atw 2018-05v6

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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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