Journal of Nuclear Materials 419 (2011) 140–144

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Journal of Nuclear Materials

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Molecular dynamics simulation of Xe bubble nucleation in nanocrystallineUO2 nuclear fuel

Emily Moore a, L. René Corrales a,b, Tapan Desai c, Ram Devanathan d,⇑a Department of Material Science and Engineering, The University of Arizona, Tucson, AZ 85721, USAb Department of Chemistry and Biochemistry, The University of Arizona, Tucson, AZ 85721, USAc Advanced Cooling Technologies, Lancaster, PA 17601, USAd Chemical & Materials Sciences Division, Pacific Northwest National Laboratory, Richland, WA 99352, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 27 May 2011Accepted 31 August 2011Available online 10 September 2011

0022-3115/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.jnucmat.2011.08.052

⇑ Corresponding author. Address: MS K2-01, PO B99352, USA. Tel.: +1 509 371 6487.

E-mail address: ram.devanathan@pnl.gov (R. Deva

We have performed molecular dynamics (MD) simulations to investigate the dynamical interactionsbetween vacancy defects, fission gas atoms (Xe), and grain boundaries in a model of polycrystallineUO2 nuclear fuel with average grain diameter of about 20 nm. We followed the mobility and aggregationof Xe atoms in the vacancy-saturated model compound for up to 2 ns. During this time we observed theaggregation of Xe atoms into nuclei, which are possible precursors to Xe bubbles. The nucleation was dri-ven by the migration of Xe atoms via vacancy-assisted diffusion. The Xe clusters aggregate faster thangrain boundary diffusion rates and are smaller than experimentally observed bubbles. As the systemevolves towards equilibrium, the Xe atom cluster growth slows down significantly, and the lattice relaxesaround the cluster. These simulations provide insights into fundamental physical processes that are inac-cessible to experiment.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

There is renewed interest in nuclear energy to meet burgeoningglobal energy demand and to address concerns about the environ-mental impact of energy use, especially CO2 emissions. The mostwidely used nuclear fuel is the fluorite-structured oxide UO2.When fission fragments dissipate their energy in the fuel, heat,atomic-level defects, and defect clusters are produced. Duringreactor operation, the thermal and mechanical properties of nucle-ar fuel are degraded as a result of the combined effects of elevatedtemperature, radiation damage, and fission gas production. Xe andKr atoms are generated at the rate of one gas atom for every threeor four fission events, with Xe being nearly seven times morenumerous than Kr [1]. These large atoms are insoluble in UO2, giverise to local lattice strain, and interact with point defects and defectclusters produced by radiation and with grain boundaries [1]. Infact, the interaction of defect clusters with fission gas atoms playsan important role in the structural evolution and performance ofthe fuel. In UO2, radiation-induced structural reorganization as aresult of fission gas release can lead to fuel swelling, deviation fromstoichiometry, and defect clustering [2]. From the applicationstand point, it is essential to understand the fundamental mecha-nisms governing the microstructural and property changes in fuel

ll rights reserved.

ox 999, PNNL, Richland, WA

nathan).

in order to extend the lifetime of existing nuclear reactors, achievehigher fuel burnup, and design fuel for the next generation of ad-vanced reactors. At the same time, the interaction of lattice defectsand impurities with grain boundaries in nanocrystalline or micro-crystalline materials is of fundamental scientific interest. More-over, the formation of nuclei from isolated atoms distributed in asystem is an issue of longstanding interest to many fields ofphysics.

Much research has been performed to advance the developmentof fuel for future generations of reactors [3–5]. Experimentally it isobserved that there is a relationship between defect accumulationand fission gas bubble formation [6]. Fission gas atoms, being over-sized, accumulate initially within vacancy clusters created underreactor operation. Computational studies have shown that singleXe atoms prefer to sit in a neutral vacancy cluster consisting ofone U vacancy and two O vacancies [7]. Aggregation of fissiongas products into macroscopic bubbles in nuclear fuels is generallyassociated with intra- and inter-granular diffusion and grainboundary sweeping mechanisms [8]. The sweeping mechanism in-volves collection of fission gas atoms by moving grain boundaries[8]. The movement of a boundary can occur because of graingrowth or shrinkage and can be locally influenced by mobilevacancies, defect clusters, pores, and impurities [9]. What is lesswell characterized is the coupled diffusive behavior of fission gasatoms in the presence of vacancies at early stages of the fuel life-time. In general, grain boundary migration occurs at time scalesmuch longer than that for vacancy migration. Thus, it may be ex-pected that vacancies and fission gas atoms will relax to metasta-

Table 1Parameters of the UO2 potential [16,17] used in the present work [see Eq. (1)].

Parameter U–U O–O U–O U–Xe

A (eV) 187.03 883.12 432.18 4887.7q (Å) 0.3422 0.3422 0.3422 0.415C (eVÅ6) 0.0 3.996 0.0 0.0D (eV) 0.0 0.0 0.5055 0.0b (Å�1) 0.0 0.0 1.864 0.0r0 (Å) 0.0 0.0 2.378 0.0

E. Moore et al. / Journal of Nuclear Materials 419 (2011) 140–144 141

ble equilibrium states faster than migration of the grain bound-aries. The nanometer length scale and transient nature of the pro-cess of Xe atom aggregation prohibit the study of Xe bubblenucleation by experiment, which creates a need for studying thefundamental defect processes in UO2 using computer simulation.

The use of theory and modeling to study radiation damage ismore than half a century old [10]. Modeling and simulation of nu-clear fuel materials has been reviewed recently [11]. Given thelarge system sizes (millions of atoms) needed to model a polycrys-talline ceramic material, molecular dynamics (MD) using empiricalinteratomic potentials has emerged as the method of choice tostudy the interaction of radiation-produced defects with grainboundaries. MD simulations of high energy recoils in UO2 haveprovided evidence of grain restructuring [11,12]. The MD methodfalls in between the more accurate but highly computationallyintensive ab initio methods and coarse grained mesoscale schemes,such as phase field modeling. Large scale MD simulations bridgethe information gap between the quantum mechanical scale andthe mesoscopic scale. This is needed to develop multiscale modelsfor nuclear fuel processes [11].

Reliable simulation of a UO2 system under the relevant condi-tions requires an interaction potential capable of accurately cap-turing the melting and premelting transition temperatures andjust as importantly the defect energies. This has been the focusof many models in recent decades [13–20]. Interatomic potentialsthat use the shell model to account for polarizability can provideaccurate results for defect properties, but have a high computa-tional cost for large scale dynamics simulations that run to tensof picoseconds. Govers et al. [18,19] have found that rigid ionpotentials with non-formal charges provide good agreement withexperimental data for dynamical simulations of UO2. Devanathanet al. [20] have shown that the model of Yakub et al. [16] is the ri-gid ion potential that best simulates equilibrium properties as wellas the necessary dynamic properties at high temperatures.

In this work, we have used atomistic simulations to examinethe coupled migration of vacancies and xenon gas atoms in a mod-el polycrystalline uranium dioxide fuel. The MD simulations per-formed in this work are designed to give insights into theexperimentally inaccessible early stages of fission gas accumula-tion in nuclear fuel by providing an understanding of the defectprocesses at the atomic level–information that is critically lacking.However, some caveats are in order about the present work. First,the grain diameter of about 20 nm studied here is much smallerthan the real lm grain size in nuclear fuel, which is beyond thereach of MD simulations with current computing power. Second,the time scale studied here, of the order of ns, is very short com-pared to the life of nuclear fuel in a reactor. Finally, the empiricalpotential models used were obtained by fitting only to equilibriumproperties. Nonetheless, this work represents the first effort to ap-ply molecular dynamics to the study of the coupled processes ofdefect migration, Xe bubble nucleation, and interaction of defectsand Xe with grain boundaries in nuclear fuel.

Given the limited time accessible to MD simulations, there is aneed to accelerate the processes of interest without losing accu-racy. We have attempted to enhance the dynamical processesusing two strategies, namely the use of elevated temperature andan extremely high concentration of vacant lattice sites. A high con-centration of defects is needed to observe Xe migration, given thelarge size and poor mobility of the Xe atom in UO2. We investigatedthe structural rearrangement that occurs as a result of fission gasrelease by using neutral Schottky defects and Xe atoms to repre-sent the defects and fission products created during fission events.We have gained atomic level insight of the mechanism of Xe atomaccumulation at early times (ns) that provides information notreadily obtainable by experiment. The results of this work reveala prior unknown mechanism of Xe bubble nucleation, as described

in the following sections that can serve as input for mesoscalemodels of nuclear fuel.

The rest of the manuscript is organized as follows. We describethe simulation details in the next section, discuss the results in thefollowing section, and finally present the conclusion regarding thenew understanding obtained from this work.

2. Details of the simulation

Ideal fully stoichiometric UO2 assumes the fluorite-structure asa single crystal, where the cations are arranged in a face centeredcubic structure and the anions are located at tetrahedral sites, cre-ating a simple cubic sublattice within the unit cell. The systemused in this study is a rectangular cell comprised of six hexagonalcolumnar grains of uniform grain size and shape with the columnsrunning parallel to the z-axis ([001] crystallographic direction) ofthe simulation cell. The orientation of atoms in each grain is ro-tated by 30 (degrees) with respect to adjacent grains to producehigh-energy grain boundaries. We used periodic boundary condi-tions to repeat this arrangement. The equilibration of such UO2

nanostructures has been previously discussed by Desai et al. [21].We initially annealed the simulation cell with the potential of Yak-ub et al. [16] for 2 ns at 2000 K in Berendsen’s isothermal isobaricensemble [22] at zero external pressure, and then relaxed the sys-tem at 300 K for 50 ps in the isothermal isobaric ensemble.

We used the potential of Yakub et al. [16] to model the interac-tions between the ions (U–U, U–O and O–O). This potential pro-vides a good fit to the properties of both solid and molten UO2,and shows a pre-melting transition, associated with enhanced an-ion mobility, at around 2500 K [20]. The pair potential betweenions i and j with charges qi and qj, respectively, separated by dis-tance rij is

VijðrijÞ ¼qiqj

rijþ Aije

�rijqij � Cij

r6ij

þ Dij e�2bijðrij�r0ijÞ � 2e�bijðrij�r0

ijÞ

h i: ð1Þ

The charges (q) of U and O are 2.2208 and�1.1104, respectively,in units of |e|, where e is the electron charge. The first term on theright hand side is the point charge electrostatic interaction, thesecond term is the short-range repulsion, the third term representsdispersion, and the fourth term represents covalent interaction. Aij,qij, Cij, Dij, bij, and r0

ij are parameters of the potential obtained byfitting to known experimental properties of UO2 and are listed inTable 1. In order to avoid unrealistic attraction due to the domi-nance of the dispersion term � Cij

r6ij

� �at short interionic distances

that can occur in radiation damage simulations, we joined thethe pair potential in Eq. (1) by a spline function to the repulsiveZiegler-Biersack-Littmark (ZBL) potential [23]. The ZBL potentialis a screened Coulombic repulsion that is given by

UðxÞ ¼ 0:1818e�3:2x þ 0:5099e�0:9423x þ 0:2802e�0:4029x

þ 0:02817e�0:2016x; ð2Þ

where x ¼rijðZ0:23

i þ Z0:23j Þ

0:8854a0: ð3Þ

Fig. 1. Initial configuration in which 10% Schottky vacancy defects have beenrandomly introduced to a cell of 244,182 atoms and of which 25% of those vacancieswere randomly filled with Xe atoms.

142 E. Moore et al. / Journal of Nuclear Materials 419 (2011) 140–144

a0 is the Bohr radius (0.5292 Å) and Zi and Zj are the atomic num-bers of ions i and j, respectively.

The Xe atom carries no charge. Xe–Xe, O–Xe, and U–Xe interac-tions are represented by the short-range pair potentials of Genget al. [17]. The Xe–Xe and O–Xe potentials have the Lennard-Jonesform,

VijðrijÞ ¼ eijrij

rij

� �12

� rij

rij

� �6" #

: ð4Þ

The U–Xe interaction is given by

VijðrijÞ ¼ Aije�

rijqij : ð5Þ

The parameters of the Xe–U interaction are listed in Table 1. TheXe–Xe and Xe–O interaction parameters are listed in Table 2.

The equilibrated stoichiometric supercell configuration wasapproximately 300 Å � 512 Å � 22 Å in size and contained244,182 atoms. We modified this cell to incorporate a supersatura-tion of point defects by introducing a 10% Schottky defect concen-tration (10% of cations and 10% of anions were removed). To createthe defects, we randomly introduced neutral trivacancies in theform of UO2 Schottky defects. Typically, we selected a cation atrandom for removal along with two anion neighbors chosen at ran-dom from all available anion neighbors. Due to random selection,the removal of ions took place from the grain boundary as wellas the grain interior. We randomly chose 25% of these trivacancydefects for the introduction of Xe atoms, so that each Xe atomoccupied the space formerly occupied by one cation and two adja-cent anions. Experimentally, it is known that fission product gasatoms accumulate at vacancy clusters [6]; theoretical studies [7]have also provided support for Xe accommodation by Schottky de-fects. The remaining trivacancies were not filled with Xe to facili-tate Xe migration by coupling to vacancy diffusion during theshort time scale of the simulation.

We employed the DL_POLY computer code [24], version 3.0.9,to perform MD simulations and the VMD program [25] for visual-ization of atomic configurations. After the introduction of vacan-cies and Xe atoms, we equilibrated the simulation cell inBerendsen’s isothermal isobaric ensemble [22] at 300 K and zeroexternal pressure for 10 ps. The configuration after defect intro-duction is shown in Fig. 1. We used the Berendsen barostat andthermostat [22], with barostat relaxation constant of 1 ps and ther-mostat relaxation constant of 0.1 ps. We calculated the electro-static interactions using the particle mesh Ewald summationwith a cut off radius of 10 Å. After the initial room temperatureequilibration, we instantaneously raised the temperature of thesystem to 2500 K and equilibrated for 20 ps in the isothermal iso-baric ensemble at zero external pressure. We performed subse-quent simulations in Berendsen’s canonical ensemble [22] at2500 K.

Fig. 2. A set of RDF plots of the system at 10 ps (blue curve), 300 ps (green curve)and 1.9 ns (red curve) of simulation time. Each curve is a single time snapshotmeasuring the Xe–Xe radial distance distribution. At early times there are some

3. Results and discussion

Fission gas migration in the UO2 system is made possiblethrough a supersaturation of vacancy concentrations and hightemperatures. Previous investigations [20,26] also indicate thatthe oxygen sublattice is mobile at temperatures around 2500 K,whereas the uranium sublattice remains in its crystalline phase.

Table 2Lennard-Jones parameters [17] for Xe–Xe and Xe–O interactions.

Parameter Xe–Xe Xe–O

e (eV) 0.017 0.0099r (Å) 4.29 2.50

In our simulations it is found that Xe atoms migrate with intrinsicdefects to form pre-coarsened gas clusters.

After less than 10 ps of simulation time the nearly uniform dis-tribution of the Xe gas is evident in the radial distribution function(RDF), see Fig. 2, that shows a small first nearest neighbor peakindicative of some clustering and otherwise a near uniform distri-bution. In the initial distribution some nearest neighbor pairs areformed statistically and others are close enough to find each otherat very early times. Given that the covalent radius of a xenon atomis 1.5 Å and the Van der Waals radius is 2.1 Å, a first nearest neigh-bor distance peak can be expected at around 3.5 Å. Initial observa-tions of nearest-neighbors at early simulation time (Fig. 2) indicatethat the nearest-neighbor distance is indeed on average approxi-mately 3.7 Å apart.

As time progresses, vacancy diffusion assists the migration of Xeatoms. Subsequent RDF snapshots at longer time reveal the aggre-gation of Xe gas atoms, as follows. As the simulation evolves the300 ps RDF of Fig. 2 shows individual Xe atoms still present inthe system as the tail does not decrease revealing a large number

near neighbor pairs at short distances as evidenced by the fist peak at 3.8 Å wheremost atoms are dispersed as evidenced by the leveling off of the tail of the curve. Atthe longest time clustering has occurred as evidenced by the broadened first peakwith evidence of a second peak forming at 6.7 Å. This second peak is due to the nextnearest neighbor. The broadened first peak is due to the extra vacancy space withinwhich xenon atoms can rattle around. This series of RDFs reveal the clusteringbehavior of this system. (For interpretation of the references to colour in this figurelegend, the reader is referred to the web version of this article.)

E. Moore et al. / Journal of Nuclear Materials 419 (2011) 140–144 143

of persisting monodispersed xenon atoms. Nonetheless, the firstnearest neighbor peak of the RDF grows as seen in Fig. 2 after300 ps. There are many Xe atoms well beyond the cutoff of theRDF that as the simulation proceeds enter into the range of theRDF because of aggregation. Monitoring the evolution of xenonclustering through pair distribution analysis, reveals clustering asthe first nearest-neighbor peak grows and depletion of isolatedatoms as the tail starts to fall off well before 2 ns of simulationtime. The RDF plot at 1.9 ns shown in Fig. 2 shows an increase inxenon aggregation as the nearest neighbor peak centered at 3.7 Åincreases; it also becomes apparent that a second nearest neighborpeak is emerging around 6.7 Å. The latter separation distance iscomparable to second nearest-neighbor xenon atoms in a cluster,thus establishing a clear coarsened structure as fission gas agglom-erates. Together these observations imply that the clusters arecoarsening to a finite distribution and stabilizing as they grow indi-cating that pre-coarsening is a more stable state or geometry for Xein the UO2 system rather than a random uniform distribution with-out clustering.

The clustering of xenon atoms was further investigated by ana-lyzing the bubble size distribution within the system. In Fig. 3,plots of the abundance of the single, quadruple, and sextuplecluster sizes over time are shown. An increase in larger cluster

Fig. 3. Distributions as a function of time of single (isolated) Xe atoms, quadrupleclusters, and sextuple clusters that clearly show the increasing number of largeclusters as the isolated population is depleted.

sizes with simultaneous decrease of single atoms validates the for-mation of second neighbor peak formation seen in the 1.9 ns RDFof Fig. 2 because sextuplet clusters are large enough to havenext-nearest neighbor packing.

An average size of a cluster containing three xenon atoms isfound to be approximately 4.4 Å in diameter, that of four Xe atomsis 6 Å in diameter, and that of six xenon atoms is 6.7 Å. The largestcluster found over the 2 ns simulation period consisted of eight xe-non atoms and was approximately 9.8 Å in diameter. Experimentalobservations report bubbles on the order of microns in size. Ourfindings of these precursory clusters are not considered in experi-mental findings, as they are not physically discernable in the spentfuel. Hence, the precursory coarsened fission gas clusters are a un-ique atomic level insight into the mechanism of bubble formationobserved by Ref. [3].

Further discernment of the mechanism by which the fission gasclustering occurs is of key interest to advancements in nuclear fueluse. We can learn more from these simulations by considering atime reversal analysis that is carried out by first locating the centerof mass coordinate of the largest Xe clusters at the end of the sim-ulation run. Snapshots of a spherical region of a radius correspond-ing to the average distance that the atoms forming the clustermigrated, reveals how the clusters aggregate from a random distri-bution to a central point. These are shown in forward time in Fig. 4revealing that over the simulation period isolated xenon atoms de-plete to form larger clusters by combining with other single diffuseatoms or other clusters and are driven by vacancy assisted diffu-sion occurring at elevated temperature. As the size of the clusterincreases a metastable cluster size distribution is reached such thatfluctuations in cluster size occur due to individual atoms diffusingbetween nearest neighbor clusters. Small pre coarsened clustersthat are considered to be a precursor to the larger fission gas bub-bles observed experimentally characterize this metastable state.

It is of interest to establish a mechanism of formation and com-bination of the precursory clusters. A center of mass analysis of thesample region depicted in Fig. 4 indicates a coalescence mecha-nism where diffuse Xe atoms are combining into the coarsenedclusters observed in the size distribution plots. At early stage theatoms are diffuse and the fuel matrix is pushed away from the fis-sion gas atoms, seen as changes in the matrix arrangement as thesimulations continue. Note that the center of the sphere is at thesame point in all snapshots.

The Xe atoms are clustering by vacancy-assisted diffusion toalleviate lattice strain. The insoluble Xe atom is slightly larger thanthe UO2 Schottky defect and is therefore more stable in aggregatedvacancies or as aggregated clusters [27]. As shown in Fig. 4 the Xeatoms are aggregating into a void created by the diffusion andagglomeration of an excess of defects. The accumulation of defectscreates a void where the rest of the fuel matrix is separated fromthe fission gas atoms. The metastable state of the coarsened clus-ters alleviates the lattice strain with the existence of the defectvoid. The individual clusters are isolated and generally less mobilebeing essentially self-trapped and will require simulation timeswell beyond the time scale of these MD simulations to reach theexperimentally observed bubble formed state.

Compared to the initial configuration of Fig. 1, the final state ofthe system Fig. 5 consists of grain boundaries that are less distinctand are essentially swallowed up by the crystal. There are indica-tions that the initial amorphous grain boundaries are locallymigrating with early time system evolution by vacancy reconstruc-tion events. The point defect migrations contribute to this restruc-turing of the grain boundaries leading to the locally reformedgrains. These events occur congruently with vacancy-assistedmigration of the xenon atoms and represent a subset of the earlytime events that contribute to grain boundary diffusion. Thisshows that simulations at the next larger scale need to account

Fig. 4. Reverse analysis leads to observing the aggregation behavior of isolated Xe atoms into one larger cluster. From left to right the snapshots were taken at 130 ps, 700 ps,1.6 ns and 1.9 ns. Ordering of the UO2 is also apparent with increasing time.

Fig. 5. Final configuration after 2 ns of simulation time in which the clustering intoa pre-coarsened state has occurred. Note that there has been some movement of thegrain boundary in some regions most likely driven by vacancy reconstructionevents that locally reforms the grains.

144 E. Moore et al. / Journal of Nuclear Materials 419 (2011) 140–144

for not only the migration of defects and fission products but alsograin restructuring.

It has been proposed [3] that grain boundary motion (e.g.sweeping) contributes to the fission gas bubble formation. In con-trast, this work reveals that at the nanosecond timescale grainboundary mechanisms do not contribute to fission gas atom aggre-gation. If grain boundary sweeping contributed to fission gas pre-cursory cluster formation, a higher concentration of clusters nearwhere the grain boundary had migrated would be apparent inthe system; however this is not observed. What does appear is arelatively homogenous distribution of xenon clusters within thegrain boundaries and within the grains even though some grainboundary movement is occurring in the simulation. Due to systemsize and simulation time restrictions arising from computationalresource limitations, the formation of micron sized coarsened bub-bles cannot be studied at present using MD simulations. However,we can conclude that the sweeping mechanism proposed in earlierwork [3] is not a contributor to bubble formation at the nanosec-ond timescale. The precursory fission gas clusters observed in thiswork are likely to contribute to the development of experimentallyobserved bubbles, where grain boundary sweeping could involvefurther aggregation of clustered fission gas atoms, not isolatedones, and with a well-developed relaxed host matrix. This studyshows the importance of studying the cooperative processes of de-fect migration, fission gas atom clustering, and grain boundary mo-tion in order to understand the evolution of the microstructure andproperties of irradiated nuclear fuel.

4. Conclusions

Schottky point defect populations, as used in this work, are foundto dictate the migration of fission gas atoms. Molecular dynamicsstudies have been carried out to track the early time Xe bubblemigration within a UO2 nuclear fuel matrix. From this work it isfound that the mechanism likely to contribute to fuel swelling mustinclude a metastable pre-coarsened fission gas cluster state that

cannot be observed experimentally in post-irradiated fuel pellets.This work shows that single diffuse atoms form sub-nanometer sizeclusters via a vacancy assisted diffusion mechanism. Clustering andcoarsening of these clusters were found to occur at simulation timesunder 1 ns after which little change occurred. Thus we infer thatthese early time clusters are what over longer timescales evolve intothe larger fission gas bubbles. This could have interesting conse-quences in the design of future nuclear fuel matrices to controlmicrostructure evolution and fission gas release.

Acknowledgments

This work was supported by the US Department of Energy(DOE) Computational Material Science Network (Multiscale Simu-lation of Thermo-mechanical Processes in Irradiated Fission-Reac-tor Materials) under the DOE Grant DE-FG02-07ER46369 and theDOE Grant DE-FG02-07ER46368. RD was supported by the Divisionof Materials Science and Engineering, Office of Basic Energy Sci-ences, DOE under Contract DE-AC05-76RL01830. EM was partlysupported by the DOE Science Undergraduate Laboratory Intern-ship program at Pacific Northwest National Laboratory (PNNL).This work was performed using the Molecular Science ComputingFacility in the Environmental Molecular Sciences Laboratory, a na-tional scientific user facility sponsored by the DOE’s Office of Bio-logical and Environmental Research and located at PNNL.

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