An austenitic stainless steel canister functions as a containment barrier for spent nuclear fuel and radioactive materials. The canister on the spent fuel storage system near the coastal area has several welding lines in the wall and lid, which have high residual tensile stresses after welding procedure. Interaction between austenitic stainless steel and chloride environment from a sea forms a detrimental condition causing chloride induced stress corrosion cracking (CISCC) in the canister. The South Korea is concerned with the dry storage of high-level spent nuclear fuel and radioactive wastes to be built on the site of a nuclear power plant. The importance of aging management has recently emerged for mitigating CISCC of dry storage canisters. When a corrosive pit is created by a localized corrosion in a sea water atmosphere, it initiates and grows as CISCC crack. Surface stress improvement works by inducing plastic strain which results in elastic relaxation that generates residual compressive stress. Surface stress improvement methods such as roller burnishing process can effectively mitigate the potential for CISCC of the canister external surfaces. The generation of compressive stress layer can inhibit the transition to cracking initiation. In this study, a flat roller burnishing process was applied as a prevention technology to CISCC of stainless steel canisters. Roller burnishing process parameters have been selected for 1:3 scale canister model having a diameter of 600 mm, a length of 1,000 mm and a thickness of 10 mm on the basis of the burnishing conditions available to control residual tensile stress of austenitic stainless steel plate specimens. The surface roughness of the scaled canister model was investigated using a surface roughness measurement equipment after roller burnishing treatment. The surface residual stresses of the scaled canister model were measured by a hole drilling contour method attached with strain gauge. The burnishing test results showed that the surface roughness of the scaled canister model was considerably improved with flat rollers having the tip width of 4 mm. The surface of the scaled canister model had significant residual compressive stress after burnishing treatment. The roller burnished canister with good surface roughness could reduce the number of crack initiation sites and the residual compressive stress formed on the welded surface might prevent the crack initiation by reducing tensile residual stress in the weld zone, finally leads to CISCC resistance.

The spent fuel storage canister is generally made of austenitic stainless-steel and has the role of an important barrier to encapsulate spent fuels and radioactive materials. Canister near coastal area has welding lines, which have high residual tensile stresses after welding process. Interaction between austenitic stainless steel and chloride environment forms detrimental condition causing chloride induced stress corrosion cracking (CISCC) in canister. Reducing or eliminating tensile stress on canister can significantly decrease probability of crack initiation. Surface stress improvement works by inducing plastic strain which results in elastic relaxation that generates compressive stresses. Surface stress improvement methods such as burnishing process can effectively prevent for CISCC of canister surfaces. In this study, burnishing treatment has been evaluated to control residual tensile stress practically applicable to atmospheric CISCC for aging management of steel canisters. Burnishing process was selected as a prevention technology to CISCC of stainless steel canisters to improve resistance of CISCC through enhancement of surface roughness and generation of compressive residual stress. SUS 316 SAW (Submerged Arc Welding) specimens were burnished with flat roller and round roller after manufactured and assembled on CNC machine using base plate. The burnishing test results showed that the surface roughness of SUS 316 SAW welded specimens after roller burnishing of pass No. 5 was improved with 85% with flat roller and 93% with round roller, individually. Surface roughness showed the best state when burnished at pressure of 115 kgf, feeding rate of 40 m/stroke and pass No. of 5 turns with round roller. The surface of SUS 316 SAW welded specimens had much high residual compressive stress than yield stress of SUS 316 materials with roller burnishing treatment, independently of kinds of roller. The surface of the welded specimen by round roller burnishing showed smaller compressive stress and deeper stress region than in the surface of flat round roller burnishing. The roller burnished canister with good surface roughness could reduce the number of crack initiation sites and the high residual compressive stress formed on the welded surface might prevent the crack initiation by reducing or eliminating tensile residual stress in the weld zone, finally leads to excellent CISCC resistance. The crack growth behavior of SUS 316 welded specimens will need to investigate to evaluate the corrosion integrity of the canister materials under chloride atmosphere according to burnishing treatment.

The spent fuel dry storage canister is generally made of austenitic stainless-steel and has the role of an important barrier to encapsulate spent fuels and radioactive materials. The canister on the dry storage system has several welding lines in the wall and lid, which have high residual tensile stresses after welding procedure. Interaction between stainless steel and chloride environment from a sea results in an aged-related degradation phenomenon causing chloride induced stress corrosion cracking (CISCC) in the dry storage system. A pending issue to the interim storage of spent fuel awaiting repository disposal is their susceptibility to CISCC of stainless steel canisters. The available mitigation technology should be studied sufficiently to prevent the degradation phenomenon. This paper assesses stress-based mitigation to control residual tensile stress practically applicable to the atmospheric CISCC for the aging management of the stainless steel canisters. There are major components, that is, elevated tensile stress, susceptible material and corrosive environment that must be simultaneously present for CISCC degradation to occur. Surface stress improvement can effectively mitigate the potential for CISCC of the canister external surfaces. The potential deleterious effect of the additional work is negated by the presence of compressive residual stress, which removes the tensile stress needed for CISCC to occur. Surface stress improvement methods such as shock-based peening, shot peening and low plasticity burnishing can be applied for surface stress improvement of the canisters. Stress relaxation processes and advanced welding methods such as laser beam welding and friction stir welding can be also available to mitigate the susceptibility to CISCC. As the result assessing the stress-based mitigation technologies, promising candidate methods could be selected to reduce the residual tensile stresses and to control an aged-related degradation condition causing CISCC in the spent fuel dry storage canister.