atw 2018-05v6

inforum

atw Vol. 63 (2018) | Issue 5 ı May

RESEARCH AND INNOVATION 326

| | Fig. 1.

Irradiation set-up for heavy ions irradiation of small fuel samples with I-127

at the MLL accelerator facility. The sample holder offers 3 irradiation positions

with temperature sensors, electrical heating, air cooling and a faraday blend.

An IR camera is used for monitoring the samples and for adjusting the beam.

studies of UMo dispersion fuel, where

small fuel particles of a few dozen

micrometre diameters are dispersed

in an aluminium matrix. Model

systems normally consist of an Al

substrate, an optional coating layer of

a few 100nm and a ≤5 µm thin UMo

layer on top. In certain cases, the

order is reversed and UMo serves as

substrate with a ≤13 µm thick Al layer

on top. The model systems have

been produced by Physical Vapor

Deposition (PVD) in the Uranium lab

of FRM II, dispersion plates were

manufactured by AREVA-CERCA.

The irradiations have been performed

at the Maier-Leibnitz Laboratorium

in a dedicated irradiation setup

(Figure 1). The MLL houses a

tandem accelerator that provides

I-127 ions at the desired 80 MeV with

high intensity. Ions are targeted perpendicular

on the sample surface.

Penetration depth ranges between

5 µm and 13 µm, depending on the

material of the top layer. The sample

can be cooled and heated in a temperature

range between 30 °C and

~200 °C. Optionally, a wobbler can be

hooked up to the beam to irradiate a

larger area with a beam that is still

focused. In that case, a quantitative

evaluation is only possible for specific

applications. Otherwise, quantitative

analysis is possible whenever the

geometric beam intensity profile can

be determined.

Uranium-Molybdenum and

the interdiffusion layer

To minimize the usage of highly

enriched Uranium (HEU) in the civil

nuclear cycle, the High Performance

Research Reactors (HPRR) that are

currently using HEU are seeking to

convert their cores to low enriched

Uranium (LEU). As the core geometry

in these reactors usually can only be

varied to a small extent, the current

fuels need to be replaced by an alloy

with considerably higher chemical

Uranium density to account for the

lower fraction of fissionable isotopes

in LEU. In this context, the most

promising candidates for such a

conversion are fuels based on the

Uranium-Molybdenum alloy with

7-10wt % Mo, either as dispersion or

as monolithic fuel. These two variants

commensurate to the two sample

variants discussed beforehand.

One particularly important application

for the ions in this context has

been the study of the growth of the

interdiffusion layer (IDL) that forms

between UMo and the aluminium

matrix and/or the Al cladding during

irradiation [1]. Due to its inferior

irradiation properties, the IDL can

lead to exponential fuel swelling and

therefore needs to be avoided [2, 3,

6]. Therefore, numerous experiments

have been carried out to understand

the development of this IDL and to

test countermeasures like the addition

of silicon to the Al matrix, the application

of diffusion barriers between

UMo and Al and even the substitution

of the Al matrix by Magnesium. Ionbased

experiments have con siderably

accelerated this develop ment; some

exemplary results will be discussed in

the following.

Understanding the IDL

Growth dynamics

Kim and Hofman [11] have developed

an Arrhenius-like formula to predict

the thickness d IDL of interaction layer

formation between UMo and Al

during irradiation, based on the data

of several in-pile experiments:

(1)


Here, A = 2.6 ∙ 10 -8 µm 2 cm 3p s p-1 is the

proportionality factor, p = 0.5 the

power of the fission rate f that has

been averaged over the irradiation

time t and q = 3,850 K the fit parameter

for the average irradiation temperature

T. f Mo and f Si are correction

factors for the molybdenum content

of the fuel and the silicon content the

matrix.

Based on a SRIM/TRIM [11]

dataset for the deposition of energy

and the creation of vacancies by

in-pile fission products as well as the

I-127 ions, a conversion of ion flux

and fluency to the corresponding

fission rate and -density equivalents is

possible. Jungwirth [8], has discussed

that the irradiation enhanced diffusion

which leads to the creation of this

layer is mainly driven by thermal spiking,

from ionization as well as recoils.

Therefore, in this case, φξ, where

ξ is the relative total energy deposition

that was calculated using SRIM/

TRIM. The factor 0.5 originates from

a real fission producing 2 fission

products. The calculation of ξ is laid

out in detail in [17].

An inverse Al/UMo system with

the Bragg peak near the interface was

irradiated [18, 22, 35] to study the

growth dynamics of the IDL and to

verify the flux-fission rate conversion

| | Fig. 2.

Deviation of the measured IDL thickness compared to the value expected from in-pile irradiations,

i.e. y-axis is difference between expected and measured value, divided by measured value.

The use of a cubic fit is arbitrary to visualize a trend.

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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