Out-of-reactor studies of fission product-silicon carbide interactions in HTGR fuel particles

Journal

OUT-OF-REACTOR STUDIES OF FISSION PRODUCT-SILICON IN HTGR FUEL PARTICLES *

of Nuclear

Materials 120 (1983) 6 30 North-Holland, Amsterdam

CARBIDE INTERACTIONS

R.J. LAUF

and T.B. LINDEMER Chemical Technoiqq Received

6 April

and R.L. PEARSON Division, Oak Ridge Natronul Loboratory.

1983; accepted

1 October

Ouk Ridge. Tenne.wee 37831). i.‘S.-f

1983

The interactions of several high-yield fission products with the SiC coating were studied in laboratory experiments by dopmg simulated fuel kernels with selected fission product elements before Triso coating. The resulting part&s were annealed in a thermal gradient and SK-fission product interactions were observed and quantified using metallography, radiography. scanning and transmission electron microscopy, and electron microprobe analysis. The results of these studies are discussed In terms of predicting Sic performance and fuel behavior during irradiation.

1. Introduction Triso-coats fuel particfes for the high-temperature gas-cooled reactor (HTGR **) contain a layer of pyrolytic Sic (fig. 1) which serves as the primary containment for the fission products. Corrosion or thinning of the SK could lead to fracture of the coating Iayers (“pressure-vessel failure”) or provide a localized path for the escape of fission products by diffusion from the fuel particle. These radionuclides might reach the primary cooling circuit, and ultimately result in maintenance and safety problems. Experience has demonstrated that the fission products must migrate from the kernel through the “buffer” and inner low-temperature isotropic (ILTI) pyrocarbon layers to the Sic before corrosion can occur. It has also been shown that SK corrosion rates in irradiated particles can be duplicated in laboratory experiments with nonradioactive, Trisocoated particles containing kernels made from oxides. * Research sponsored by the High Temperature Reactor Division. Office of Nuclear Power Systems, US Department of Energy under contract W-7405~eng-26 with the Union Carbide Corporation. ** See nomenclature listing at end of article.

~022-3~15/84/$03.~ 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

carbides, metals. and their mixtures. This eases the task of determining the SiC corrosion rates, which are one parameter used in the design of HTGR fuels. This paper presents the results of several such investigations given in greater detail in laboratory reports [l---4]. The Triso-coated kernels in these studies contained one or more of the following materials: UO,, UC,, SrO. ZrO,. La,O,, LaC,, Nd20,, Sm,O,, Pd. Ag, MO, Ru, Rh. C.

and CdO.

Interactions of fission products with the SK layer have been observed in over ten years of postirradiation examination (PIE) of UOz. UC:. and UO,-UC‘, fuel particles. Several years ago, some form of strontium was proposed to be interacting with Sic in Triso-coated UO, fuel (51. Concurrently, PIE of HEU UC2 fuel revealed that phases containing La, Ce, Pr, Nd, Sm, and Eu migrated down the temperature gradient through the inner pyrocarbon layers to the Sic layer and had, in some cases, interacted with the Sic [6,7J. Collectively. the rare earths are high-yield fission products, with a total yield of about 50%, i.e., one-fourth of all fission products [8]. Silver-IlOm has been detected in the

B.V.

R.J. L.auf et al. / Fission product -silicon carbide interactions

Fig. 1. Section of a Triso-coated HTGR fuel particle.

primary HTGR circuit [9]; it is now known that both silver and cesium diffuse through intact SIC [lO,ll]. Later, palladium was detected as a fission product involved with SIC corrosion in low-enriched uranium (LEU) fuel [6,12,13]. Several other investigators have reported the presence of one or more of the elements Xe, Cs, Sr, Ba, Y, Ce, Nd, Zr, Rh, and Ru within the Sic layer of irradiated Triso-coated particles [13-211, often at the very low concentrations more typical of diffusion through the Sic layer, rather than of gross interactions with Sic. Usually, the actual chemical state of combination of these fission products was not reported. A multitude of previous investigations have revealed two major factors that influenced the direction of the present work. One is the absolute amounts and relative proportions of 233U, 235U, 239U, and 241Pu that fission in the kernel. The second factor is the initial proportion, in the kernel, of actinide oxide to actinide carbide. This oxide/carbide content strongly affects the chemical

potential of oxygen in the fissioning kernel, and thus determines whether the fission products are present as metals, carbides, or oxides. Consider first the effect of the actinide isotopes. Triso-coated particles containing only ThC, or ThO, (fertile particles) incur a maximum of 7% FIMA (fissions per initial heavy-metal atom) from transmutation of thorium to the fissile 223U. As far as is known, Sic corrosion does not occur in these particles, possibly because the low burnup results in low concentrations of fission products. In the fissile kernels, it appears that the Pd, Ag, and Cd content is by far most affected by the initial 235U enrichment of the kernel. The high-enriched uranium (HEU) fuel (93% 235U) incurs - 72% FIMA (70.42% 235U, 1.15% 239Pu, and 0.36% 241Pu), while a medium-enriched uranium (MEU) fuel incurs - 25% FIMA (15.8% 235U, 6.63% 239Pu, and 2.48% 241Pu). In the latter fuel, over one-third of the fissions are from plutonium, for which the cumulative yields [8] of the long-lived isotopes of Pd, Ag, and Cd are about

R.J. LuuJ et al. / Fusion product-srbcon curhde mteractiom

x Table 1 Comparison

of relevant

fission product inventory (based on 100 initiai U atoms)

product

MEU fuel ‘) (atoms)

HEU fuel hJ (atoms)

MO Ku Rh Pd Ag Cd Rare earths

5.88 3.42 1.09 I .87 0.166 0.075 11.87

17.53 8.12 2.24 1.40 0.050 0.058 35.41

Fission

MEU/HEU

ratio

Case 1 ‘)

Case 2 d’

Case 3 ”

0.335 0.422 0.487 1.34 3.32 1.29 0.335

1.13 1.42 1.64 4.52 11.20 4.35 1.13

1.80 2.26 2.61 7.18 11.79 6.91 1.80

‘f i

- 25% FIMA (235U = 15.80%, 239Pu = 6.638, 24’Pu = 2.48%). h! - 72% FIMA (235U = 70.42%. 239Pu= 1X5%, 2*1Pu = 0.36%). ‘) MEU kernel diam. = 200 pm; HEW kernel diam. = 200 pm ‘) MEU kernel diam. = 300 pm; HEU kernel diam. = 200 pm. ” MEU kernel diam. = 350 pm: HEU kernel diam. = 200 pm.

ten times higher than for uranium. Table 1 lists the in-particle inventories for several typical cases and reveals that the relative amounts of Pd, Ag, and Cd are higher in MEU fuel than in HEU fuel, although the absolute concentrations are low in both fuels. In view of the more limited irradiation experience with MEU fuel, and the higher actual concentrations of Pd, Ag, and Cd in this fuel as compared with the HEU fuel kernel having the same total actinide content, experiments with these elements were included in this investigation. The effect of the initial actinide oxide/carbide ratio has also been studied in considerable detail. Measurements of CO pressure in irradiated UO, and ThO, particles and assumption of the C-CO-O, equilibrium permit the calculation of the chemical potential of oxygen (oxygen potential) in oxide fuel [22,23]. The oxygen potential is po*(kJ/mol)

= AG” = - RT I*( po,/&)

where R is the gas constant (J mall’ K-‘), T is the temperature (K), pO, is the oxygen partial pressure :MPa), and &, is the oxygen pressure in the standard state, 0.101 MPa (1.00 atm). The oxygen potential values are conveniently plotted versus temperature, with the plot often being called an Ellingham diagram or Richardson-Jeffes chart. The inparticle oxygen potential values in oxide fuel are shown as the shaded area in fig. 2, and represent actual CO pressures of approximately 0.1 to 20 MPa. These pressures result because the fission products do not combine with all the oxygen released by the fissioned actinide oxide, leaving the oxygen to combine with a miniscule portion of the pyrocarbon. In the UO,-UC, fuels, the

oxygen potential is a function of the initial oxide/carbide ratio, burnup, and temperature [13,24]. The oxygen potential can range from that of an oxide

0

-ioo -200 REGION

A

-300

-400 5 E 2-500 -is z -600

-700

-800

/ / /

-900 I

I 40-T

‘I

---3

I

-

I I

ALL

sro PRESSURES IN MPo

I I I I i

I

1 -.I

I

I

40-s

I 40-’

I 40-S

i

-(OOo

rota

4500

2500

2000 TEMPERATUTURE

(K)

Fig. 2. Ellingham diagram of the Sr-Zr-O-C system. Shaded area is the oxygen potential measured in irradiated UO, and ThO, fuel particles.

R.J. Lauf et al. / Fission product-silicon carbide interactions

fuel to that of the equilibria between rare-earth sesquioxides, rare-earth carbides, and carbon, as shown in fig. 3 for the La-C-O system. Specific oxide/carbide ratios were developed in these fuels to overcome two possibly fuel-design-limiting phenomena, i.e., the migration of 100% UO, kernels into the coatings and, in kernels containing large percentages of UC,, the interactions of rare-earth carbides with Sic [20,24-271. Both the irradiations and thermodynamic calculations indicate that rare earths do not diffuse to and react with the SIC as long as they are present in the kernel as rare-earth oxides entirely. In carbide fuels, the oxygen potential is below that of the rare-earth oxide-carbide equilibria. Previous investigations of HTGR fuels also give some indication of the chemical state of the fission products of interest here [13,14,20,24,28,29]. Cesium and, by inference, its homolog rubidium, are calculated [28] to be either in the elemental state or adsorbed on the pyrocarbon layer for all kernel compositions, a finding consistent with PIE observations. Barium, strontium, and zirconium apparently exist mainly as oxides dissolved in oxide fuel or as zirconates, but barium also migrates

9

into the pyrocarbon. Silver and cadmium undoubtedly exist as elements. Palladium is observed to diffuse from the kernel and accumulate at the Sic independent of kernel composition; it is apparently not a component of metallic or carbide phases. Fission-product MO, Ru, Rh, and Tc are present in one or more metallic or carbide phases that may also contain rare earths and uranium. 1.2. Chemical thermodynamics within the particle Extensive chemical thermodynamic information specific to the fuel and fission product system of the present study was calculated and is presented in figs. 2-4. The required thermodynamic data was obtained from tables A and D of Kubaschewski and Alcock [30]. The information in these figures is used primarily to ascertain which condensed-phase and gas-phase species may be present, and which gas-phase species may be TEMPERATURE (s@y

,600

1700

~~

‘500

(*c) 1400

1300

1200

0

-200

2

-6

I

-400 5

n-

,E 2

:

-6

0” a -600

-600

-16

- IO00 I500

2000 TEMPERATURE

2500 IK)

Fig. 3. Ellingham diagram for the b-C-0 system. Shaded area is the oxygen potential measured in irradiated UO* and ThO, fuel particles.

4.60

5.00

6.00

7.00

1QOOO/TIK)

Fig. 4. Calculated partial vapor pressures of monoatomic metal gases in HTGR fuel particles. The in-particle (Cd) pressure is d 0.3 MPa, see section 1.2.

most important in gas-phase transport of materials within the particles. Fig. 2 is an Ellingham diagram for the Sr-Zr-O-C’ system and serves as an example of the information that can be gained by this analysis. In region A of the strontium-containing system (ZrO,) *. (SrZrO,), and (C) are in equilibrium: the vapor pressures of (Sr) and (SrO) result from the equilibrium (SrZrO,) ti (Sr) + OS(0,) + (ZrO,) and (SrZrO,) @ (SrO) i- (ZrO,) respectively. In region B, (SrZrO,). (ZrO). and (C) are m equilibrium. and the vapor pressures can be calculated from the equilibria (SrZrOJ) + (C) s (Sr) + 1.5(0,) + (ZrC) and (SrZrO,) + (C) 2 (SrO) + 0,) + (ZrC). In region C, (SrC,). (ZrC), and (C) are present, and the vapor pressure of (Sr) is obtained from the equilibrium (SrC,) * (Sr) + 2(C). If (SrO) went into solution in (UO,) instead of forming (SrZrO,). as it apparently does above 1573 K, the pressures of (Sr) and (90) would be even less than those shown in fig. 2. at a given oxygen potential and temperature, because It5 chemical activity would be reduced. Examination of fig. 2 reveals that (Sr) is dominant over (SrO) as long as the oxygen potential < -- 2OU kJ/mol. Since the oxygen potential of the oxide fuels (the shaded area in figs. 2 and 3) and of the carbide fuels is i -200 kJ/mol. (9) is the principal strontium-containing gaseous species in all HTGR fuels. Comparison of fig. 2 with the corresponding Ba-Zr-O-C Ellingham diagram [Z] shows that the pressure of (Ba) is less than the (Sr) pressure at the same temperature and oxygen potential. The isobars for (La) and (Lao) for the La--O-C system are shown in fig. 3. In region A. (La,O,) is the condensed phase, and (LaC2) is the condensed phase in region B. The gas-phase equilibrium (Lao) ti (La) + 0.5(0,) was considered in calculating the oxygen potential at which (Lao) became dominant over (La), as shown near the bottom of fig. 3. (Since this equilibrium involves only gases. this boundary is independent of the condensed La-O-C species present.) It can be seen that (Lao) is the dominant gaseous species for most oxygen potential conditions likely to be present in HTGR fuels. Thus, in region A, consideration of the equilibrium (LazO,) it 2(LaO) + 0.5(0,) leads to the (Lao) isobars for that region. In region B. the equilibria (LaC,) + 0.5(02) $ (Lao) + 2(C) and (LaC2) * (La) + 2(C) were utilized to calculate the corresponding isobars for (Lao) and (La). The pressures shown in fig. 3 are the maximum expected. since the known solutions of La,O, in UO, and LaC, in UC, would lower their activity * ( ) denotes a gas and ( ) a solid.

below unity. The other rare-earth fission product, and yttrium would be expected to behave similarly. The calculated partial pressures of Cd. Ag. Pd. Rh. Ru. Ll, and MO are shown in fig. 4 for particles containing either UOz or UC2 kernels. The pressure of Cd. Ag. and Mo for both fuels was assumed to be that for the respective element. In actuality. the inventory of cadmium in a MEU particle is so low that it exists onlv as (Cd) at a pressure of about 0.3 MPa in the free volume within the particle, a pressure that precludes the presence of liquid cadmium. In oxide particles. the pressure of Pd. Rh, and Ru is approximated as that over the metal. In carbide particles the pressure is calculated from the typical equilibrium (UMe,) + 2(C) F? 3(Me) + (UC,). where Me represents Pd. Ku. and Rh. These U--Me compounds are very stable thermodynamically and are calculated to be the equilibrium form of Pd. Ru. and Rh in carbide fuels. The (U) pressure in carbide particles is that over (UC?) and (C>.

2. Experimental procedure

The Triso-coated particles used in this study contained either SrO--ZrOzpUO,. SrZrO,, La,O;. La,O,-LaC,, Nd,O,-UO,. Nd,O,-UO,~ L1C‘2~ Sm,O,. Pd-UO,, Ag-UO,. MO-Ru Kh~ Pd. Mo-Ru-Rh-Pd-UC,--C. Mo- Ru-Pdm UC, -C. Mo-Ru-Pd-La-UC:-C. Mo+RuPd-LIO1. Mo-Ru--Rh-UO,. Mo-Ru-Pd-UO,mUC’,. OI CdO-SrO-UO,. As noted in table 1. Mo. Ru. Rh, Pd. Ag. and Cd are of particular interest in low- and medium-enriched uranium fuels, so these systems were studied in some detail. -7.2. Kernel prepuration The oxides of the Sr-Zr-U mix (sample 3-1) were made by heating oxalates coprecipitated from nitrate solution. The concentrations of strontium (1.5 wt%) and zirconium (3.3 wt%) in the UOz were equal to the fission yields of strontium plus barium and of zirconium. respectively, in MEU fuel irradiated to 205%FIMA. The SrZrO, (sample 3-2) was purchased from a commercial source. These powders were hot-pressed, and the kernels obtained by crushing and sieving. Kernels of La,O, (sample 3-3) Sm,O, (sample 3-7). and Nd,O, - UO, (sample 3-5) were prepared by bading a weak-acid resin (WAR-Amberlite IRC-72) with the appropriate cations from an acid-deficient nitrate

R.J. Luuf et al. / Fission produci -silicon carbide interactions solution [31,32]. The carbonizing and partial conversion of the loaded resin to a carbide was carried out under controlled conditions [33]. Several kernels containing MO, Ru, Rh, and Pd were prepared to study the differences shown in table 1 for HEU and MEU fuels. Sample l-1 (table 2) was an attempt to duplicate just the alloyed metallic inclusions for a MEU UO, particle irradiated to 20% FIMA. Sample l-2 is the same fission-product mixture, but added to a UC, kernel. Uranium carbide forms compounds with these fission products, which may influence their interaction with SIC, as discussed in more detail in section 4. It was discovered that identification of rhodium and palladium individually was impossible because of their nearly identical X-ray characteristics in the electron microprobe (EMP). Thus, in samples 2-l and 2-2, rhodium and palladium were separated in order to positively identify the behavior of each. Lanthanum was added to sample 2-3 to study the interaction of a UC,-based kernel containing ruthenium, palladium, and lanthanum with the Sic layer. In samples 2-4 and 2-5, the metallic inclusion alloy was dispersed in UO, to more closely simulate an irradiated UO, particle than did sample l-l. Samples 2-6 and 2-7 were prepared to investigate the effect a UO,/UC, fuel mixture would have on the fission product interaction with Sic. Silver and cadmium additions were made to UO, kernels to study the differences shown in table 1 for HEU and MEU fuels. In sample l-3, one hundred times more silver was added than will be present in an MEU UO, particle irradiated to 20% FIMA. The purpose of the extra silver was twofold. First, one may need to compensate for silver loss because of its high vapor pressure (- low3 MPa) at coating temperatures. Second, the normal amount of silver in an MEU UO, fuel irradiated to full bumup is barely enough to detect with our X-ray equipment. In sample 2-8, the silver concentration more closely simulated the MEU UO, particle at full burnup. Finally, sample 2-9 was an attempt to include cadmium in a UO, fuel particle. All the kernels containing the MO, Rh, Ru, Pd, Ag, and Cd were made in the following way: Each compound or element was obtained as a powder and sieved through a 325-mesh screen. The powder mixture was blended intimately in a vibratory mill as an ethyl alcohol slurry, which was then dried in a vacuum. All of the mixtures that contained UO, were hot pressed into pellets. The UC, plus fission product samples were made into buttons in an arc-melting furnace in an argon atmosphere. The desired kernel sizes were obtained by crushing and sieving the buttons or pellets.

11

Effects of Sic deposition parameters on silver and palladium retention in UOz-containing particles were studied with particle batches 4-l and 4-2. Spherical kernels were used so that the SIC layer would be more typical of that deposited on fuel microspheres. Two batches of calcined uranium oxide microspheres produced by the chemical gelation process were obtained before sintering. One batch was sintered to about 60% of theoretical density by heating to 973 K in flowing hydrogen and then to 1473 K in argon. After cooling, the particles were infiltrated with aqueous silver nitrate solution, were rinsed, and were dried overnight at about 383 K in air. Some metallic silver was visible on the particle surfaces at this stage. To fully reduce the silver and remove any remaining nitrate ions, the particles were heated to 873 K in flowing hydrogen and held for 1 h. After coating with the pyrocarbon layers and preparing a metallographic section, a fine distribution of silver was observed optically within the kernel and confirmed by EMP analysis [l]. The palladium-containing batch was sintered to about 30% of theoretical density by heating to 1275 K in flowing argon. The particles were infiltrated with aqueous PdCl,, then rinsed and dried as before. The dried spheres were heated to 775 K in flowing hydrogen, were held for 1 h, and then were heated to 1675 K in argon for final sintering. (This sequence was possible because of the higher melting point of palladium.) Some of the particles developed a rough surface during sintering, but this did not affect the subsequent coating steps. Coating the silver- and cadmium-containing kernels introduced a special set of problems. These elements have very high vapor pressures (fig. 4) at 1473 to 1573 K, the optimum temperature range used to form the buffer and inner low-temperature isotropic pyrocarbon layers [34]. The silver-containing particles in the first set of samples were successfully coated by first depositing a thick, gas-tight pyrolytic carbon sealer layer around the kernel. This layer was first deposited at 1073 K, then at 1173 K to avoid a high silver vapor pressure during the deposition. The remaining layers were then formed at optimum conditions. Cadmium could not be retained during the coating process, however, and this aspect of the study was not continued. 2.3. Particle characterization 2.3. I. Kernel and pyrocarbon layers Samples from each of the batches listed in table 2 were mounted, polished, and analyzed by energy-dispersive X-ray (EDX) analysis in an electron microprobe or in a scanning electron microscope (SEM) to ensure that

R.J. L.auf’ em al. / Fission producr

I? Table 2 Triso-coated

particles

prepared

silicon carbrde rnwacrions

for evaluation

Sample

Batch

Kernel composition

1-l l-2 1-3

OR-2172 OR-2173 OR3-2715

52.1 MO. 28.5 Ru. 9.0 Rh. 10.4 Pd 12.5 MO. 6.0 Ru, 1.4 Rh. 0.9 Pd. 77.0 UC2. 1.9 C 3.3 Ag. 96.7 UOz

2-l 2-2 2-3 2-4 2-5 2-6 2-7 2-8 2-9 2-10

OR-2809 OR-2812 OR-2814 OR-2822 OR-2806 OR 2823 OR-2807 OR-2815 OR-2808 GA-VSM-6151-00-035

8.2 MO. 4.9 Ru. 3.0 Pd. 80.2 UC,. 1.8C 8.4 MO. 5.4 Ru, 1.8 Rh, 82.5 UC,. 1.9 C 7.7 MO. 6.5 Ru, 2.8 Pd. 5.5 La, 75.8 UC,. 1.7 C 8.1 MO. 6.8 Ru. 2.9 Pd, 82.2 UOz 8.3 MO. 5.3 Ru, 1.8 Rh, 84.6 UOz 5.4 MO. 4.6 Ru. 2.0 Pd. 79.5 UOz, 8.5 UC, 5.5 MO. 4.6 Ru, 2.0 Pd, 57.9 UO,. 30.0 UC, 0.4 Ag, 99.6 UO, 3.7 CdO. 6.8 SrO, 89.5 UO, 100 UC,

3-l 3-2 3-3 3-4

OR-2770 OR-2771 OR-2700 OR-2699

3-5

OR-2696

1.5 SrO. 3.3 ZrO. 95.2 UO, 100 SrZrO, 100 La,O, La ?O,. LaCL 5 Nd,O,. 95 UO,

3-6 3-7

OR-2691 OR-2698

Nd,O,. UO,. UC, 100 Sm,O,

4-l 4-2

OR-2830 OR-2831

1.26 Ag, 98.74 UO, 0.54 Pd. 99.46 UO,

all of the simulated fission products could be identified qualitatively. The detection limit of the elements by EDX analysis is approximately 0.1%. All of the fission products except those in samples 2-3. 2-8. and 2-9 were easily identified, as described more fully below. Those kernels containing UO, were coated with the iLTI layer in a gas mixture containing 10 ~01% CO, which was added to maintain O/U = 2.000 via the C-CO-O, equilibrium. The purpose of this treatment was to coat kernels having an oxygen chemical potential [35] approximately the same as that in irradiated oxide fuel particles. Attempts to determine the composition of the resinloaded oxide or oxide/carbide kernels by Debye-Scherrer X-ray patterns were unsuccessful. The patterns were too diffuse to analyze, probably because the crystallites were too small in either the “as made” or in the 2173 K heat-treated WAR particles. From the X-ray spectrum obtained with a scanning electron microscope, it was estimated that 95% of the Nd,O,-U02 mixture was UO,. Extended hot chlorine leach tests [36], performed prior to application of the SIC layer, revealed that all but four of the batches listed in table 2

(wt%)

had impermeable iLT1 layers. This conclusion was drawn because, except for these four, no change could be observed in radiographs taken before and after an 18 h chlorine treatment at 1773 K and because no chlorine was detected in the SEM X-ray spectra of the completed particles. However, 5% of the SrZrO, kernels (sample 3-2) were removed by the chlorine treatment, as were 10% of the Mo-Ru-Rh-Pd kernels (sample l-l). 4% of the Mo-Ru-Pd-UC,-C kernels (sample 2-l). and 10% of the CdO-SrO-UO, kernels (sample 2-9). No chlorine could be detected in SEM X-ray spectra of any of the completed Triso-coated particles, not even in the four batches whose iLTI layers were identified as porous. The latter four batches probably contained some chlorine, below EDX detection limits (- 0.1%). (The presence of chlorine in the fuel particle must be avoided. since the formation of volatile metal chlorides, including actinide chlorides, will greatly enhance the mobihty of the heavy metals within the particles. Because chlorine is present in the SIC deposition process, the iLTI layer must be impermeable to prevent chlorine contamination of the kernel [21,36,37].)

R.J. L.uuf et al. / Fission pro duct-silicon

2.3.2. Silicon carbide layer The condition of the as-deposited SIC was assessed by mounting several particles from each, batch, pohshing, and etching in a boiling ‘1 : 1 mixture of two solutions, saturated K,Fe(CN), and saturated NaOH. Typical microstructures are shown in fig. 5. The results were compared to the microstructures found in a previous SIC coating study [38], and while these Sic batches are not quite optimum, the only atypical SIC coating was in sample l-l, in which the Sic coating contained

carbide interactions

13

circumferential porosity. No elements other than silicon could be detected by EDX anaiysis within any of the SIC layers. (Carbon is not detectable with these units.) 2.4. Heat treatment About 20 particles from each batch were enclosed at the midplane of a 12.5 mm diam. carbon disk and heated in a graphite-resistance furnace where a known temperature and a temperature gradient of 278 K/cm

Fig. 5. Optical rnicrogmphs showing microstructural details of the as-made SIC layers: (a) Triso-coated Mo-Ru-Rh-Pd alloy (batch OR-2772); (b) Triso-doated UOr plus sib& (QR-2775); (c) Triso-coated UC, plus MO-Ru-Pd Particle (OR-2809); (dJ Triso-coated 90% UO,/lO% UC, #is Mo-Ru-Pd (OR-2823).

R.J.

14

Lmf et ul. / Fmtonproduct

-stkon

carbide interacttons

were maintained. Thus, difference across a 500 ple, a disk heated at contain particles with

there was a 14 K temperature pm diam. particle. As an exama maximum of 2175 K would temperatures of 182552175 K. The details of the procedure used to prepare particles for heat treatment have been published previously [26]. 2.5. Examination

techniques

The fission product interaction with Sic was observed by several methods. The extent of interaction at specific times during heat treatment was estimated by periodically turning off the furnace and radiographing the wafer. After the heat treatment was completed, the samples were mounted and polished to midplane. The condition of the polished SIC surface was observed with an optical microscope under bright-field illumination and sometimes with an SEM. The condition of the SIC layer for some distance (perhaps 20 pm) below the plane of polish could also be evaluated because the Sic was translucent and could be examined in a stereomicroscope under either polarized or oblique light. Both the identity and the movement of the heavy metals within the particles, and through the coating layers, were determined by using the X-ray capabilities of either the SEM or the EMP. A limited number of samples were prepared for transmission electron microscopy (TEM) by mechanically grinding and polishing a thin section from the midplane of the wafer. Thinning to electron transparency was done by ion milling using 3 kV argon ions. TEM examinations were carried out using JEM-100C and JEM-100CX microscopes in the transmission and scanning-transmission modes respectively [39].

1645K

- 3528

hr

Fig. 6. Photomicrograph of Triao-coated SrZrO, part& heat treated for 3528 h at 1645 K in a thermal gradient of 278 K/cm. Note attack of Sic at tip of crack through inner pyrocarbon layers [2].

interaction occurred at the ends at cracks leading through the inner pyrocarbon layers. Microprobe examination showed both strontium and zirconium in the kernels, but only strontium in the area of Sic attack. The SIC corrosion rate as a function of temperature (21 is shown in fig. 7. 3.1.2. Rare-earth oxides The Triso-coated La,O, and Sm,O, particles were heat treated for 232 h at 1850-2175 K in a gradient of 278 K/cm. A comparison of the radiographs of as-made and heat-treated La,O, particles (fig. 8) showed some movement of lanthanum into the buffer pyrocarbon layer during heat treatment. The Sm,O, particles showed similar redistribution, but optical microscopy revealed no attack of the Sic in either the La,O, or the Sm,O, particles.

3. Results 3.1. Triso-coated particles containing only fission product oxldes 3.1.1. SrZrO_, A carbon disk containing Triso-coated particles of SrZrO, was heat treated for 94 h at 1910-2200 K in a thermal gradient of 278 K/cm. No kernel-Sic interaction was evident from the radiograph. However, when the disk was polished to midplane, several particles showed localized areas where complete penetration of the Sic had occurred. Two more runs were made with SrZrO, particles, 260 h at 1725-2023 K, and 3528 h at 1473-1773 K. At the lower temperatures, the Sic layer showed only partial penetration (fig. 6). In all cases, the

3.2. Triso-coatedparticles alloy

containing only Mo-Ru-Rh-Pd

Twenty particles of sample l-l were embedded in three separate wafers. Each was heat treated in a temperature gradient of 278 K/cm - one at 1823-2173 K for 25 h, another at 1773-2023 K for 24 h, and the third at 1423-1773 K for 3528 h. The Mo-Ru-Rh-Pd alloy was extremely corrosive. AI1 of the Sic layers in the particles heated at 1823-2173 K and 1773-2023 K were breached, usually on the hot side of the particles, and many of the Sic layers in the particles heated at 1423-1773 K were also breached. An example of a particle that was heated at - 1490 K for 3582 h is shown in fig. 9. The material deposited along the cir-

R.J. Luau/et al. / Fission product-silicon carbide interactions TEMPERATURE 1600

1

111

I

LABORATORY

10-3

1200

)

I

DATA

IN-REACTOR X-

l-LaZ03-LoC2 A -uop-

DATA

UO2

(4%

ref.

5)

FIMA.

3.3. Triso-coated particles containing UO,

sro-zro*

-_

I

I

O-U02-MO-Ru-Rh

\

carbon layer. Most of the palladium, which has the highest vapor pressure, escaped. Except for molybdenum, all of these fission products are apparently quite mobile [3].

PC)

1400

95%

CONFIDENCE

GA-ORNL

10-a

LIMITS

IRRADIATED

3.3.1. uo, onry The coated particles that contained only UO, kernels showed no BC degradation after 2000 h of annealing in the thermal gradient furnace. Some kernels, particularly those at 1725 to 1775 K, exhibited slight shrinkage. This evidently resulted from further sintering during the anneal. Even in these cases the Sic and pyrocarbon layers were unaffected [l].

OF FUEL

DATA

\

\" : : L 5

15

10-5

In z s " Tl 10-G

10-7

4.5

I

I

I

I

I

5.0

5.5

6.0

6.5

7.0

-\

I\

7.5

6.0

lO.OOO/?‘(K)

Fig, 7. Sic corrosion rate versus reciprocal temperature for several Triso-coated particles.

cumferential lines in the SIC layer was identified by EDX analysis to be mainly ruthenium and rhodium. Some palladium and rhodium remained within the inner carbon layers, and some ruthenium and rhodium reached the outer low-temperature isotropic (oLT1) pyro-

3.3.2. .!JO, plus SrO and ZrO, A carbon disk containing Triso-coated kernels of SrO-ZrO,-UO, was heat-treated at 1910-2200 K in a temperature gradient of 278 K/cm. No Sr, Zr, or U movement from the kernel after 94 h heat treatment was detected in either the radiographs or by SEM EDX analysis, nor was interaction seen at the iLTI/SiC interface by optical metallography. A second disk containing Triso-coated SrO-ZrO,-UO, was heated under identical conditions for 364 h. Again no interaction was observed in the radiograph. The limit of detection of SIC interaction in a radiograph is 2 to 3 pm; therefore, the maximum possible rate of SIC corrosion in this sample would be 2.3 x 10W6 pm/s, and is plotted in fig. 7. 3.3.3. UO, plus rare-earth oxides Particles of Nd,Os-UO, (sample 3-5) were heated at 1850-2175 K and a temperature gradient of 278 K/cm for 581 h. Radiography indicated some movement of heavy metal from the kernel into the inner pyrocarbon layers on the cold side of the particles. EDX analysis showed that only uranium had migrated, and metallographic examination showed no damage to the Sic layer

PI.

Ok

13SlK-232hr

Fig. 8. Radiographs of Triso-coated La,O, particles before and after heat treatment for 232 h at 1867 Kjn a thermal gradient of 278 K/cm. Note lanthanum redistribution in the buffer layer 121.

3.3.4. UO, plus Mo-Ru-Rh Twenty particles of sample 2-5 were embedded in a graphite matrix and heat treated at 1823-2173 K for 215 h in a temperature gradient of 278 K/cm. Some Sic interaction occurred randomly in a few of the particles. An example of such a reaction can be seen in fig. 10 for a particle heated at 2008 K. EDX analysis identified only rhodium in the nodules within the SIC. The white phase within the iLTI layer and/or at the iLTI-SiC interface contained both ruthenium and rhodium [3].

Fig. 9. Triso-coated Mo-Ru-Rh-Pd alloy particle (batch OR-2772) polished to rnidplane after being heated at 1490 K for 3528 h in
Fig. 10. Triso-coated UO, plus Mo-Ru-Rh particle (batch OR-2806) polished to midplane after being heated at 2008 K for 215 h m a 278 K/cm temperature gradient. (a) Optical micrograph; (b) backscattered electron image of Sic layer: (c) Ru L, X-rays: (d) Rh L,j X-rays [3].

R. J. L.auj et al. / Fission product *silicon carbide interactions 3.3.5. (10, plus Pd Twelve batches of Triso-coated UO, containing palladium (sample 4-2) were produced in a parametric study of the effect of SIC deposition variables on noble metal retention [l]. These were heat treated for 2000 h at 1473-1773 K in the usual thermal gradient. Several observations were reported, based on optical metallography: (a) Localized attack was noted in all batches, particularly on the cold sides of the particles. The attack was characterized by partial penetration of the BC layer, generally associated with a buildup of palladium at the Sic-iLT1 interface. Free palladium and possibly an unidentified reaction product usually formed a nodule at the attack site, and the area was nearly always optically anisotropic. The overall appearance of the reaction zone, shown in fig. 11, is identical to that of

25 pm

Fig. 11. Palladium attack of SIC after thermal gradient anneal between 1473-1773 K for 2000 b. (a) Bright field; (b) polarized light [l].

17

similar attack seen in irradiated microspheres [12]. (b) Small bright specks that appear to be palladium occurred at some distance into the coating thickness in some specimens where no localized attack would be seen. It is not known whether these are accumulations following solid-state diffusion or protuberances on the surface of an attack nodule that is too far below or above the plane of polish to be visible. (c) Metallic spots (probably palladium) distributed along circumferential striations in the SIC were seen in coatings deposited at 1773 K. 3.3.6. U02 plus Mo-Ru-Pd Twenty particles of sample 2-4 were embedded in a graphite matrix and heat treated at 1823-2173 K.for 215 h in a temperature gradient of 278 K/cm. Interaction between the kernel and the SIC layer occurred in most of the particles. In an optical microscope under bright-field illumination, a white phase was seen in the polished SIC surface. The attack points appeared to be randomly distributed between the hot and cold sides of the particle. In a stereomicroscope using polarized light, these white phases were seen to extend into the SIC. They are more accurately described as irregularly shaped nodules within the translucent Sic layer. The white phases seen on the surface were only portions of the nodules that had been exposed by polishing. Palladium and ruthenium were identified by EDX analysis of the nodules. Palladium and ruthenium - but not uranium were found in the white phases lying within the iLTI and/or at the iLTI-SIC interface. 3.3.7. 110, plus Ag Twelve sets of Triso-coated UO,-Ag particles (sample 4-l) were annealed for 2000 h at 1473-1773 K in a temperature gradient of 278 K/cm. In general, the silver-doped particles exhibited damage to the SIC layer after annealing. The three modes of interaction seen in palladium-doped particles were also observed in silver-doped specimens. As a rule there was greater penetration by silver than by palladium. This probably results from the following causes: (a) Silver melts at 1233 K, and thus at 1573 to 1773 K one would expect considerable silver mobility as well as a high vapor pressure (fig. 4). On the other hand, palladium was not even at its melting point (1823 K) in the temperature range used in this experiment. (b) The silver-doped particles contained more free metal than did the palladium-doped particles. This made individual attack sites larger and more readily observed. It should be emphasized again that the silver concentrations used here were 100 times those expected

18

R.J. Lauf et al. / Fission product -silicon carbide interactions

in HTGR MEU fuel at full burnup. Localized attack of the SIC by silver alone has not been observed in irradiated fuel. 3.4. Triso-coated

particles containing UC,

3.4.1. UC, only Twenty particles of sample 2-10 were embedded in a graphite matrix and heat treated at 1823-2173 K for 215 h in a temperature gradient of 278 K/cm. NO interaction in any of the Sic layers could be detected I31. 3.4.2. UC, plus Mo-Ru-Rh-Fd Twenty particles of sample l-2 were embedded in three separate graphite wafers. Each was heat treated in a temperature gradient of 278 K/cm - one at 1823-2173 K for 50 h, another at 1673-2023 K for 260 h, and the third at 1423-1773 K for 3528 h. After mounting and polishing the wafers to midplane, they were examined with an optical microscope under bright-field illumination. All of the particles had undergone interaction all the way around the Sic layer. The appearance of the attack differed markedly from the oxide fuel interactions described in section 3.3. Optical micrographs of the Sic layer on the hot and cold sides of a particle heated at 1729 K for 3528 h are shown in figs. 12a and 12b. The depth of penetration appeared to be equal, but the size of the white phase within the Sic on the cold side was larger than that on the hot side. The difference in attack between the cold and hot sides of a particle was more pronounced in particles heated at higher temperatures. Under the microscope, it became clear at higher magnification that on the hot side of the particle, the second, white phase had extended entirely across the Sic layer. Complete penetration was observed on the hot side of a particle heated at 2173 K for 50 h. A thorough examination of the Sic layer in a particle heated at 1948 K for 260 h was made using a SEM. On the cold side of the particle, the white phase had penetrated 18 gm into the Sic layer, and the nodules were 2-3 pm in diameter. On the hot side of the particle, the second, white phase had penetrated all the way through the SIC layer, but the nodules were only 0.5 to 1.0 pm in diameter. All of the particles in the wafer heated at 1423-1773 K for 3528 h were examined with a stereomicroscope under polarized light. The largest area of the white phase shown in fig. 12a had extended below the surface and had formed nodules much like those described in section 33.6, except that there was a change in the color and translucency of the Sic in the vicinity of the

Fig. 12. Optical

micrographs of Triso-coated UC, plus Mo-Ru-Rh-Pd particles (batch OR-2773) polished to midplane after heat treatment at - 1729 K for 3528 h [3]: (a) SIC layer on cold side of particle; (b) Sic layer on hot side of particle.

nodule. Each nodule was surrounded by a black, cloudy area that was somewhat opaque. Palladium and uranium were detected in these areas by EMP X-ray analyses. As the size of the white phase became smaller, the nodules in the Sic became undetectable either by radiography or microscopy. However, the dark opaqueness still existed beyond this white phase, and the depth to which palladium and uranium had reached into the Sic layer could still be measured with the EMP. Sample 1-2 gave good SiC corrosion data, but we were unabie to identify precisely the interacting species because we could not distinguish between the rhodium and palladium X-ray profiles. When these two elements co-exist, their L, X-ray lines overlap. Transmission electron microscopy (391 showed very small (c 1 pm) nodules of a noble metal compound occurring along Sic gram boundaries (fig, 13). EDX analysis of the nodules in fig. 13 indicated the presence of Pd, Rh, U, and Si. This is consistent with EMP results [3], except that distinct Ka lines for palladium

R.J. Lmf et al. / Fission product -silicon carbide interactions 3.4.4.

19

UC, plus Mo-Ru-Pd

Twenty particles of sample 2-1 were embedded in three separate graphite wafers. Each was heat treated in a temperature gradient of 278 K/cm - one at 1823-2173 K for 25 h, another at 1823-2173 K for 90 h, and the third at 1523-1873 K for 50 h. Portions of the polished surfaces of all the particles in both wafers heated at 1823-2173 K contained a white phase that penetrated all the way through the Sic layers. The wafer heated at 1523-1873 K for 50 h contained some particles whose Sic layers had been penetrated partially and some whose SIC layers had been breached. In a particle heated at 1790 K for 50 h, both palladium and nranium were associated with the white phases in the Sic and at the iLTf-SiC interface. Ruthenium and uranium were found within the iLTI layer and molybdenum only within the kernel. The particle shown in fig. 14 was heat treated at 1873 K for 50 h. The Sic layer was breached, and the backscattered electron image (fig. ‘14a) indicated that a portion of the oLT1 was filled with heavy elements. The X-ray displays in figs. 14b, 14c, and 14d identify the heavy elements as ruthenium, palladium, and uranium.

Fig. 13. Transmission electron microscope photograph of noble metal nodules(N) in the Sic coating of a UC, -Mo-Ru-Rh-Pd particle annealed in the thermal gradient furnace for 260 h at 1870 K. Arrow indicates radial outward direction of the Sic coating.

and rhodium

were seen when EDX analysis was done in

the TEM. 3.4.3.

UC,

ph

MO-Ru-Rh

Graphite wafers filled with 20 particles of sample 2-2 were heat-treated in a gradient of 278 K/cm - one at 1823-2173 K for 25 h, one at 1823-2173 K for 90 h, and one at 1523-1873 K for 100 h. All of the particles heated in the 1823-2173 K test had portions of the SIC layers penetrated by a white phase. The particles in the wafer heated at 1523-1873 K for 100 h had some SIC layers that had been penetrated partially and some that had been breached. In the particles heated at 1523-1873 K for 100 h, ruthenium, rhodium, and uranium were associated with the white phases in the BC and also with the white phase within the iLT1. A particle heated at 1896 K for 100 h had a breached Sic layer, and ruthenium, rhodium, and uranium were found in portions of the iLTI [2].

3.4.5. UC, plus Mo-Ru-Pd-SC Work was done to explore the possibility that an intentional addition of Sic to the fuel kernel itself might immobilize the palladium in the kernel as a U-Pd-Si-C compound. A literature search for palladium-cont~ning compounds not previously considered in the system of fuel, fission products, and Sic revealed MPdr Si, and MRh,Si,, in which M represents yttrium and lanthanum through erbium of the lanthanide series [40]. Because of the chemical similarity of lanthanide and actinide compounds, UPd,Si, was suggested. Previously, the most thermodynamically stable known palladium-containing compound was UPd,, which was calculated to be the equilibrium form of palladium in carbide-containing HTGR fuels [3]. If UPd,Si, were more stable than UPd, in the U-Pd-Si-C system, one might expect the reaction 0.67 UPd,

+ 2 Sic + 0.33 UC, + UPd,Si,

-t 2.67 C.

To test this hypothesis, amounts of U, C, Pd, SIC, and UC, were arc melted and annealed at 1800-2200 K. If the phase set on the left in the above reaction were stable, it should have been present in the sample. It was not, nor was UPd,Si,. This conclusion was reached by the following reasoning. Quantitative metallography was used to measure the proportion of the two phases present. The overall elemental composition was known because there was insignificant weight loss of the in-

Fig. 14. X-ray displays of Triso-coated UC, plus Mo-Ru-Pd particle (batch OR-2809) polished to midplane after being heated at 1873 K for 50 h in a 278 K/cm temperature gradient. (a) Backscattered electron image of breached SIC: (b) Ru L,; (c) Pd L,: (d) U

M<,

gredients used to make the arcmelted annealed sample. Since the molar volume of each element is essentially conserved in any of the phases in which it is a component, it was possible to calculate that there was no combination of UPd,, BC, UC,, UPd,Si,, and carbon that could produce the proportion of the observed two phases. Since the sample did not contain significant free carbon, it appeared that at least two new U-Pd-Si-C compounds had formed. These are, therefore, more stable than either UPd, or UPd,Si,, offering the possibility of reducing the palladium chemical activity in the particle to less than that required for interaction with the SIC coating. The approximate melting point for each of several arc-melted U-Pd-Si-C samples was determined. One of the samples, with a U : Pd : Si : C molar ratio of 1 : 3 : 3 : 5, had a melting point > 2225 K. This is much higher than the melting point of palladium or any other known palladium compounds. Palladium stabilization or gettering may, therefore, be possible by intentionally adding SiC to UC, or UO,-UC, kernels; the UO,-UC, kernel is favored (see discussion below). Stabilization of palladium in the kernels of Trisocoated particles was investigated at 1670-1920 K. Silicon and carbon were added to the kernel compositions of samples 2-1 and 2-3 of the earlier study [3]. The new

kernel compositions and the kernel composition of sample l-2, which was used as a standard, are listed in table 3. Sample l-2 simulates particles with kernels having 93% *j5U enrichment and irradiated to 64% FIMA. Samples 5-l and 5-5 simulate particles with kernels having 20% 23sU enrichment and 25% FIMA (see ref. [12]). The amounts of molybdenum, ruthenium, and palladium added to the kernels of samples 5-l and 5-5 were increased by a factor of four over those present in-reactor, to assure detection by EDX analysis. Trisocoated particles of each sample were enclosed in graphite wafers and heated at 1573-1923 K for 165 h in a 278 K/cm temperature gradient. The general observations made in these experiments are summarized in table 4 and given in full detail below. The results of the heat treatment for sample 2-2. the standard, are seen in fig. 15a. The as-produced particle exhibited little or no heavy metal at the iLTI-SiC interface. After heat treatment (fig. 15a), there are heavy metals at the interface, and some have penetrated into the Sic layer. Uranium, palladium, and rhodium were identified in the iLTI layer. At the iLTI-SiC interface, the two main elements were uranium and silicon. The nodules within the SiC layer contained uranium, palladium, and silicon. The nodules probably contained

R.J. Luuf et al. / Fission product -silicon carbide interactions

21

Table 3 Kernel composition used for fission product immobilization studies Sample a’

UC,

MO

Rh

Pd

6.0 7.0

1.4

0.9 3.1

6.0

2.7

Ru

Kernel composition (wt 8) 2-2 17.0 12.5 5-l 71.9 8.3 5-5 71.7 1.2 Kernel composition (molar ratio) 5-l 75 24 5-5 75 24

19 19

LaC,

6.0

8 8

‘) The iLT1 coating of OR-2773 was chlorine leached contamination of the buffer layer did not occur during

12

in Triso-coated

Original

2-2 5-l 5-5

UC, + MO, Ru, Rh, Pd UC, + MO, Ru, Pd, Sic UC, + MO, Ru, Pd. LaC,,

kernel a)

by Robinson

12.9

3.1

50 97

1 2 so that

particles

Sample

‘) See table 3. b, Elements detected

1.9 2.2

1790 K for 50 h, some of the metals had penetrated the iLT1 layer and concentrated at several points along the iLTI-BC interface. At these points, the structure and appearance of the iLTI layer had changed; the light gray, dense pyrolytic carbon had changed to a darker gray area flecked with a white phase. No optical activity in these areas could be seen under polarized light. Immediately adjacent to these metal concentration points, a second phase which had a much whiter appearance, had begun to penetrate the Sic layer. Ruthenium, palladium, and uranium were identified in the white phase within the darker gray areas in the Sic-iLT1 interface, and palladium and uranium were detected in the white phase in the Sic layer. The particle shown in figs. 16a and 16b was heated at 1827 K for 50 h. The metals passed through the BC layer and began to enter the oLT1 layer. The appearance and structure of this area had changed; it appeared darker and less dense. Palladium, ruthenium, and uranium were found within the area. The interior surface of the SIC, at the location where the metals passed through the oLT1, showed signs of erosion. The integrity of the Sic layer of a particle that was heated at 1859 K for 50 h

3.4.6. UC, plus Mo-Ru-Pd-La A graphite wafer filled with 20 particles of sample 2-3 was included in each of the three heat treatments described in section 3.4.4. With some lanthanum added to the UC,-Mo-Ru-Pd kernel, the SIC interaction was more severe than the reaction with the UC,-Mo-Ru-Pd kernel, and the photomicrographs of the reaction zones seemed to show more contrast. In a particle treated at

of elements

1.5

51 102

Si Mo+Ru+Pd

C

at 1773 K for 18 h to insure that the iLT1 was impermeable SIC coating. Batches 5-l and S-5 were not chlorine leached.

carbon also, but the X-ray detector was unable to identify carbon. The elements remaining in the kernel after heat treatments were ruthenium, molybdenum, and uranium. The results of the heat treatment of sample 5-1, where Sic was added to the kernel, are seen in fig. 15b. Very little heavy element movement can be detected even after heat treatment. The ILTI-SIC interface of a heat-treated particle appeared very similar to that of the as-produced particle. Fig. 15b is a magnification of the kernel-buffer interface. Within the buffer layer, there were only a few faint areas containing uranium, ruthenium, and silicon. All of the palladium was associated with U, Si, Ru, and MO within the kernel.

Table 4 Location

Sic

After 165 h at 1670 < T < 1920 K b,

Sic

backscattered

Kernel

Buffer layer

iLT1 layer

SiC layer

U, MO, Ru U, MO, Ru, Pd. Si U, MO, Ru, Si

Not determined U, Ru, Si U, La

U, Pd, Rh None Pd. La

U, Pd, Si None Pd, La, Si

electron

detector

coupled

with an X-ray energy-dispersive

spectrometer.

7’

R.J. Lauf et ul. / Ft.won

product -srhcon curhrde rnteruc’ttom

Fig. 16. Trlso-coated UC7 partwle containing Mo-Ru-Pd -IA after 50 h at 1827 K in a gradient of 278 K/cm. Cold side is at the top of the photos. (a) Whole particle, showing buildup of metals toward hot side. (b) Detail showing discoloration oi oLTl layer.

had been destroyed. The metals appeared to have diffused along most of the SiCoLTI interface and then outward into the oLT1 layer itself. Palladium, ruthenium. and uranium were still within the oLT1. In a particle heated at 1934 K for 25 h, the heavy metals completely penetrated the oLTI, and parts of the Sic layer were consumed. Finally, in a particle that was heated at 2098 K for 90 h. most of the Triso coating had disintegrated. Covering half of the ends of the remaining SIC layer was the second (white) phase containing mainly uranium with a little silicon. The darker phase - the remaining SIC - contained mainly silicon; a little ruthenium. palladium, and uranium were identified in the white material accumulated in the destroyed oLT1. No lanthanum was found in any of the heat-treated particles in this series.

3.4.7. UC, plus Mo- Ru-Pd-La-SK‘ The purpose of this test was to determine if Sic could immobilize noble metals in the kernel in the presence of rare earths. The composition of these particles, sample 5-5. is given in table 3. The results of the heat treatment of sample 5-5 are summarized in table 4 and seen in fig. 17. Fig. 17a shows the iLTI-SiC interface of a heat-treated particle. Both palladium and lanthanum have permeated the iLT1 layer and accumulated at the iLTI-SiC interface.

Fig. 15. Triso-coated particles of UC, containing MO, Ku. Rh. and Pd. (a) Without Sic addition to the kernel, attack OCCURS at the Sic-iLT1 interface: (1) Pd; (2) U-Pd; (3) U-Pd-Rh; (4) U-Si; (5) U-Pd-Si. (b) With SIC in the kernel, no attack of the SiC coating occurs and redistribution of metal is confined (1) U-Si-Ru; (2) the buffer-iLT1 interface: to U-Si-Pd-Ru-Mo.

R. J. Lauf et al. / Fission product-silicon carbide interactions

Fig. 17, T&-coated interface;

particle of UC,-Mo-Ru-Pd-LaC,-SiC

(b) kernel-buffer

interface.

(1) Pd-La;

(2) Si-Pd-La;

nodules within the SIC layer contained palladium, lanthanum, and silicon. In fig. 17b, which is a magnification of the kernel-buffer layer interface, some uranium and lanthanum can be seen in the buffer layer. Uranium, ruthenium, molybdenum, and silicon remained in the kernel. Thus, the results seen in figs. 15 and 17 indicate that, under the right conditions, the addition of Sic to the kernel can reduce or eliminate the attack of SIC coatings by palladium. It appears from fig. 17, however, that the presence of lanthanide carbides is deleterious. This can be alleviated in-reactor by using UO,-UC, kernels having the required UO, content 113,241. Therm~hemical calculations indicate that the U-Pd-Si-C compounds would be stable in these kernels and would result from irradiation of an initial UO,-UC,-Sic kernel fabricated from a UO,-SiO,-C mixture instead of the usual UO,-C mixture.

(batch 5-5) heat-treated 165 h at 1886 K, 278 K/cm.

23

(a) Sic-iLTI

(3) U + La; (4) U + Si-Ru-Mo.

The

RAMGRAPH 21OOK - 41hr

3.5. Trim-coated particles containing a rare earth oxide/ carbide mixture A carbon disk was filled with Triso-coated LaC,-La,O, particles and heat-treated at NO-2175 K in a temperature gradient of 278 K/cm. Radiographs were taken after 41 h. A typical Sic-La interaction is shown in fig. 18. The penetration depth is approximately 12 pm. Both the radiograph (fig. 18a) and the SEM verified that there had been lanthanum movement

OmcALMIcRoGRApH 198oK - 41hr

Fig. 18. Triso-coated

La@,-LaC, particles after 41 h at 1980-2100 K in a gradient of 278 K/cm. (Cold side of each particle is at the top.) (a) Radiograph showing extensive migration of lanthanum; (b) optical micrograph showing corrosion of the SIC.

to the cold side of the particle. In comparing figs. 8b and 18a, it becomes evident that lanthanum has diffused to the Sic-iLTI interface in the La@-LaC, particles, where it can interact with the SIC layer. The Sic corrosion rate as a function of l/T for Triso-coated La@-LaC, particles is plotted in fig. 7: it is the same as that previously measured for Triso-coated LaC, particles 125,411. For comparison, in the Triso-coated particles containing rare earths as oxides (sections 3.1.2. and 3.3.3). no rare-earth reaction with the SIC was observed. -1.6. Tnso-coated particles containing UO, f UC, mixtures 3.6. I. 65 % UO, / 35 $ UC, plus Mo- Ru-Pd Twenty particles of sample 2-7 were embedded in a graphite matrix and heat treated at 1823-2173 K for 215 h in a temperature gradient of 278 K/cm. Some SIC interaction occurred in several of the particles. One such particle heated at 2025 K is shown in fig. 19. X-ray analysis identified both palladium and uranium, but not ruthenium, within the Sic layer. 3.6.2. 90%1 L/0,/10% UC,plus Mo-Ru-Pd Twenty particles of sample 2-6 were embedded in a graphite matrix and heat treated at 1823-2173 K for 215 h in a temperature gradient of 278 K/cm. Interaction with Sic occurred in several of the particles. Like the particles reported in section 3.6.1, palladium and uranium, but not ruthenium, were identified within the Sic layer.

Fig. 19. Triso-coated 65% UO,/35% UC, plus Mo-Ru-Pd particle (batch OR-2808) polished to midplane after being heated at 2025 K for 215 h in a 278 K/cm temperature gradient. Backscattered electron image of Sic layer; light area contains uranium and palladium.

4. Analysis and Discussion 4.1. Detection of fission-product

inteructrons with SIC’

Optical metallography and SEM techniques could be used to detect palladium and other noble-metal interactions, while radiography could not. The present investigation from 1673 to 2173 K revealed that radiography cannot be used to measure SIC thinning accurately in either UC, plus Mo-Ru-RhPd, UO, plus Mo-Ru-Pd. UO, plus MO-Ru-Rh, or UO,/UC, plus Mo-Ru--Pd particles. Unlike the rare-earth interaction with Sic. which occurred only on the cold side of the particles and resulted in a sharp interface, the Sic-Pd and Sic-Rh interactions occurred anywhere within the particle. The interactions were also deeper into the SIC on the hot side than on the cold side of the particle. Unfortunately. the interface on the hot side was diffuse and could not be distinguished clearly in a radiograph. At 2173 K. however. the penetration front on the hot side could be seen easily by the metallographic technique. Nodules containing either palladium. rhodium. or palladium with uranium were distributed throughout the Sic layer. They could be seen at the polished surface in bright-field illumination, or under the surface in polarized or oblique light. since unirradiated SIC is translucent. As the experimental temperature decreased from 2173 K, the nodules in the SIC became smaller and smaller until they could no longer be detected with the optical microscope. However, metal penetration into the SIC could still be measured by SEM. Silver. in very high concentrations, will attack SIC’. displaying Arrhenius behavior with about the same activation energy as palladium [1.2]. At low concentrations. typical of operating fuel particles, silver can escape from the particle, apparently diffusing through intact Sic. The TEM examination of several UO,-Ag particles after 2000 h of annealing revealed no microstructural changes on either the hot or cold sides of the particles. No second-phase nodules were detected, nor were there any obvious grain boundary films [39]. Analysis by EDX detected a large concentration of silver accumulating at the inner surface of the Sic, but no silver was detected along several grain boundaries probed with a 100 A spot. These results suggest that. if a grain boundary layer of silver (or an Ag-Si alloy) exists in the SIC, it is exceedingly thin. It might be possible to use lattice fringe imaging or higher resolution EDX to try to resolve this layer.

R.J. Lauf et al. / Fission product -silicon carbide interactions

4.2. Time dependence of Sic corrosion

10-4

There was some uncertainty concerning whether SIC corrosion by rare-earth carbides that diffuse from the kernel to the SiC layer [41] is proportional to time or the square root of time. Early work at General Atomic Company (GA) indicated the square-root dependence [42], but the linearity with time was adopted later [25]. The data shown in fig. 20 also establish proportionality with time, and thus a rate that is calculated from the amount of SIC corroded away divided by time. It should be noted that rare-earth interaction with Sic is corrosion in the usual sense; it results in actual thinning of the BC layer. The temperature dependence of the Sic corrosion rate calculable from the data shown in fig. 20 falls within the 95% confidence limits of earlier studies [25,41]. The time dependence of palladium interaction with Sic was also established. Experiments were performed at 485,995,2000, and 4016 h and at 1670 to 1920 K. An extensive analysis of the data revealed that palladium migrated into the Sic linearly with time [4]. The Sic corrosion rate from this limited set of experiments is shown in fig. 21. No silver penetration into the Sic layer, other than an occasional large nodule where the Sic had been completely replaced by silver, could be detected by SEM in the heat-treated, Ttiso-coated, silver-containing particles. Consequently, no time-temperature dependence of silver interactions with Sic could be determined.

14

I

I

I

I

,

I

2 2 =

10 -

0

: 0 *

8-

f

6

2 iii

4

5 x

2

o LaC2-(796 l LaC2 - 9544 0 NdC2 - 1760 9 NdCp -1590

I

I

K K K K

0 0

3000

6000 TIME

25

9000

(h)

Fig. 20. The corrosion of the SIC layer by NdC, or LaC, versus time.

-

95%

----

(Simulated

CONFIDENCE

particles

LIMITS by SEMI

-.*-.-**-

95% CONFIDENCE LIMITS (lrradiatrd particle8 by optical microscope 1

to-7 5.5

6.0

6.5

6.9

10,000/T(K)

Fig. 21. SC corrosion rate for palladium-containing Trisocoated particles versus reciprocal temperature. All of the particles were heated in a 278 K/cm temperature gradient.

4.3. Sic corrosion rates The Sic corrosion rates determined here for all simulated fission products excepting palladium are shown in fig. 7, while those for palladium are shown in figs. 21 and 22. The results of a systematic study of 12 Sic coating batches containing UO,-Pd kernels were in agreement with the data shown in figs. 21 and 22. The slope change in the confidence limits shown in fig. 7 is taken from ref. [43] and may result from different Sic corrosion phenomena. The confidence limits for data above 1673 K are undoubtedly for the interaction of rare-earth carbides with BC in particles containing irradiated UC, kernels. Reviewing this briefly, GO obtained data by irradiating HEU UC, fuel particles at low temperature and then heating them in a temperature gradient in a laboratory furnace [42]. The interaction was measured radiographically. In parallel ORNL work [41], Triso-coated rare-earth carbide kernels were heated in a temperature gradient, and the radiographically-determined Sic corrosion rates all fell within the GA 95% confidence limits. Thus, it appears that the

corrosmn rate is not influenced by the kernel type. The data in fig. 7 generally indicate that the interaction ~11 other chemical systems with SiC give corrosion ratcx that are about the same as those for the rare-earth carbides.

Noble metals of the palladium group w’erc shov n h\ TEM (fig. 13) as small nodules along Sit’ grain boundaries. There does not appear to be significant restructuring behind the nodules. which suggests that the nodules move by dissolving Sic at the leading edge and forming Sic at the trailing edge. As shown in fig. 23. the newly formed Sic could grow directly on the original material. maintaining the same orientatrnn and leaving the Sic microstructure relatively unaffected. The presence of other fission product elements in an operating fuel particle could. of course. inhibit the rrparr process and lead to local corrosion and thinning of the Sic‘. A study of the kinetics of palladium attack 141 incii.45

5”

55

60

65

70

75

8 0

RS

to,ooG/:IK~

Fig. 22. SIC corrosion rate due to palladium-SiC interaction\ versus reciprocal temperature. Data were obtained from a variety of Triso-coated particles heat-treated in the laboratnr> and in-reactor. The points with arrows above them indicate complete penetration of the Sic layer sometime during heat treatment.

Sic corrosion rates above 1673 K measured from the radiographs at GA were due to rare-earth carbide interactions with SIC. Below 1673 K. the data were determined metallographically at GA and ORNL from irradiated particles in which the interacting in which the interacting species was identified as palladium. indicating that the lower-temperature confidence limits represent the bounds on the corrosion of Sic by palladium. These lower-temperature confidence limits are also plotted in fig. 22. identified as in-reactor data. Also shown are the 95% confidence limit for the ORNL laboratory data above - 1673 K. and for all the data. It is unclear at the present time which set of confidence limits should be used. Comparison of the results in figs. 7 and 22 indicates that palladium interactions with Sic give the highest Sic corrosion rates. Comparison of the data from the several palladium-containing kernels indicates that the

-

I./,!

,’

!,il, ,,,, !,/,/

Fig. 23. Schematic illustration of possible noble metal transport process in the Sic coating.

R.J. Lauf et al. / Fission product -silicon carbide interactions

cated that palladium penetration into SIC follows an Arrhenius relation with an activation energy, Q, of about 70 to 190 kJ/mol depending on the original SIC deposition conditions. Specifically, coatings deposited at higher temperatures had a lower apparent activation energy than coatings deposited at lower temperatures. This suggests that microstructural features play a role. Since previous work [44] showed that high deposition temperatures tend to favor large, columnar grains, it is possible that grain boundary transport is the controlling process. Also, the activation energy for bulk (vacancyassisted) diffusion in Sic would be at least a factor of four greater than the observed values of Q [45]. This is further evidence suggesting transport along grain boundaries or some other path of enhanced mobility. The value of Q may also represent that for reactions at the Sic-Pd interfaces, or for metal self-diffusion across the palladium-containing nodule. On the other hand, typical values of the activation energy for carbon diffusion in a liquid metal are generally much less than the values of Q reported here. This suggests that carbon diffusion in the noble metal nodule is probably not the rate-limiting process. Consider next the rare-earth interactions with Sic. The lack of interaction with Sic in the Nd,O,-UO,-UC, particles implies that an irradiated UO,-UC, fuel will not result in Sic attack by the rare earths. In the mixed fuel, all rare-earth fission products will exist as solid oxides as long as the initial UC, content is kept below a specified maximum value [24]. The observation that no Sic interaction occurs in a Triso-coated La,O, particle, while the rate of Sic thinning in a Triso-coated La@-LaC, particle is the same as was seen in a Triso-coated LaC, particle supports the earlier conclusion that the presence of rare-earth carbides is necessary for their reaction with Sic [41]. However, (La,O,) may dissociate to form (La) and (Lao) to give the partial pressures shown in fig. 3. The transfer of lanthanum into the porous buffer layer, fig. 8, may have resulted from the (Lao) partial pressure. The other rare earths are likely to have such gaseous forms because of similar chemistry. This assumption may explain why a variety of rare earths were observed to have distributed themselves quite widely within fuel particles containing UO,-UC, kernels irradiated to 85% FIMA in HFIR capsule HRB-9 [46,47]. Mass-balance and thermodynamic considerations suggest the lack of rare-earth interactions when only rare-earth oxides are present. Clearly, the presence of rare-earth carbides in the kernel provides substantial amounts of material that are observed to migrate down the temperatuie gradient and corrode the Sic [25,41]. Rare earths present only as

27

oxides must dissociate, however, to provide rare earths to the Sic. Oxygen from the dissociation remains in the particle and would be present mainly as CO. It can be shown that if only ten percent of the rare-earth oxides dissociated in a full-burnup MEU fuel, the CO pressure would be about 1 MPa at normal HTGR operating temperatures, and the oxygen potential would be within the shaded area shown in fig. 3. The chemical potential of the rare earths, however, would now be much lower in the kernel than in any rare-earth carbides at the iLTI-SIC interface, and would result in diffusion of rare earth down their chemical potential gradient, i.e., back to the kernel. The rare-earth-oxide dissociation process is thus self-limiting in its capability to supply rare earths to the SIC. The SrO-ZrO,-UO, and SrZrO, samples were used in an attempt to simulate the performance of these oxide fuel components. Strontium was chosen to simulate both strontium and barium because the literature indicates the possibility of strontium interactions with the coatings. Sufficient strontium was added to the UO, sample to equal the combined yield of both elements. The proportion of zirconium to strontium plus barium exceeds unity; thus, even if zirconates form, extra ZrO, is available to form a solid solution in the UO, matrix. The lack of interaction of the SrO-ZrO,-UO, sample with Sic is not in agreement with postirradiation results reported by Brown et al. [5] Their approximate Sic corrosion rate (fig. 7) falls closer to the data obtained from the SrZrO, particles in this study. In these particles, interactions occurred at the ends of cracks through the carbon layers, which resulted in direct exposure of the BC to the gaseous species, probably (Sr), in equilibrium with the SrZrO, kernel. The cause of the cracks is unknown, but the chlorine leach test did show that some of the iLTI layers were defective; therefore, the cracks may have resulted from flaws in the iLTI layer. Interestingly, the interaction points are not temperature-gradient oriented; rather, they occur wherever the cracks intercept the Sic layer. In the Triso-coated SrO-ZrO,-UO, particles heated at 2200 K for 94 h, the iLTI layers seemed impermeable to (Sr) because we saw no interaction. The (Sr) pressure at 2200 K (fig. 2) inside these particles was about lo-’ MPa, compared to 10e9 MPa at 1645 K in the SrZrO, particle shown in fig. 6. Earlier work on the UO,-C-N, reaction at 1975 K demonstrated that a (HCN) pressure of 10m6 MPa was sufficient to transport substantial amounts of carbon as (HCN) from graphite, across a 0.02 cm gap to a UO,-U,N,-U(C, N) pellet [48]. It appears that the (Sr) pressure should be adequate for gas-phase transport in the present experiments. Because the Sr-SiC interaction

reported by Brown et al. [5] falls closer to our SrZrO, data than to our SrO-ZrO,-UOz data. it may be that their observed Sr-SiC interaction was due to a permeable or defective iLTI layer in the Triso-coated UOz fuel. 4.5. Potential methods for the reduction

faults. These changes might reduce the potential benefits of an optimum microstructure. The planned TEM examination after irradiation of systemically varied SiC coatings will yield much needed information on this aspect.

nf Sic corrosiorl

If Sic thinning could be reduced or eliminated. there would be more latitude for the design of an HTGR with higher core outlet temperatures. Both palladium and rare earth interactions with Sic may be alleviated at practical HTGR temperatures, as discussed below. 4.5.1. Chemical methods From early work [2], it was learned that the movement of rare earth fission products within Triso-coated particles was practically eliminated if the rare earths existed as oxides. This is accomplished through stoichiometric control of the kernel and has been demonstrated in-reactor [2,13,24]. The addition of SIC to the fuel kernel, discussed in section 3.4.5. can reduce or eliminate attack of the Sic by palladium. It does appear that the presence of rareearth carbides is deleterious. This can be alleviated in-reactor by using UO,-UC, kernels having the required UO, content (2.241. 4.5.:. Coating optimization Microstructural observations (fig. 13) and kinetic considerations suggest that noble metal attack or transport involves the SIC grain boundaries almost exclusively. This implies that large, columnar grains with highly disordered boundaries would be undesirable from the standpoint of noble metal retention. Large. columnar grains would provide a more direct diffusion path than would small, equiaxed grains having a more interlocking structure. This might explain the decrease in activation energy for palladium attack with increasing Sic deposition temperature. The boundary between two heavily faulted Sic grains is probably much more disordered and is effectively “wider” than a boundary between two nearly perfect grains. Voids or gaps at grain boundaries or grain boundary junctions are probably also deleterious. This is consistent with observations that increasing the Sic coating rate tends to increase the palladium attack rate [1.12]. In all cases, the microstructural effect is rather weak compared to the effect of annealing temperature or irradiation temperature. Furthermore, irradiation tends to cause numerous microstructural changes, such as void formation and the elimination of some stacking

5. Conclusions (1) Strontium-Sic interactions were observed in strontiumand zirconium-containing Triso-coated particles only if there was a direct contact between the strontium-containing atmosphere and the Sic. Such conditions were achieved in particles with defective iLT1 layers. No reaction occurred (even at 2200 K) when the iLTI layers were impermeable. The approximate SIC thinning rate observed by Brown et al. 151 in irradiated Triso-coated UO,, and attributed by them to be a strontium-SiC interaction, falls close to the SIC corrosion rate we obtained here in strontiumand zirconium-bearing Triso-coated oxide particles with defective iLT1 layers. It is possible that their strontium--SiC interaction was the result of a permeable or defective iLT1 layer in the Triso-coated UOz fuel. (3) No Sic interactions were observed in Trisocoated Sm,O,. LazO,, Nd203-UOZ. or Nd,O,-UO,-UC,. even at extreme temperatures. These particles may have a rare-earth monoxide partial pressure of about 10e6 MPa at 2000 K. Even so, mass-balance considerations apparently limit the amount of rare earths that can diffuse through the iLT1 layer and interact with the Sic; however. some redistribution of rare earths within the buffer layer may result. (3) The SIC layer corrodes at the same rate in Trisocoated La-#-LaC, particles as in Triso-coated LaC, particles under the same heating conditions. Rare-earth carbide interaction with the SIC appears to be linear with time. (4) Palladium migrates through the SiC coating without actually corroding this layer. The migration appears to be linear with time, i.e., the migration rate has units of pm/s. The migration characteristics are consistent with a grain-boundary-diffusion mechanism. (5) Palladium-SiC interactions were prevented at 1670 c T < 1920 K by doping kernels to form previunknown U-Pd-Si-C compounds. A ously UO,-UC,-Sic kernel is preferred. (6) Silver-Sic interactions sufficient to determine the reaction time dependence could not be detected by the present techniques, nor was silver observed in the Sic by high-resolution microscopy. (7) The palladium and rare-earth interactions and

R.J. Lauf et al. / Fission product -silicon carbide interactions

their time dependence should be confirmed in-reactor, as should the proposed retention of paIladium by Sic additions to UO,-UC2 kernels.

Nomenclature EDX EMP FIMA GA HEU HFIR HTGR iLT1 LEU MEU OLTI ORNL PIE SEM TEN

Energy-dispersive X-ray Electron microprobe Fissions per initial heavy-metal atom General Atomic Company Defined as fuel with greater than 90% 235U High Flux Isotope Reactor High-temperature gas-cooled reactor Inner low-temperature isotropic (pyrocarbon layer on a HTGR particle) Low-ennched uranium fuel (less than 10% 235U) Medium-enriched uranium fuel (less than 20% 23sU, but more than 10%) Outer low-temperature isotropic (pyrocarbon layer on a HTGR particle) Oak Ridge National Laboratory Post-irradiation examination Scanning electron microscope Transmission electron microscope

Acknowledgements

The authors wish to thank the following individuals For the contributions to this study: CA. Culpepper, P.A. Kuehn, J.H. Chaffer, G.W. Weber, C.S. Morgan, Jr., and W.R. Johnson for helping to prepare the kernel compositions; J M .Robbins, C. Hamby, Jr., and B.R. Chilcoat for Triso-coating the kernels; CA. Culpepper O.B. Cavin, R.S. ‘Grouse, B.C.Leslie, N.W. Atchley, W.H. Warwick, and W.J. Mason for their assistance in examining the particles. Betty Drake, Donna Amburn, and Amy Harkey helped prepare the manuscript for publication.

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1401 R. Ballestracci. CR Acad. Sci. Paris B282 (1976) 291. [41] R.L. Pearson and T.B. Lindemer, The Interaction of LaC, and NdC, with Sic in HTGR Particles, Proc. ANS Thermal Reactor Safety Meeting, July 31 -August 4. 1977. Sun Valley. Idaho (CONF-770708) pp. 3-357 to 3-371. [42] General Atomic Company, General Atomic Report GAA15842 (1980). [43] General Atomic Company. General Atomic Report GAA15216 (1979). [44] R.J. Lauf and D.N. Braski. Oak Ridge National Laboratory, Oak Ridge Report ORNL/TM-7209 (1980). [45] J.D. Hong, Self-Diffusion of Carbon-14 and Silicon-30 in Alpha Silicon Carbide Single Crystals, PhD Thesis, North Carolina State University at Raleigh (1978). [46] F.J. Homan et al.. Oak Ridge National Laboratory, Oak Ridge Report ORNL-5254 (1978). [47] T.N. Tiegs and T.J. Henson, Oak Ridge National Laboratory. Oak Ridge Report ORNL/TM-6780 (1979). 1481 T.B. Lindemer. J. Am. Cer. Sot. 55 (1972) 601.