Erfahrungs- und Forschungsbericht 2012 - Ensi

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Erfahrungs- und Forschungsbericht 2012 - Ensi

As we discussed before [3], this cladding originally

has two families of precipitates: the first one is

β-Nb and the second type Zr 0.5 Nb 0.3 Fe 0.2 [6]. The

second family of precipitates already showed

a considerable depletion of Fe in the fresh Zirlo,

which was irradiated up to 53.4 MWd/kgU [3].

This Fe depletion has continued in the high burnup

material and although, in some precipitates a

small amount of Fe is encountered (< 1 at.), it is

concluded that the Fe depletion has started at the

early stages of irradiation and has been extensive

at medium fluences, reaching a maximum in the

case of high burnup material. The depletion of Fe

and Nb from the precipitates should imply a certain

increase of concentration of these two elements

in the matrix. Such enrichment of the matrix has

been observed around certain precipitates. In the

absence of an archive material of this cladding

grade, it is not possible to separate the distribution

of the alloying elements in the matrix and in the

grain boundaries, prior to irradiation, however, it

cannot be excluded that a part of this alloying element

has been already in the matrix in the original

unirradiated cladding.

This material has also precipitates in the oxide layer,

the Nb/Zr ratio of these precipitates in the oxide is

in the range of 0.4 and no sign of amorphization

is observed in these precipitates. These precipitates

show a substoichiometric oxide, as it was observed

for fresh Zirlo.

In this material once more, the crescent shaped

crack of the precipitates close to the metal-oxide

interface, indicate delayed oxidation behavior

previously observed in unirradiated binary Zr-Nb

alloys [9].

Discussion and Conclusions

As it was mentioned in the previous report, this

research project contributes to the large amount

of data already available on the IFA-638 campaign.

With observation of the two materials in

this part of the project, all materials selected for

this research project have been examined. The

TEM investigations on the selected materials

namely fresh M5, alloy E635, ZIRLO and Alloy

A, together with the pre-irradiated M5 and Zirlo,

provide interesting information about these materials

regarding their oxidation behavior under

irradiation. One of the important findings is that

all examined materials contain precipitates near

the metal-oxide interface and in the oxide, after

medium term irradiation and for the two materials

studied in pre-irradiated state (M5 and Zirlo)

after long term irradiation.

The comparison of the four materials including the

results from the new analysis in 2012 indicates

that practically all types of precipitates dissolve under

irradiation. However, βNb precipitates show

much slower dissolution and no amorphization in

comparison with the other types of precipitates.

Precipitates containing both Fe and Nb, do not

show the same resistance to dissolution and to

amorphization. This observation is in the favor of

cladding grades with more βNb precipitates, if a

high resistance to dissolution of SPP is desired.

In the case of Alloy A, studied in 2012, another

family of precipitate with the formula of (Cr,Fe) 2

Zr,Nb is also observed. This type of precipitate

has a resistance to amorphization. It is interesting

to note that the Cr containing precipitates show

amorphization in Zircaloy-2 irradiated in BWR [12].

This precipitate type shows a delayed oxidation

as is the case of Cr containing precipitates in the

Figure 3: (a) TEM

bright field contrast

of the metal side of

the pre-irradiated zirlo,

some precipitates are

indicated with arrows.

(b) TEM dark field

contrast of the metaloxide

interface of the

pre-irradiated zirlo,

hydrides can be observed

in the metal side of

the interface.

ENSI Erfahrungs- und Forschungsbericht 2012 119

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