The EBR-II X501 Minor Actinide Burning Experiment Page: 3 of 11
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resulting microstructure are given in reference'". Considerable americium was lost due to volatilization
during the fabrication process, which was not designed for use with Am-bearing alloys.
Materials and methods
The typical injection casting temperature for U-20%Pu-10%Zr is 14650C. The superheat is about
1500C which is required to fill the molds which are 4.3 mm x 460 mm. The feed stock used was elemental
metallic Np, U, Zr, and Pu. The Am feed stock was a metallic alloy of Pu-20%Am. Potential concerns in
casting metallic fuels with minor actinide elements were the wide variations in vapor pressure between Am,
Np, Pu and U, and the very highly negative free energy of oxide formation for Am. Over the range of
melting temperatures, Am has nearly three orders of magnitude higher vapor pressure than Pu, and five
orders of magnitude higher than Np or U'". This implies that a significant amount of Am could be lost as a
vapor at casting temperatures. The potential for Am oxidation arises because AmO is considerably more
stable than ZrO, UO, or PuO, at all temperatures'". It is possible that the Am could become part of the
casting dross and not be incorporated into the fuel. The challenge becomes how to cast these fuel alloys
with minimum Am loss under the characteristic range of processing conditions.
In order to reduce Am loss through evaporation and reaction with oxygen, the time at elevated
temperatures, where the Am evaporation and reaction rates are high, should be constrained to as short as
possible. The fabrication facility (the Experimental Fuels Laboratory at Argonne National Laboratory in
Idaho) was therefore modified to include a bucket device with a lid allowing the Pu-Am feed material to be
added to molten fuel within the crucible late in the melt cycle to minimize the time at temperature.
Furthermore, the atmosphere above the melt was extremely pure Ar (99.9997%), containing less than 1.5
ppm oxygen. The elements (U, Zr, Np and some Pu) were loaded into a Y203 coated graphite crucible and
induction heated under an Ar atmosphere. After reaching a temperature of 14950C, the melt was stirred
causing the temperature to drop to 14650C. The melt was reheated and stirred again. At this point, the Pu-
Am was added to the crucible via the bucket which sat next to and above the crucible inside the furnace.
When the Pu-Am feed stock was added, "sparks" were observed emerging from the melt. Casting
proceeded and three full-length pins were successfully cast into ZrO, coated quartz molds. The entire
crucible charge was consumed during casting and the pins were removed from the molds. Two pins were
encapsulated and made ready for in-reactor (EBR-II) testing while the remaining pin was sacrificed for
metallographic and chemical characterization.
Figure 1 shows optical images of fuel cross-sections taken from locations near the top and bottom of a
pin from the same casting batch as the irradiated fuel pins. Figure 1(a) was taken from the top section of
the fuel pin, and shows the inhomogeneous structure typical of U-Pu-Zr fuel produced by this casting
method. Figure 1(b) shows a micrograph of a section taken from the lower part of the fuel pin, in a region
where cooling is slower after solidification due to proximity with the furnace internals.
(a) (b) -
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Carmack, Jon; Hayes, S. L.; Meyer, M. K. & Tsai, H. The EBR-II X501 Minor Actinide Burning Experiment, article, June 1, 2008; [Idaho Falls, Idaho]. (digital.library.unt.edu/ark:/67531/metadc893878/m1/3/: accessed December 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.