SCC of stainless steel

Salt deposits on the canister surface open to the environment may be sig­nificant in coastal areas. SCC of the stainless steel canister needs to be considered when the relative humidity (RH) in air is appropriately high, the amount of salt deposits is sufficient to form aggressive and sufficient aqueous conditions at welds, and when a sufficient tensile stress is present. If the RH is too low, the aqueous condition would not exist. On the other hand, if RH is too high, the chloride concentration would not be high enough to initiate SCC. The weld area could have residual tensile stress and sensitized microstructure which is prone to SCC.

The RH of the environment surrounding the canister surface and salt deposits depends on the canister surface temperature. Over a long time, the surface temperature will decrease as the radioactivity inside the canister gradually decays. This will result in increasing RH of the environment immediately adjacent to the canister surface. Also, temperature, RH and the amount of salt deposits will not be homogeneous on the canister surface because of the SNF storage configuration and air flow surrounding the canister. In addition, primarily the weld areas will be susceptible to SCC. Considering these environmental and materials factors, the probability associated with SCC could be low enough for it to be screened out from performance assessment (PA), especially with appropriate remediation.

If SCC were to occur, radionuclide releases may be primarily caused by the release of aerosol radioactive materials, which may in turn be driven by the pressure of inert fill gas and fission gas inside the canister (from prior release from failed cladding). The release rates are also affected by the opening area of the canister surface caused by SCC. The SCC area density per weld area of the canister may be estimated conservatively (the estimate was originally under seismic events) by the following equation (Gwo et al., 2011):

5 = C a/E [7.1]

where 8 is crack areal density (m2/m2), a is applied stress (MPa), E is Young’s modulus (MPa) and C is geometric constant.

For example assuming no inspections and remediation, a calculation for stainless steel using Eq. [7.1] suggests that the crack areal density per unit weld area is approximately 1.2 x 10-3 at 170-310 MPa (25-45 ksi) of applied stress, (193-207) x 103MPa [(28-30) x 103ksi] of Young’s modulus (Gwo et al., 2011). The weld area fraction is about 10-2-10-1 of total surface area (ASM International, 1993). In a canister surface area of about 30 m2 (4.6 x 104 inch2), the surface opening area will become 3.6 x (102-103) mm2 (0.56­5.6 inch2). The model in Eq. [7.1] is conservative, assuming a distribution of uniform crack size. In reality, the number and size of cracks are likely to be smaller. This calculated area is obviously larger than that allowed for leak tightness (Institute for Nuclear Materials Management, 1997).

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