Nuclear Science and Engineering Research

8-3Life Prediction of Material in Nuclear Reprocessing Plants

Cooling condenser

Insulated cylindricalelectrode
Electrochemicalbridge

Corrosion potential Ecorr (V)－ Corrosion Characteristics of Stainless Steel in Boiling Nitric Acid Solution Including Np －3 mol/dm 3 HNO3+ initial Np(VI)

SUS310Nb
Np: 5.3x10 -3 mol/dm3

1.0

3 mol/dm 3 HNO3+ initial Np(V)

0.83 mol/dm 3 HNO3
0.6

0.4

0.2300

320

340

360

380

Temperature (K)Fig.8-8 Effect of temperature on corrosion potential ofstainless steel (SUS310Nb)
The increasing corrosion potential caused by a slight amountof Np accelerates corrosion; the behavior depends on thevalence of the Np and the temperature.

Optical fiber for
remote spectrum analyzer
Fig.8-7 Appearance of developed electrochemicalcell including optical cell for spectral analysis
The test cell is combined with a transparent heater,a potential probe, and an optical cell. It is operated
remotely. The amount of solution is approximately 15 cm3.

Nuclear reprocessing has been promoted for the reuseof uranium (U) and plutonium (Pu) in spent reactor fuels
worldwide. A corrosive nitric acid solution is used in nuclear
reprocessing, which involves materials containing metallic ionsthat accelerate corrosion. Therefore, accelerated corrosion ofstainless steel, one of the component materials, is an issue innuclear reprocessing plants.
It is well known that among metallic ions, Pu and neptunium(Np), which are oxidized in boiling nitric acid solution tometallic ions in higher oxidation states, accelerate corrosion
the most. Furthermore, the amount of Pu and Np that canbe handled in an experiment is restricted because they areradioactive elements.
We developed a specially designed small electrochemical test
cell integrated with an optical cell for spectroscopic analysis
in order to concomitantly evaluate the oxidation states of Np
ions and the polarization curves of stainless steel in boiling
nitric acid solution. The goal of this study is to understand thecorrosion acceleration mechanism of stainless steel.
The test cell shown in Fig.8-7 enables (1) the handling of asmall amount (ca.15 cm3) of test solution, (2) simultaneous

Np(V)
Np(VI)

100

Existence ratio (%)

Optical cell forspectral analysis
Quartz glass cell withtransparent heater

80604020
Initial Np(V): 5.3x10-3 mol/dm30300

320340360380

Temperature (K)Fig.8-9 Effect of temperature on valence change in Np
Pentavalent Np(V) is oxidized to hexavalent Np(VI) byincreasing the temperature of the solution. This suggeststhat the oxidized Np(VI) ions accelerate corrosion.

spectroscopic characterization and electrochemical monitoringof the solution, and (3) stable electrochemical measurements ina boiling nitric acid solution.
Fig.8-8 shows that the corrosion potential Ecorr of thestainless steel increases with increasing temperature whenthe solution contains Np ions. The presence of Np ionsaccelerates the corrosion of stainless steel by increasing thecorrosion potential. Further, the corrosion potential in thesolution containing Np(VI), in a higher valence state, does notchange with increasing temperature, whereas that of a solution
containing Np(V), in a lower valence state, increases.
Fig.8-9 shows the existence ratios of Np(VI) and Np(V) in3 mol/dm 3 HNO 3 with initial Np(V) ions at various
temperatures. The existence ratio of Np(VI) increases withincreasing temperature.
In conclusion, Np(VI) oxidized by nitric acid was found toraise the corrosion potential and accelerate the corrosion ofstainless steel.
This work was part of work commissioned by the Japan
Nuclear Energy Safety Organization (JNES).

Reference
Kato, C. et al., Electrochemical Measurements in Boiling Nitric Acid Solutions Containing Radioactive Elements by using Small Cell with a Portion forSpectral Analysis Function, Zairyo to Kankyo, vol.60, no.2, 2011, p.69-71 (in Japanese).

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JAEA R&D Review 2013