atw Vol. 63 (2018) | Issue 5 ı May
mechanism in eq. 1. The beam had a
2D-Gaussian profile and was not
wobbled, the six samples therefore
cover a large variety of fission rate and
burn-up equivalents. The samples
were irradiated at six different
temperatures between 383 K and
548 K and for different time spans
between 5 h and 27.85 h. For this
irradiation geometry, ξ = 0.21 µm -1
was calculated.
Figure 2 shows the deviation
between expected and measured IDL
thickness. Even though significant
scattering is present, which is attributed
to the general fluctuation of IDL
thicknesses that is also observed
in-pile and to suboptimal sample
quality, prediction and measurement
match well within the uncertainties
of the experiment and the assumed
uncertainties of the equation of Kim
and Hofman (~15 %). The proportionality
constant A was reproduced
with an accuracy of 10 %. It was
furthermore shown that p is constant
over several orders of fission rates and
the dependency d IDL ∝√t was established
also for the early growth of the
IDL [18]. The temperature normalization
q, which includes the activation
energy, was verified by comparing the
value obtained from the fit results of
ion irradiation data (3,906K ±30 K)
and the one from in-pile data
(3,850K).
In summary, the matching thicknesses
and growth dynamics of outof-pile
produced IDLs with predictions
based on in-pile data support
the current understanding of the
Arrhenius like in-pile IDL growth as
well as the conversion between ion
flux and fission rate.
fission density, therefore exact
comparisons are difficult. The same
observations were made for ground as
well as atomized UMo powder [6, 8].
Ion and in-pile experiments did
not show ternary U x Mo y Al z phases
like purely thermal-driven diffusion
couple experiments [24, 25], which
underlines the necessity of irradiation
to correctly reproduce IDL growth.
Nearest neighbour distances in
neutron irradiations were found
between 0.239 nm and 0.251 nm [2,
23], which is in agreement with
0.248 nm measured after Iodine
irradiation [20].
Fission gas behaviour
The experiments presented above
cover the early phase of the fuel in the
reactor, up to a fission density of
n = 5.8 ∙ 10 20 , about 15 % of the
target burn-up for that fuel. Even
though fission gases play a less pronounced
role at this point, their
behaviour already can provide insight
into the mechanism that leads to
the break-away swelling that was
observed around n ≈ 2.5 ∙ 10 21
in early in-pile UMo irradiation
experiments.
To extend the ion irradiation technique
to this field, Krypton-82 ions
with 45 MeV have been implanted
on a UMo/Al sample at the GANIL
facility. The sample was irradiated
with I-127 beforehand to a burn-up
equivalent of n = 1.2 ∙ 10 20 [9, 10]
at the MLL. Kr was preferred over
the more common fission gas Xenon
as it is easier to accelerate. At
c Kr = 2.68 ∙ 10 19 , the concentration
of Kr ions reached the desired
equivalent of 25 % of “virtual” fission
products. Even though the two irradiations
have been carried out consecutively
and the irradiation environment
in the second did not reach the fission
rate equivalent of an in-pile irradiation
due to limited beam intensity,
a similar gas bubble growth was
observed as in-pile (Figure 3): Fission
gases have accumulated at the IDL
interfaces in macroscopic bubbles.
Secondary Ion Mass Spectrometry
(SIMS) revealed that the bubbles are
indeed filled with Kr and that the Kr
has accumulated in the IDL [9].
Avoiding the IDL
As outlined above, several approaches
are available to limit or prevent IDL
growth in UMo/Al fuel. Silicone
added to the matrix can form a protective
Si layer around the particles,
since U x Si y preferably forms over UAl x .
From a manufacturing perspective,
such an addition would be superior
over the coating of particles with a
diffusion barrier material. A more
rigorous approach would be to replace
the Al matrix of dispersion fuel by
Magnesium, which does neither interact
with Uranium nor Molybdenum.
Additions to the matrix
If Si is added to the Al matrix, a Si rich
diffusion layer (SiRDL) will form
around the UMo matrix during production
of the plate if the Si fraction
exceeds 2-5 %. This layer consists of a
mixture of U x Si y -phases. During irradiation,
the SiRDL is consumed and
finally a conventional IDL grows, i.e.
its presence does not prevent the
formation of an IDL (Figure 4). The
RESEARCH AND INNOVATION 327
Composition
IDLs generated in-pile usually consist
of UAl 3 with far smaller amounts of
UAl 2 and UAl 4 . Early ion experiments
lead to a partially crystalline IDL.
UAl 3 was identified using XRD [8]
but SEM/EDX showed a higher Al
fraction (3.5 – 5), which was attributed
to partial amorphization.
Improved sample cooling and the
usage of monolithic samples allowed
reproducing a fully amorphous IDL
[19]. RBS measurements showed a
U/Al ratio of ½, while EDX yielded ²∕ 3
[20]. The discrepancy is attributed to
the measurement techniques. Latest
experiments without beam wobbling
led to IDLs with a U/Al ratio between
¹∕ 3 and ¼ as identified by EDX [22].
This fairly agrees with in-pile results;
IDL composition depends on irradiation
temperature, fission rate and
| | Fig. 3.
Kr implantation in an Iodine pre-irradiated UMo/Al layer system: The Kr beam targets pre-irradiated as
well as as-fabricated parts of the sample (A). Fission gas bubbles can only be found inside the IDL (B),
not in the as-fabricated part (C). No bubbles have been found in locations that were not previously
irradiated with Iodine but only exposed to Kr (D). The white areas in (A) were analyzed using SIMS to
confirm the presence and depth profile of Kr.
Research and Innovation
Heavy Ions Irradiation as a Tool to Minimize the Number of In-Pile Tests in UMo Fuel Development ı H. Breitkreutz, J. Shi, R. Jungwirth, T. Zweifel, H.-Y. Chiang and W. Petry
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