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.

According to the ‘Basic Plan for High-Level Radioactive Waste Management (draft)’, the total amount of CANDU spent nuclear fuel is expected to be approximately 660,000 bundles. To safely and efficiently transport this amount to interim storage facilities, it is essential to develop a large-capacity transport cask. Therefore, we have been developing a large-capacity PHWR spent nuclear fuel transport cask, called the KTC-360 transport cask. According to the transport-cask related regulations, the KTC-360 transport cask was classified as a Type B package, and such packages must be able to withstand a temperature of 800°C for a period of 30 min. It is desirable to conduct a test using a fullscale model of a shipping package when performing tests to evaluate its integrity. However, it is costly to perform a test using a full-scale model. Therefore, to evaluate the thermal integrity of the KTC-360 transport cask, the fire test was conducted using a slice model. For comparison purposes, the fire test was also carried out using a 1/4 scale model. In the fire test using a slice model and in the fire test using a 1/4 scale model, the maximum temperature of the cask body was lower than the permitted maximum temperature limit. Therefore, the thermal integrity of the KTC-360 transport cask could be considered to be maintained. The temperature results from the fire test using a slice model were higher than those of the fire test using a 1/4 scale model. Therefore, the effect of flame on a transport cask without combustible materials, such as the KTC-360 transport cask, seems to be affected by the reduction in the time rather than the size reduction.

The safety of a KTC-360 transport cask, a large-capacity pressurized heavy-water reactor transport cask that transports CANDU spent nuclear fuel discharged from the reactor after burning in a pressurized heavy-water reactor, must be demonstrated under the normal transport and accident conditions specified under transport cask regulations. To confirm the thermal integrity of this cask under normal transport and accident conditions, high-temperature and fire tests were performed using a one-third slice model of an actual KTC-360 cask. The results revealed that the surface temperature of the cask was 62°C, indicating that such casks must be transported separately. The highest temperature of the CANDU spent nuclear fuel was predicted to be lower than the melting temperature of Zircaloy-4, which was the sheath material used. Therefore, if normal operating conditions are applied, the thermal integrity of a KTC-360 cask can be maintained under normal transport conditions. The fire test revealed that the maximum temperatures of the structural materials, stainless steel, and carbon steel were 446°C lower than the permitted maximum temperatures, proving the thermal integrity of the cask under fire accident conditions.

Currently, the HI-STAR 63 transport cask, developed to transport CANDU spent nuclear fuel from the wet storage pool to the dry storage facility which is called the MACSTOR/KN-400, has a transport capacity of 120 bundles, which is unfavorable when considering transportation costs and other related aspects. According to the ‘Basic Plan for High-Level Radioactive Waste Management (draft)’, the total amount of CANDU spent nuclear fuel is expected to be approximately 660,000 bundles. To safely and efficiently transport this amount to interim storage facilities, it is essential to develop a large-capacity transport cask. Therefore, we have been developing a large-capacity PHWR spent nuclear fuel transport cask, called the KTC-360 transport cask. According to the transport-cask related regulations, the KTC-360 transport cask was classified as a Type B package, and such packages need to maintain integrity under the normal transport and accident conditions described in these regulations. To prove the thermal integrity of this cask under the normal transport and accident conditions, high-temperature and fire tests were performed using a one-third slice model of an actual KTC-360 cask. The results revealed that the surface temperature of the cask was 62°C, indicating that such casks need to be transported exclusively. The highest temperature of the CANDU spent nuclear fuel was predicted to be lower than the melting temperature of Zircaloy-4, which was the sheath material used. Therefore, if normal operating conditions are applied, the thermal integrity of a KTC- 360 cask could be maintained under normal transport conditions. The fire test revealed that the maximum temperatures of the structural materials, stainless steel, and carbon steel, were 446°C lower than the permitted maximum temperatures, proving the thermal integrity of the cask under fireaccident conditions.