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NATIONAL BUREAU OF STANDARDS - 1963

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LEGAL NOTICE
This report was prepared as an account of
Government sponsored work. Neither the
United States, nor the Commission, nor any
person acting on behalf of the Commission:
i A. Makes any warranty or representa-
tion, expressed or implied, with respect to
the accuracy, completeness, or usefulness
of the information contained in this report,
or that the use of any information, appa-
ratus, method, or process disclosed in this
report may not infringe privately owned
rights; or
B. Assumes any liabilities with respect
to the use of, or for damages resulting from
the use of any information, apparatus,
method, or process disclosed in this report.
As used in the above, “person acting on
behalf of the Commission" includes any em-
ployee or contractor of the Commission, or
employee of such contractor, to the extent
that such employee or contractor of the
Commission, or employee of such contractor
prepares, disseminates, or provides access
to, any information pursuant to his employ-
ment or contract with the Commission, or
his employment with such contractor.
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ORNr-Porov
CONF-651101-18 0
Summary for presentation at the 1965 Winter Meeting of the American
Nuclear Society, Washington, D. C., November 15–18, 1965.
ORNI – AEC - OFFICIAL
SEP 1 6 1965
MATHEMATICAL MODEL FOR PREDICTING COATED-PARTICLE BEHAVIOR*
..ORN.AE
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•
Si
J. W. Prados** and J. L. Scott
Metals and Ceramics Division
Oak Ridge National Laboratory
Oak Ridge, Tennessee .
internet
MASTER
R
We have developed a mathematical model for predicting
stresses and strains in pyrolytic-carbon coatings surrounding
fuel microspheres under irradiation. These stresses arise from
differential thermal expansion of the fuel particle against the
coating, fuel swelling, fission-gas release, damage from recoils,
and anisotropic dimensional changes in pyrolytic carbon resulting
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from thermal expansion and fast-neutron damage. With this model,
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we can evaluate the relative merits of low- or high-density fuel
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particles, single- or multiple-layer coatings, and low or high
RELEASED POR ANNOUNCEMENT
IN NUCLEAR SCIENCE ABSTRACT'S
values for coating thickness, density, and strength. Details of
the model and a computer program, STRETCH, for performing the
necessary calculations have been reported elsewhere. 2
We have performed calculations with this model to investigate
the effects of configuration and environment on the expected
service life of coated particles typical of those of interest in
current reactor technology. 2, 3, 4, 5 Conditions investigated have
included the following: fuel particle radius, .100 ; single- and
two-layer coatings with total thicknesses from 100 to 200 m;
-1
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ORNI - AEC - OFFICIAL
* Research sponsored by the U. S. Atomic Energy Commission
• under Contract with the Union Carbide Corporation.
**Speaker.
LEGAL NOTICE
- AEC - OFFICIAL

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TU. report w propund um 100ml of Gover sponsord wort, Maider the UMM
matau, nor the Commission, wory pro scuns a hall of the Contatou
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minately owned notes ar
8. As may tahluss with monet to the wed, or for denne monthung from the
w of my budounen, wanto, method, o parcon coclauod in ho rto .
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plegno ar contractor of the Commission, womplesso nulla corretor, to
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mod nyployee or contractor of the Constantin, plus at wall content Manarna,
dondates, or montan noon w, we were the per la playa contrar
vide the Counterton, or wo employment will media contractor
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ORNI - AEC-OFFICIAL
inner-coating porosity from 15 to 50%; fuel material, 102 and
(UC2) with varying enrichmenta; tuel density, 90 and 100% of
theoretical; Irradiation temperature, 250 to 1450°C; fast neutron
doses, 0 to 4 x 1021 neutrons/cm²; fuel burnup, 0 to 50 at. 26;
fission-gas release from fuel, 1 and 100%. The results of these
calculations lead us to the following predictions.
1. Two distinct modes of coating failure may occur: (a) fail-
ure initiated at the inner surface from combined effects of fuel-
particle swelling and fission-gas pressure and (b) failure
initiated at the outer surface from anisotropic thermal expansion
and fast-neutron damage in the pyrolytic carbon.
2. For enriched-uranium-based fuel particles, operating to
high burnup, failure will normally occur by the first mode above.
To ensure adequate coating life, free volume of 2 to 3% of the
fuel volume per atom percent burnup expected, should be provided. .
Free volume may be obtained by use of a low-density fuel, a porous
inner coating which densifies rapidly when subjected to fission-
recoil bombardment, by manufacturing techniques which provide
empty space between fuel and coating, or by a combination of
these.
3. For fuels with low-uranium enrichment or high-thorium
content, failure may occur by the second mode above; that is,
from fast neutron damage in the coating, before fuel swelling or
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fission-gas pressure become significant. Hence, there is an
incentive to use isotropic outer coatings in such cases; however,
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free volume must still be provided in accordance with the burnup
ORNI - AEC - OFFICIAL
expected.
ORNL - ACC - OFFICIAL
ORNL - AEC - OFFICIAL
4. Increasing outer-coating thickness beyond the level at
which the ratio of its inner to outer diameter 18 2/3 will not
appreciably increase its life, provided that the inner surface 18
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adequately protected against fission recoils.
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5. The release of gaseous Pission products from the fuel has
3
a minor effect on coating life, so long as adequate free volume is .
.
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provided (see 2 above), and coating rupture stress 18 above about
20,000 psi. This effect is illustrated in Fig. 1. Hence, the use
of a low-density fuel particle to help provide free volume should
have little adverse effect on coated-particle life in spite of
attendant higher fission-gas-release rates.
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• References
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1. J. W. Prados and J. L. Scott, Analysis of Stress and Strain
in Spherical Shells of Pyrolytic Carbon, USAEC 'Report ORNL-3553,
Oak Ridge National Laboratory, June 1964.
2. E. S. Bomar and R. J. Gray, "Thorium-Uranium Carbide for
Coated-Particle Graphite Fuels," pp 703–28 in International Symposium
on Compounds of Interest in Nuclear Reactor Technology, ed. by
J. T. Waber, P. Chiotti, and W. N. Miner, AIME Metallurgical Society,
1964.
3. R. W. Dayton, Ji H. Oxley, and C. W. Townley, "Ceramic
Coated-Particle Nuclear Fuels," J. Nucl. Mater. 11, 1–31 (1964).
4. W. V. Goeddel, "Development and Utilization of Pyrolytic-
Carbon-Coated Fuel for the High-Temperature Gas-Cooled Reactor,"
Nucl. Sci. Engr. 20, 201—18 (1964).
5.-J. B. Sayers, K. S. B. Rose, J. A. Coobs, G. P. Hausner,..
and C. Vivante, "The Irradiation Behavior of Coated Particle
Fuels," pp 912-62 in Symposium on Carbides in Nuclear Energy,
Harwell, England, November 57, 1963, Macmillan, London, 1964.
ORNL - AEC - OFFICIAL
ORNL - AEC - OFFICIAL
FIL
CIAL
ORNI-
ENCENE DIELICENCO
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ORNL - AEC - OFFICIAL

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END

SAATLAR,Mg
DATE FILMED
10/ 20 / 65
"a de dos