Computational Studies in the UNLV. Phil F. Weck and Eunja Kim December 2009

Transcription

1 Computational Studies in the UNLV Radiochemistry Group Phil F. Weck and Eunja Kim December 2009

2 Outline I. Introduction 1. Objectives and Areas of Research 2. Computing Facilities Overview 3. Computational Methods II. Examples 1. Nuclear Waste Forms: Tc Zr Alloys, Ln Technetate Pyrochlores 2. Advanced Fuels: Actinide Nitrides 3. Separation: Uranyl Complexes, Actinyl Calixarenes 4. Fuel Cladding Materials: Steels 5. Coordination Chemistry of Tc III. Conclusion and Future Work

3 I. Introduction Computational modeling and simulation: the latest addition to the UNLV Radiochemistry Program Timeline: 2007: Informal theory/experiment collaboration (Weck, Kim/Czerwinski, Poineau, Gong, Yeamans, Silva) July 2008: Beginning of the computational effort in the Radiochemistry Group (Prof. Phil Weck joins; Chemistry of materials and molecular complexes) March 2009: Development of the research effort on fuel cladding materials (Prof. Eunja Kim joins; Condensed matter Physics, matter under extreme conditions and nanosciences)

4 1. Objectives and Areas of Research Main objectives: Designing new materials for advanced nuclear fuels and transmutation targets Understanding the electronic structure and properties of systems containing actinides and fission products. Usingatomistic/nanoscale models and simulationsof of candidate materials as an early way to avoid using expensive and timeconsuming real materials and to complement experiments. Trainingthe next generation of theorists and experimentalists Main Areas of Research: NuclearWaste Forms Nuclear Fuels Separation Fuel Cladding Materials & Radiation Damage

5 2. Computing Facilities Overview The new RadChem computer UNLV (November 2009): Quick specs: 8 nodes, d with h twin AMD 6 core Opteron CPUs per node d (total ( l off 96 cores))

6 3. Computational Methods Quantum Mechanics Ab initio quantum mechanical methods: Calculations performed using all electron spin polarized scalar relativistic and planewave Density Functional Theory Molecular Mechanics Empirical Methods Computer Codes: First principles: VASP, Dmol, CASTEP, Gaussian03, CPMD, Abinit, Molecular mechanics: GULP, MS Discover, Additional homemade codes for properties calculations and analysis.

7 II. Examples: 1. Nuclear Waste Forms The long lived isotope of technetium, 99 Tc (t 1/2 = 2.1 x 10 5 years), is produced in large quantity from spent nuclear fuel (6% of the fission yield). Need for chemical and physical waste forms for immobilizing 99 Tc. Two principal approaches for Tc nuclear waste: 1) transmutation of metallic 99 Tc target by neutron capture in a reactor to give stable Ru; 2) incorporation of 99 Tc metal in a very stable disposal form (Tc alloys) placed in a geological repository. Option 2): Tc Zr alloys, Rare Earth Technetate pyrochlores,

8 II. Examples: 1. Nuclear Waste Forms Technetium zirconium waste forms: Tc 6 Zr SEM image of sample A Sample Phase Tc(at. %) Zr(at. %) Composition A Light Tc 6.24 Zr Phase Bonds Distances (Å) Tc 6 Zr (I4-3m) i (Å) α-mn-typem t XRD DFT Lattice constant: DFT: a = 9.93 Å Tc3-Tc , , Exp.: a = Å Tc 6.24 Zr Tc2-Tc Zr1-Tc Tc-Zr

9 II. Examples: 1. Nuclear Waste Forms Technetium zirconium waste forms: Tc 2 Zr and Tc 4 Zr SEM image of sample B Tc 4 Zr (I4-3m) Sample Phase Tc(at. %) Zr(at. %) Composition B Light Tc 4.52 Zr Dark Tc 2.04 Zr Tc 2 Zr (P6 3 /mmc) Tc 2 Zr: DFT: a =5.33 Å c =8.80 Å XRD: a =5.216Å c =8.635 Å Tc 4 Zr: DFT: a =9.99 Å XRD: a =9.749Å Phase Bonds Distances (Å) XRD DFT Tc 2 Zr Tc1-Tc Tc2-Tc Zr1-Zr

10 II. Examples: 1. Nuclear Waste Forms Technetium zirconium waste forms: Tc 6 Zr HOMO Clear metallic behavior Valence and conduction bands dominated by Tc and Zr 4d orbitals s and p orbitals play minor roles s-p and p-d hybridization in valence and conduction bands Zr: [Kr]4d 2 5s 2 ; Tc: [Kr]4d 5 5s 2

11 II. Examples: 1. Nuclear Waste Forms Technetium zirconium waste forms: Tc 2 Zr HOMO Similarities with Tc 6 Zr (i.e., metallic behavior, importance of 4d-orbitals, s-p and p-d hybridization) (110 ) electron charge density

12 II. Examples: 1. Nuclear Waste Forms Technetium zirconium waste forms: Tc 4 Zr HOMO Similarities with Tc 6 Zr and Tc 2 Zr (i.e., metallic behavior, importance of 4d-orbitals, s-p and p-d hybridization) Density of states at the Fermi surface: 72.3 states/ev for Tc 4Zr 85.6 states/ev for Tc 6 Zr (Larger number of 4d electrons in Tc-rich alloys compared with Zr-rich alloy systems)

13 II. Examples: 1. Nuclear Waste Forms Rare Earth Tc pyrochlores

14 II. Examples: 2. Advanced Fuels Actinide Nitride Fuels Advanced fuels for fast breeder reactors (FBR) and target materials for the transmutation of Pu and minor actinides (Np,Am,Cm) in fast reactor cores andaccelerator drivenaccelerator driven sub critical systems (ADS). Potentially enhanced fuel performance in Generation IV fission reactors: superior themophysical properties (high melting point, high thermal conductivity, high metal density), favorable neutronic properties (high energy neutron spectrum), good sodium compatibility, high solubility in nitric acid (fuel reprocessing).

15 II. Examples: 2. Advanced Fuels Actinide Nitride Fuels U N β-u 2 N 3 P3-m1 1 space group Trigonal, La 2 O 3 -type (Stable above 1323 K) UN 2 Fm3m space group FCC, CaF 2 -type U 2 N 3 Ia-3 space group BCC, Mn 2 O 3 -type (Stable up to 1323 K) UN Fm3m space group FCC, NaCl-type

16 II. Examples: 2. Advanced Fuels Actinide Nitride Fuels: UN and UN 2 Lattice constants: DFT: UN 2 : a = 5.28 Å; UN: a = 4.90 Å XRD: UN 2 : a = Å; UN : a = Å

17 II. Examples: 2. Advanced Fuels UN 2 LUMO Valence bands near the Fermi level: N 2p orbitals dominate Small contribution of U 5f and 6d orbitals Some degree of p-f hybridization Conduction bands near the Fermi level: U 5f orbitals dominate 0.9 ev indirect band gap U:[Rn]5f 3 6d 1 7s 2 N:[He]2s 2 2p 3 HOMO

18 II. Examples: 2. Advanced Fuels UN LUMO Valence bands near the Fermi level: U5f and d6d orbitals dominate Strong d-f hybridization Negligible contribution of N 2p orbitals Conduction bands near the Fermi level: U 5f orbitals dominate Some degree of d-f hybridization No band gap (better orbital overlap) HOMO

19 II. Examples: 2. Advanced Fuels Actinide Nitride Fuels: Heat capacity of UN and UN 2 DFT calculations reproduce calorimetric data within 4%

20 II. Examples: 3. Separation Uranyl Nitrate Complexes Molecular orbital diagrams of the highest lying states of the stable UO (H O) 2+ Molecular orbital diagrams of the highest lying states of the stable UO 2 (H 2 O) 5, UO 2 (H 2 O) 4 (NO 3 ) +, UO 2 (H 2 O) 2 (NO 3 ) 2, UO 2 (NO 3 ) 3, and UO 2 (NO 3 ) 4 2 complexes calculated using DFT (top), with the corresponding relaxed geometries (bottom).

21 II. Examples: 3. Separation Coordination of uranyl calix[3]arenes complexes

22 II. Examples: 3. Separation Electron charge densities calculated with LDA/PWC DFT Molecular orbital (MO) energy level diagram calculated with LDA/PWC DFT

23 II. Examples: 4. Fuel Cladding Materials Steels: modeling and simulation of HT 9 cladding

24 II. Examples: 4. Fuel Cladding Materials Steels: Ferritic to martensitic transition in HT 9 cladding Characterize HT 9 samples theoretically a=5.65å c=5.71å a=5.67å Analyze the damage after irradiation Elucidate the evolution of defects Recover the samples by annealing Fe (purple) Cr (red) Composition: 12% Cr

25 II. Examples: 5. Coordination Chemistry of Tc Crystal Structure of Technetium Halides: TcCl 4 b c a TcCl 4 crystal unit cell (orthorhombic Pbca) TcCl 4 polymeric chain (distorted octahedral coordination) Lattice constants (in Å): DFT: a= 12.15; b= 14.68; c= 6.29 a/c= 1.93; b/c= 2.33 Exp.: : a= 11.65; b= 14.06; c= a/c= 1.93; b/c= 2.33 (Elder & Penfold, 1966) Bond lengths (in Å): DFT: Tc Cl= 2.26; 2.37; 2.45 Exp.:Tc Cl= 2.24; 2.38; 2.49

26 II. Examples: 5. Coordination Chemistry of Tc TcF 4 crystal unit cell (Pbca) Lattice constants (in Å): a= 10.01; b= 11.92; c= 5.60 a/c= 1.79; b/c= 2.13 TcBr 4 crystal unit cell (Pbca) Lattice constants (in Å): a= 12.82; b= 15.25; c= 6.53 a/c= 1.96; b/c= 2.33 TcI 4 crystal unit cell (P1) Lattice constants (in Å): a= 13.18; 18 b= 16.21; c= 6.93 a/c= 1.90; b/c= 2.34

27 II. Examples: 5. Coordination Chemistry of Tc Technetium oxides: Tc 2 O 7, TcO 2, TcO 3 Structures of (a) Tc 2 O 7 and (b) TcO 2. Structure of TcO 3 calculated using all electron scalar relativistic DFT: (c) electron charge density (in e/å 3 ); (d) HOMO; (e) LUMO; (f) Ab initio MD simulation of TcO 3 (Pm3 m) in a unit cell at T=200K with GGA/PW91 DFT in the NVT ensemble

28 III. Conclusion and Future Work Conclusion: Computational modeling and simulation effort carried out in the UNLV Radiochemistry Rdi i Program covers Nuclear Fuel, Waste Forms, Separation, Fuel Cladding Materials and Radiation Damage Unique on campus synergy between the computational and experimental research effort Active collaborations with National Laboratories and the Industry (LANL, ANL, INL, ORNL and TerraPower) Future Work: Pursue research work on Tc waste forms (DOE NEUP), methacrylates for the synthesis of ion selective resins (DOE NEUP), and fundamental Tc chemistry (DOE BES) New research directions (14 white papers submitted to DOE NEUP with a computational component) Acknowledgment: funding provided by DOE (NEUP and BES)

29 Publications Weck P. F., Kim E., Masci B., Thuery P., Czerwinski K. R., Density Functional Analysis of the Trigonal Uranyl Equatorial Coordination in Hexahomotrioxacalix [3]arene based Macrocyclic Complexes Inorg. Chem., submitted. Weck P. F., Kim E., Poineau F., Czerwinski K. R., "Structural evolution and properties of subnanometer Tc n (n = 2 15) clusters", Phys. Chem. Chem. Phys. 11, (2009) Weck P. F., Kim E., Poineau F., Rodriguez E., Sattelberger A. P., Czerwinski K. R., "Technetium(IV) halides predicted from first principles", Inorg. Chem. 48, 6555 (2009) Poineau F., Weck P. F., Forster P., Sattelberger A. P., Czerwinski K. R., "Synthesis, structure, and first principles calculations of the TcBr 2 (PMe 3 ) 4 and Tc 2 Br 4 (PMe 3 ) 4 complexes", Dalton Trans. 46, (2009). Poineau F., Rodriguez E. E., Weck P. F., Sattelberger A. P., Forster P., Hartmann T., Mausolf E., Silva G. W. C., Jarvinen G. D., Cheetham A. K., Czerwinski K. R., "Review of technetium chemistry research conducted at the University of Nevada Las Vegas", J. Rad. Nucl. Chem. 282, 605 (2009). Weck P. F., Kim E., Balakrishnan N., Poineau F., Yeamans C. B., Czerwinski K. R., "First principles study of single crystal uranium mono and dinitride", Chem. Phys. Lett. 443, 82 (2007).