Silicon Carbide in TRISO particle Fuel

Transcription

1 Silicon Carbide in TRISO particle Fuel Vienna, 8 th December 2015 Nadia Rohbeck, Han Liu, Ping Xiao Ceramic Coatings Group, School of Materials, University of Manchester, UK

2 Overview of TRISO Particle Research 2 SiC coatings in TRISO Particle Fuel Tomography Fission product retention The issue of Silver release Diffusion vs Reaction model Thermodynamic understanding Mechanical Integrity at elevated temperatures High Temperature Nanoindentation: PYCASSO experiment Hardness and Plasticity in indentation of SiC

3 Microstructural Analysis of TRISO Particle 3 High resolution X-ray tomography at Henry Moseley Facility Voxel size ~0.5 micron Tracking of layer dimensions and porosity in carbon layers

4 Outer pyrolytic carbon coating 4 Porosity of the OPyC coating is highly interconnected some pores are several hundred microns long

5 Variation of Thickness in the buffer layer 5

6 The Issue of Silver release 6 TRISO fuel retains radiotoxic fission products reliably up to 1800 C except for Silver that is found to be released in large quantities even at regular HTR operating conditions (1000 C) Potential release mechanisms Grain boundary diffusion Nanopore/Nanotube assisted diffusion Palladium-assisted diffusion Wetting-assisted transport

7 Wetting-assisted Transport 7 Thermodynamic Analysis 1 Ag+SiC = Ag x Si 1-x +C 2 Ag+Si = Ag x Si 1-x 3 Ag+C = Ag x C 1-x If free Silicon is present, R2 is preferable to R1 Thermodynamic calculations confirmed that an interaction of Ag with SiC can take place in the relevant temperature range. The dissolution of SiC is related to the SiC stoichiometry. Ag+SiC Ag+C+SiC Ag+Si+SiC Ag wt.% SiC wt.% Si% wt.% C% wt.% Reactiontemp (ºC)

8 Thermocal SiC/Ag/(Si) system 8 Phase diagrams indicate strong temperature dependency of SiC-Ag reaction depending on stoichiometry

9 Load, P Nanoindentation - Theory 9 Nanoindentation set-up Load-displacement curves loading unloading P max S E r * 2 A i E E E r i S H P A max h Displacement, h h max

10 High Temperature Nanoindentation 10

11 Nanoindentation of PYCASSO Fuel 11 PYCASSO irradiation at the High Flux Reactor (NRG Petten, The Netherlands) 4 months; gas-cooled environment; simulated fuel samples fabricated by CEA (France)

12 TRISO Fuel Coatings 12 Grain size: 0.86 ± 0.57 µm 1.38 ± 1.14 µm 0.8 ± 0.46 µm

13 Commercial bulk CVD SiC 13 Fabricated by static chemical vapour deposition (sold by Rohm & Haas, USA) Large, randomly oriented grains Fully dense E = 460 GPa H = 35 GPa

14 Nanoindentation of different TRISO Fuel 14 Strong variation in the Elastic modulus of the different specimen Hardness and its evolution with temperature similar

15 Raman Spectroscopy of SiC 15 SiC polytype splitting of the TO bands Crystallinity band width Stoichiometry Carbon, silicon, silica additional bands Temperature/pressure Peak position Stress/strain Peak position

16 Raman spectroscopy of deformed SiC 16 A B TO and LO bands shift to higher values at point of strongest deformation, peak width increases at higher temperatures increases plasticity

17 Y Y Y Y Y Y Y Maps of LO position and width C 300 C 500 C X X X Y Y X X X

18 Summary + Outlook 18 Fission product retention Thermodynamic calculations confirmed that an interaction between silver and SiC can occur in the relevant temperature range for HTR application. Initial experiments detected proof for this reaction with surrogate samples at temperatures of 1400 C and above. Further work will investigate, if by modifying the SiC coatings characteristics the reaction rate can be reduced.

19 Summary + Outlook 19 Mechanical Performance Successful measurement of the elastic modulus and hardness of different SiC coatings up to 500 C by insitu nanoindentation Elastic modulus decreases slightly with temperature; drop in hardness more pronounced due to a pronounced plasticity in SiC at elevated temperatures Microstructural differences of the SiC show some effect in the elastic modulus values Neutron-irradiated SiC exhibited slight hardening effect

20 Acknowledgements 20 Funding: EPSRC (NNUMAN) and European Commission through the FP7 Archer project UMAN: Xin Geng, Dimitrios Tsivoulas, Ian Shapiro, Andrew Forrest NRG: Steven Knol, Tien Pham CEA: Jean-Michel Escleine, Marc Perez, Thierry Lambert INET: Bing Liu

21 Relevent Publications 21 X. Geng, F. Yang, N. Rohbeck & P. Xiao. An original Way to Investigate Silver Migration Through Silicon Carbide Coating in TRISO Particles. J. Am. Cer. Soc. 97, (2014) H. Liu, X. Geng, N. Rohbeck, M. Bai & P. Xiao. An Investigation into Interaction between the Fission Product Silver and Silicon Carbide. Proceedings of ICAPP 2015, Nice, France, 2015 N. Rohbeck, D. Tsivoulas, I. P. Shapiro, P. Xiao, S. Knol, J.-M. Escleine & M. Perez. In-situ nanoindentation of irradiated silicon carbide in TRISO Particle Fuel up to 500 C. J. Nucl. Mat. 465, (2015)