RESULTS OF TESTS TO DEMONSTRATE A SIX-INCH DIAMETER COATER FOR PRODUCTION OF TRISO-COATED PARTICLES FOR ADVANCED GAS REACTOR EXPERIMENTS Page: 4 of 10
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Buffer-coated particle samples were taken for density
analysis from 22 B&W coating runs. For these runs the coating
gas fraction varied from 0.54 to 0.60, the total gas flow rate
from 102 to 186 slpm, the average coating rate from 12 to 25
pm/min, and the control temperature from 1400 to 15200C. For
three runs in which the buffer layer was applied with low
coating rates and low temperatures, the buffer density was
found to be 0.88-0.90 g/cm3. However at all other conditions,
buffer densities were in the range 0.94-1.16 g/cm3.
PYROLYTIC CARBON LAYERS
AGR fuel specifications for pyrocarbon layers include
layer densities (1.90+0.05 g/cm3), thicknesses (40+4 pm), and
anisotropies (mean equivalent BAFo < 1.045 for IPyC and <
1.035 for OPyC). The mean equivalent BAFo, calculated as 1+
3 times the diattenuation, is used in order to compare results to
historical German particle anisotropy values. Although not a
specification, the surface connected porosity of the pyrocarbon
layers is also an important property and was measured for
samples from many of the coating tests.
AGR-1 fuel includes particles with IPyC layers deposited
at three different coating conditions:
Due to the IPyC layer being applied at a higher temperature,
variant 1 particles have a lower IPyC density and a lower
anisotropy than baseline particles. Increasing the coating gas
fraction, as was done for variant 2 particles, is an alternative
means of increasing the coating rate and generally results in a
lower anisotropy . However, for the AGR-1 particles, the
anisotropies of baseline particles and variant 2 particles were
nearly equivalent (0.0074 diattenuation for baseline particles
compared to 0.0075 for variant 2 particles).
Prior to fabrication of AGR-1 particles, a study was
performed in which IPyC coating temperatures and gas
fractions were systematically varied . Consistent with
previous fuel coating literature [6, 12-13], pyrocarbon density
was found to be primarily dependent on bed temperature. A
comparison of the trends of pyrocarbon density versus
temperature for the two-inch and six-inch diameter coaters is
shown in Figure 1.
Interestingly, for the IPyC layer, the trend lines for the two
coaters intersect very near to a density of 1.9 g/cm3, which is
the target density for AGR fuel. The trend line for the IPyC
density of particles produced in the six-inch diameter coater has
a lower slope than the IPyC density trend line for the two-inch
coater; hence acceptable IPyC densities can be obtained with
the larger coater over a wider temperature range. The same is
true for the OPyC layer. These trends likely reflect differences
in temperature profiles between the two coaters.
R2= 0.94 2 0 6
* Z OPyC
* 6', oPyc
a 2', IPyC
r 6', IPyC
1150 1175 1200 1225 1250 1275 1300 1325 1350
Temperature, degees C
Figure 1. Comparison Of Pyrocarbon Density Versus
Temperature For 2" And 6" Diameter Coaters
Figure 1 also shows that to achieve the same OPyC density
as IPyC density, a higher temperature is required, at least up to
a temperature of about 13250C. The scatter in the data shown
in Figure 1, such as the range of OPyC densities for the two-
inch diameter coater at 13000C, illustrates the fact that coater
temperature measurements have uncertainties. Internal
thermocouples, external thermocouples and pyrometers all have
Figure 2 shows data only for the B&W coater. Densities
plotted against bed temperature are the same as shown on
Figure 1. The other two data sets shown on Figure 2 are
densities plotted versus coater control temperatures. Figure 2
shows that to have high confidence in meeting the density
specification, a control temperature in the range 1175-13400C
is needed for the IPyC layer, while the narrower range of 1335-
13650C is needed for the OPyC layer.
Past studies have shown that the anisotropy of pryrocarbon
layers decreases with increasing coating temperature and
increasing coating rate [12, 14, 16]. Figure 3 shows trends of
equivalent anisotropy versus temperature, coating gas fraction,
and average coating rate for the two-inch diameter coater used
to produce AGR-1 particles. The AGR-1 coating data confirms
the trend of decreasing anisotropy with increasing coating
temperature, but indicates that for any temperature, there is an
optimum coater rate that gives a minimum anisotropy.
Furthermore, there is an optimum coating gas fraction that
appears to be nearly independent of temperature. For the AGR-
1 coater, this optimum coating gas fraction is about 0.35.
' Coating rate varies with time during any coating. Coating rates plotted in
Figure 3 and later figures have not been adjusted for small variations in layer
thicknesses observed in the difference tests.
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Marshall, Douglas W. RESULTS OF TESTS TO DEMONSTRATE A SIX-INCH DIAMETER COATER FOR PRODUCTION OF TRISO-COATED PARTICLES FOR ADVANCED GAS REACTOR EXPERIMENTS, article, September 1, 2008; Idaho Falls, Idaho. (digital.library.unt.edu/ark:/67531/metadc832189/m1/4/: accessed January 20, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.