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A Review of Corrosion and Water Chemistry Aspects Concerning the Tokamak Cooling Water Systems of Iter

Anders Molander, Studsvik Nuclear AB, Nykφping, SE-61182, Sweden

This paper summarizes a critical review of several aspects concerning the materials behavior, water chemistry and water side corrosion in ITER environment that was recently performed. The goal of this work was to make a review of corrosion aspects for ITER, to summarize the system design from a corrosion engineering perspective and to review relevant corrosion mechanisms that are identified as important for the systems. These parts were aimed to justify requirements concerning corrosion and water chemistry and/or to point out any lacking knowledge and any need for additional work. Some key issues were identified which need further considerations and investigations to assure the operational lifetime of ITER.

Corrosion has historically caused huge costs for repair and loss of productivity in the electric industry. It seems adequate to compare the development of ITER with the pioneer construction of nuclear power plants. A number of significant failures in light water reactors caused huge problems in both pressurized water reactors (PWRs) and boiling water reactors (BWRs) historically. Steam generator corrosion in PWRs and intergranular cracking of stainless steel in BWRs are two well-known examples of very costly failures caused by corrosion. As the water environment in ITER is similar to BWRs the lessons learned should be considered for the ITER design and construction.

Important key issues high-lighted in the study were:

• Radiolysis in the ITER systems and its consequences on the redox conditions. The radiolysis conditions will settle the corrosion potential and the corrosion potential is the prime parameter for stress corrosion cracking and also for other corrosion phenomena.

• The corrosion of Cu-alloys used at least in the divertor system and the indirect influence of copper corrosion products on the stress corrosion on stainless steel..

• Stress corrosion cracking of the stainless steel Type316L(N)-IG used as structural material in most ITER systems.

• Crevice corrosion and general corrosion of shielding block materials in the vacuum vessel.

Special attention was also recommended regarding the following items:

• Corrosion of Type 316L(N)-IG stainless steel in cold worked and welded conditions. Grinding effects should be taken into account if applied during manufacturing as well as effect of other post-weld treatments. An important issue is whether or not cracks that have been initiated in cold worked material continue to grow when the cracks meet unaffected material

• General corrosion of stainless steel and copper alloys during slightly oxidizing conditions.

• Introduction and planning for water chemistry and corrosion monitoring during ITER operation.

Improvements and stricter requirements were suggested concerning a number of issues. For example:

• Revised water chemistry specifications aiming at a high purity, better controlled and less corrosive environment.

• Calculation of release rates and activity build-up caused by the large but less irradiated surfaces of the shielding plates.

• Re-consideration of hydrogen addition and incorporation of a CVCS for the vacuum vessel PHTS.

Another important finding is also that IASCC is not identified as a key issue for ITER as is the case for BWRs. However, relevant crack growth data are not available and the temperature effect is unclear. A better knowledge of the environment and the ECP would be valuable for better prediction of the risk for IASCC. Relevant crack growth data are also missing for unirradiated materials. Reliable crack growth rates are needed for flaw tolerance analyses and safety assessments. For the most important pressure boundary material of ITER such data might be needed in the future.

Work concerning some of the items mentioned above has been initiated and some results will be included. However, the main scope of this paper is to summarize the performed review.