RRFM 2009 Transactions - European Nuclear Society

THE THIRTEENTH INTERNATIONAL TOPICAL MEETING ON RESEARCH REACTOR
FUEL MANAGEMENT
March 22-25, 2009
Vienna International Center
Vienna, Austria
TEM CHARACTERIZATION OF IRRADIATED RERTR DISPERSION
FUEL
J. GAN, D. D. KEISER, JR., D. M. WACHS, A. B. ROBINSON, B.D. MILLER 1 , AND T. R.
ALLEN 1
Nuclear Fuels and Materials Division, Idaho National Laboratory
P. O. Box 1625, Idaho Falls, Idaho 83403 USA
1
University of Wisconsin
Madison, Wisconsin 53706
Australia
ABSTRACT
The irradiation performance of RERTR dispersion fuels depends on the radiation stability of the various
phases that comprise a fuel plate. Transmission electron microscopy was performed on a sample taken
from an irradiated U-7Mo dispersion fuel plate with Al-2Si matrix to investigate how the presence of Si in
the fuel meat matrix impacts the radiation stability of the U-7Mo/matrix interaction layer phase(s). A
similar interaction layer that forms in irradiated U-7Mo dispersion fuels with pure Al matrix has been found
to exhibit poor irradiation stability. This behavior seems to be due to the fact that it cannot adequately
retain fission gases that are generated during irradiation. For the sample from the irradiated U-7Mo/
Al-2Si fuel plate, the interaction layer was observed to be amorphous, like was case for the layer in
irradiated U-7Mo/Al dispersion fuel. However, unlike the U-7Mo/Al fuel, this amorphous layer was found
to effectively retain fission gases in areas of high Si concentration. The fission gases were observed to
be distributed as small (< 2 nm diameter) fission gas bubbles throughout the interaction layer, which were
not observed in the interaction layer in irradiated U-7Mo/Al fuel. The presence of these fission gas
bubbles seems to correlate to the Si concentration within the interaction layer. When the Si concentration
becomes relatively low, the fission gas bubbles agglomerate into fewer large pores. Within the U-7Mo
fuel particles, a bubble superlattice was observed where the bubbles were 2 to 4 nm in size.
1. Introduction
Low-enriched uranium (LEU) fuels are being developed by the United States Reduced
Enrichment for Research and Test Reactors (RERTR) program to enable research and test
reactors to discontinue the use of highly-enriched uranium (HEU) fuel [1].
A big part of the development of LEU fuels has involved using the Advanced Test Reactor (ATR)
to test different fuel types to investigate irradiation performance. The first experiment that was
run to test dispersion fuels with Si-containing Al alloy matrices was the RERTR-6 experiment
[2]. Si is added to the matrix of a U-Mo dispersion fuel to enable the formation of stable U-Mo/
matrix interaction layers that behave well during irradiation. Similar interaction layers in U-7Mo
dispersion fuels with only Al as the matrix do not exhibit good irradiation performance [1]. TEM
characterization of the layers in an irradiated U-7Mo/Al dispersion fuel showed that the
interaction layer that was present around the U-7Mo fuel particles was amorphous and did not
retain fission gases as stable bubbles [3]. Instead, the fission gases migrated to the interaction
layer/matrix interface, where they developed into relatively large pores that could link up and
cause fuel plate failure. By adding Si to the matrix of a U-Mo dispersion fuel, it is predicted that
a more stable interaction layer will develop that can better accommodate fission gases [4].
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