Advanced plutonium fuels program. Quarterly report, April 1 through June 30, 1974 and eighth annual report, FY 1974 Page: 49 of 62
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Fig. 463-13. Photomosaic of a high-density, sodium-
bonded, carbide fuel element showing swell-
ing resulting from a bubble trapped by an
eccentrically-located pellet. (Fuel element
U195, Section N; Mount No. 3C92T).
boundaries of the high-porosity regions are not influenced
by the major cracks in the fuel pellets, i.e., the bound-
aries cross the cracks with little perturbation. If it is
assumed that the observed grain-boundary swelling occurs
only above some critical temperature, the boundary be-
tween the region of high swelling and the region of low
swelling should parallel isotherms in the fuel.
Heat transfer calculations were performed to obtain
the shape of isotherms in a transverse plane of a fuel
element with various size gas bubbles in the sodium bond
between the fuel and cladding. The fuel, bond, and clad-
ding were divided into eight radial regions, 24 circum-
ferential regions (15 each), and three axial layers for
numerical heat transfer calculations. Sodium-bond bub-
bles subtending angles of 300 to 1200 were simulated by
replacing liquid sodium conductivity with argon gas con-
ductivity in the bubble nodes. Radiation was allowed
from the fuel surface to the inside cladding surface
through the bubble. Calculations were performed for bub-
bles with a ratio of circumferential width to axial width
of 2, 1, and 0 (infinitely long bubble). Figure 463-14
shows the isotherms at 800C intervals in a fuel pelletFig. 463-14. Calculated isotherms at 800C intervals in
a carbide fuel pellet from an element with
a very long, SO , gas bubble in the sodium
bond.
from an element with a very long, 600, gas bubble in the
sodium bond. A comparison of the isotherm shape with
the shape of the high swelling region of Fig. 463-12
shows excellent agreement. Figure 463-15 shows the
isotherms at 800C intervals in a fuel pellet from an ele-
ment with a short (axial/circumferential width = 1/2) 1200
gas bubble in the sodium bond. The isotherms are from
the central plane of the bubble. Their shape compares
well with the high porosity region of Fig, 463-13. There
is no unique correspondence between the bubble size (cir-
cumferential and axial widths) and the isotherm shape.
Isotherms similar to those in Fig. 463-14 could also re-
sult from an approximately 750 square bubble in the sodium
bond. Thus, for any observed high-swelling pattern, a
unique bubble size cannot be determu.'ed, only a possible
size range.
The high temperature in the fuel under a gas bubble
can also affect fission product migration. Figure 463-16
shows a 9 - y autoradiograph (the dark areas are high
activity) of the section shown in Fig. 463-13. There is a
depletion of activity in the center, and in one sector the
activity depletion approaches the fuel surface. This45
60 bond
bubble
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Baker, R. D. Advanced plutonium fuels program. Quarterly report, April 1 through June 30, 1974 and eighth annual report, FY 1974, report, November 1, 1974; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1018429/m1/49/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.