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159

ENSI Erfahrungs- und Forschungsbericht 2011

ve the applicability of measuring the Seebeck co-

efficient

. The accuracy of is mainly determi-

ned by the measuring method itself (accuracy of

measured temperatures and voltage), the offset

which is given by the electronic components of

the TEP-measuring system, the environment (am-

bient temperature and humidity) and the material

dependent scatter of

which may vary in space

on the micro- or macroscopic scale. Dependent on

the material type, the scatter of

can be rather

large.

In order to improve the precision and repeatabili-

ty, a new TEP-device was developed and tested at

PSI. The device shown in Fig. 1 allows to apply a

constant temperature gradient on Charpy speci-

mens and to measure the electric voltage over this

gradient.

is then calculated by dividing the ther-

moelectric voltage by the temperature difference.

The advantage of applying a constant temperature

gradient is that

is averaged over a larger volu-

me while other instruments yield very local valu-

es which may result in large scatter due to materi-

al inhomogeneity. The so achieved repeatability is

smaller than 10 nV/K.

Fig. 2 shows the increase in repeatability of the

new device (TSP) compared to the old one (TEP).

The results stem from specimens made of unir-

radiated RPV base material (GW), material from

weld (SG) and heat affected zone (WEZ). A clear

difference between GW and SG was observed,

whereas the difference between SG and WEZ

is small. It was demonstrated that the TSP de-

vice provides more precise results than the TEP

device.

3.1.2. Application of the TEP method to the

assessment of irradiated RPV material

The main goal of this subproject is to develop a

method that allows to measure the Seebeck co-

efficient on Surveillance specimens and to use the

results for the direct determination of the shift

of the ductile-to-brittle transition temperature,

which is a measure for the material embrittle-

ment. For this purpose the knowledge of materi-

al dependent calibration curves are necessary. For

this reason the Seebeck coefficient of a set irra-

diated Charpy specimens taken from the Surveil-

lance program of the KKG (base material) were de-

termined. Fig. 3 shows that the change of

bet-

ween unirradiated and irradiated material up to

a fluence of 2.6 × 10

n/cm

is about 400 nV/K.

Note that in the Charpy impact test the measu-

red specimens were broken at different tempera-

tures and are therefore partially plastic deformed.

Therefore time consuming preparation of the sam-

ples surface was needed before precise measure-

ments were possible.

3.1.3. Results and discussions

The test results of the new TSP-device are very sa-

tisfying since both the repeatability and accuracy

are excellent. Furthermore, the in-situ processing

of the measured parameters as temperatures, vol-

tage and Seebeck coefficient allows insight to the

dynamics of the measurement. This is an impor-

tant advantage towards more reliable results.

The measurement of irradiated Surveillance spe-

cimens confirmed former measurements. Howe-

ver, the inherent scatter of the material properties,

which is also reflected in the conventional Char-

py impact test results, remains relatively large. It

is therefore recommended to use the average va-

lue of a set of specimens at each fluence to ob-

serve the change of

. Single measurements on ir-

radiated, deformed and broken specimens show

a scatter that makes it difficult to distinguish the

change of

due to small increase of the fluence.

Nevertheless, the monotonic increase of

due to

Figure 3:

Seebeck coefficient

vs. fluence for KKG

specimens.

Figure 2:

Repeatability of the new

device TSP compared to

the old one TEP.

1 2 ... 156 157 158 159 160 161 162 163 164 165 166 ... 303 304

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