NSUF 16-CINR-10181: Effects of High Dose on Laser Welded, Irradiated AISI 304SS
The objective of this project is to assess the mechanical integrity of laser weld repairs of highly irradiated, He-containing AISI 304 stainless steel under extended LWR service conditions.Life extension of commercial light water reactors (LWR) raises concerns about possible cracking of critical in-core or near-core components. Cracks, accelerated by and often attributed to, irradiation damage, have already been observed in BWR shrouds. There is a long-term concern that PWR baffle-former plates may also crack. Attempts have been made to repair such cracks by welding [1], but weld repairs of in-core or near-core components are complicated by helium accumulation in the material during reactor operation. Heat input from welding causes He bubbles to coalesce on the weld melt boundary and on grain boundaries. Thermal stresses during weld solidification result in He-induced cracks outside the heat affected zone. An additional complication in the study of weld repairs is that LWR internals span a range of irradiation damage levels and He concentrations, so weld technologies must be adaptable to a variety of conditions. BWR shrouds, for example, experience a low irradiation damage accumulation rate, whereas PWR baffle plates experience a higher irradiation damage rate, with void swelling and greater radiation-induced segregation. These microstructural factors impact post-weld cracking. Thus, there is a critical need to (1) demonstrate crack-free weld repairs on representative BWR and PWR internal materials, then (2) assess the integrity of these welds over extended irradiation service. The proposed study will utilize AISI 304 stainless steel hexagonal blocks irradiated in the EBR-II reflector. These hex blocks are ideal for this study because they were produced using technology typical of the period during which the baffle-former plates in current PWRs were produced. Their microstructure is well-characterized [2], their He concentration spans the critical range over which conventional welding techniques will produce He-induced cracks (Fig. 2), and they have manageable radioactivity. The hex blocks are stored in the Westinghouse Materials Center of Excellence (MCOE) hot cells, a NSUF partner facility. The team hypothesizes that low-energy input laser welding will minimize stresses driving cracking and He coalescence at the weld boundary. Fortuitously, MCOE is the only facility worldwide with in-hot-cell laser welding capabilities. This project will make laser welds on the hex blocks over a range of He and swelling conditions, without the need for extensive pre-characterization. Weld cross-sections will be examined for cracking. Since the welds must maintain integrity under further irradiation, weld cross-sections will be subjected to additional irradiation at PWR-relevant temperatures using self-ions to doses as high as 200 displacements per atom (dpa). Subsequently, welds will be reexamined for microstructure, mechanical properties, and cracking.
Additional Info
| Field | Value |
|---|---|
| Abstract | The objective of this project is to assess the mechanical integrity of laser weld repairs of highly irradiated, He-containing AISI 304 stainless steel under extended LWR service conditions.Life extension of commercial light water reactors (LWR) raises concerns about possible cracking of critical in-core or near-core components. Cracks, accelerated by and often attributed to, irradiation damage, have already been observed in BWR shrouds. There is a long-term concern that PWR baffle-former plates may also crack. Attempts have been made to repair such cracks by welding [1], but weld repairs of in-core or near-core components are complicated by helium accumulation in the material during reactor operation. Heat input from welding causes He bubbles to coalesce on the weld melt boundary and on grain boundaries. Thermal stresses during weld solidification result in He-induced cracks outside the heat affected zone. An additional complication in the study of weld repairs is that LWR internals span a range of irradiation damage levels and He concentrations, so weld technologies must be adaptable to a variety of conditions. BWR shrouds, for example, experience a low irradiation damage accumulation rate, whereas PWR baffle plates experience a higher irradiation damage rate, with void swelling and greater radiation-induced segregation. These microstructural factors impact post-weld cracking. Thus, there is a critical need to (1) demonstrate crack-free weld repairs on representative BWR and PWR internal materials, then (2) assess the integrity of these welds over extended irradiation service. The proposed study will utilize AISI 304 stainless steel hexagonal blocks irradiated in the EBR-II reflector. These hex blocks are ideal for this study because they were produced using technology typical of the period during which the baffle-former plates in current PWRs were produced. Their microstructure is well-characterized [2], their He concentration spans the critical range over which conventional welding techniques will produce He-induced cracks (Fig. 2), and they have manageable radioactivity. The hex blocks are stored in the Westinghouse Materials Center of Excellence (MCOE) hot cells, a NSUF partner facility. The team hypothesizes that low-energy input laser welding will minimize stresses driving cracking and He coalescence at the weld boundary. Fortuitously, MCOE is the only facility worldwide with in-hot-cell laser welding capabilities. This project will make laser welds on the hex blocks over a range of He and swelling conditions, without the need for extensive pre-characterization. Weld cross-sections will be examined for cracking. Since the welds must maintain integrity under further irradiation, weld cross-sections will be subjected to additional irradiation at PWR-relevant temperatures using self-ions to doses as high as 200 displacements per atom (dpa). Subsequently, welds will be reexamined for microstructure, mechanical properties, and cracking. |
| Award Announced Date | 2019-12-19T00:00:00 |
| Awarded Institution | Idaho National Laboratory |
| Facility | Advanced Test Reactor |
| Facility Tech Lead | Alina Zackrone |
| Irradiation Facility | None |
| PI | Janelle Wharry |
| PI Email | [email protected] |
| Project Type | CINR |
| RTE Number | 3045 |