The improvement of manufacturing efficiency of overpack, HLW disposal container, was studied. Cast steel is a promising alternative to forged steel that is the current reference material. NUMO has been studying the applicability of cast steel to overpack. Cast steel contains casting defects and the required corrosion allowance may not be ensured if defects are distributed in the corrosion allowance layer. We therefore produced the full-scale cast steel overpack experimentally and investigated the distribution of casting defects. The corrosion test regarding corrosion rate and stress corrosion cracking (SCC) susceptibility were also conducted using samples taken from the full-scale cast steel overpack and the corrosion resistance of cast steel was compared with that of forged steel.
For detecting the distribution of casting defects, magnetic particle testing (MT) and ultrasonic testing (UT) were conducted as non-destructive testing (NDT) of the full-scale cast steel overpack (Shape: Cylindrical, Outer diameter: 820 mm, Height: 1750 mm, Thickness: 190 mm). As a result of NDT, although the size of detected defects was so small that the size was lower than the acceptance criteria used in codes for manufacturing of nuclear reactor pressure vessel of Japan, small casting defects were distributed from the outer surface to the center of the thickness. This result means that there are defects in the corrosion allowance layer of overpack.
Regarding the experiment of corrosion rate, immersion tests of cast steel samples were performed in compacted bentonites saturated with synthetic sea water as a simulated groundwater for a 1-year duration at 80°C under anaerobic condition. Prior to corrosion tests, heterogeneities of chemical composition and metallographic microstructure in the full-scale cast steel overpack were examined and no significant difference of them was confirmed. Corrosion rates of cast steel samples for a 1-year duration were less than 2 μm/y and almost the same with that of forged steel.
SCC susceptibility was examined by means of the slow strain rate testing (SSRT). SSRT were conducted in carbonate-bicarbonate solutions, which are known as the representative environment where SCC occurs on carbon steel and many SCC data have been acquired subject to forged steel so far. The test solutions were prepared from 1 M NaHCO3 and 0.5 M Na2CO3 solution. Although the carbonate concentration of the disposal environment will not be such high, to compare SCC susceptibility of cast steel and that of forged steel, highly concentrated carbonate solution that is known to promote SCC of carbon steel was used in this study. Temperature was set to 80°C. The applied potential ranged from -775 mVSCE to -625 mVSCE, corresponding to the active-passive transition zone of anodic polarization curves. The ratio of strain-to-failure in a carbonate-bicarbonate solution to that in silicone oil, corresponding to SCC susceptibility was compared with regard to cast steel and forged steel. As a consequence, the SCC susceptibility of cast steel found to be almost the same with forged steel on condition that the concentrations of carbon in both steels are the same.
From above two corrosion tests, it can be said that the corrosion resistance of cast steel is mostly the same as that of forged steel. However, to ensure the required corrosion allowance, the detailed size and shape of casting defects should be determined using such as X-ray computed tomography (CT) and the additional thickness might have to be provided to corrosion allowance.
Cast steel, Overpack, Corrosion rate, SSRT, X-ray CT