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Crush strength of silicon carbide coated TRISO particles: Influence of test method and process variables
Journal of Nuclear Materials 445 (2014) 30–36
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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Crush strength of silicon carbide coated TRISO particles: Influence of test method and process variables R.D. Cromarty ⇑, G.T. van Rooyen, J.P.R. de Villiers Department of Materials Science and Metallurgical Engineering, University of Pretoria, South Africa
h i g h l i g h t s Crush strength is strongly dependent on the anvil hardness. Soft anvil crush strength is more sensitive to particle properties than hard anvil crush strength. Crush strength of SiC coated particles is influenced by the inner pyrocarbon. Crush strength depends on SiC thickness, deposition temperature and coater design. MTS concentration and carrier gas flow rate did not influence crush strength.
a r t i c l e
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Article history: Received 22 May 2013 Accepted 19 October 2013 Available online 30 October 2013
a b s t r a c t The influence of deposition temperature, methyl trichlorosilane (MTS) concentration, hydrogen carriergas flow rate and gas inlet design on the strength of silicon carbide coated TRISO particles was investigated using whole particle crushing strength. Crush strength was measured using soft aluminium anvils. For comparison a selection of particles were also measured with hard anvils. The influence of silicon carbide thickness was determined to allow for normalisation of all crush strength measurements to a crush strength at an equivalent thickness of 35 lm. It was found that the strength of the underlying pyrocarbon coated particles had a significant influence on the crush strength of the silicon carbide coated particles. Deposition temperature and gas inlet design were the only process parameters that influenced the coated particle crush strength. No evidence was found for MTS concentration and hydrogen flow rate having any influence on particle crush strength. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction High Temperature Gas Cooled Reactors (HTGR) have been proposed as a viable option for next generation nuclear reactors to be used for electricity production as well as potential heat sources for industrial applications and hydrogen production. Previously operated HTGR research reactors (e.g. Peach Bottom, Dragon, AVR) as well as commercial reactors (e.g. Fort St. Vrain, THTR) made use of fuel particles coated with a single layer of pyrocarbon, Bistructural Isotropic (BISO) or Tristructural Isotropic (TRISO) coated particles. Currently operational HTGR research reactors in Japan (High Temperature Test Reactor – HTTR) and China (HTR10) make use of TRISO coated particles. TRISO coated fuel particle will also be used for the Chinese HTR-PM commercial prototype reactor. In addition to the programs in Japan and China investigation of the manufacturing and behaviour of TRISO particles is also ongoing in the USA, Russia and Republic of Korea. TRISO particles consist of a fuel kernel containing fissile elements; a buffer layer of low-density, porous, pyrocarbon; a dense pyrocarbon layer; a ⇑ Corresponding author. Tel.: +27 12 420 2955. E-mail address: [email protected] (R.D. Cromarty). 0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.10.041
silicon carbide layer and a final coating of dense pyrocarbon. The layers making up the TRISO coating are applied to the kernel by means of a spouted bed chemical vapour deposition (CVD) coater. Processing conditions are known to influence the properties of the coating layers. In this paper, the influence of process conditions on the crush strength of the silicon carbide coating will be investigated. Each of the layers within the TRISO coating performs specific tasks. Silicon carbide acts as a diffusion barrier preventing the escape of fission products and provides the mechanical strength required to resist internal gas pressure. During use, pressure builds up within the particle due to the release of gaseous fission products and, in the case of oxide fuels, the release of oxygen. At normal operating temperatures, oxygen released from the fuel will react with carbon to form CO and CO2. The magnitude of the internal pressure depends on many variables including fissile elements, chemical form of the fuel, operating temperature and level of burnup. In the case of UO2 fuels Minato et al. [1] calculated fission gas pressures in the order of 10 MPa while CO pressure varied between approximately 0.1 MPa and 100 MPa depending on burnup and operating temperature.
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In addition to the stresses resulting from internal gas pressure, coated particles may be damaged during handling and manufacture of fuel elements. Although the risk of damaging the coated particles during these operations may be minimised by careful design of particle handling and fuel manufacturing operations, the particles will need to exceed a minimum strength to prevent damage. Various methods have been used to measure the mechanical properties of CVD silicon carbide layers within TRISO coatings. Methods reported in the literature include: 1. Crush testing of whole particles or hemispherical shells using flat anvils [2–12]. 2. Crushing of hemispherical shell by means of a punch placed inside a hemispherical shell [13]. 3. Compression or tensile testing of rings polished out of coated particles [14–16]. 4. Internal pressurisation of hemispherical shells or tubes using gas pressure [17] or elastomeric inserts [18]. 5. Bend testing of micro-beams produced by ion beam milling [19]. Crush testing of whole particles using hard flat anvils has been the most commonly used. For the majority of crush tests hard anvils made of steel, alumina or silicon carbide were used. Recently the use of soft anvils has been reported [5,9,10,12]. Van Rooyen et al. [9] showed that for an anvil hardness above 270 HV crush strength was consistently low while for an anvil hardness below 38 HV particle crush strength was consistently high. Between 38 HV and 270 HV crush strength increased with decreasing anvil hardness. This change in measured strength was due to the fracture mechanism changing with changes in anvil hardness. In the case of a hard anvil, fracture was due to Hertzian cracking at the particle/anvil contact area. For a soft anvil, fracture was due to tensile fracture at a point away from the contact area. Typical values of published crush strength of TRISO particles are presented in Table 1. Direct comparison of previously reported results is complicated by the differences in the particles tested. It
has been shown that crush strength is dependent on kernel properties [3], properties of the pyrocarbon layers [4] and silicon carbide thickness [3]. Importantly the crush strength has been shown to depend on the properties of the silicon carbide [3], being sensitive to both deposition conditions and post deposition annealing [3,6,9]. Not all studies have found a link between process conditions and crush strength; Ogawa and Ikawa [6] found that whole particle crush strength did not depend on silicon carbide properties. Byun et al. [5] found a correlation between crush strength and roughness of the silicon carbide inner surface as well as the pore density of the silicon carbide. They did not find any correlation between silicon carbide grain structure and crush strength. Kim et al. [8] found that inner surface roughness effects overshadowed any effects of microstructure or porosity in the silicon carbide. Heat treatment of the particles has ambiguous effects. Researchers have reported a decrease in crush strength [3], an increase in crush strength [9] or behaviour that depends on annealing temperature [10]. Several studies have noted the large spread of crush strength results. Reported Weibull modulus values range between 2.2 [10] and 19 [4]. Particle to particle coefficient of variation (r= x) has been reported to range between 12% [8] and 18% [3]. 2. Materials and methods Particles were coated in a spouted bed coater. The process tube consisted of a graphite tube (50 mm ID, 70 mm OD, 339 mm long) with a 60° conical base. Power was supplied by a 10 kW induction power supply operated at 119 kHz. Temperature control was by means of a Type B thermocouple mounted outside the conical section of the process tube. The thermocouple was mounted in an alumina protection sheath inserted into a graphite ring that fitted over the conical base of the tube. Coating was carried out using hydrogen as a carrier gas and methyl trichlorosilane, CH3SiCl3, (MTS) as a precursor. MTS was supplied to the coater by means of a bubbler maintained at 0 °C by means of an ice bath. Hydrogen flow to the bubbler and for fluidization was controlled by mass flow controllers. The bubbler and
Table 1 Published crush strength of TRISO particles. Description 1
2
3
4
5
Hard anvil Converted resin kernel Buffer, IPyC, SiC outer layer Buffer, IPyC, SiC, pyrocarbon outer layer Hard anvil UO2 kernel SiC outer layer. CH4 derived pyrocarbon SiC outer layer. C3H6 derived pyrocarbon Pyrocarbon outer layer. CH4 derived pyrocarbon Pyrocarbon outer layer. C3H6 derived pyrocarbon Hard anvil SiC outer layer Kernel: 493 lm, Buffer: 43.3 lm, IPyC: 31.7 lm, SiC: 27.9 lm Kernel: 603 lm, Buffer: 58.9 lm, IPyC: 31.4 lm, SiC: 28.0 lm Pyrocarbon outer layer Soft anvil ZrO2 kernel Buffer: 112 lm, IPyC: 64 lm, SiC: 32 lm, OPyC: 48 lm Buffer: 113 lm, IPyC: 72 lm, SiC: 39 lm, OPyC: 48 lm Soft anvil UO2 kernel Buffer: 88 lm, 1.0 g cm3 IPyC: 36 lm, 1.92 g cm3 Buffer and IPyC as above. SiC: 36 lm, 3.20 g cm3
Crush strength (g) [3] x ¼ 1000 to 1400 x ¼ 1900 to 2500 (a) x ¼ 2000 x ¼ 2200 x ¼ 4200 x ¼ 3600
[4] m ¼ 19 m ¼ 19 m ¼ 10 m ¼ 9:6 [6]
x ¼ 740 x ¼ 1100 x ¼ 2200 [9] Median = 6100 Median = 5600 (b) x ¼ 5173; r ¼ 286 r ¼ 1510 x ¼ 6990; r ¼ 296 r ¼ 1257
). a: Mean ( x) and Weibull parameter (m) reported in [4]. b: Mean ( x), between batch standard deviation (r), and average within lot standard deviation (r
[12]
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R.D. Cromarty et al. / Journal of Nuclear Materials 445 (2014) 30–36
Table 2 Starting material properties.
Table 3 Test run summary.
Buffer pyrocarbon
G130 G140 a
Inner pyrocarbon
Thickness (lm)
Density (g cm3)
Thickness (lm)
Density (g cm3)
129 117
1.03 0.97
72 93
Not measureda 1.58
Based on known parameters the IPyC density was estimated to be 2.03 g cm3.
fluidization gas streams were mixed prior to being introduced into the process tube. Two designs of gas inlet were investigated; one using a watercooled feed tube while the other used an alumina feed tube without any cooling. These will be referred to as the cold and hot inlet respectively. In the case of the hot inlet, severe problems were experienced with blockage of the gas inlet due to deposition in the gas feed tube and inlet. These problems were reduced, but not fully solved, by changing the design of the inlet to minimise heat conduction to the gas feed tube. When viewed using an optical microscope deposits in the gas feed tube appeared to consist of two phases, possibly silicon and silicon carbide. Only silicon carbide was deposited during this test work. The starting material consisted of 500 lm zirconia particles coated with a buffer layer and inner pyrocarbon layer. Two lots of particles were used for the tests; details of the starting material are given in Table 2. The starting material was produced by South African Nuclear Energy Corporation (NECSA) as part of the commissioning and characterisation of a production scale coater. As the carbon coating was not performed under standard coating conditions the thickness and material properties of the buffer and inner pyrocarbon layer were not within normal TRISO specifications. Several batches of particles were available; two of these (G130 and G140) were used for test runs. The choice of starting material was based on the size uniformity of the carbon coated particles. Particles were heated to the deposition temperature in argon. Gas flow was then switched to hydrogen, the temperature stabilized and finally the MTS introduced. After silicon carbide deposition, the gas flow was switched back to argon while the furnace was cooled. No outer pyrocarbon was deposited. Four groups of test runs were carried out. A summary of these are presented in Table 3. Test groups DR, HF and CR were based on a Central Composite Design of experiment. Group TT was run under fixed process condition; only deposition time was varied to achieve a range of SiC thicknesses. In the case of groups HF and CR initial tests were conducted using G130 starting material. These tests were repeated using starting material from G140. This allowed for a direct comparison of the effects of starting material. Initially no attempt was made to control the thickness of group DR, all tests being run at a fixed time. This resulted in a wide spread of SiC thickness. For later tests in group DR a SiC thickness of 35 lm was targeted by varying the deposition time. For groups HF and CR deposition time was varied to achieve a silicon carbide thickness of 35 lm ± 5 lm. Test runs with a silicon carbide thickness outside of the 30 lm to 40 lm range were included in the crush tests. This allowed for a better understanding of the effect of SiC thickness on the crush strength of the particles. All test runs formed part of a larger study of the silicon carbide deposition process. 3. Experimental The equipment used in these tests was previously described by van Rooyen et al. [9]. In this work, the hard anvils consisted of alumina plates while the soft anvils were of annealed aluminium of approximately 20 HV.
Start material: Run #
Comment
G130: DR1–DR26 Hot gas inlet Determine deposition rate for various process conditions Variables: Deposition temperature, MTS concentration, Hydrogen flow rate, Mass of particles Deposit thickness not controlled G130: HF1–HF6 G140: HF7–HF25
Hot gas inlet Determine the impact of process conditions on SiC properties Variables: Deposition temperature, MTS concentration, Hydrogen flow rate Deposition time varied to obtain SiC thickness of 35 ± 5 lm
G130: CR1–CR3 G140: CR4–CR28
Cold gas inlet Determine the impact of process conditions on SiC properties Variables: Deposition temperature, MTS concentration, Hydrogen flow rate Deposition time varied to obtain SiC thickness of 35 ± 5 lm
G140: TT1–TT5
Cold gas inlet Determine effect of SiC thickness for fixed process conditions Deposition time varied to obtain SiC thickness of 25–55 lm
For each test run, at least 50 particles were crushed. When using the soft anvils each particle needed to be placed on a separate position on the anvil as the particles left an indent in the soft aluminium. In the case of the hard anvils, each particle was placed in the centre of the alumina plate. All test runs and the starting material (i.e. pyrocarbon coated zirconia particles) were tested using the soft anvils, only a selection of test runs were tested using the hard anvils. Equipment was calibrated each day before use. The test equipment was also characterised to test the variability of the crush test equipment in terms of equipment calibration, anvil used and position on the anvil. Long-term stability of the test procedure was also checked. No statistically significant differences were found between the aluminium plates, between measurement positions on the plates or between daily set up.
4. Results For the starting material and all the test runs it was found that the crush strength of individual particles was equally well described by the normal and Weibull distributions. Although Weibull statistics have often been used to describe failure probability of brittle materials, including silicon carbide in TRISO coated particles (for example [4,7–10,15,16,18,20,21]), the normal distribution may be a better option [22]. In this study it was accepted that the data was normally distributed. Normal probability plots of the crush strength of the carbon coated particles measured using hard and soft anvils is presented in Fig. 1. The crush strengths obtained using the hard anvils are significantly lower than those obtained using the soft anvils. A summary of these results is presented in Table 4. Of note is the clear difference (95% confidence interval: 4726 g ± 526 g) between G130 and G140 when soft anvils are used compared to the small difference (95% confidence interval: 173 g ± 151 g) when hard anvils are used. In Fig. 2 the crush strength of two test runs coated using similar process condition and having similar silicon carbide layer
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R.D. Cromarty et al. / Journal of Nuclear Materials 445 (2014) 30–36 Table 5 Coefficient of variation for starting particles and coated particles.
Z (Standard Deviation)
3 2
Soft anvil
1 G130 G140 G130 G140
0 -1
start start coated coated
G130 Soft Anvil G130 Hard Anvil G140 Soft Anvil G140 Hard Anvil
-2 -3 0
5000
10000
15000
Mean
Minimum
Maximum
Mean
Minimum
Maximum
0.183 0.193 0.090 0.092
– – 0.056 0.059
– – 0.183 0.119
0.166 0.136 – 0.212
– – – 0.133
– – – 0.327
12000
20000
Crush Load (g)
TT Soft Anvil TT Hard Anvil
Table 4 Starting material crush strength.
Crush Load (g)
10000
Fig. 1. Crush strength normal probability plot of starting material prior to silicon carbide coating.
8000 6000 4000 2000
Soft anvil
G130 G140
Hard anvil
Hard anvil
x ðgÞ
r (g)
x ðgÞ
r (g)
12,072 7346
2203 1572
2508 2681
417 364
0 20
25
30
35
40
45
50
55
60
Thickness (µm) Fig. 3. Crush load as a function of SiC thickness for constant process conditions. Markers represent the median value for each test run error bars represent 3r values.
2
G130: HF3 G140: HF16
12000
1 0 -1 -2 -3 3000
G140 Soft Anvil G140 Hard Anvil
10000
Crush Load (g)
Z (Standard Deviation)
3
4000
5000
6000
7000
8000
Crush Load (g) Fig. 2. Normal probability plot of soft anvil crush strength of coated G130 and G140 particles. Test run HF3 and HF16 were coated using similar process conditions and had similar silicon carbide thickness. HF3 : x ¼ 6470 g, r = 503 g; HF16 : x ¼ 5487 g and r = 651 g.
thickness, but different starting material, are compared. The difference in crush strength between G130 and G140 starting material is reflected in the differences in crush strength between test HF3 and HF16 (95% confidence interval: 983 g ± 231 g), the difference is however only 21% of the difference measured on uncoated particles. From Figs. 1 and 2 it is seen that for both HF3 and HF16 the crush strength of the particles is reduced by silicon carbide coating. This reduction in crush strength after SiC deposition was seen for 40 of 47 test runs using G140 starting material and all 25 of the runs using G130 starting material. It was found that there was considerable variation of the crush strength of individual particles within each test. Although the particle to particle standard deviation of the particles tested using soft anvils was higher than that of particles tested using hard anvils the coefficient of variation was lower when soft anvils were used to test coated particles. For starting material the type of anvil used did not have a large impact on the coefficient of variation. A summary of the coefficient of variation measured for the starting material and all test runs is presented in Table 5.
8000 6000 4000 2000 0 20
30
40
50
60
70
Thickness (µm) Fig. 4. Impact of silicon carbide thickness on crush strength of coated G140 particles for varying process conditions. Data scatter around the trend line is due to differing process conditions, coater set up and measurement variations. Each marker represents the average of approximately 50 particles from each test run.
For a number of the test runs a few (5 or less) individual particles were found to have crush strengths significantly lower (3.2– 7.2 standard deviations below the mean value) than expected for normally distributed data. As the majority (74%) of the test runs did not have any outliers, and were well described by a normal distribution, it was assumed that these particles were in some way defective and did not represent the true strength of the material. Two test runs had outliers with crush strengths significantly (3.2–9.0 standard deviations above mean) higher than the normally distributed data. All outliers were excluded from the data analysis. The impact of silicon carbide layer thickness on the crush strength of the particles is shown in Figs. 3 and 4. Test runs TT1–TT5 were specifically intended to investigate the influence of silicon carbide thickness on crush load. These test runs were conducted under similar process conditions with only deposition time being varied in order to obtain deposit thicknesses ranging
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10000
Hot Cold Hard
Crush Load (g)
8000 6000 4000 2000 0 1100
1200
1300
1400
1500
1600
Deposition Temperature (°C) Fig. 5. Crush strength of G140 particles as a function of deposition temperature. Markers represent median value of all test runs coated at each deposition temperature; error bars represent 3r values. Only particles coated using the cold inlet were tested using the hard anvils (lower data points) while both hot and cold inlet were tested using the soft anvils (upper data points).
from approximately 25 lm to 55 lm. In Fig. 3, the lower slope of the line fitted to the hard anvil results indicate that the hard anvil test may be a less sensitive test than the soft anvil test. This effect is also seen in Figs. 4 and 5. Result obtained using hard anvils are less sensitive to silicon carbide thickness and process conditions than those obtained using soft anvils. To facilitate analysis of the impact of deposition process conditions the crush strength of the particles was normalised to the crush strength of particles with a 35 lm layer of silicon carbide. Analysis of these results was based on batch average rather than individual measurements. Deposition temperature was found to have virtually no effect on the crush strength for temperatures above 1310 °C. Using soft anvils it was found that the average crush strength of runs coated using the hot inlet and cold gas inlet was 5788 g and 6325 g, respectively. For test runs coated at 1310 °C and higher Analysis of Variance (ANOVA) testing found no influence of deposition temperature for both the hot (P = 0.85) and cold inlet (P = 0.93). As can be seen from Fig. 5 the crush strength of particles coated using the cold gas inlet was higher than the crush strength of particles coated using the hot gas inlet. Using the one sided t-test these differences were found to be statistically significant for deposition temperatures of 1490 °C (P = 0.005) and 1400 °C (P = 0.006) but not for 1310 °C (P = 0.29). For deposition temperatures below 1310 °C the crush strength increased to approximately 7620 g for particle coated using either the hot or the cold inlet. From Fig. 5 it is also apparent that when hard anvils were used deposition temperature appeared to have no influence on crush strength, even at the lowest deposition temperatures. This again highlights the higher sensitivity of the crush test when using soft anvils. MTS concentration and hydrogen flow rate were both found to have no influence on the crush strength of the particles.
5. Discussion As shown in Fig. 2, it is clear that the buffer and IPyC layers have an influence on the crush strength of the coated particles. This may result from the properties of the pyrocarbon/silicon carbide interface, the pyrocarbon density or thickness of the pyrocarbon layers. Lower density IPyC will have a lower strength and, assuming larger pores, will have a rougher surface. A rough outer surface will result in better interlocking between the pyrocarbon and silicon carbide and so improving strength. However, depending on pore size distribution, low density pyrocarbon may result in a rough inner surface of the silicon carbide resulting in higher stress concentrations.
Inner surface stress concentration has been implicated in reduced crush strength of particles [8]. Finite element analysis has shown that when hard anvils are used the maximum stress is on the inner surface of the silicon carbide while when soft anvils are used the stress is more uniformly distributed through the thickness of the silicon carbide [9]. In either case, inner surface roughness may reduce the fracture load. It is clear that soft anvils result in a higher crush load than hard anvils. For coated G140 particles, the average crush load was 1506 g for hard anvils and 6380 g for soft anvils. As was shown by van Rooyen et al. [9] use of soft anvils results in a lower maximum stress as well as a shift in the position of maximum stress. When hard anvils are used the volume of material effectively tested is very small and concentrated on the inner surface of the silicon carbide layer close to the point of contact between the anvil and particle. In the case of soft anvils a larger volume of material, evenly distributed through the thickness of the silicon carbide layer, is stressed. It may be argued that the larger volume of stressed material would result in failure at a lower load due to the increased probability of critical defect within the volume of material stressed. However, the maximum stress levels within the silicon carbide are much higher when hard anvils are used resulting in fracture at lower loads in comparison to soft anvils. It is interesting to note that when hard anvils are used, complete TRISO particles (i.e. including the outer pyrocarbon) fail at a higher load than particles with a silicon carbide outer layer [6]. This may be related to the outer pyrocarbon layer reducing the stress concentration at the particle/anvil interface or because of compressive residual stresses in the silicon carbide due to the outer pyrocarbon layer. Soft anvil testing appears to be more sensitive to changes in particle strength than testing with hard anvils. For example, in Fig. 5 when hard anvils were used little variation was seen as deposition temperature was varied. When soft anvils were used, there was a very clear increase in crush strength at low deposition temperatures. Similarly, as seen in Figs. 3 and 4, the crush strength showed a greater sensitivity to silicon carbide thickness when soft anvils were used. The lower coefficient of variation of the soft anvil tests also allow for improved confidence in any results. At temperatures of 1310 °C and higher it was found that the cooled inlet consistently resulted in higher crush strengths. Although the difference between the hot and cold inlet were relatively small (average difference of 467 g) they were found to be statistically significant. The cause of these differences is uncertain. It is unlikely that it is purely an effect of the temperature of the gas entering the particle bed. If this were the case it would have been expected that the crush load versus deposition temperature curves for the two gas inlets in Fig. 5 would have been horizontally offset. An alternative explanation is that in the case of the hot inlet the gas entering the particle bed has changed in composition due to deposition taking place in the hot areas of the inlet tube. This will result in a reduced MTS concentration, increased concentration of by products and possibly a change in silicon/carbon ratio. Deposition at low temperatures in the gas feed tube would have reduced the silicon/carbon ratio of the feed gas as a silicon rich deposit is formed at the relatively low temperatures in the gas feed tube. Deposition temperature has been identified as a key process variable, affecting a number of material properties as well as crush strength. Stinton and Lackey [11] found a quadratic relationship between crush strength and deposition temperature, with a maximum predicted crush strength at a deposition temperature of 1500 °C. This is consistent with the findings of Lackey et al. [3] who found that crush strength decreased when deposition temperature was increased from 1600 °C to 1700 °C. However, Kim et al. [8] found no clear relationship between crush strength and deposition temperature for deposition temperatures between 1400 °C
R.D. Cromarty et al. / Journal of Nuclear Materials 445 (2014) 30–36
Fig. 6. Effect of silicon carbide thickness on crush strength. Solid line: Expected crush load of silicon carbide shell without underlying pyrocarbon. Dashed line: estimated crush load of complete particle. Reduction in strength due to thin SiC indicated by ‘‘a’’, increase in strength due to pyrocarbon indicated by ‘‘b’’.
and 1600 °C. In this study, it was found that between 1310 °C and 1550 °C deposition temperature did not influence crush strength. For deposition temperatures of 1250 °C and 1200 °C the crush strength increased significantly above that for higher deposition temperatures. Currently there is no explanation for the increase in crush strength at low deposition temperatures. Changes in crush strength do not correlate well with changes of other material properties investigated in the larger study of silicon carbide properties. Silicon carbide properties investigated included density; micro hardness; nano hardness; fracture toughness; grain size; crystallite size and phase composition. Although silicon carbide properties do change with changing deposition temperature the changes are gradual or the temperatures of step changes do not correspond to the temperature where crush strength increases. One tenuous correlation is between the silicon content and crush strength. As part of the wider study of the impact of process conditions the silicon content of the deposits were measured using electron microprobe, X-ray diffraction and Raman spectroscopy. Microprobe analysis indicated that there is a rapid increase in silicon content of the deposit at deposition temperatures below 1310 °C. Raman spectroscopy supports these results but indicates that the increase in silicon content may be more gradual rather than the sudden increase measured by microprobe analysis. When coated particles are tested, the silicon carbide shell is highly stressed in comparison to the pyrocarbon. This is due to the high elastic modulus of silicon carbide in comparison to that of pyrocarbon. The direct contribution of pyrocarbon to the strength of the silicon carbide coated particle is therefore small. Considering a silicon carbide shell without a pyrocarbon coated kernel, it would be expected that the crush strength of the shell would decrease linearly to zero as the silicon carbide thickness is decreased to zero. When the pyrocarbon layers and kernel are also taken into account the crush strength would be expected to decrease to the strength of the uncoated particle (i.e. the pyrocarbon coated kernel). Results from this study however indicate that for particles coated with a thin layer of silicon carbide the crush strength is lower than the crush strength of the uncoated particles. This is shown in Fig. 6. A similar effect was reported by Lackey et al. [3] for hard anvils. A possible explanation for the decrease in crush strength when the silicon carbide layer is below a critical value may be that the silicon carbide layer fractures at an applied load below the crush load of the carbon coated kernel. Cracks in the silicon carbide then propagate through the carbon layers resulting in failure of the particle. Once the silicon carbide layer thickness exceeds a critical value, the load required to initiate cracking of the silicon carbide exceeds the crush strength of the underlying parti-
35
cles. In Fig. 6 the dashed line hypothetically represents the crush load of the silicon carbide coated particles while the solid line is the hypothetical crush strength of a silicon carbide shell without any underlying particle. The reduction in crush strength of the coated particles, indicated by ‘‘a’’, results from the silicon carbide layer fracturing at a load less than the crush load of the carbon coated particles. The increase in strength of the coated particles above that expected for a silicon carbide shell, indicated by ‘‘b’’, is due to the effect of the underlying carbon coated particle. In the absence of measurements for silicon carbide layers less than 25 lm the actual crush strength thickness relationship is not known. If the above-mentioned explanation for the decrease in crush strength is correct, it is likely that the crush strength would initially decrease very sharply as even a thin layer of silicon carbide would fail and initiate failure of the complete particle. Relating material properties to TRISO fuel performance is complicated by the variety of failure mechanisms. Crush testing only relates to the mechanical properties of the coated particles limiting its utility to the prediction of ‘‘pressure vessel’’ failures where stress due to internal pressure exceeds the fracture stress of the silicon carbide layer. Calculation of the fracture stress of the particles may be accomplished by means of finite element analysis. This approach was followed by van Rooyen et al. [10] to convert fracture load to fracture stress. Fracture stress data has been used in a number of models to predict the failure probability of TRISO coated particles. Both analytical models and finite element models have been developed for TRISO particles as well as particles with defects such as asphericity, pyrocarbon cracking, pyrocarbon debonding and silicon carbide thinning [23–27].
6. Conclusion The crush strength of TRISO particles tested using soft anvils is dependent on the properties of the pyrocarbon layers as well as silicon carbide thickness and properties layer. This makes it impossible to draw conclusions about any single layer from the crush test results of complete particles. As a result; crush testing of whole particles is a poor test for the study of individual layers within the TRISO coating. Hemispherical silicon carbide shells may be isolated by polishing away half the particle and burning off the pyrocarbon as used by Byun et al. [5] and Kim et al. [8]. This procedure however negates the crush test advantage of minimal sample preparation. As an alternative, a single batch of carbon coated particles can be used for a number of silicon carbide test runs. Due to limitations on the size of a single batch of pyrocarbon coated kernels this will only be practical for relatively small silicon carbide test runs. Crush testing using soft anvils always resulted in a significantly higher crush load than when hard anvils were used. Within-run standard deviation also increased when soft anvils were used. However the coefficient of variation obtained using the hard anvils was higher than that of the soft anvils. Detail design of the coater system was found to have an impact on the strength of the silicon carbide. The exact reason for this is not clearly understood. It is speculated that this may be due to a change in gas chemistry as deposition of silicon carbide occurs in the gas inlet when the hot inlet is used. This will result in a lower MTS concentration and by-products in the gas stream. Deposition temperature alone has little impact on crush strength over the range of deposition temperatures where this effect is observed. MTS concentration and hydrogen flow rate were found to have no influence on crush strength. Deposition temperature was found to have no effect on crush strength for temperatures above 1310 °C. Below 1310 °C, the crush strength of the particles showed a step change increase. MTS
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R.D. Cromarty et al. / Journal of Nuclear Materials 445 (2014) 30–36
concentration and total hydrogen flow rate were found to have no statistically significant influence on crush strength. As a test of the strength of the complete TRISO coating, without the need to measure the properties of the individual layers, crush testing using soft anvils may be a useful test method. As such, this test may be a useful quality test of production particles where conformity of the entire coating process is checked. However due to its inability to determine the strength of the individual layers it is a poor test for investigating individual layers within the complete deposition process due to an inability to isolate the effect of individual layers. Use of soft anvils appears to provide a more sensitive test than the use of hard anvils. A disadvantage of the soft anvils is the need to position each particle to avoid the indents formed in the anvils. Also soft anvils introduce another variable, anvil hardness, into the measurement system. It appears that if the anvils are sufficiently soft anvil hardness will not have an influence on the result. Due to the variability of crush test results and the lack of sensitivity of the crush test to variations in process parameters this test will only be of utility for detecting gross coating errors. These errors may be better detected using other existing tests rather than implement additional new tests.
Acknowledgement The authors wish to express their gratitude to Pebble Bed Modular Reactor (Propriety) Limited for the supply of carbon coated particles used in this study.
References [1] K. Minato, T. Ogawa, K. Fukuda, M. Shimizu, J. Nucl. Mater. 208 (1994) 266. [2] E.H. Voice, The Formation and Structure of Silicon Carbide Pyrolytically Deposited in a Fluidised Bed of Microspheres, Winfrith, 1970. [3] W.J. Lackey, D.P. Stinton, L.E. Davis, R.L. Beatty, Nucl. Technol. 31 (1976) 191. [4] A. Briggs, R. Davidge, C. Padgett, S. Quickenden, J. Nucl. Mater. 61 (1976) 233. [5] T.-S. Byun, J. Hunn, J. Miller, L. Snead, J.W. Kim, Int. J. Appl. Ceram. Technol. 7 (2009) 327. [6] T. Ogawa, K. Ikawa, J. Nucl. Mater. 98 (1981) 18. [7] K. Minato, K. Fukuda, K. Ikawa, H. Matsushima, S. Kurobane, J. Nucl. Mater. 119 (1983) 326. [8] W.-J. Kim, J.N. Park, M.S. Cho, J.Y. Park, J. Nucl. Mater. 392 (2009) 213. [9] G.T. van Rooyen, R.D. Preez, J.D. Villiers, R. Cromarty, J. Nucl. Mater. 403 (2010) 126. [10] I. van Rooyen, J.H. Neethling, P.M. van Rooyen, J. Nucl. Mater. 402 (2010) 136. [11] D.P. Stinton, W.J. Lackey, Am. Ceram. Soc. Bull. 57 (1978) 568. [12] J. Hancke, G. van Rooyen, J.P. de Villiers, Nucl. Technol. 182 (2013) 49. [13] A. Evans, C. Padgett, R. Davidge, J. Am. Ceram. Soc. 56 (1973) 36. [14] K. Bongartz, E. Gyarmati, H. Nickel, H. Schuster, W. Winter, J. Nucl. Mater. 45 (1972) 261. [15] K. Bongartz, E. Gyarmati, H. Schuster, K. Tiiuber, K.F.A.J. Gmbh, I. Reaktonverkstoffe, F. Republic, J. Nucl. Mater. 62 (1976) 123. [16] S. Xu, J. Zhou, B. Yang, B. Zhang, J. Nucl. Mater. 224 (1995) 12. [17] K.E. Gilchrist, J.E. Brocklehurst, J. Nucl. Mater. 43 (1972) 347. [18] S.-G. Hong, T.-S. Byun, R. Lowden, L. Snead, Y. Katoh, J. Am. Ceram. Soc. 90 (2007) 184. [19] X. Zhao, R.M. Langford, J. Tan, P. Xiao, Scripta Mater. 59 (2008) 39. [20] H. Wang, R.N. Singh, J.S. Goela, J. Am. Ceram. Soc. 78 (1995) 2437. [21] D. Petti, J. Buongiorno, J. Maki, R. Hobbins, G. Miller, Nucl. Eng. Des. 222 (2003) 281. [22] C. Lu, R. Danzer, F. Fischer, Phys. Rev. E 65 (2002) 1. [23] G.K. Miller, R.G. Bennett, J. Nucl. Mater. 206 (1993) 35. [24] G.K. Miller, D.C. Wadsworth, J. Nucl. Mater. 211 (1994) 57. [25] G. Miller, J. Nucl. Mater. 295 (2001) 205. [26] G. Miller, D. Petti, J. Maki, J. Nucl. Mater. 334 (2004) 79. [27] G.K. Miller, D.A. Petti, J.T. Maki, D.L. Knudson, J. Nucl. Mater. 355 (2006) 150.
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Crush strength of silicon carbide coated TRISO particles: Influence of test method and process variables R.D. Cromarty ⇑, G.T. van Rooyen, J.P.R. de Villiers Department of Materials Science and Metallurgical Engineering, University of Pretoria, South Africa
h i g h l i g h t s Crush strength is strongly dependent on the anvil hardness. Soft anvil crush strength is more sensitive to particle properties than hard anvil crush strength. Crush strength of SiC coated particles is influenced by the inner pyrocarbon. Crush strength depends on SiC thickness, deposition temperature and coater design. MTS concentration and carrier gas flow rate did not influence crush strength.
a r t i c l e
i n f o
Article history: Received 22 May 2013 Accepted 19 October 2013 Available online 30 October 2013
a b s t r a c t The influence of deposition temperature, methyl trichlorosilane (MTS) concentration, hydrogen carriergas flow rate and gas inlet design on the strength of silicon carbide coated TRISO particles was investigated using whole particle crushing strength. Crush strength was measured using soft aluminium anvils. For comparison a selection of particles were also measured with hard anvils. The influence of silicon carbide thickness was determined to allow for normalisation of all crush strength measurements to a crush strength at an equivalent thickness of 35 lm. It was found that the strength of the underlying pyrocarbon coated particles had a significant influence on the crush strength of the silicon carbide coated particles. Deposition temperature and gas inlet design were the only process parameters that influenced the coated particle crush strength. No evidence was found for MTS concentration and hydrogen flow rate having any influence on particle crush strength. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction High Temperature Gas Cooled Reactors (HTGR) have been proposed as a viable option for next generation nuclear reactors to be used for electricity production as well as potential heat sources for industrial applications and hydrogen production. Previously operated HTGR research reactors (e.g. Peach Bottom, Dragon, AVR) as well as commercial reactors (e.g. Fort St. Vrain, THTR) made use of fuel particles coated with a single layer of pyrocarbon, Bistructural Isotropic (BISO) or Tristructural Isotropic (TRISO) coated particles. Currently operational HTGR research reactors in Japan (High Temperature Test Reactor – HTTR) and China (HTR10) make use of TRISO coated particles. TRISO coated fuel particle will also be used for the Chinese HTR-PM commercial prototype reactor. In addition to the programs in Japan and China investigation of the manufacturing and behaviour of TRISO particles is also ongoing in the USA, Russia and Republic of Korea. TRISO particles consist of a fuel kernel containing fissile elements; a buffer layer of low-density, porous, pyrocarbon; a dense pyrocarbon layer; a ⇑ Corresponding author. Tel.: +27 12 420 2955. E-mail address: [email protected] (R.D. Cromarty). 0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.10.041
silicon carbide layer and a final coating of dense pyrocarbon. The layers making up the TRISO coating are applied to the kernel by means of a spouted bed chemical vapour deposition (CVD) coater. Processing conditions are known to influence the properties of the coating layers. In this paper, the influence of process conditions on the crush strength of the silicon carbide coating will be investigated. Each of the layers within the TRISO coating performs specific tasks. Silicon carbide acts as a diffusion barrier preventing the escape of fission products and provides the mechanical strength required to resist internal gas pressure. During use, pressure builds up within the particle due to the release of gaseous fission products and, in the case of oxide fuels, the release of oxygen. At normal operating temperatures, oxygen released from the fuel will react with carbon to form CO and CO2. The magnitude of the internal pressure depends on many variables including fissile elements, chemical form of the fuel, operating temperature and level of burnup. In the case of UO2 fuels Minato et al. [1] calculated fission gas pressures in the order of 10 MPa while CO pressure varied between approximately 0.1 MPa and 100 MPa depending on burnup and operating temperature.
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In addition to the stresses resulting from internal gas pressure, coated particles may be damaged during handling and manufacture of fuel elements. Although the risk of damaging the coated particles during these operations may be minimised by careful design of particle handling and fuel manufacturing operations, the particles will need to exceed a minimum strength to prevent damage. Various methods have been used to measure the mechanical properties of CVD silicon carbide layers within TRISO coatings. Methods reported in the literature include: 1. Crush testing of whole particles or hemispherical shells using flat anvils [2–12]. 2. Crushing of hemispherical shell by means of a punch placed inside a hemispherical shell [13]. 3. Compression or tensile testing of rings polished out of coated particles [14–16]. 4. Internal pressurisation of hemispherical shells or tubes using gas pressure [17] or elastomeric inserts [18]. 5. Bend testing of micro-beams produced by ion beam milling [19]. Crush testing of whole particles using hard flat anvils has been the most commonly used. For the majority of crush tests hard anvils made of steel, alumina or silicon carbide were used. Recently the use of soft anvils has been reported [5,9,10,12]. Van Rooyen et al. [9] showed that for an anvil hardness above 270 HV crush strength was consistently low while for an anvil hardness below 38 HV particle crush strength was consistently high. Between 38 HV and 270 HV crush strength increased with decreasing anvil hardness. This change in measured strength was due to the fracture mechanism changing with changes in anvil hardness. In the case of a hard anvil, fracture was due to Hertzian cracking at the particle/anvil contact area. For a soft anvil, fracture was due to tensile fracture at a point away from the contact area. Typical values of published crush strength of TRISO particles are presented in Table 1. Direct comparison of previously reported results is complicated by the differences in the particles tested. It
has been shown that crush strength is dependent on kernel properties [3], properties of the pyrocarbon layers [4] and silicon carbide thickness [3]. Importantly the crush strength has been shown to depend on the properties of the silicon carbide [3], being sensitive to both deposition conditions and post deposition annealing [3,6,9]. Not all studies have found a link between process conditions and crush strength; Ogawa and Ikawa [6] found that whole particle crush strength did not depend on silicon carbide properties. Byun et al. [5] found a correlation between crush strength and roughness of the silicon carbide inner surface as well as the pore density of the silicon carbide. They did not find any correlation between silicon carbide grain structure and crush strength. Kim et al. [8] found that inner surface roughness effects overshadowed any effects of microstructure or porosity in the silicon carbide. Heat treatment of the particles has ambiguous effects. Researchers have reported a decrease in crush strength [3], an increase in crush strength [9] or behaviour that depends on annealing temperature [10]. Several studies have noted the large spread of crush strength results. Reported Weibull modulus values range between 2.2 [10] and 19 [4]. Particle to particle coefficient of variation (r= x) has been reported to range between 12% [8] and 18% [3]. 2. Materials and methods Particles were coated in a spouted bed coater. The process tube consisted of a graphite tube (50 mm ID, 70 mm OD, 339 mm long) with a 60° conical base. Power was supplied by a 10 kW induction power supply operated at 119 kHz. Temperature control was by means of a Type B thermocouple mounted outside the conical section of the process tube. The thermocouple was mounted in an alumina protection sheath inserted into a graphite ring that fitted over the conical base of the tube. Coating was carried out using hydrogen as a carrier gas and methyl trichlorosilane, CH3SiCl3, (MTS) as a precursor. MTS was supplied to the coater by means of a bubbler maintained at 0 °C by means of an ice bath. Hydrogen flow to the bubbler and for fluidization was controlled by mass flow controllers. The bubbler and
Table 1 Published crush strength of TRISO particles. Description 1
2
3
4
5
Hard anvil Converted resin kernel Buffer, IPyC, SiC outer layer Buffer, IPyC, SiC, pyrocarbon outer layer Hard anvil UO2 kernel SiC outer layer. CH4 derived pyrocarbon SiC outer layer. C3H6 derived pyrocarbon Pyrocarbon outer layer. CH4 derived pyrocarbon Pyrocarbon outer layer. C3H6 derived pyrocarbon Hard anvil SiC outer layer Kernel: 493 lm, Buffer: 43.3 lm, IPyC: 31.7 lm, SiC: 27.9 lm Kernel: 603 lm, Buffer: 58.9 lm, IPyC: 31.4 lm, SiC: 28.0 lm Pyrocarbon outer layer Soft anvil ZrO2 kernel Buffer: 112 lm, IPyC: 64 lm, SiC: 32 lm, OPyC: 48 lm Buffer: 113 lm, IPyC: 72 lm, SiC: 39 lm, OPyC: 48 lm Soft anvil UO2 kernel Buffer: 88 lm, 1.0 g cm3 IPyC: 36 lm, 1.92 g cm3 Buffer and IPyC as above. SiC: 36 lm, 3.20 g cm3
Crush strength (g) [3] x ¼ 1000 to 1400 x ¼ 1900 to 2500 (a) x ¼ 2000 x ¼ 2200 x ¼ 4200 x ¼ 3600
[4] m ¼ 19 m ¼ 19 m ¼ 10 m ¼ 9:6 [6]
x ¼ 740 x ¼ 1100 x ¼ 2200 [9] Median = 6100 Median = 5600 (b) x ¼ 5173; r ¼ 286 r ¼ 1510 x ¼ 6990; r ¼ 296 r ¼ 1257
). a: Mean ( x) and Weibull parameter (m) reported in [4]. b: Mean ( x), between batch standard deviation (r), and average within lot standard deviation (r
[12]
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R.D. Cromarty et al. / Journal of Nuclear Materials 445 (2014) 30–36
Table 2 Starting material properties.
Table 3 Test run summary.
Buffer pyrocarbon
G130 G140 a
Inner pyrocarbon
Thickness (lm)
Density (g cm3)
Thickness (lm)
Density (g cm3)
129 117
1.03 0.97
72 93
Not measureda 1.58
Based on known parameters the IPyC density was estimated to be 2.03 g cm3.
fluidization gas streams were mixed prior to being introduced into the process tube. Two designs of gas inlet were investigated; one using a watercooled feed tube while the other used an alumina feed tube without any cooling. These will be referred to as the cold and hot inlet respectively. In the case of the hot inlet, severe problems were experienced with blockage of the gas inlet due to deposition in the gas feed tube and inlet. These problems were reduced, but not fully solved, by changing the design of the inlet to minimise heat conduction to the gas feed tube. When viewed using an optical microscope deposits in the gas feed tube appeared to consist of two phases, possibly silicon and silicon carbide. Only silicon carbide was deposited during this test work. The starting material consisted of 500 lm zirconia particles coated with a buffer layer and inner pyrocarbon layer. Two lots of particles were used for the tests; details of the starting material are given in Table 2. The starting material was produced by South African Nuclear Energy Corporation (NECSA) as part of the commissioning and characterisation of a production scale coater. As the carbon coating was not performed under standard coating conditions the thickness and material properties of the buffer and inner pyrocarbon layer were not within normal TRISO specifications. Several batches of particles were available; two of these (G130 and G140) were used for test runs. The choice of starting material was based on the size uniformity of the carbon coated particles. Particles were heated to the deposition temperature in argon. Gas flow was then switched to hydrogen, the temperature stabilized and finally the MTS introduced. After silicon carbide deposition, the gas flow was switched back to argon while the furnace was cooled. No outer pyrocarbon was deposited. Four groups of test runs were carried out. A summary of these are presented in Table 3. Test groups DR, HF and CR were based on a Central Composite Design of experiment. Group TT was run under fixed process condition; only deposition time was varied to achieve a range of SiC thicknesses. In the case of groups HF and CR initial tests were conducted using G130 starting material. These tests were repeated using starting material from G140. This allowed for a direct comparison of the effects of starting material. Initially no attempt was made to control the thickness of group DR, all tests being run at a fixed time. This resulted in a wide spread of SiC thickness. For later tests in group DR a SiC thickness of 35 lm was targeted by varying the deposition time. For groups HF and CR deposition time was varied to achieve a silicon carbide thickness of 35 lm ± 5 lm. Test runs with a silicon carbide thickness outside of the 30 lm to 40 lm range were included in the crush tests. This allowed for a better understanding of the effect of SiC thickness on the crush strength of the particles. All test runs formed part of a larger study of the silicon carbide deposition process. 3. Experimental The equipment used in these tests was previously described by van Rooyen et al. [9]. In this work, the hard anvils consisted of alumina plates while the soft anvils were of annealed aluminium of approximately 20 HV.
Start material: Run #
Comment
G130: DR1–DR26 Hot gas inlet Determine deposition rate for various process conditions Variables: Deposition temperature, MTS concentration, Hydrogen flow rate, Mass of particles Deposit thickness not controlled G130: HF1–HF6 G140: HF7–HF25
Hot gas inlet Determine the impact of process conditions on SiC properties Variables: Deposition temperature, MTS concentration, Hydrogen flow rate Deposition time varied to obtain SiC thickness of 35 ± 5 lm
G130: CR1–CR3 G140: CR4–CR28
Cold gas inlet Determine the impact of process conditions on SiC properties Variables: Deposition temperature, MTS concentration, Hydrogen flow rate Deposition time varied to obtain SiC thickness of 35 ± 5 lm
G140: TT1–TT5
Cold gas inlet Determine effect of SiC thickness for fixed process conditions Deposition time varied to obtain SiC thickness of 25–55 lm
For each test run, at least 50 particles were crushed. When using the soft anvils each particle needed to be placed on a separate position on the anvil as the particles left an indent in the soft aluminium. In the case of the hard anvils, each particle was placed in the centre of the alumina plate. All test runs and the starting material (i.e. pyrocarbon coated zirconia particles) were tested using the soft anvils, only a selection of test runs were tested using the hard anvils. Equipment was calibrated each day before use. The test equipment was also characterised to test the variability of the crush test equipment in terms of equipment calibration, anvil used and position on the anvil. Long-term stability of the test procedure was also checked. No statistically significant differences were found between the aluminium plates, between measurement positions on the plates or between daily set up.
4. Results For the starting material and all the test runs it was found that the crush strength of individual particles was equally well described by the normal and Weibull distributions. Although Weibull statistics have often been used to describe failure probability of brittle materials, including silicon carbide in TRISO coated particles (for example [4,7–10,15,16,18,20,21]), the normal distribution may be a better option [22]. In this study it was accepted that the data was normally distributed. Normal probability plots of the crush strength of the carbon coated particles measured using hard and soft anvils is presented in Fig. 1. The crush strengths obtained using the hard anvils are significantly lower than those obtained using the soft anvils. A summary of these results is presented in Table 4. Of note is the clear difference (95% confidence interval: 4726 g ± 526 g) between G130 and G140 when soft anvils are used compared to the small difference (95% confidence interval: 173 g ± 151 g) when hard anvils are used. In Fig. 2 the crush strength of two test runs coated using similar process condition and having similar silicon carbide layer
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R.D. Cromarty et al. / Journal of Nuclear Materials 445 (2014) 30–36 Table 5 Coefficient of variation for starting particles and coated particles.
Z (Standard Deviation)
3 2
Soft anvil
1 G130 G140 G130 G140
0 -1
start start coated coated
G130 Soft Anvil G130 Hard Anvil G140 Soft Anvil G140 Hard Anvil
-2 -3 0
5000
10000
15000
Mean
Minimum
Maximum
Mean
Minimum
Maximum
0.183 0.193 0.090 0.092
– – 0.056 0.059
– – 0.183 0.119
0.166 0.136 – 0.212
– – – 0.133
– – – 0.327
12000
20000
Crush Load (g)
TT Soft Anvil TT Hard Anvil
Table 4 Starting material crush strength.
Crush Load (g)
10000
Fig. 1. Crush strength normal probability plot of starting material prior to silicon carbide coating.
8000 6000 4000 2000
Soft anvil
G130 G140
Hard anvil
Hard anvil
x ðgÞ
r (g)
x ðgÞ
r (g)
12,072 7346
2203 1572
2508 2681
417 364
0 20
25
30
35
40
45
50
55
60
Thickness (µm) Fig. 3. Crush load as a function of SiC thickness for constant process conditions. Markers represent the median value for each test run error bars represent 3r values.
2
G130: HF3 G140: HF16
12000
1 0 -1 -2 -3 3000
G140 Soft Anvil G140 Hard Anvil
10000
Crush Load (g)
Z (Standard Deviation)
3
4000
5000
6000
7000
8000
Crush Load (g) Fig. 2. Normal probability plot of soft anvil crush strength of coated G130 and G140 particles. Test run HF3 and HF16 were coated using similar process conditions and had similar silicon carbide thickness. HF3 : x ¼ 6470 g, r = 503 g; HF16 : x ¼ 5487 g and r = 651 g.
thickness, but different starting material, are compared. The difference in crush strength between G130 and G140 starting material is reflected in the differences in crush strength between test HF3 and HF16 (95% confidence interval: 983 g ± 231 g), the difference is however only 21% of the difference measured on uncoated particles. From Figs. 1 and 2 it is seen that for both HF3 and HF16 the crush strength of the particles is reduced by silicon carbide coating. This reduction in crush strength after SiC deposition was seen for 40 of 47 test runs using G140 starting material and all 25 of the runs using G130 starting material. It was found that there was considerable variation of the crush strength of individual particles within each test. Although the particle to particle standard deviation of the particles tested using soft anvils was higher than that of particles tested using hard anvils the coefficient of variation was lower when soft anvils were used to test coated particles. For starting material the type of anvil used did not have a large impact on the coefficient of variation. A summary of the coefficient of variation measured for the starting material and all test runs is presented in Table 5.
8000 6000 4000 2000 0 20
30
40
50
60
70
Thickness (µm) Fig. 4. Impact of silicon carbide thickness on crush strength of coated G140 particles for varying process conditions. Data scatter around the trend line is due to differing process conditions, coater set up and measurement variations. Each marker represents the average of approximately 50 particles from each test run.
For a number of the test runs a few (5 or less) individual particles were found to have crush strengths significantly lower (3.2– 7.2 standard deviations below the mean value) than expected for normally distributed data. As the majority (74%) of the test runs did not have any outliers, and were well described by a normal distribution, it was assumed that these particles were in some way defective and did not represent the true strength of the material. Two test runs had outliers with crush strengths significantly (3.2–9.0 standard deviations above mean) higher than the normally distributed data. All outliers were excluded from the data analysis. The impact of silicon carbide layer thickness on the crush strength of the particles is shown in Figs. 3 and 4. Test runs TT1–TT5 were specifically intended to investigate the influence of silicon carbide thickness on crush load. These test runs were conducted under similar process conditions with only deposition time being varied in order to obtain deposit thicknesses ranging
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R.D. Cromarty et al. / Journal of Nuclear Materials 445 (2014) 30–36
10000
Hot Cold Hard
Crush Load (g)
8000 6000 4000 2000 0 1100
1200
1300
1400
1500
1600
Deposition Temperature (°C) Fig. 5. Crush strength of G140 particles as a function of deposition temperature. Markers represent median value of all test runs coated at each deposition temperature; error bars represent 3r values. Only particles coated using the cold inlet were tested using the hard anvils (lower data points) while both hot and cold inlet were tested using the soft anvils (upper data points).
from approximately 25 lm to 55 lm. In Fig. 3, the lower slope of the line fitted to the hard anvil results indicate that the hard anvil test may be a less sensitive test than the soft anvil test. This effect is also seen in Figs. 4 and 5. Result obtained using hard anvils are less sensitive to silicon carbide thickness and process conditions than those obtained using soft anvils. To facilitate analysis of the impact of deposition process conditions the crush strength of the particles was normalised to the crush strength of particles with a 35 lm layer of silicon carbide. Analysis of these results was based on batch average rather than individual measurements. Deposition temperature was found to have virtually no effect on the crush strength for temperatures above 1310 °C. Using soft anvils it was found that the average crush strength of runs coated using the hot inlet and cold gas inlet was 5788 g and 6325 g, respectively. For test runs coated at 1310 °C and higher Analysis of Variance (ANOVA) testing found no influence of deposition temperature for both the hot (P = 0.85) and cold inlet (P = 0.93). As can be seen from Fig. 5 the crush strength of particles coated using the cold gas inlet was higher than the crush strength of particles coated using the hot gas inlet. Using the one sided t-test these differences were found to be statistically significant for deposition temperatures of 1490 °C (P = 0.005) and 1400 °C (P = 0.006) but not for 1310 °C (P = 0.29). For deposition temperatures below 1310 °C the crush strength increased to approximately 7620 g for particle coated using either the hot or the cold inlet. From Fig. 5 it is also apparent that when hard anvils were used deposition temperature appeared to have no influence on crush strength, even at the lowest deposition temperatures. This again highlights the higher sensitivity of the crush test when using soft anvils. MTS concentration and hydrogen flow rate were both found to have no influence on the crush strength of the particles.
5. Discussion As shown in Fig. 2, it is clear that the buffer and IPyC layers have an influence on the crush strength of the coated particles. This may result from the properties of the pyrocarbon/silicon carbide interface, the pyrocarbon density or thickness of the pyrocarbon layers. Lower density IPyC will have a lower strength and, assuming larger pores, will have a rougher surface. A rough outer surface will result in better interlocking between the pyrocarbon and silicon carbide and so improving strength. However, depending on pore size distribution, low density pyrocarbon may result in a rough inner surface of the silicon carbide resulting in higher stress concentrations.
Inner surface stress concentration has been implicated in reduced crush strength of particles [8]. Finite element analysis has shown that when hard anvils are used the maximum stress is on the inner surface of the silicon carbide while when soft anvils are used the stress is more uniformly distributed through the thickness of the silicon carbide [9]. In either case, inner surface roughness may reduce the fracture load. It is clear that soft anvils result in a higher crush load than hard anvils. For coated G140 particles, the average crush load was 1506 g for hard anvils and 6380 g for soft anvils. As was shown by van Rooyen et al. [9] use of soft anvils results in a lower maximum stress as well as a shift in the position of maximum stress. When hard anvils are used the volume of material effectively tested is very small and concentrated on the inner surface of the silicon carbide layer close to the point of contact between the anvil and particle. In the case of soft anvils a larger volume of material, evenly distributed through the thickness of the silicon carbide layer, is stressed. It may be argued that the larger volume of stressed material would result in failure at a lower load due to the increased probability of critical defect within the volume of material stressed. However, the maximum stress levels within the silicon carbide are much higher when hard anvils are used resulting in fracture at lower loads in comparison to soft anvils. It is interesting to note that when hard anvils are used, complete TRISO particles (i.e. including the outer pyrocarbon) fail at a higher load than particles with a silicon carbide outer layer [6]. This may be related to the outer pyrocarbon layer reducing the stress concentration at the particle/anvil interface or because of compressive residual stresses in the silicon carbide due to the outer pyrocarbon layer. Soft anvil testing appears to be more sensitive to changes in particle strength than testing with hard anvils. For example, in Fig. 5 when hard anvils were used little variation was seen as deposition temperature was varied. When soft anvils were used, there was a very clear increase in crush strength at low deposition temperatures. Similarly, as seen in Figs. 3 and 4, the crush strength showed a greater sensitivity to silicon carbide thickness when soft anvils were used. The lower coefficient of variation of the soft anvil tests also allow for improved confidence in any results. At temperatures of 1310 °C and higher it was found that the cooled inlet consistently resulted in higher crush strengths. Although the difference between the hot and cold inlet were relatively small (average difference of 467 g) they were found to be statistically significant. The cause of these differences is uncertain. It is unlikely that it is purely an effect of the temperature of the gas entering the particle bed. If this were the case it would have been expected that the crush load versus deposition temperature curves for the two gas inlets in Fig. 5 would have been horizontally offset. An alternative explanation is that in the case of the hot inlet the gas entering the particle bed has changed in composition due to deposition taking place in the hot areas of the inlet tube. This will result in a reduced MTS concentration, increased concentration of by products and possibly a change in silicon/carbon ratio. Deposition at low temperatures in the gas feed tube would have reduced the silicon/carbon ratio of the feed gas as a silicon rich deposit is formed at the relatively low temperatures in the gas feed tube. Deposition temperature has been identified as a key process variable, affecting a number of material properties as well as crush strength. Stinton and Lackey [11] found a quadratic relationship between crush strength and deposition temperature, with a maximum predicted crush strength at a deposition temperature of 1500 °C. This is consistent with the findings of Lackey et al. [3] who found that crush strength decreased when deposition temperature was increased from 1600 °C to 1700 °C. However, Kim et al. [8] found no clear relationship between crush strength and deposition temperature for deposition temperatures between 1400 °C
R.D. Cromarty et al. / Journal of Nuclear Materials 445 (2014) 30–36
Fig. 6. Effect of silicon carbide thickness on crush strength. Solid line: Expected crush load of silicon carbide shell without underlying pyrocarbon. Dashed line: estimated crush load of complete particle. Reduction in strength due to thin SiC indicated by ‘‘a’’, increase in strength due to pyrocarbon indicated by ‘‘b’’.
and 1600 °C. In this study, it was found that between 1310 °C and 1550 °C deposition temperature did not influence crush strength. For deposition temperatures of 1250 °C and 1200 °C the crush strength increased significantly above that for higher deposition temperatures. Currently there is no explanation for the increase in crush strength at low deposition temperatures. Changes in crush strength do not correlate well with changes of other material properties investigated in the larger study of silicon carbide properties. Silicon carbide properties investigated included density; micro hardness; nano hardness; fracture toughness; grain size; crystallite size and phase composition. Although silicon carbide properties do change with changing deposition temperature the changes are gradual or the temperatures of step changes do not correspond to the temperature where crush strength increases. One tenuous correlation is between the silicon content and crush strength. As part of the wider study of the impact of process conditions the silicon content of the deposits were measured using electron microprobe, X-ray diffraction and Raman spectroscopy. Microprobe analysis indicated that there is a rapid increase in silicon content of the deposit at deposition temperatures below 1310 °C. Raman spectroscopy supports these results but indicates that the increase in silicon content may be more gradual rather than the sudden increase measured by microprobe analysis. When coated particles are tested, the silicon carbide shell is highly stressed in comparison to the pyrocarbon. This is due to the high elastic modulus of silicon carbide in comparison to that of pyrocarbon. The direct contribution of pyrocarbon to the strength of the silicon carbide coated particle is therefore small. Considering a silicon carbide shell without a pyrocarbon coated kernel, it would be expected that the crush strength of the shell would decrease linearly to zero as the silicon carbide thickness is decreased to zero. When the pyrocarbon layers and kernel are also taken into account the crush strength would be expected to decrease to the strength of the uncoated particle (i.e. the pyrocarbon coated kernel). Results from this study however indicate that for particles coated with a thin layer of silicon carbide the crush strength is lower than the crush strength of the uncoated particles. This is shown in Fig. 6. A similar effect was reported by Lackey et al. [3] for hard anvils. A possible explanation for the decrease in crush strength when the silicon carbide layer is below a critical value may be that the silicon carbide layer fractures at an applied load below the crush load of the carbon coated kernel. Cracks in the silicon carbide then propagate through the carbon layers resulting in failure of the particle. Once the silicon carbide layer thickness exceeds a critical value, the load required to initiate cracking of the silicon carbide exceeds the crush strength of the underlying parti-
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cles. In Fig. 6 the dashed line hypothetically represents the crush load of the silicon carbide coated particles while the solid line is the hypothetical crush strength of a silicon carbide shell without any underlying particle. The reduction in crush strength of the coated particles, indicated by ‘‘a’’, results from the silicon carbide layer fracturing at a load less than the crush load of the carbon coated particles. The increase in strength of the coated particles above that expected for a silicon carbide shell, indicated by ‘‘b’’, is due to the effect of the underlying carbon coated particle. In the absence of measurements for silicon carbide layers less than 25 lm the actual crush strength thickness relationship is not known. If the above-mentioned explanation for the decrease in crush strength is correct, it is likely that the crush strength would initially decrease very sharply as even a thin layer of silicon carbide would fail and initiate failure of the complete particle. Relating material properties to TRISO fuel performance is complicated by the variety of failure mechanisms. Crush testing only relates to the mechanical properties of the coated particles limiting its utility to the prediction of ‘‘pressure vessel’’ failures where stress due to internal pressure exceeds the fracture stress of the silicon carbide layer. Calculation of the fracture stress of the particles may be accomplished by means of finite element analysis. This approach was followed by van Rooyen et al. [10] to convert fracture load to fracture stress. Fracture stress data has been used in a number of models to predict the failure probability of TRISO coated particles. Both analytical models and finite element models have been developed for TRISO particles as well as particles with defects such as asphericity, pyrocarbon cracking, pyrocarbon debonding and silicon carbide thinning [23–27].
6. Conclusion The crush strength of TRISO particles tested using soft anvils is dependent on the properties of the pyrocarbon layers as well as silicon carbide thickness and properties layer. This makes it impossible to draw conclusions about any single layer from the crush test results of complete particles. As a result; crush testing of whole particles is a poor test for the study of individual layers within the TRISO coating. Hemispherical silicon carbide shells may be isolated by polishing away half the particle and burning off the pyrocarbon as used by Byun et al. [5] and Kim et al. [8]. This procedure however negates the crush test advantage of minimal sample preparation. As an alternative, a single batch of carbon coated particles can be used for a number of silicon carbide test runs. Due to limitations on the size of a single batch of pyrocarbon coated kernels this will only be practical for relatively small silicon carbide test runs. Crush testing using soft anvils always resulted in a significantly higher crush load than when hard anvils were used. Within-run standard deviation also increased when soft anvils were used. However the coefficient of variation obtained using the hard anvils was higher than that of the soft anvils. Detail design of the coater system was found to have an impact on the strength of the silicon carbide. The exact reason for this is not clearly understood. It is speculated that this may be due to a change in gas chemistry as deposition of silicon carbide occurs in the gas inlet when the hot inlet is used. This will result in a lower MTS concentration and by-products in the gas stream. Deposition temperature alone has little impact on crush strength over the range of deposition temperatures where this effect is observed. MTS concentration and hydrogen flow rate were found to have no influence on crush strength. Deposition temperature was found to have no effect on crush strength for temperatures above 1310 °C. Below 1310 °C, the crush strength of the particles showed a step change increase. MTS
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concentration and total hydrogen flow rate were found to have no statistically significant influence on crush strength. As a test of the strength of the complete TRISO coating, without the need to measure the properties of the individual layers, crush testing using soft anvils may be a useful test method. As such, this test may be a useful quality test of production particles where conformity of the entire coating process is checked. However due to its inability to determine the strength of the individual layers it is a poor test for investigating individual layers within the complete deposition process due to an inability to isolate the effect of individual layers. Use of soft anvils appears to provide a more sensitive test than the use of hard anvils. A disadvantage of the soft anvils is the need to position each particle to avoid the indents formed in the anvils. Also soft anvils introduce another variable, anvil hardness, into the measurement system. It appears that if the anvils are sufficiently soft anvil hardness will not have an influence on the result. Due to the variability of crush test results and the lack of sensitivity of the crush test to variations in process parameters this test will only be of utility for detecting gross coating errors. These errors may be better detected using other existing tests rather than implement additional new tests.
Acknowledgement The authors wish to express their gratitude to Pebble Bed Modular Reactor (Propriety) Limited for the supply of carbon coated particles used in this study.
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