Description
Even though additive manufacturing methods have been widely used in various industries such as in aviation and aerospace fields, their usability has not been recognized in the nuclear field. The nuclear industry has strict requirements for materials and their manufacturing methods. The acceptability of these choices is strongly based on the field experience in similar operating environments. Lack of standardization and usage in nuclear environment complicates the approval of additive manufacturing. However, the adoption of these methods as an alternative way of manufacturing can provide several advantages to nuclear power plants. Additive manufacturing allows the production of parts which are no longer commercially available for purchase or are replaceable only by re-fabrication. The number of these obsolete parts commonly increases as the power plant ages.In this thesis, materials for pressure purposes, such as austenitic stainless steels, and their operating environment in Teollisuuden Voima Oyj’s (TVO) nuclear power plants were studied. TVO has two types of light water reactors: Olkiluoto 1 and 2 are boiling water reactors (BWR) and Olkiluoto 3 is a European pressurized water reactor (EPR). The main difference between these aqueous environments is that in the BWR oxygen is present in the reactor coolant, while in the EPR oxygen is reduced with hydrogen. In addition, the EPR reactor has higher radiation levels and operating temperatures than the BWR reactor. Based on the literature survey, the typical failure mechanism of pressure equipment materials is stress corrosion cracking. An experimental study was performed on additively manufactured 316L tensile and impact test rods. The test rods were manufactured by three different manufacturers and were either stress relief annealed or solution annealed. The mechanical properties were compared to EN 10222-5 Steel forgings for pressure purpose standard values of the corresponding material. The aim was to show that the requirements for mechanical properties based on the aforementioned standard are fulfilled when the rods are solution annealed. Based on various studies, structural defects in
additively manufactured parts, such as porosity and lack of fusion defects, increased the stress corrosion cracking susceptibility. The results obtained from the experimental study encourage to utilize additive manufacturing for pressure equipment. The yield and tensile strengths are much higher and the elongation to fracture values are almost as good or better than the standard requires. Solution annealing is an adequate heat treatment for additively manufactured parts. It was found that the part can be manufactured with a rather high layer thickness without deteriorating the mechanical properties. The susceptibility to stress corrosion cracking is assumed to be low because of the low internal
porosity and small number of lack of fusion defects. At the nuclear power plant additive manufacturing could be used to secure the availability of obsolete parts, especially when they are critical to the plant’s operation. A good starting point is to make non-safety classifield parts and then as the operational experience increases proceed to the safety-classified parts.
| Aikajakso | 19 huhtik. 2021 |
|---|---|
| Tutkittava | |
| Tunnustuksen arvo | National |