Nuclear Science and Engineering Research

4-3 Measurement of the Corrosive Environment in High-Temperature, High-Pressure Water within Light Water Reactors
ᴪ Development of an Electrochemical Measuring Technique and Evaluation of the Corrosive Condition in High-Temperature Pure Water ᴪTop view
Inner diameter
0.8 cmTest vessel

60

Test
specimen

Counter
electrodeDistance 0.3 cm

30 cm(Innerdiameter3 cm)

PTFE jigs and test specimen
Test specimen

Hightemperaturewater
circulation
systeminstalled

40
30
2010

Counter
electrode
Jigs made of PTFE

Corrosion rate (μm/year)

501 cm

8 cm

Measured corrosion rate
Fitting curve

0050100150

200250300

Hydrogen peroxide concentration (ppb)
H2O2 addition

Fig.4-7 Electrochemical-test vesselThis vessel is designed for use under high-pressure; hightemperature water is caused to flow inside it via a circulation
system. The size of the vessel was logically minimized andPTFE jigs have been installed. Such special designs could
permit electrochemical measurements with high accuracy undersimulated BWR conditions. The test pressure was 80 atm, larger
than that in the BWRs, to prevent the boiling of the water.

Fig.4-8 The relationship between the corrosion rate
determined by the resistance measurements and thehydrogen peroxide concentration
The relationship confirmed that the corrosion rate of stainlesssteel in high-temperature water containing hydrogen peroxide
was proportional to the hydrogen peroxide concentration.

Degradation of materials owing to corrosion (e.g., stresscorrosion cracking) is a major problem in boiling water reactors(BWRs). Electrochemical measurement in simulated BWR
cooling-water conditions is a major approach to understandthe mechanism of corrosion. The cooling water in a BWR
is pure light water with high temperature (288 °C) and highpressure (70 atm). Hydrogen peroxide (H2O2), which has highoxidizability, is generated owing to irradiation. The hot purewater has a high electric resistance, while H2O2 decomposes
easily at high temperature (it disappears after approximately
1 min) and this decomposition is further enhanced by contact
with the surfaces of the metals. Thus, electrochemicalmeasurements in simulated BWR conditions are difficult
to perform. Therefore, we developed a system to measureelectrochemical data under such conditions.
A test vessel designed to maintain high pressure is shown in
Fig.4-7. High-temperature water flows into the test vessel inwhich the vessel specimen was installed for electrochemical
examinations. The high electric resistance of water caused apotential drop in the water when the current ﬂowed. To reduce
this drop, we minimized the path of current ﬂow between thetest specimen and the counter electrode to a distance of 0.3 cm.
To prevent the disappearance of H2O2 through decomposition,
we performed the measurements as quickly as possible after
the H2O2 was added to the test specimen. We also minimized
the distance between the addition point of the H2O2 and thetest vessel to approximately 8 cm as well as the volume of thetest vessel. To prevent the decomposition of H2O2 at the vessel
surface, we installed jigs composed of polytetraﬂuoroethylene(PTFE), in which the decomposition was much smaller than thatof the metals. These enabled us to obtain the electrochemical
data with greater precision in simulated BWR conditions.
The measurements of the impedances, which areelectrochemical parameters, were performed. The impedanceswere measured by supplying a sinusoidal AC voltage between
two stainless-steel specimens installed inside the vessel. The
impedances corresponding to the conductivity, the composition,and the thickness of the oxide ﬁlm as well as the corrosion rate
could be measured by changing the frequency of the supplied
AC voltages. Obtained impedances were inversely proportional
to the current that flowed at the surface of the stainless steel,which was proportional to the stainless steel’s corrosion
rate. Therefore, the corrosion rate could be determined bymeasurements of the impedances. The obtained stainlesssteel corrosion rates are shown in Fig.4-8 alongside H2O2
concentrations; we confirmed that the two quantities wereproportional. This is expected to be important informationin understanding the mechanism of degradation due to thecorrosion. Moreover, we will improve this electrochemical test
equipment such that it can be applied to the measurements ofother electrochemical parameters.

Reference
Satoh, T. et al., In-situ Measurement of Corrosion Environment in High Temperature Water without Electrolyte Utilizing Electrochemical ImpedanceSpectroscopy, Zairyo to Kankyo (Corrosion Engineering of Japan), vol.64, no.3, 2015, p.91-97 (in Japanese).

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JAEA R&D Review 2016-17