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134

ENSI Erfahrungs- und Forschungsbericht 2011

of Fe content, ZIRLO has twice the amount of Fe

of M5 and alloy E635 has 6 times the Fe content

of M5. The Sn content of M5 is negligible, and can

be considered as impurity, where as the amount of

Sn in alloy E635 is 1.25 wt%; the fresh ZIRLO has

1.0 wt% of Sn. In the composition range of the

claddings studied here, the Sn does not induce any

precipitation and can be considered to remain in

solid solution.

The behaviour of precipitates in the different alloys

is as follows: From the observations in this study

and in agreement with the data available in the lit-

erature, it can be observed that in the Fe contain-

ing alloys of this series, two families of precipitates

are present. A Nb-rich, -Nb type and a precipitate

containing Fe. These precipitates are present both

in the ZIRLO and in E635 alloys. The amount of Fe

in the M5 seems to be below the threshold of for-

mation of such precipitates, or their frequency is

statistically very low; therefore they have not been

encountered in the present study.

The Fe containing precipitate type seems to amor-

phize under irradiation (already in the metal), and

their Fe content reduces, therefore a dissolution

can be observed in these precipitates. This behav-

iour is observed both in the ZIRLO and the E635,

however in the case of M5 in the absence of these

precipitates, no amorphization of precipitates un-

der irradiation is observed. The -Nb type precip-

itate does not amorphize in all alloys studied in

this part.

Behaviour of precipitates in the metal and

in the oxide

Precipitates in the metal: these precipitates show

dissolution under irradiation, the phenomenon is

present both for the -Nb and the Fe containing

precipitates, however, the rate of dissolution of

precipitates seems to be slower, or the amount of

alloying element larger for the -Nb in compari-

son with the Fe containing precipitates. The pre-

cipitates containing Fe are more frequent in E635,

this alloy contains more Fe and thus it is not sur-

prising to find them more frequent in comparison

with the ZIRLO.

Precipitates in the oxide: the trend observed in the

metal is accentuated in the oxide. The dissolution

of the precipitates continues and oxidation can

lead to a further amorphization. The dissolution of

the alloying element is also accentuated. This lat-

ter occurs in both types of precipitates.

The comparison of the Pre-irradiated M5 with

fresh M5, confirms that the effect of irradiation

on the dissolution increases as a function of flu-

ence, thus, the depletion of Nb takes place in the

-Nb precipitates and it can be expected that it will

cause a final exposure of the cladding to rapid ox-

idation, this finding brings a new understanding

about the role of Nb containing precipitates in the

claddings used in the reactor.

National Cooperation

The Service of Microscopy and Nanoscopy (SMN)

of the «Centre Suisse d’Electronique et Micro-

technique» (CSEM) has provided access in case of

special needs, for the use of TEM.

International Cooperation

This project has been started in collaboration with

the OECD Halden reactor project, and the IFE

Kjeller laboratory.

Assessment 2011 and

Perspectives for 2012

The examination of the present irradiated cladding

segments should be considered as a great success,

as access to such cladding materials from the re-

actor and the preparation of such materials for

TEM studies needs a great amount of effort. Not

only the sample decontamination and preparation

is demanding, but also the transport of such ma-

terial from Norway has been a very lengthy pro-

cess. As mentioned before, results obtained in this

study allow a better understanding of the oxida-

tion behavior of the modern claddings. The study

of the metal-oxide interface of the three types of

cladding and the possibility of comparing the M5

fresh with pre-irradiated M5 adds to this success.

The metal-oxide interface of the TEM thin foils,

were not sufficiently homogenous to obtain the

oxygen profile along the metal-oxide interface, in

view of determining the diffusion parameters of

the materials [2]. However, this was compensat-

ed by the fact that the presence of precipitates in

these materials, allowed the chemical analysis of

these secondary phases in the metal and in the ox-

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