atw Vol. 63 (2018) | Issue 5 ı May
RESEARCH AND INNOVATION 328
| | Fig. 4.
IDL growth around UMo particles in a UMo/AlSi7 miniplate, protected by a SiRDL, in three steps, from low to high ion dose.
Left: The UMo particle located at the sample surfaces is surrounded by first a SiRDL and then an IDL. Center: In the upper part of UMo
particle embedded in the Al matrix, the SiRDL has been destroyed by the heavy ion irradiation and the IDL is directly in contact with
the UMo particle core. Right: A standard UMo/Al interaction occurs and the shape of the UMo particle indicates clearly that UMo has
been consumed in the interaction.
same effect was observed in the IRIS-3
and RERTR-7 test [28, 29]. It was also
found that the IDL is free of Si except
when precipitates were present from
manufacturing using blended instead
of alloyed AlSi-powder.
Ion irradiations were carried out
on samples from real test plates of the
E-FUTURE II (EF2) irradiation experiment
with 7 and 12 wt% Si addition to
the Al matrix. E-FUTURE [7] samples
were not irradiated due to their
likeness to other samples irradiated
before. For the EF2 samples, it was
found that the SiRDL similarly gets
consumed during irradiation [31].
Yet, extrapolation predicted sufficient
Si around the particles and in the
matrix until very high burn-up. In the
end, the EF2 test failed unexpectedly
due to macroscopic deformation of
the plates at comparably low burn-up
[4]. One of the possible reasons is the
very low creep of the hard AlSi12
matrix. Thereby, EF2 is an experiment
demonstrating the limits of out-of-pile
ion irradiation.
Other additions to the matrix like
2-5 wt % Bi and 2 wt % Ti have been
tested using ions but not in-pile as no
beneficial effect was found [8].
Several other coating materials
would be available from a reactor
physics and manufacturing point of
view, e.g. Mo, Nb and Ti. None of
these has been tested in-pile so far,
but in a series of ion experiments
[20] and analysed using Rutherford
Backscattering and µ-XRD. The
experiments have correctly predicted
the efficacy of several coating
materials as well as their interaction
with surrounding matrix and
fuel materials. Several interactions
between the coating, UMo and Al
were found: For Ti, the presence of
Ti 0.04 U 0.96 indicates a transformation
of γ-UMo to α-UMo. Nb forms
compounds on both interfaces that
have poor crystallinity, Nb 3 Al and
Mo 0.1 Nb 0.45 U 0.45 .
For the current standard coating of
monolithic fuel, Zr, it was found that it
forms Mo 2 Zr with the Mo from the
UMo fuel, leaving behind a small area
that is poor in Mo in the fuel zone. In
this area, the UMo would no longer be
sufficiently stabilized, i.e. α-U would
form and macroscopic bubbles would
appear – a precursor of plate failure.
Such an effect has indeed turned up
in the RERTR-12 in-pile irradiation
at very high fission densities
(n > ∼ 6 ∙ 10
21
[33].
No detrimental effects could
be identified for a Molybdenum
coating, which is now tested in-pile
in the SEMPER FIDELIS experiment
since September 2017. Most important,
it could be shown that the
driver for the interaction between
the barrier material, the UMo fuel
and the Aluminium matrix is
indeed the chemical potential of
the respective constituents. This
allows for a prediction of the protective
properties of coating material
candidates [20].
Magnesium as matrix material
If the Aluminium matrix was replaced
by Magnesium, no chemical reaction
between UMo and the matrix would
take place. Instead, TEM analysis
showed that spinodal decomposition
would occur at elevated temperatures
(~200 °C, Figure 5), which favours
embrittlement. However, this temperature
is at the upper limit of what is
usually found in research reactor fuel
under nominal conditions (~140 °C),
where no spinodal decomposition
could be detected. At the lower
temperature, only a ~50 nm thin
interaction layer at the interface with
decreased crystallinity was found. Mg
would therefore be a viable alternative
for the matrix material once all
manufacturing-specific difficulties are
solved.
Besides its use in the 50’s and 60’s 1 ,
an UMo/Mg fuel has already been
tested in the RERTR-3 and -8 experiments
under increasingly aggressive
irradiation conditions up to temperatures
between 145 °C and 171 °C
[27]. In agreement with the ion
1) See [21] for a
comprehensive
overview
Coating of UMo particles
A coating aims on physically – and
thereby chemically – separating UMo
and Aluminium. For monolithic UMo,
a layer of Zr usually is applied between
foil and cladding. UMo particles in
dispersion fuel are coated with ZrN
and Si. In-pile, a Si coating has shown
little to no benefit over the Al-Simatrix
[5, 31]. The protective properties
of ZrN were confirmed on actual
dispersion samples from SELENIUM.
Cracks in the coating led to formation
of a conventional IDL in ion and
neutron experiments [4, 30].
| | Fig. 4.
Spinodal decomposition in the UMo/Mg system at 200°C. No decomposition and only very little
amorphous interaction was found at 140°C, the typical operating temperature for HPRR cores.
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
Change language