Effect of Re-Oxidation Rate of Additive Cation on Corrosion Rate of Stainless Steel in Boiling Nitric Acid Solution.
Authors: Yamamoto Masahiro , Kato Chiaki , Motooka Takafumi , Ban Yasutoshi , Ueno Fumiyoshi 
Published: February 2020

Fuel reprocessing plant is important for atomic energy cycle which is aimed to re-use uranium (U) and plutonium (Pu) from the spent nuclear fuels after barning-out in light water reactor. PUREX process, in which boiling nitric acid is used for dissolving fuel elements, is adopted in Japan. Many structural materials are used stainless steels protected by passive layers of the surface. At the beginning of developing the plant, many troubles of intergranular corrosion of stainless steels were experienced. According to the investigations of these problems, very low carbon type austenitic stainless steels were utilized for the materials.

However, these stainless steels were corroded in some cases. For example, addition of hexavalent chromium (Cr (VI)) is heavily accelerated corrosion rate of these stainless steels in boiling nitric acid solutions. The corrosion potential of stainless steel in Cr (VI) added solution is much higher than that in pure nitric acid solution.

The authors introduced that heptavalent vanadium (V (V)) was also accelerated corrosion rate of these stainless steels but the tendency of acceleration process was different from the case of containing Cr (VI). We analyzed the valence stats of Cr and V during corrosion test and showed that Cr (VI) reduced to Cr (III) with corrosion of stainless steel propagating. On the other hand, amount of V (V) was not changed during the corrosion test. It was concluded that V (IV) was easily re-oxidized in boiling nitric acid solution. Namely, V (V) is reduced to V (IV) on stainless steel surface by cathodic reaction of corrosion and then V (IV) is re-oxidized immediately to V (V) in the balk nitric acid solution.

These data were obtained using non-radioactive solutions in the ordinary laboratory. The present work is aimed to compare the data of re-oxidation rate of actinides which are included in the actual solutions used in a reprocessing plant. We implemented a specially designed electrochemical test cell used for analyzing the valence states of actinide cations, i.e. neptunium (Np) and Pu, in a boiling nitric acid solution. The cell was set into glovebox in a hot laboratory which could be handled nuclear fuel materials. The cell was designed for minimizing the solution. The amounts of solution used are about 15 cm3 and sample size of working electrode is about 4 mm2. Electrochemical measurements and spectrum analyses were conducted with this apparatus. Analyzed cations were Cr (VI), V (V), Pu (VI) and Np (VI).

Polarization curves showed that corrosion potentials of stainless steels changed to higher direction by addition of these cations and also corrosion current densities increased. The higher order of those is Cr (VI), Np (VI), V (V) and Pu (VI). Re-oxidation rates were obtained by the changes of absorption spectra with time using lower valence state ions added solutions, i.e. Cr (III), V (IV), Pu (IV) and Np (V). These species are re-oxidized to Cr (VI), V (V), Pu (VI) and Np (VI), respectively. Re-oxidation rates of them were very much different among them. Very fast re-oxidation rates were obtained by the V (IV) and Np (V) added solutions. A very low re-oxidation rate was obtained by the Cr (III) added solution. The Pu (IV) added solution was in the middle of them.

The corrosion of stainless steel is controlled by corrosion potential of cation added solution at the initial stage and controlled by re-oxidation rate of cation as time passed. The element Np which indicates high corrosion potential and has fast re-oxidation rate influences too much the corrosion of stainless steel in boiling nitic acid solution among these elements in consideration.