Interim dry cask storage systems comprising AISI 304 or 316 stainless steel canisters have become critical for the storage of spent nuclear fuel from light water reactors in the Republic of Korea. However, the combination of microstructural sensitization, residual tensile stress, and corrosive environments can induce chloride-induced stress corrosion cracking (CISCC) for stainless steel canisters. Suppressing one or more of these three variables can effectively mitigate CISCC initiation or propagation. Surface-modification technologies, such as surface peening and burnishing, focus on relieving residual tensile stress by introducing compressive stress to near-surface regions of materials. Overlay coating methods such as cold spray can serve as a barrier between the environment and the canister, while also inducing compressive stress similar to surface peening. This approach can both mitigate CISCC initiation and facilitate CISCC repair. Surface-painting methods can also be used to isolate materials from external corrosive environments. However, environmental variables, such as relative humidity, composition of surface deposits, and pH can affect the CISCC behavior. Therefore, in addition to research on surface modification and coating technologies, site-specific environmental investigations of various nuclear power plants are required.

One of the options for spent fuel dry storage systems is to store them in canisters using metal or concrete casks close to shore. The interaction between the austenitic stainless steel and the chloride atmosphere generated from the sea creates detrimental conditions leading to chloride induced stress corrosion cracking (CISCC) in the canister. The corrosion integrity of the canister in the concrete cask is very important because the canister is sealed and used for a long period of time. A canister made of austenitic stainless steel has several welding lines on the wall and lid, which are generated during the welding process and have high residual tensile stress. The interaction between the austenitic stainless steel and the chloride atmosphere generated from the sea creates detrimental conditions leading to chloride induced stress corrosion cracking (CISCC) in the canister. The corrosion integrity of the canister in the concrete cask is very important because the canister is sealed and used for a long period of time. In order to evaluate such soundness, an accelerated test capable of simulating the CISCC crack propagation phenomenon of the canister weld is required. In this study, a test device for performing the CISCC simulation test was constructed using the DCPD device. The direct current potential drop (DCPD) technique is a widely accepted method of monitoring crack initiation and growth in controlled laboratory tests. Total 10 types of test specimens with varying welds, base metal, salinity and stress were selected and a sealed chamber with DCPD test apparatus were designed and constructed to evaluate them. The chamber for CISCC simulation was manufactured as a sealed with a solution containing 10% MgCl2. A 1/2 CT specimen with precracked pre-cracks was loaded into the prepared container, and gauze was attached from the bottom for smooth delivery to the specimen to facilitate penetration of chloride. After the test, the measured DCPD data were correlated with Electron Back scattered Diffraction (EBSD) data.

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.

Safe management of spent nuclear fuel (SNF) is a key issue to determine sustainability of current light water reactor (LWR) fleet. However, none of the countries are actually conducting permanent disposal of SNFs yet. Instead, most countries are pursuing interim storage of spent nuclear fuels in dry cask storage system (DCSS). These dry casks are usually made of stainlesssteels for resistibility against cracking and corrosion, which can be occurred over a long-term storage period. Nevertheless, some corrosion called Chloride-Induced Stress Corrosion Cracking (CISCC) can arise in certain conditions, exacerbating the lifetime of dry casks. CISCC can occur if the three conditions are satisfied simultaneously: (i) residual tensile stress, (ii) material sensitization, and (iii) chloride-rich environment. A residual tensile stress is developed by the two processes. One is the bending process of stainless-steel plates into a cylindrical shape, and the other is the welding process, which can incur solidification-induced stress. These stresses provide a driving force of pit-to-crack transition. Around the fusion weld areas, chromium is precipitated at the grain boundary as a carbide form while it depletes chromium around it, leading to material susceptible to pitting corrosion. It is called sensitization. Finally, coastal regions, where nuclear power plants usually operate, tend to have a higher relative humidity and more chloride concentration compared to inland areas. This high humidity and chloride ion concentration initiate pitting corrosion on the surface of stainless-steels. To prevent initiation of CISCC, at least one of the three conditions should be removed. For this, several surface engineering techniques are under investigation. One of the most promising approaches is surface peening method, which is the process that impacts the surface of materials with media (e.g., small pins, balls, laser pulse). By this impact, plastic deformation on the surface occurs with compressive stress that counteracts with pre-existing residual tensile stress, so this approach can prevent pit-to-crack transition of stainless-steels. Also, cold spray deposition can prevent CISCC. Cold spray deposition is a method of spraying fine metal powder to a substrate by accelerating them to supersonic velocity with propellant gas. As a result, a thin coating composed of the feedstock powders can protect the substrate from outer corrosive environments. In addition, the impact of the feedstock powder on the substrate during the process provides compressive stress, similar to the peening method.

One of the options for spent fuel dry storage systems is to store them in canisters using metal or concrete casks close to shore. The interaction between the austenitic stainless steel and the chloride atmosphere generated from the sea creates detrimental conditions leading to chloride induced stress corrosion cracking (CISCC) in the canister. The corrosion integrity of the canister in the concrete cask is very important because the canister is sealed and used for a long period of time. A canister made of austenitic stainless steel has several welding lines on the wall and lid, which are generated during the welding process and have high residual tensile stress. The interaction between the austenitic stainless steel and the chloride atmosphere generated from the sea creates detrimental conditions leading to chloride induced stress corrosion cracking (CISCC) in the canister. The corrosion integrity of the canister in the concrete cask is very important because the canister is sealed and used for a long period of time. In order to evaluate such soundness, an accelerated test capable of simulating the CISCC crack propagation phenomenon of the canister weld is required. In this study, a test device for performing the CISCC simulation test was constructed using the DCPD device. The direct current potential drop (DCPD) technique is a widely accepted method of monitoring crack initiation and growth in controlled laboratory tests. In its simplest form it involves passing a constant current through the test piece and accurately measuring the electrical potential across the crack plane, and it is a suitable device to measure crack growth in real time. The requirements for the CISCC simulation test selected based on the literature search results include test material 316 L, load range 1.75YS, positive displacement load, and 7% MgCl2 concentration. In order to smoothly evaluate these various conditions, it was determined that it is advantageous to collect crack length data in real time using a DCPD device, rather than receiving and analyzing specimens maintained for a certain time in the chamber. Therefore, in this study, 4 types of test conditions in real time was built, and data collection on crack propagation could be performed in real time by using it.

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.

One of the options for spent fuel dry storage systems is to store them in canisters using metal or concrete casks close to shore. The interaction between the austenitic stainless steel and the chloride atmosphere generated from the sea creates detrimental conditions leading to chloride induced stress corrosion cracking (CISCC) in the canister. The corrosion integrity of the canister in the concrete cask is very important because the canister is sealed and used for a long period of time. A canister made of austenitic stainless steel has several welding lines on the wall and lid, which are generated during the welding process and have high residual tensile stress. The interaction between the austenitic stainless steel and the chloride atmosphere generated from the sea creates detrimental conditions leading to chloride induced stress corrosion cracking (CISCC) in the canister. The corrosion integrity of the canister in the concrete cask is very important because the canister is sealed and used for a long period of time. In order to evaluate such soundness, an accelerated test capable of simulating the CISCC crack propagation phenomenon of the canister weld is required. In this study, the current status of CISCC simulation tests performed around the world to build a test equipment for the CISCC simulation accelerated test is investigated, and based on this, the test conditions suitable for the simulation test and specimen specifications are selected to establish the test equipment. The settings were performed. In consideration of the set device requirements, the essential limiting conditions for device manufacturing were derived, and detailed design was performed to satisfy them, and it was used to build a CISCC simulation test device for welding parts. The CISCC simulation test equipment requires performance to maintain the test temperature range of room temperature to 80°C and humidity 35 to 95%. In addition, it should be manufactured in consideration of humidity and temperature maintenance in the chamber of the complex corrosion tester, measures to prevent leakage of the connection part between the chamber and the salt water tank of the complex corrosion tester, and measures to supply stable salt water and maintain temperature in the salt water tank. Based on these contents, detailed specifications and design contents of the chloride stress corrosion cracking simulation test apparatus were presented in this study.

국내의 사용후핵연료가 증가함에 따라 사용후핵연료 저장조는 곧 포화가 될 것으로 예상된다. 따라서 사용후핵연료 건식저장 운영 및 관리 방안에 대해 연구하는 것은 매우 중요하다. 미국에서는 오랜 기간 건식저장을 운영해왔으며 이를 바탕으로 사용후핵연료 건식저장 운영 및 관리 방안에 대해 많은 연구가 수행되고 있다. 그러나 우리나라에서는 경수로 사용후핵연료 건식저장 경험이 없으며 관련 관리방안 및 구체적인 기준이 매우 부족한 현실이다. 건식저장기간 동안 주요한 이슈중의 하나는 건식저장용기 열화현상이며 대표적으로 응력부식균열에 의한 부식현상이 있다. 미국에서는 U.S. DOE, U.S. NRC, 그리고 EPRI 주관 아래 건식저장 캐니스터에서의 염화물 응력부식균열에 관한 많은 연구들을 수행하고 있다. 또한 건식저장 캐니스터의 염화물 응력부식균열 현상을 설명하기 위해 SNL에서는 확률론적 응력부식균열 모델을 제시하였다. 본 논문에 서는 SNL에서 제시한 확률론적 응력부식균열 모델을 검토하였으며 모델에 제시된 주요인자들을 세세하게 분석하였다. 본 논문은 우리나라에서 스테인리스 스틸로 제작된 캐니스터를 경수로 사용후핵연료 건식저장으로 이용할 경우, 건식저장 운영 및 관리 방안을 구축하는 대에 좋은 참고문헌이 될 것이라 사료된다.