Periodic Reporting for period 1 - SCORPION (SiC composite claddings: LWR performance optimization for nominal and accident conditions)
Reporting period: 2022-09-01 to 2024-02-29
The Fukushima Daiichi event in 2011 demonstrated the need for enhanced nuclear energy safety, becoming a major driving force for global investments in accident-tolerant fuels (ATFs) over the past decade. Candidate ATF cladding material concepts that are being developed in replacement of the standard zirconium-based alloy (zircaloy) fuel cladding materials used in light water reactors (LWRs) must outperform commercial zircaloys under nominal operation, high-temperature transient (<1200°C) and accident (>1200°C) conditions. SiC/SiC composites are a rather ‘revolutionary’ ATF cladding material concept exhibiting inherent refractoriness, pseudo-ductility, and a lack of accelerated oxidation during a loss-of-coolant scenario. Due to their unique potential in meeting the stringent property requirements of the ATF cladding application, SiC/SiC composites have already claimed large global investments. Despite these investments, all state-of-the-art variants of the SiC/SiC composite cladding material concept must still overcome inherent shortcomings prior to their perspective deployment. Two important weaknesses are their inadequate compatibility with the coolant (water and steam) and the early (<2 dpa) saturation of radiation-induced swelling during nominal operation. SCORPION strives for a radical improvement in the performance of SiC/SiC composite fuel claddings by highly innovative material tailoring on the nanoscale to limit hydrothermal corrosion and radiation swelling, whilst also modifying the fibre/matrix interface for better stability under irradiation and in high-temperature oxidizing environments. SCORPION is an ATF application-driven international collaboration between Europe (including the UK and Switzerland), the USA and Japan, which combines multidisciplinary scientific excellence, stakeholder know-how, and cutting-edge manufacturing approaches to produce proof-of-concept (PoC) SiC/SiC composite fuel cladding materials with a radically optimized performance for Gen-II/III LWR service environments. If SCORPION succeeds in achieving its scientific & technical objectives, it will become an important stepping stone to the prospective deployment of SiC/SiC composite ATF claddings in Gen-II/III LWRs on a global scale.
The SCORPION achievements are:
WP1 – Processing of bulk PoC materials
• Grain boundary (GB) engineered & doped SiC bulk samples were produced by means of spark plasma sintering (SPS) to improve the hydrothermal stability of SiC. Rare-earth (RE) silicates/garnets decorated the GBs, showing good wetting with the SiC grains.
• High-purity coating materials (RE silicates or garnets) were produced in bulk by pressureless sintering; the samples were milled into powders and converted into dense samples via SPS.
• Porous SiC ceramics with uniform or gradient porosity were produced (different routes) and characterised vis-à-vis microstructure, porosity, properties.
• State-of-the-art SiC/SiC composites were produced by CVI (chemical vapour infiltration) and LPS (liquid phase sintering).
WP2 – Deposition of PoC coatings
• RE silicate coatings were deposited by magnetron sputtering from compound & elemental targets.
• The coatings were characterised with various techniques (XRD, ERDA, SEM/EDS).
WP3 – Joining of PoC materials & testing of joints
• Pressure-less joining of SiC/SiC composites was achieved with the help of localised laser heating; the candidate joining materials were RE silicates & garnets.
• Joining SiC by preceramic polymer precursors has also been attempted.
• The joint quality was controlled by computed tomography (CT), SEM/EDS.
WP4 – Characterization of PoC materials & joints
• RE silicates & garnets (bulk samples & coatings) & doped SiC were characterized by SEM/WDS, XRD, FIB, STEM/EELS.
WP5 – Cladding/coolant interaction tests
• The hydrothermal stability of RE silicates & garnets was assessed in autoclave (360°C, 187 bar, PWR water with 1000 ppm B & 2 ppm Li).
• All materials showed negligible weight changes; minor weight loss was observed in 2 samples that contained small fractions of residual SiO2.
• The steam oxidation resistance of RE silicates/garnets was assessed:
o Isothermally at 1200°C for 1 h – negligible weight change
o Isothermally at 1600°C for 1 h – negligible weight change except from 1 sample containing residual SiO2.
• The steam oxidation resistance of GB engineered & doped SiC was assessed during slow transient heating to 1600°C – small weight change except from few samples that reacted with the sample holder.
WP6 – Ion/proton irradiation campaigns
• The radiation tolerance of RE silicates/garnets was assessed by means of in situ ion irradiation in the TEM (600 keV Kr+, 350-1000°C) – despite the harsh test conditions, all materials behaved well (@350°C, amorphization occurred <3.5 dpa; @800°C, crystallinity was maintained to >40 dpa).
• Synergistic proton irradiation/aqueous corrosion tests (5.4 MeV p+, 320°C, 48 h, PWR water with 3 ppm H2) on RE silicates/garnets (2 samples) showed superior resistance to radiation-enhanced hydrothermal corrosion as compared to CVD SiC tested under identical conditions.
WP7 – Advanced PIE of BR2-irradiated SiC-based ATF cladding materials
• The organisation of hot transport of BR2-irradiated SiC-based materials from SCK CEN to JRC has started.
WP8 – Predictive modelling activities
• The thermodynamic stability of candidate coating materials has been assessed by:
o rough screening of the complete chemical compound space, and
o development of CALPHAD databases for 2 complete ternary systems, each comprising a different RE oxide, Al2O3 and SiO2.
WP9 – Dissemination, communication & training
Two (scientific) Workshops were organised:
• Workshop on application-driven coating methodologies
• Workshop on multiscale engineering of advanced materials
WP1 – Processing of bulk PoC materials
• Grain boundary (GB) engineered & doped SiC bulk samples were produced by means of spark plasma sintering (SPS) to improve the hydrothermal stability of SiC. Rare-earth (RE) silicates/garnets decorated the GBs, showing good wetting with the SiC grains.
• High-purity coating materials (RE silicates or garnets) were produced in bulk by pressureless sintering; the samples were milled into powders and converted into dense samples via SPS.
• Porous SiC ceramics with uniform or gradient porosity were produced (different routes) and characterised vis-à-vis microstructure, porosity, properties.
• State-of-the-art SiC/SiC composites were produced by CVI (chemical vapour infiltration) and LPS (liquid phase sintering).
WP2 – Deposition of PoC coatings
• RE silicate coatings were deposited by magnetron sputtering from compound & elemental targets.
• The coatings were characterised with various techniques (XRD, ERDA, SEM/EDS).
WP3 – Joining of PoC materials & testing of joints
• Pressure-less joining of SiC/SiC composites was achieved with the help of localised laser heating; the candidate joining materials were RE silicates & garnets.
• Joining SiC by preceramic polymer precursors has also been attempted.
• The joint quality was controlled by computed tomography (CT), SEM/EDS.
WP4 – Characterization of PoC materials & joints
• RE silicates & garnets (bulk samples & coatings) & doped SiC were characterized by SEM/WDS, XRD, FIB, STEM/EELS.
WP5 – Cladding/coolant interaction tests
• The hydrothermal stability of RE silicates & garnets was assessed in autoclave (360°C, 187 bar, PWR water with 1000 ppm B & 2 ppm Li).
• All materials showed negligible weight changes; minor weight loss was observed in 2 samples that contained small fractions of residual SiO2.
• The steam oxidation resistance of RE silicates/garnets was assessed:
o Isothermally at 1200°C for 1 h – negligible weight change
o Isothermally at 1600°C for 1 h – negligible weight change except from 1 sample containing residual SiO2.
• The steam oxidation resistance of GB engineered & doped SiC was assessed during slow transient heating to 1600°C – small weight change except from few samples that reacted with the sample holder.
WP6 – Ion/proton irradiation campaigns
• The radiation tolerance of RE silicates/garnets was assessed by means of in situ ion irradiation in the TEM (600 keV Kr+, 350-1000°C) – despite the harsh test conditions, all materials behaved well (@350°C, amorphization occurred <3.5 dpa; @800°C, crystallinity was maintained to >40 dpa).
• Synergistic proton irradiation/aqueous corrosion tests (5.4 MeV p+, 320°C, 48 h, PWR water with 3 ppm H2) on RE silicates/garnets (2 samples) showed superior resistance to radiation-enhanced hydrothermal corrosion as compared to CVD SiC tested under identical conditions.
WP7 – Advanced PIE of BR2-irradiated SiC-based ATF cladding materials
• The organisation of hot transport of BR2-irradiated SiC-based materials from SCK CEN to JRC has started.
WP8 – Predictive modelling activities
• The thermodynamic stability of candidate coating materials has been assessed by:
o rough screening of the complete chemical compound space, and
o development of CALPHAD databases for 2 complete ternary systems, each comprising a different RE oxide, Al2O3 and SiO2.
WP9 – Dissemination, communication & training
Two (scientific) Workshops were organised:
• Workshop on application-driven coating methodologies
• Workshop on multiscale engineering of advanced materials
In the 1st reporting period, SCORPION made significant progress with respect to the state-of-the-art (i.e. CVD-coated CVI & LPS) SiC/SiC composites for the ATF application. Such composites are susceptible to hydrothermal attack under irradiation (confirmed by synergistic proton irradiation/aqueous corrosion tests on CVD SiC). Moreover, LPS SiC/SiC composites are more susceptible to radiation-induced degradation in PWR water due to the high solubility of certain sintering additives (Al2O3, Y2O3) in warm water. The in-pile degradation of SiC/SiC composites is the reason why many technology drivers attempt to protect them with metallic coatings (i.e. Cr or Ti/Cr). SCORPION produced two different types of PoC materials to improve the compatibility of SiC with water/steam: (a) RE silicates/garnets to be used as protective coatings, and (b) GB engineered & doped SiC. The former are chemically more compatible with SiC, while the latter are significantly more resistant to radiation-accelerated aqueous corrosion as compared to CVD SiC tested under identical conditions.