atw 2018-05v6

inforum

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

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