Type 304 Stainless Steel With 0.5% Boron for Storage of Spent Nuclear Fuel

Summary

For many years, Type 304 stainless steel with 1–2 wt.% boron has been used for control rods and thermal shielding of nuclear reactors, and for spent-fuel storage racks. There is current interest in Type 304 with 0.5–0.7% boron for the latter application; this paper presents a metallurgical evaluation at 0.53% boron. As measured by Gleeble hot tensile testing, strength of the alloy is no more than 25% higher than standard 304, but ductility is reduced over the 900–1180°C range and hence would require special care in hot working. A uniform density and concentration of particles occur at grain boundaries which are 1 or 6 microns, and which analyze ∼50% chromium via EDAX/SEM. These chromium boride particles are very distinct and dominate TEM sections which reveal no “common” sensitization, as no carbides are present at grain boundaries. However, this does not exclude chromium depletion adjacent to the particles. Polarization measurements in 2.5N and 1.0N H₂SO₄ are presented along with results in oxalic acid and HNO₃−HCl etching solution. From the electrochemical measurements, it is apparent that Type 304 with 0.53% boron is susceptible to intergranular attack. In non-sensitized specimens, the presence of boride particles may either accelerate intergranular corrosion or tend to suppress it, depending upon the environment.