PSI - Issue 19

Lloyd Hackel et al. / Procedia Structural Integrity 19 (2019) 346–361 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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6. Stress Corrosion Cracking Tests

A series of accelerated CISCC tests was performed on 316 stainless steel panels prepared by Holtec to evaluate the benefit of laser peening to prevent stress corrosion cracking (CISCC) of spent fuel storage canisters. Tests were performed using a setup in the manner of ASTM G36-94 (Reapproved 2013) Standard Practice for Evaluating Stress Corrosion-Cracking Resistance of Metals and Alloys in a Boiling Magnesium chloride Solution [10]. Test plates of 316 stainless steel were rolled to canister shape, machined by Holtec using their specific joint geometry and finally joined using the Holtec welding process. Strongbacks were stitch-welded to the back of the plate to help hold the cylindrical shape of the canister from straining during the laser peening. Figure 5a shows one of the test plates oriented with the weld running left to right. Figure 5b shows a side-on view of a test plate in which the stich-welding of the stiffening ribs can be clearly seen. Because the corrosive exposure of the spent-fuel canisters in actual storage use will be to their exterior surfaces only, a more appropriate and acceptably modified version of an ASTM G36 apparatus as shown in Figure 10 was used in which a glass cell with flanged open bottom was sealed with rubber o- rings and bolts to the “exterior” face of the test plate. Magnesium chloride crystals (MgCl2-6H2O) were inserted into the glass cell and heat applied to melt the crystals. To accelerate the corrosion exposure rate, a hot plate was placed underneath and heating coils were wrapped around the glass container to provide additional heating of the chloride liquid. A condenser cooled by separated flow from a chiller was inserted into the opening on top of the glass cell. A vapor trap containing a 25 weight percent solution of magnesium chloride was placed on top of the condenser to trap and reflux and thus minimize liquid loss. Power levels were adjusted on the hot plate and heating coils to melt the salt and bring the solution to a boil. The water-salt concentration was adjusted by adding water or allowing it to evaporate by temporarily removing the condenser until the boiling temperature reached a steady 155°C liquid level stabilized. Once the test apparatus was settled in, continuous operation at 155°C was straightforward to maintain and operate for long durations. Although there is a much discussion in the literature about CISCC crack growth rates on temperature, tensile stress and other environmental conditions, in our work we simultaneously exposed unpeened and laser peened areas of the same samples. We observed extensive cracking to initiate in the unpeened area after 18 hours and reassembled and continued the simultaneous exposure for approximately 340 hours, a factor of 19 times longer, and observed no cracking in the laser peened area.

Figure 10. Test setup using MgCl 2 at elevated temperature (150-160

o C). Only top face of the steel plate is exposed to the corrosive chemistry.

Tensile stress from the welding is present at the weld joint.

In the initial run, the cell was bolted to a section of plate that had a weld joint running across the diameter. This first test was of an area not peened with the intent that it would give an indication of the minimum exposure time for cracking to occur. The test was run for 7 days and when cooled and the cell removed, extensive cracks, were observed as shown in in Figure 11, Figure 12, and Figure 13. The scale length of the most distinct crack in Figure 11 (the white