur only as the results of simultaneous 

response to irradiation, temperature, stress, and corrosive environment. The absence of any of those 

variables can, remarkably, preclude crack initiation. Although the degradation mechanism is known from 

an engineering viewpoint, there remains a lack of understanding of the physical and chemical processes, 

and the interplay between them, that ultimately are responsible for crack initiation and propagation. For 

example, recent progress in understanding liquid metal embrittlement along grain boundaries suggests 

that a simple reduction in the interfacial energy due to the embrittling species may by itself be 

insufficient, and that structural unit changes in the interface and plastic deformation processes may 

contribute to cracking. Similarly, in irradiation-assisted stress corrosion cracking, it is understood that 

deformation processes at grain boundaries are necessary to rupture the protective native oxide; but the 

variation in the susceptibility of specific grain boundary locations is not understood. 

There remains a need to uncover the fundamental physical and chemical processes that occur at the 

atomic level, and to link them through the mesoscale to understand the factors that ultimately control the 

degradation response. That is, the physical and chemical processes that govern the window of 

susceptibility to degradation need not only to be defined for the operation conditions but also to be 

understood at a fundamental level. Doing so will require integrating multiple analytical tools within a 

single experiment to probe the compositional and structural changes simultaneously at appropriate spatial 

and temporal resolution. The experimental approaches must be coupled with computational methods that 

allow for comprehensive interrogation of the data.

Figure 42. Compositional modification at an interstitial 

loop produced in NiCoFeCrMnPd equi-molar alloy 

following 1250 keV electron irradiation at 400°C. Iron 

and palladium are deficient and cobalt and nickel 

are enhanced at the loop. | Reprinted from Acta Mater, 

126, M.-R. He et al., Mechanisms of radiation-induced 

segregation in CrFeCoNi-based single-phase concentrated 

solid solution alloys, 182–193, copyright 2017, with permission 

from Elsevier
Basic Research Needs for Future Nuclear Energy

Crosscutting Themes 93

Understanding chemistry and structures at interfaces

Understanding chemical and structural changes at 

interfaces is critical to the development of new coolants 

and materials for future nuclear energy systems. 

Analytical techniques have improved significantly. It is 

now possible to determine atomic-scale structure and 

chemistry in interfacial regions at sub-angstrom 

resolution with in situ and operando x-ray, neutron, and 

even microscopy techniques. As an example, the layering 

of nonaqueous room-temperature ionic liquid electrolytes 

at a potential controlled electrode/electrolyte interface 

can be observed directly using x-ray reflectivity (XR in 

Figure 43). [Zhou et al. 2012] These interface-sensitive 

methods thus have the potential to help determine the 

reaction products of corrosion in nuclear materials. By

direct comparison with MD simulations, as shown in 

Figure 43, they also allow validation of simulations and 

the potentials used in the simulations. However, the use 

of other spectroscopic and microscopic probes, in 

conjunction with reflectometry measurements, are needed 

to examine additional molecular changes in liquids at the 

interface. In addition, in situ irradiation techniques, such as high-energy electrons, ions, or neutrons, could 

be used to simulate radiation damage in these studies. 

Conclusion

The Crosscut panel identified a range of recent advances in characterization and computational 

capabilities that are highly relevant to understanding the fundamental chemical and physical processes 

that occur under extreme conditions in order to enable the development of future reactor concepts. 

However, current capabilities will not be sufficient to elucidate chemical and physical processes that 

occur over broad spatial and temporal scales—from atomic to macroscopic length scales and from 

femtoseconds to years. New in situ and multidimensional characterization tools are needed to probe both 

chemical reactions and structural changes simultaneously. These experimental techniques must be closely 

coupled with emerging data analysis tools to assimilate and interpret large amounts of data, providing 

unprecedented information on the highly coupled processes that occur under extreme environments. 

Advances in computational tools are needed to advance the scalability of computational approaches to 

yield realistic systems models. Together, these new capabilities provide the underpinning knowledge 

required to understand and predict chemical and physical changes to materials, coolants, and fuels in 

future nuclear energy systems.

Figure 43. The molecular-scale vertical 

distribution of room-temperature ionic liquid 

ions at a graphene electrode under potential 

control obtained by x-ray reflectivity is 

compared directly with molecular dynamics 

simulations. | Reprinted with permission from H. 

Zhou et al. Nanoscale perturbations of room 

temperature ionic liquid structure at charged and 

uncharged interfaces,” ACS Nano. 6, 9812–9827, 

copyright 2012 American Chemical Society

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Basic Research Needs for Future Nuclear Energy

Appendix A A-1

APPENDIX A: WORKSHOP PARTICIPANTS

Basic Research Needs for Future Nuclear Energy Workshop

Chair: Kelly Beierschmitt (Idaho National Laboratory)

Co-chairs: Michelle Buchanan (Oak Ridge National Laboratory)

Aurora Clark (Washington State University)

Ian Roberston (University of Wisconsin–Madison)

Basic Energy Sciences Team: Linda Horton, Materials Sciences and Engineering

Bruce Garrett, Chemical Sciences, Geosciences, and Biosciences

John Vetrano, Materials Sciences and Engineering

Philip Wilk, Chemical Sciences, Geosciences, and Biosciences

Katie Runkles, BES Administrative Lead

Plenary Speakers:

Kevan Weaver, TerraPower

John Herczeg, DOE –Office of Nuclear Energy

Sheng Dai, Oak Ridge National Laboratory

Laura Gagliardi, University of Minnesota

Robin Grimes, Imperial College

Panel A: Design and Discovery of Coolants and Liquid Fuels

Chairs: Phillip Britt, Oak Ridge National Laboratory; Alexandra Navrotsky, University of California–

Davis

William (Bill) Casey, University of California–Davis

Sue Clark, Pacific Northwest National Laboratoryw

Richard Crooks, University of Texas–Austin

Nicolas Dacheux, University of Montpellier

Dave Dixon, University of Alabamaw

David Grills, Brookhaven National Laboratory

Ed Maginn, University of Notre Dame

Vic Maroni, Argonne National Laboratory (retired)w

Amy Marschilok, SUNY Stony Brook

Jenifer Shafer, Colorado School of Minesw

Kumar Sridharan, University of Wisconsin–Madison

Michael Toney, SLAC National Accelerator Laboratoryc

Panel B: Physics and Chemistry of Interfaces

Chairs: Amit Misra, University of Michigan; James Wishart, Brookhaven National Laboratory

Niaz Abdolrahim, University of Rochester

Peter Anderson, Ohio State University

Ying Chen, Rensselaer Polytechnic Institutec

Samrat Choudhury, University of Idaho

Tanja Cuk, University of California–Berkeley/Lawrence Berkeley National Laboratory

Vinayak Dravid, Northwestern Universityc

Jian Gan, Idaho National Laboratory

Simerjeet Gill, Brookhaven National Laboratory

Khalid Hattar, Sandia National Laboratoriesw

Jessica Krogstad, University of Illinois, Urbana-Champaign

Jay LaVerne, University of Notre Dame

Emmanuelle Marquis, University of Michigan

Thom Orlando, Georgia Institute of Technologyw

c indicates Crosscutting Themes; w indicates writer
Basic Research Needs for Future Nuclear Energy

A-2 Appendix A

Nikolai Petrik, Pacific Northwest National Laboratory

Simon Pimblott, University of Manchester

Guangwen Zhou, SUNY Binghamton

Panel C: Understanding Behavior at Coupled Extremes

Chairs: Bruce Mincher, Idaho National Laboratory; Izabela Szlufarska, University of Wisconsin

Jeremy Busby, Oak Ridge National Laboratory

Christopher Dares, Florida International Universityw

Andreas Geist, Karlsruhe Institute of Technology

Peter Hosemann, University of California–Berkeleyw

Yutai Katoh, Oak Ridge National Laboratory

Maik Lang, University of Tennessee–Knoxvillec

Laurence Marks, Northwestern University

Mathew Mathew, University of Illinois–Chicago

Steve Mezyk, California State University–Long Beach

Mikael Nilsson, University of California–Irvine

Subramanian Sankaranarayanan, Argonne National Laboratory

John Scully, University of Virginia

Gary Was, University of Michiganc

Clara Wren, Western University

Peng Xu, Westinghouse

Panel D: Design and Discovery of Structural Materials and Solid Fuels

Chairs: Peter Burns, Notre Dame University; Peter Tortorelli, Oak Ridge National Laboratory (retired)

Todd Allen, University of Wisconsin/Idaho National Laboratoryc

Mark Asta, University of California–Berkeley/Lawrence Berkeley National Laboratory

Pascal Bellon, University of Illinois, Urbana-Champaign

Don Brown, Los Alamos National Laboratoryc

T. S. Byun, Pacific Northwest National Laboratory

Laura Gagliardi, University of Minnesota

Michele Manuel, University of Florida

Mitchell Meyer, Idaho National Laboratory

Albert Migliori, Los Alamos National Laboratory

Tina Nenoff, Sandia National Laboratoriesw

Kurt Terrani, Oak Ridge National Laboratoryw

Jason Trelewicz, SUNY Stony Brookc

James Warren, National Institute of Standards and Technology

Yanwen Zhang, Oak Ridge National Laboratory

Steve Zinkle, University of Tennessee–Knoxville

c indicates Crosscutting Themes; w indicates writer

Panel E: Crosscutting Themes

Chairs: Paul Fenter, Argonne National Laboratory; Andy Gewirth, University of Illinois, UrbanaChampaign; Brian Wirth, University of Tennessee–Knoxville

Todd Allen, University of Wisconsin/Idaho National Laboratoryd

Don Brown, Los Alamos National Laboratoryd

Ying Chen, Rensselaer Polytechnic Instituteb

Vinayak Dravid, Northwestern Universityb

Daniel Gianola, University of California–Santa Barbara

Maik Lang, University of Tennessee–Knoxvillec

Michael Toney, SLAC National Accelerator Laboratorya/w

Jason Trelewicz, SUNY Stony Brookd/w

Gary Was, University of Michiganc

a, b, c, and d indicate the theme panel that the individual represents; w indicates writer
Basic Research Needs for Future Nuclear Energy

Appendix A A-3

Invited Participants

Niaz Abdolrahim, University of Rochester 

Todd Allen, University of Wisconsin/Idaho National Laboratory 

Peter Anderson, Ohio State University

Mark Asta, University of California–Berkeley/Lawrence Berkeley National Laboratory

Kelly Beierschmitt, Idaho National Laboratory

Pascal Bellon, University of Illinois, Urbana-Champaign 

Phillip Britt, Oak Ridge National Laboratory

Don Brown, Los Alamos National Laboratory

Michelle Buchanan, Oak Ridge National Laboratory

Peter Burns, University of Notre Dame 

Jeremy Busby, Oak Ridge National Laboratory

T. S. Byun, Pacific Northwest National Laboratory

William (Bill) Casey, University of California–Davis 

Ying Chen, Rensselaer Polytechnic Institute

Samrat Choudhury, University of Idaho 

Aurora Clark, Washington State University

Sue Clark, Pacific Northwest National Laboratory

Richard Crooks, University of Texas–Austin

Tanja Cuk, University of California–Berkeley/ Lawrence Berkeley National Laboratory

Nicolas Dacheux, University of Montpellier 

Sheng Dai, Oak Ridge National Laboratory

Christopher Dares, Florida International University 

Dave Dixon, University of Alabama

Vinayak Dravid, Northwestern University 

Paul Fenter, Argonne National Laboratory

Laura Gagliardi, University of Minnesota 

Jian Gan, Idaho National Laboratory

Andreas Geist, Karlsruhe Institute of Technology

Andy Gewirth, University of Illinois, Urbana-Champaign 

Daniel Gianola, University of California–Santa Barbara 

Simerjeet Gill, Brookhaven National Laboratory

David Grills, Brookhaven National Laboratory

Robin Grimes, Imperial College London 

Khalid Hattar, Sandia National Laboratories

John Herczeg, DOE–Nuclear Energy

Peter Hosemann, University of California–Berkeley 

Yutai Katoh, Oak Ridge National Laboratory

Jessica Krogstad, University of Illinois, Urbana-Champaign 

Maik Lang, University of Tennessee–Knoxville

Jay LaVerne, University of Notre Dame 

Ed Maginn, University of Notre Dame 

Michele Manuel, University of Florida

Laurence Marks, Northwestern University 

Vic Maroni, Argonne National Laboratory (retired)
Basic Research Needs for Future Nuclear Energy

A-4 Appendix A

Emmanuelle Marquis, University of Michigan 

Amy Marschilok, SUNY Stony Brook

Mathew Mathew, University of Illinois–Chicago 

Mitchell Meyer, Idaho National Laboratory

Steve Mezyk, California State University–Long Beach 

Albert Migliori, Los Alamos National Laboratory

Bruce Mincher, Idaho National Laboratory

Amit Misra, University of Michigan

Alexandra Navrotsky, University of California–Davis 

Tina Nenoff, Sandia National Laboratories

Mikael Nilsson, University of California–Irvine 

Thom Orlando, Georgia Institute of Technology

Nikolai Petrik, Pacific Northwest National Laboratory

Simon Pimblott, University of Manchester 

Ian Robertson, University of Wisconsin 

Subramanian Sankaranarayanan, Argonne National Laboratory

John Scully, University of Virginia

Jenifer Shafer, Colorado School of Mines 

Kumar Sridharan, University of Wisconsin

Izabela Szlufarska, University of Wisconsin 

Kurt Terrani, Oak Ridge National Laboratory

Michael Toney, SLAC National Accelerator Laboratory

Peter Tortorelli, Oak Ridge National Laboratory (retired)

Jason Trelewicz, SUNY Stony Brook 

James Warren, National Institute of Standards and Technology

Gary Was, University of Michigan 

Kevan Weaver, TerraPower

Brian Wirth, University of Tennessee–Knoxville 

James Wishart, Brookhaven National Laboratory

Clara Wren, Western University 

Peng Xu, Westinghouse 

Yanwen Zhang, Oak Ridge National Laboratory

Guangwen Zhou, SUNY Binghamton

Steve Zinkle, University of Tennessee–Knoxville

Invited Observers

Tom Arsenlis, Lawrence Livermore National Laboratory

Ilke Arslan, Pacific Northwest National Laboratory

Jeff Binder, Argonne National Laboratory

Daniel Clark, DOE–Fusion Energy Science

Peter Collins, Ames Laboratory

Doug Crawford, Idaho National Laboratory

Glen Crawford, DOE–High Energy Physics

Teresa Crockett, DOE–Basic Energy Sciences

Matthew Denman, Sandia National Laboratories
Basic Research Needs for Future Nuclear Energy

Appendix A A-5

Remi Dingreville, Sandia National Laboratories

Janelle Eddins, DOE–Nuclear Energy

Phil Edmondson, Oak Ridge National Laboratory

Greg Fiechtner, DOE–Basic Energy Sciences

Peter Fischer, Lawrence Berkeley National Laboratory

Luke Fletcher, SLAC National Accelerator Laboratory

Brenda Garcia-Diaz, Savannah River National Laboratory

Marc Garland, DOE–Nuclear Physics

Bruce Garrett, DOE–Basic Energy Sciences

Mary Gillespie, DOE–Nuclear Energy

George Goff, Los Alamos National Laboratory

Matthias Graf, DOE–Basic Energy Sciences

Linda Horton, DOE–Basic Energy Sciences

Jim Horwitz, DOE–Basic Energy Sciences

David Hurley, Idaho National Laboratory

Erfan Ibrahim, National Renewable Energy Laboratory

Alan Icenhour, Oak Ridge National Laboratory

Tatiana Ivanova, Nuclear Science Committee, Organisation for Economic Co-operation and Development

Hesham Khalifa, General Atomics

Stephen Kung, DOE–Nuclear Energy

Harriet Kung, DOE–Basic Energy Sciences

Randall Laviolette, DOE–Advanced Scientific Computing Research

Susan Lesica, DOE–Nuclear Energy

Meimei Li, Argonne National Laboratory

Diana Li, DOE–Nuclear Energy

Apurva Mehta, SLAC National Accelerator Laboratory

Tom Miller, DOE–Nuclear Energy

Raul Miranda, DOE–Basic Energy Sciences

Jamie Morris, Oak Ridge National Laboratory

Gene Nardella, DOE–Fusion Energy Science

Philip Parilla, National Renewable Energy Laboratory

Chuck Peden, DOE–Basic Energy Sciences

Mark Pederson, DOE–Basic Energy Sciences

Mike Pellin, Argonne National Laboratory

James Rhyne, DOE–Basic Energy Sciences

Katie Runkles, DOE–Basic Energy Sciences

Tom Russell, DOE–Basic Energy Sciences

Jim Rustad, DOE–Basic Energy Sciences

Suibel Schuppner, DOE–Nuclear Energy

Andy Schwartz, DOE–Basic Energy Sciences

Viviane Schwartz, DOE–Basic Energy Sciences

Tom Settersen, DOE–Basic Energy Sciences

Nick Smith, Southern Company 

Mark Spitler, DOE–Basic Energy Sciences

Dave Sprouster, Brookhaven National Laboratory

Chris Stanek, Los Alamos National Laboratory
Basic Research Needs for Future Nuclear Energy

A-6 Appendix A

Petr Sushko, Pacific Northwest National Laboratory

Thiyaga Thiyagarajan, DOE–Basic Energy Sciences

Patrice Turchi, Lawrence Livermore National Laboratory

John Vetrano, DOE–Basic Energy Sciences

Philip Wilk, DOE–Basic Energy Sciences

Kevin Wilson, Lawrence Berkeley National Laboratory

Jason Wilson, Savannah River National Laboratory

Brenda Wyatt, Oak Ridge National Laboratory

Zhong, Brookhaven National Laboratory

Jane Zhu, DOE–Basic Energy Sciences
Basic Research Needs for Future Nuclear Energy

Appendix B B-1

APPENDIX B: WORKSHOP AGENDA

BASIC RESEARCH NEEDS FOR 

Future Nuclear Energy Workshop

Hilton Washington DC/Rockville Hotel ● August 9–11, 2017

Wednesday, August 9, 2017

7:00 – 8:00 a.m. Registration and breakfast

Opening Plenary Session — Plaza Ballroom

8:00 – 8:15 a.m. BES Welcome and Workshop Charge

Harriet Kung, Associate Director of Science for Basic Energy Sciences

8:15 – 8:30 a.m. Chair Welcome and Workshop Structure

Kelly Beierschmitt, Idaho National Laboratory (workshop chair)

8:30 – 8:45 a.m. Introduction to Plenaries

Aurora Clark, Washington State University (workshop co-chair)

8:45 – 9:15 a.m. The Role of American Innovation in a Global Marketplace

Kevan Weaver, TerraPower

9:15 – 9:45 a.m. Nuclear Energy Needs in Basic Science to Support the Next Generation of 

Advanced Reactors

John Herczeg, DOE–Office of Nuclear Energy

9:45 – 10:15 a.m. Chemistry for Molten Salt Reactors—History and Perspectives

Sheng Dai, Oak Ridge National Laboratory

10:15 – 10:45 a.m. Break

10:45 – 11:15 a.m. Session chair: Ian Robertson, University of Wisconsin–Madison (workshop co-chair)

Theoretical and Computational Challenges for the Future of Nuclear Energy

Laura Gagliardi, University of Minnesota

11:15 – 11:45 a.m. The Future for Fuels and Materials

Robin Grimes, Imperial College

11:45 – 12:00 p.m. Panel introductions—Michelle Buchanan, Oak Ridge National Laboratory 

(workshop co-chair)

12:00 – 1:30 p.m. Working lunch

1:30 – 5:30 p.m. Parallel Panel Sessions

Panel A Design and Discovery of Coolants and Liquid Fuels

Phil Britt, Oak Ridge National Laboratory, and Alexandra Navrotsky, University of 

California–Davis

Panel B Physics and Chemistry of Interfaces

Amit Misra, University of Michigan, and Jim Wishart, Brookhaven National 

Laboratory

Panel C Understanding Behavior at Coupled Extremes

Bruce Mincher, Idaho National Laboratory, and Izabela Szlufarska, University of 

Wisconsin–Madison

Panel D Design and Discovery of Structural Materials and Solid Fuels

Peter Burns, University of Notre Dame, and Pete Tortorelli, Oak Ridge National 

Laboratory (retired)

Crosscutting Themes Paul Fenter, Argonne National Laboratory; Andy Gewirth, University of Illinois, 

Urbana-Champaign; and Brian Wirth, University of Tennessee–Knoxville
Basic Research Needs for Future Nuclear Energy

B-2 Appendix B

5:30 – 7:00 p.m. Break for dinner (on own)

7:00 – 10:00 p.m. Parallel Panel Discussions (continued)

Thursday, August 10, 2017

7:00 – 8:00 a.m. Breakfast

8:00 – 10:30 a.m. Parallel Panel Sessions for discussion/preparation of preliminary reports

10:30 – 10:45 a.m. Break

Plenary Session — Plaza Ballroom 

10:45 – 11:00 a.m. Report from Panel A: Design and Discovery of Coolants and Liquid Fuels

11:00 – 11:15 a.m. Report from Panel B: Physics and Chemistry of Interfaces

11:15 – 11:30 a.m. Report from Panel C: Understanding Behavior at Coupled Extremes

11:30 – 11:45 p.m. Report from Panel D: Design and Discovery of Structural Materials and Solid Fuels

11:45 – 12:00 p.m. Report from Crosscutting Themes

12:00 – 1:30 p.m. Working lunch

Plenary Session — Plaza Ballroom 

1:30 – 2:00 p.m. Discussion of refined PRDs

2:00 – 5:30 p.m. Breakout to panel/PRD discussions (continued) 

5:30 – 7:00 p.m. Break for dinner (on own)

7:00 – 10:00 p.m. Panel/PRD discussions (continued) and preparation for final panel reports

Friday, August 11, 2017

Plenary Session — Plaza Ballroom 

7:00 – 8:00 a.m. Breakfast

8:00 – 8:30 a.m. Report from PRD 1 

8:30 – 9:00 a.m. Report from PRD 2 

9:00 – 9:30 a.m. Report from PRD 3

9:30 – 9:45 a.m. Break

9:45 – 10:15 a.m. Report from PRD 4

10:15 – 10:45 a.m. Report from PRD 5

10:45 – 11:45 a.m. Discussion

11:45 – 12:00 noon Closing Remarks

Kelly Beierschmitt, Idaho National Laboratory

12:00 noon Workshop Adjourned

Noon – 5:00 p.m. Working lunch/writing (chairs, panel leads and designated writers)
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