Journal of Nuclear Materials 445 (2014) 30–36

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Journal of Nuclear Materials

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Crush strength of silicon carbide coated TRISO particles: Influence of testmethod and process variables

0022-3115/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jnucmat.2013.10.041

⇑ Corresponding author. Tel.: +27 12 420 2955.E-mail address: robert.cromarty@up.ac.za (R.D. Cromarty).

R.D. Cromarty ⇑, G.T. van Rooyen, J.P.R. de VilliersDepartment 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 2013Accepted 19 October 2013Available online 30 October 2013

a b s t r a c t

The influence of deposition temperature, methyl trichlorosilane (MTS) concentration, hydrogen carrier-gas flow rate and gas inlet design on the strength of silicon carbide coated TRISO particles was investi-gated 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 car-bide thickness was determined to allow for normalisation of all crush strength measurements to a crushstrength at an equivalent thickness of 35 lm.

It was found that the strength of the underlying pyrocarbon coated particles had a significant influenceon the crush strength of the silicon carbide coated particles. Deposition temperature and gas inlet designwere the only process parameters that influenced the coated particle crush strength. No evidence wasfound for MTS concentration and hydrogen flow rate having any influence on particle crush strength.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction silicon carbide layer and a final coating of dense pyrocarbon. The

High Temperature Gas Cooled Reactors (HTGR) have been pro-posed as a viable option for next generation nuclear reactors tobe used for electricity production as well as potential heat sourcesfor industrial applications and hydrogen production. Previouslyoperated 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 inJapan (High Temperature Test Reactor – HTTR) and China (HTR-10) make use of TRISO coated particles. TRISO coated fuel particlewill also be used for the Chinese HTR-PM commercial prototypereactor. In addition to the programs in Japan and China investiga-tion of the manufacturing and behaviour of TRISO particles is alsoongoing in the USA, Russia and Republic of Korea. TRISO particlesconsist of a fuel kernel containing fissile elements; a buffer layerof low-density, porous, pyrocarbon; a dense pyrocarbon layer; a

layers making up the TRISO coating are applied to the kernel bymeans of a spouted bed chemical vapour deposition (CVD) coater.Processing conditions are known to influence the properties of thecoating layers.

In this paper, the influence of process conditions on the crushstrength of the silicon carbide coating will be investigated.

Each of the layers within the TRISO coating performs specifictasks. Silicon carbide acts as a diffusion barrier preventing the es-cape of fission products and provides the mechanical strength re-quired to resist internal gas pressure. During use, pressure buildsup within the particle due to the release of gaseous fission productsand, in the case of oxide fuels, the release of oxygen. At normaloperating temperatures, oxygen released from the fuel will reactwith carbon to form CO and CO2. The magnitude of the internalpressure depends on many variables including fissile elements,chemical form of the fuel, operating temperature and level of bur-nup. In the case of UO2 fuels Minato et al. [1] calculated fission gaspressures in the order of 10 MPa while CO pressure varied betweenapproximately 0.1 MPa and 100 MPa depending on burnup andoperating temperature.

R.D. Cromarty et al. / Journal of Nuclear Materials 445 (2014) 30–36 31

In addition to the stresses resulting from internal gas pressure,coated particles may be damaged during handling and manufactureof fuel elements. Although the risk of damaging the coated particlesduring these operations may be minimised by careful design of par-ticle handling and fuel manufacturing operations, the particles willneed to exceed a minimum strength to prevent damage.

Various methods have been used to measure the mechanicalproperties of CVD silicon carbide layers within TRISO coatings.Methods reported in the literature include:

1. Crush testing of whole particles or hemispherical shells usingflat anvils [2–12].

2. Crushing of hemispherical shell by means of a punch placedinside a hemispherical shell [13].

3. Compression or tensile testing of rings polished out of coatedparticles [14–16].

4. Internal pressurisation of hemispherical shells or tubes usinggas 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 beenthe most commonly used.

For the majority of crush tests hard anvils made of steel, alu-mina or silicon carbide were used. Recently the use of soft anvilshas been reported [5,9,10,12]. Van Rooyen et al. [9] showed thatfor an anvil hardness above 270 HV crush strength was consis-tently low while for an anvil hardness below 38 HV particle crushstrength was consistently high. Between 38 HV and 270 HV crushstrength increased with decreasing anvil hardness. This change inmeasured strength was due to the fracture mechanism changingwith changes in anvil hardness. In the case of a hard anvil, fracturewas due to Hertzian cracking at the particle/anvil contact area. Fora soft anvil, fracture was due to tensile fracture at a point awayfrom the contact area.

Typical values of published crush strength of TRISO particles arepresented in Table 1. Direct comparison of previously reported re-sults is complicated by the differences in the particles tested. It

Table 1Published crush strength of TRISO particles.

Description

1 Hard anvilConverted resin kernelBuffer, IPyC, SiC outer layerBuffer, IPyC, SiC, pyrocarbon outer layer

2 Hard anvilUO2 kernelSiC outer layer. CH4 derived pyrocarbonSiC outer layer. C3H6 derived pyrocarbonPyrocarbon outer layer. CH4 derived pyrocarbonPyrocarbon outer layer. C3H6 derived pyrocarbon

3 Hard anvilSiC outer layerKernel: 493 lm, Buffer: 43.3 lm, IPyC: 31.7 lm, SiC: 27.9Kernel: 603 lm, Buffer: 58.9 lm, IPyC: 31.4 lm, SiC: 28.0Pyrocarbon outer layer

4 Soft anvilZrO2 kernelBuffer: 112 lm, IPyC: 64 lm, SiC: 32 lm, OPyC: 48 lmBuffer: 113 lm, IPyC: 72 lm, SiC: 39 lm, OPyC: 48 lm

5 Soft anvilUO2 kernelBuffer: 88 lm, 1.0 g cm�3 IPyC: 36 lm, 1.92 g cm�3

Buffer and IPyC as above. SiC: 36 lm, 3.20 g cm�3

a: Mean (�x) and Weibull parameter (m) reported in [4]. b: Mean (�x), between batch stan

has been shown that crush strength is dependent on kernel proper-ties [3], properties of the pyrocarbon layers [4] and silicon carbidethickness [3]. Importantly the crush strength has been shown to de-pend on the properties of the silicon carbide [3], being sensitive toboth deposition conditions and post deposition annealing [3,6,9].Not all studies have found a link between process conditions andcrush strength; Ogawa and Ikawa [6] found that whole particlecrush strength did not depend on silicon carbide properties. Byunet al. [5] found a correlation between crush strength and roughnessof the silicon carbide inner surface as well as the pore density of thesilicon carbide. They did not find any correlation between siliconcarbide grain structure and crush strength. Kim et al. [8] found thatinner surface roughness effects overshadowed any effects of micro-structure or porosity in the silicon carbide.

Heat treatment of the particles has ambiguous effects.Researchers have reported a decrease in crush strength [3], an in-crease in crush strength [9] or behaviour that depends on anneal-ing temperature [10].

Several studies have noted the large spread of crush strength re-sults. Reported Weibull modulus values range between 2.2 [10]and 19 [4]. Particle to particle coefficient of variation (r=�x) hasbeen reported to range between 12% [8] and 18% [3].

2. Materials and methods

Particles were coated in a spouted bed coater. The process tubeconsisted 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 inductionpower supply operated at 119 kHz. Temperature control was bymeans of a Type B thermocouple mounted outside the conical sec-tion of the process tube. The thermocouple was mounted in an alu-mina protection sheath inserted into a graphite ring that fitted overthe conical base of the tube.

Coating was carried out using hydrogen as a carrier gas andmethyl trichlorosilane, CH3SiCl3, (MTS) as a precursor. MTS wassupplied to the coater by means of a bubbler maintained at 0 �Cby means of an ice bath. Hydrogen flow to the bubbler and for flu-idization was controlled by mass flow controllers. The bubbler and

Crush strength (g)

[3]

�x ¼ 1000 to 1400�x ¼ 1900 to 2500

(a) [4]

�x ¼ 2000 m ¼ 19�x ¼ 2200 m ¼ 19�x ¼ 4200 m ¼ 10�x ¼ 3600 m ¼ 9:6

[6]

lm �x ¼ 740lm �x ¼ 1100

�x ¼ 2200

[9]

Median = 6100Median = 5600(b) [12]

�x ¼ 5173;r ¼ 286�r ¼ 1510�x ¼ 6990;r ¼ 296�r ¼ 1257

dard deviation (r), and average within lot standard deviation (�r).

Table 2Starting material properties.

Buffer pyrocarbon Inner pyrocarbon

Thickness(lm)

Density(g cm�3)

Thickness(lm)

Density(g cm�3)

G130 129 1.03 72 Not measureda

G140 117 0.97 93 1.58

a Based on known parameters the IPyC density was estimated to be 2.03 g cm�3.

Table 3Test run summary.

Start material:Run #

Comment

G130: DR1–DR26 Hot gas inletDetermine deposition rate for various process conditionsVariables: Deposition temperature, MTS concentration,Hydrogen flow rate, Mass of particlesDeposit thickness not controlled

G130: HF1–HF6 Hot gas inletG140: HF7–HF25 Determine the impact of process conditions on SiC

propertiesVariables: Deposition temperature, MTS concentration,Hydrogen flow rateDeposition time varied to obtain SiC thickness of35 ± 5 lm

G130: CR1–CR3 Cold gas inletG140: CR4–CR28 Determine the impact of process conditions on SiC

propertiesVariables: Deposition temperature, MTS concentration,Hydrogen flow rateDeposition time varied to obtain SiC thickness of35 ± 5 lm

G140: TT1–TT5 Cold gas inletDetermine effect of SiC thickness for fixed processconditionsDeposition time varied to obtain SiC thickness of25–55 lm

32 R.D. Cromarty et al. / Journal of Nuclear Materials 445 (2014) 30–36

fluidization gas streams were mixed prior to being introduced intothe process tube.

Two designs of gas inlet were investigated; one using a water-cooled feed tube while the other used an alumina feed tube with-out any cooling. These will be referred to as the cold and hot inletrespectively. In the case of the hot inlet, severe problems wereexperienced with blockage of the gas inlet due to deposition inthe gas feed tube and inlet. These problems were reduced, butnot fully solved, by changing the design of the inlet to minimiseheat conduction to the gas feed tube. When viewed using an opti-cal microscope deposits in the gas feed tube appeared to consist oftwo phases, possibly silicon and silicon carbide.

Only silicon carbide was deposited during this test work. Thestarting material consisted of 500 lm zirconia particles coatedwith a buffer layer and inner pyrocarbon layer. Two lots of particleswere used for the tests; details of the starting material are given inTable 2. The starting material was produced by South African Nu-clear Energy Corporation (NECSA) as part of the commissioningand characterisation of a production scale coater. As the carboncoating was not performed under standard coating conditions thethickness and material properties of the buffer and inner pyrocar-bon layer were not within normal TRISO specifications. Severalbatches of particles were available; two of these (G130 andG140) were used for test runs. The choice of starting materialwas 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 stabi-lized and finally the MTS introduced. After silicon carbide deposi-tion, the gas flow was switched back to argon while the furnacewas cooled. No outer pyrocarbon was deposited.

Four groups of test runs were carried out. A summary of theseare presented in Table 3. Test groups DR, HF and CR were based ona Central Composite Design of experiment. Group TT was run un-der fixed process condition; only deposition time was varied toachieve a range of SiC thicknesses. In the case of groups HF andCR initial tests were conducted using G130 starting material. Thesetests were repeated using starting material from G140. This al-lowed for a direct comparison of the effects of starting material.Initially no attempt was made to control the thickness of groupDR, all tests being run at a fixed time. This resulted in a widespread of SiC thickness. For later tests in group DR a SiC thicknessof 35 lm was targeted by varying the deposition time. For groupsHF and CR deposition time was varied to achieve a silicon carbidethickness of 35 lm ± 5 lm. Test runs with a silicon carbide thick-ness outside of the 30 lm to 40 lm range were included in thecrush tests. This allowed for a better understanding of the effectof SiC thickness on the crush strength of the particles. All test runsformed part of a larger study of the silicon carbide depositionprocess.

3. Experimental

The equipment used in these tests was previously described byvan Rooyen et al. [9]. In this work, the hard anvils consisted of alu-mina plates while the soft anvils were of annealed aluminium ofapproximately 20 HV.

For each test run, at least 50 particles were crushed. When usingthe soft anvils each particle needed to be placed on a separate po-sition on the anvil as the particles left an indent in the soft alumin-ium. In the case of the hard anvils, each particle was placed in thecentre of the alumina plate. All test runs and the starting material(i.e. pyrocarbon coated zirconia particles) were tested using thesoft anvils, only a selection of test runs were tested using the hardanvils.

Equipment was calibrated each day before use. The test equip-ment was also characterised to test the variability of the crush testequipment in terms of equipment calibration, anvil used and posi-tion on the anvil. Long-term stability of the test procedure was alsochecked. No statistically significant differences were found be-tween the aluminium plates, between measurement positions onthe plates or between daily set up.

4. Results

For the starting material and all the test runs it was found thatthe crush strength of individual particles was equally well de-scribed by the normal and Weibull distributions. Although Weibullstatistics have often been used to describe failure probability ofbrittle materials, including silicon carbide in TRISO coated particles(for example [4,7–10,15,16,18,20,21]), the normal distribution maybe a better option [22]. In this study it was accepted that the datawas normally distributed.

Normal probability plots of the crush strength of the carboncoated particles measured using hard and soft anvils is presentedin Fig. 1. The crush strengths obtained using the hard anvils are sig-nificantly lower than those obtained using the soft anvils. A sum-mary of these results is presented in Table 4. Of note is the cleardifference (95% confidence interval: 4726 g ± 526 g) betweenG130 and G140 when soft anvils are used compared to the smalldifference (95% confidence interval: 173 g ± 151 g) when hard an-vils are used.

In Fig. 2 the crush strength of two test runs coated using similarprocess condition and having similar silicon carbide layer

Crush Load (g)0 5000 10000 15000 20000

Z (S

tand

ard

Dev

iatio

n)

-3

-2

-1

0

1

2

3

G130 Soft AnvilG130 Hard AnvilG140 Soft AnvilG140 Hard Anvil

Fig. 1. Crush strength normal probability plot of starting material prior to siliconcarbide coating.

Table 4Starting material crush strength.

Soft anvil Hard anvil

�x ðgÞ r (g) �x ðgÞ r (g)

G130 12,072 2203 2508 417G140 7346 1572 2681 364

Crush Load (g)3000 4000 5000 6000 7000 8000

Z (S

tand

ard

Dev

iatio

n)

-3

-2

-1

0

1

2

3G130: HF3G140: HF16

Fig. 2. Normal probability plot of soft anvil crush strength of coated G130 and G140particles. Test run HF3 and HF16 were coated using similar process conditions andhad similar silicon carbide thickness. HF3 : �x ¼ 6470 g, r = 503 g; HF16 : �x ¼ 5487 gand r = 651 g.

Table 5Coefficient of variation for starting particles and coated particles.

Soft anvil Hard anvil

Mean Minimum Maximum Mean Minimum Maximum

G130 start 0.183 – – 0.166 – –G140 start 0.193 – – 0.136 – –G130 coated 0.090 0.056 0.183 – – –G140 coated 0.092 0.059 0.119 0.212 0.133 0.327

Thickness (µm)20 25 30 35 40 45 50 55 60

Cru

sh L

oad

(g)

0

2000

4000

6000

8000

10000

12000TT Soft AnvilTT Hard Anvil

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 3rvalues.

Thickness (µm)20 30 40 50 60 70

Cru

sh L

oad

(g)

0

2000

4000

6000

8000

10000

12000G140 Soft AnvilG140 Hard Anvil

Fig. 4. Impact of silicon carbide thickness on crush strength of coated G140particles for varying process conditions. Data scatter around the trend line is due todiffering process conditions, coater set up and measurement variations. Eachmarker represents the average of approximately 50 particles from each test run.

R.D. Cromarty et al. / Journal of Nuclear Materials 445 (2014) 30–36 33

thickness, but different starting material, are compared. The differ-ence in crush strength between G130 and G140 starting material isreflected in the differences in crush strength between test HF3 andHF16 (95% confidence interval: 983 g ± 231 g), the difference ishowever only 21% of the difference measured on uncoated parti-cles. From Figs. 1 and 2 it is seen that for both HF3 and HF16 thecrush strength of the particles is reduced by silicon carbide coating.This reduction in crush strength after SiC deposition was seen for40 of 47 test runs using G140 starting material and all 25 of theruns using G130 starting material.

It was found that there was considerable variation of the crushstrength of individual particles within each test. Although the par-ticle to particle standard deviation of the particles tested using softanvils was higher than that of particles tested using hard anvils thecoefficient of variation was lower when soft anvils were used totest coated particles. For starting material the type of anvil useddid not have a large impact on the coefficient of variation. A sum-mary of the coefficient of variation measured for the starting mate-rial and all test runs is presented in Table 5.

For a number of the test runs a few (5 or less) individual parti-cles were found to have crush strengths significantly lower (3.2–7.2 standard deviations below the mean value) than expected fornormally distributed data. As the majority (74%) of the test runsdid not have any outliers, and were well described by a normal dis-tribution, it was assumed that these particles were in some waydefective 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 nor-mally distributed data. All outliers were excluded from the dataanalysis.

The impact of silicon carbide layer thickness on the crushstrength of the particles is shown in Figs. 3 and 4. Test runsTT1–TT5 were specifically intended to investigate the influenceof silicon carbide thickness on crush load. These test runs wereconducted under similar process conditions with only depositiontime being varied in order to obtain deposit thicknesses ranging

Deposition Temperature (°C)1100 1200 1300 1400 1500 1600

Cru

sh L

oad

(g)

0

2000

4000

6000

8000

10000HotColdHard

Fig. 5. Crush strength of G140 particles as a function of deposition temperature.Markers represent median value of all test runs coated at each depositiontemperature; error bars represent 3r values. Only particles coated using the coldinlet were tested using the hard anvils (lower data points) while both hot and coldinlet were tested using the soft anvils (upper data points).

34 R.D. Cromarty et al. / Journal of Nuclear Materials 445 (2014) 30–36

from approximately 25 lm to 55 lm. In Fig. 3, the lower slope ofthe line fitted to the hard anvil results indicate that the hard anviltest may be a less sensitive test than the soft anvil test. This effectis also seen in Figs. 4 and 5. Result obtained using hard anvils areless sensitive to silicon carbide thickness and process conditionsthan those obtained using soft anvils.

To facilitate analysis of the impact of deposition process condi-tions the crush strength of the particles was normalised to thecrush strength of particles with a 35 lm layer of silicon carbide.Analysis of these results was based on batch average rather thanindividual measurements.

Deposition temperature was found to have virtually no effect onthe crush strength for temperatures above 1310 �C. Using soft an-vils it was found that the average crush strength of runs coatedusing the hot inlet and cold gas inlet was 5788 g and 6325 g,respectively. For test runs coated at 1310 �C and higher Analysisof Variance (ANOVA) testing found no influence of deposition tem-perature for both the hot (P = 0.85) and cold inlet (P = 0.93). As canbe seen from Fig. 5 the crush strength of particles coated using thecold gas inlet was higher than the crush strength of particlescoated using the hot gas inlet. Using the one sided t-test these dif-ferences were found to be statistically significant for depositiontemperatures of 1490 �C (P = 0.005) and 1400 �C (P = 0.006) butnot for 1310 �C (P = 0.29). For deposition temperatures below1310 �C the crush strength increased to approximately 7620 g forparticle coated using either the hot or the cold inlet. From Fig. 5it is also apparent that when hard anvils were used depositiontemperature appeared to have no influence on crush strength, evenat the lowest deposition temperatures. This again highlights thehigher sensitivity of the crush test when using soft anvils.

MTS concentration and hydrogen flow rate were both found tohave 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 havean influence on the crush strength of the coated particles. This mayresult from the properties of the pyrocarbon/silicon carbide inter-face, the pyrocarbon density or thickness of the pyrocarbon layers.Lower density IPyC will have a lower strength and, assuming largerpores, will have a rougher surface. A rough outer surface will resultin better interlocking between the pyrocarbon and silicon carbideand so improving strength. However, depending on pore size dis-tribution, low density pyrocarbon may result in a rough inner sur-face of the silicon carbide resulting in higher stress concentrations.

Inner surface stress concentration has been implicated in reducedcrush strength of particles [8]. Finite element analysis has shownthat when hard anvils are used the maximum stress is on the innersurface of the silicon carbide while when soft anvils are used thestress is more uniformly distributed through the thickness of thesilicon carbide [9]. In either case, inner surface roughness may re-duce the fracture load.

It is clear that soft anvils result in a higher crush load than hardanvils. For coated G140 particles, the average crush load was1506 g for hard anvils and 6380 g for soft anvils. As was shownby van Rooyen et al. [9] use of soft anvils results in a lower maxi-mum stress as well as a shift in the position of maximum stress.When hard anvils are used the volume of material effectivelytested is very small and concentrated on the inner surface of thesilicon carbide layer close to the point of contact between the anviland particle. In the case of soft anvils a larger volume of material,evenly distributed through the thickness of the silicon carbidelayer, is stressed. It may be argued that the larger volume ofstressed material would result in failure at a lower load due tothe increased probability of critical defect within the volume ofmaterial stressed. However, the maximum stress levels withinthe silicon carbide are much higher when hard anvils are usedresulting in fracture at lower loads in comparison to soft anvils.

It is interesting to note that when hard anvils are used, com-plete TRISO particles (i.e. including the outer pyrocarbon) fail at ahigher load than particles with a silicon carbide outer layer [6].This may be related to the outer pyrocarbon layer reducing thestress concentration at the particle/anvil interface or because ofcompressive residual stresses in the silicon carbide due to the out-er pyrocarbon layer.

Soft anvil testing appears to be more sensitive to changes inparticle strength than testing with hard anvils. For example, inFig. 5 when hard anvils were used little variation was seen as depo-sition temperature was varied. When soft anvils were used, therewas a very clear increase in crush strength at low deposition tem-peratures. Similarly, as seen in Figs. 3 and 4, the crush strengthshowed a greater sensitivity to silicon carbide thickness when softanvils were used. The lower coefficient of variation of the soft anviltests also allow for improved confidence in any results.

At temperatures of 1310 �C and higher it was found that thecooled inlet consistently resulted in higher crush strengths.Although the difference between the hot and cold inlet were rela-tively small (average difference of 467 g) they were found to bestatistically significant. The cause of these differences is uncertain.It is unlikely that it is purely an effect of the temperature of the gasentering the particle bed. If this were the case it would have beenexpected that the crush load versus deposition temperature curvesfor 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 gasentering the particle bed has changed in composition due to depo-sition taking place in the hot areas of the inlet tube. This will resultin a reduced MTS concentration, increased concentration of byproducts and possibly a change in silicon/carbon ratio. Depositionat low temperatures in the gas feed tube would have reduced thesilicon/carbon ratio of the feed gas as a silicon rich deposit isformed at the relatively low temperatures in the gas feed tube.

Deposition temperature has been identified as a key processvariable, affecting a number of material properties as well as crushstrength. Stinton and Lackey [11] found a quadratic relationshipbetween crush strength and deposition temperature, with a maxi-mum predicted crush strength at a deposition temperature of1500 �C. This is consistent with the findings of Lackey et al. [3]who found that crush strength decreased when deposition temper-ature was increased from 1600 �C to 1700 �C. However, Kim et al.[8] found no clear relationship between crush strength and deposi-tion temperature for deposition temperatures between 1400 �C

Fig. 6. Effect of silicon carbide thickness on crush strength. Solid line: Expectedcrush load of silicon carbide shell without underlying pyrocarbon. Dashed line:estimated crush load of complete particle. Reduction in strength due to thin SiCindicated by ‘‘a’’, increase in strength due to pyrocarbon indicated by ‘‘b’’.

R.D. Cromarty et al. / Journal of Nuclear Materials 445 (2014) 30–36 35

and 1600 �C. In this study, it was found that between 1310 �C and1550 �C deposition temperature did not influence crush strength.For deposition temperatures of 1250 �C and 1200 �C the crushstrength increased significantly above that for higher depositiontemperatures.

Currently there is no explanation for the increase in crushstrength at low deposition temperatures. Changes in crushstrength do not correlate well with changes of other material prop-erties investigated in the larger study of silicon carbide properties.Silicon carbide properties investigated included density; microhardness; nano hardness; fracture toughness; grain size; crystallitesize and phase composition. Although silicon carbide properties dochange with changing deposition temperature the changes aregradual or the temperatures of step changes do not correspondto the temperature where crush strength increases. One tenuouscorrelation is between the silicon content and crush strength. Aspart of the wider study of the impact of process conditions the sil-icon content of the deposits were measured using electron micro-probe, X-ray diffraction and Raman spectroscopy. Microprobeanalysis indicated that there is a rapid increase in silicon contentof the deposit at deposition temperatures below 1310 �C. Ramanspectroscopy supports these results but indicates that the increasein silicon content may be more gradual rather than the sudden in-crease measured by microprobe analysis.

When coated particles are tested, the silicon carbide shell ishighly stressed in comparison to the pyrocarbon. This is due tothe high elastic modulus of silicon carbide in comparison to thatof pyrocarbon. The direct contribution of pyrocarbon to thestrength of the silicon carbide coated particle is therefore small.

Considering a silicon carbide shell without a pyrocarbon coatedkernel, it would be expected that the crush strength of the shellwould decrease linearly to zero as the silicon carbide thickness isdecreased to zero. When the pyrocarbon layers and kernel are alsotaken into account the crush strength would be expected to de-crease to the strength of the uncoated particle (i.e. the pyrocarboncoated kernel). Results from this study however indicate that forparticles coated with a thin layer of silicon carbide the crushstrength 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 crushstrength when the silicon carbide layer is below a critical valuemay be that the silicon carbide layer fractures at an applied loadbelow the crush load of the carbon coated kernel. Cracks in the sil-icon carbide then propagate through the carbon layers resulting infailure of the particle. Once the silicon carbide layer thickness ex-ceeds a critical value, the load required to initiate cracking of thesilicon carbide exceeds the crush strength of the underlying parti-

cles. In Fig. 6 the dashed line hypothetically represents the crushload of the silicon carbide coated particles while the solid line isthe hypothetical crush strength of a silicon carbide shell withoutany underlying particle. The reduction in crush strength of thecoated particles, indicated by ‘‘a’’, results from the silicon carbidelayer fracturing at a load less than the crush load of the carboncoated particles. The increase in strength of the coated particlesabove that expected for a silicon carbide shell, indicated by ‘‘b’’,is due to the effect of the underlying carbon coated particle. Inthe absence of measurements for silicon carbide layers less than25 lm the actual crush strength thickness relationship is notknown. If the above-mentioned explanation for the decrease incrush strength is correct, it is likely that the crush strength wouldinitially decrease very sharply as even a thin layer of silicon carbidewould fail and initiate failure of the complete particle.

Relating material properties to TRISO fuel performance is com-plicated by the variety of failure mechanisms. Crush testing onlyrelates to the mechanical properties of the coated particles limitingits utility to the prediction of ‘‘pressure vessel’’ failures wherestress due to internal pressure exceeds the fracture stress of the sil-icon carbide layer. Calculation of the fracture stress of the particlesmay be accomplished by means of finite element analysis. This ap-proach was followed by van Rooyen et al. [10] to convert fractureload to fracture stress. Fracture stress data has been used in anumber of models to predict the failure probability of TRISO coatedparticles. Both analytical models and finite element models havebeen developed for TRISO particles as well as particles with defectssuch as asphericity, pyrocarbon cracking, pyrocarbon debondingand silicon carbide thinning [23–27].

6. Conclusion

The crush strength of TRISO particles tested using soft anvils isdependent on the properties of the pyrocarbon layers as well as sil-icon carbide thickness and properties layer. This makes it impossi-ble to draw conclusions about any single layer from the crush testresults of complete particles. As a result; crush testing of wholeparticles is a poor test for the study of individual layers withinthe TRISO coating. Hemispherical silicon carbide shells may be iso-lated by polishing away half the particle and burning off the pyro-carbon as used by Byun et al. [5] and Kim et al. [8]. This procedurehowever negates the crush test advantage of minimal sample prep-aration. As an alternative, a single batch of carbon coated particlescan be used for a number of silicon carbide test runs. Due to limi-tations on the size of a single batch of pyrocarbon coated kernelsthis will only be practical for relatively small silicon carbide testruns.

Crush testing using soft anvils always resulted in a significantlyhigher crush load than when hard anvils were used. Within-runstandard deviation also increased when soft anvils were used.However the coefficient of variation obtained using the hard anvilswas higher than that of the soft anvils.

Detail design of the coater system was found to have an impacton the strength of the silicon carbide. The exact reason for this isnot clearly understood. It is speculated that this may be due to achange in gas chemistry as deposition of silicon carbide occurs inthe gas inlet when the hot inlet is used. This will result in a lowerMTS concentration and by-products in the gas stream. Depositiontemperature alone has little impact on crush strength over therange of deposition temperatures where this effect is observed.MTS concentration and hydrogen flow rate were found to haveno influence on crush strength.

Deposition temperature was found to have no effect on crushstrength for temperatures above 1310 �C. Below 1310 �C, the crushstrength of the particles showed a step change increase. MTS

36 R.D. Cromarty et al. / Journal of Nuclear Materials 445 (2014) 30–36

concentration and total hydrogen flow rate were found to have nostatistically significant influence on crush strength.

As a test of the strength of the complete TRISO coating, withoutthe need to measure the properties of the individual layers, crushtesting using soft anvils may be a useful test method. As such, thistest may be a useful quality test of production particles where con-formity of the entire coating process is checked. However due to itsinability to determine the strength of the individual layers it is apoor test for investigating individual layers within the completedeposition process due to an inability to isolate the effect of indi-vidual layers.

Use of soft anvils appears to provide a more sensitive test thanthe use of hard anvils. A disadvantage of the soft anvils is the needto position each particle to avoid the indents formed in the anvils.Also soft anvils introduce another variable, anvil hardness, into themeasurement system. It appears that if the anvils are sufficientlysoft anvil hardness will not have an influence on the result.

Due to the variability of crush test results and the lack of sensi-tivity of the crush test to variations in process parameters this testwill only be of utility for detecting gross coating errors. These er-rors may be better detected using other existing tests rather thanimplement additional new tests.

Acknowledgement

The authors wish to express their gratitude to Pebble Bed Mod-ular Reactor (Propriety) Limited for the supply of carbon coatedparticles used in this study.

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