NSUF 20-2954: Radiation-accelerated, strain-induced martensitic transformation and its potential impact on performance of 304 stainless steel irradiated to high doses in PWRs following plant life extension
Many pressurized water reactors (PWRs) are extending their licensed life from 40 years to 60 or possibly 80 years and their internal structural components will experience neutron exposures greatly exceeding any levels obtained for currently operating or decommissioned reactors. It is anticipated that some currently recognized second-order degradation phenomena may grow to first-order importance during extended lifetimes and it is prudent to explore the possibility of such growth. One of the identified phenomena is the increasing proclivity of 300 series stainless steels to progressively develop deformation-induced martensite with increasing dose. 304 stainless steel used to construct the majority of core internals is thought to be particularly vulnerable to this issue. One startling aspect of this tendency is the traveling “deformation wave” phenomenon observed during tensile testing at high dose and low temperature where necking cannot occur and a deformation front moves completely along the specimen, leaving behind a region with 25-50% martensite although the engineering ductility may be significantly restored compared to losses at lower doses. At very high dose this high-energy-density phase may form on the core-facing surface of PWR 304 stainless baffle plates as a result of relatively minor scratches or bumps occurring during outages for fuel removal or surveillance inspection, possibly producing shallow surface layers more vulnerable to corrosion and perhaps cracking. This surface concern may outweigh the impact of this phenomenon on bulk mechanical properties.
This project will employ irradiated 304 stainless steel derived from EBR-II hex-blocks at 1-28 dpa, steel produced 60 years ago with the same chemical and production specifications used to build currently operating PWRs. The specific focus of this project will be to explore the origins of this late-developing instability phenomenon, following up on clues observed in several earlier studies where the deformation wave front exhibited several very pronounced tendencies associated with irradiation dose, irradiation and test temperature, and especially crystalline orientation of each grain on the front and the irradiation and deformation microstructure within each grain. The irradiation-induced microstructure of these specimens (loops, dislocations, voids, precipitates) have already been determined for these specimens, establishing their relevance to PWR baffle plates at high doses.
The specimens currently exist as ~1cm square plates at ~1 mm thickness and will be used to produce mini-tensile specimens, which will be tensile-tested within a TESCAN MIRA3 SEM/EBSD/EDS facility. Periodically the test will be interrupted and EBSD measurements will be performed to measure the evolving deformation and crystallographic orientation. At the end of the test, areas of identified high importance will be FIBed to produce lamella specimens to obtain via TEM microstructural and microchemical information that may illuminate the operating processes in the martensite instability process.
The project can be completed in 6 months and will provide guidance concerning the necessity 1) to decide whether follow-on studies involving corrosion of hex-block material, 2) to conduct additional studies using PWR-irradiated material and 3) to consider possibly adding this concern to current surveillance requirements for operating PWRs, especially as the reactors move to ever higher neutron exposures.
Additional Info
| Field | Value |
|---|---|
| Abstract | Many pressurized water reactors (PWRs) are extending their licensed life from 40 years to 60 or possibly 80 years and their internal structural components will experience neutron exposures greatly exceeding any levels obtained for currently operating or decommissioned reactors. It is anticipated that some currently recognized second-order degradation phenomena may grow to first-order importance during extended lifetimes and it is prudent to explore the possibility of such growth. One of the identified phenomena is the increasing proclivity of 300 series stainless steels to progressively develop deformation-induced martensite with increasing dose. 304 stainless steel used to construct the majority of core internals is thought to be particularly vulnerable to this issue. One startling aspect of this tendency is the traveling “deformation wave” phenomenon observed during tensile testing at high dose and low temperature where necking cannot occur and a deformation front moves completely along the specimen, leaving behind a region with 25-50% martensite although the engineering ductility may be significantly restored compared to losses at lower doses. At very high dose this high-energy-density phase may form on the core-facing surface of PWR 304 stainless baffle plates as a result of relatively minor scratches or bumps occurring during outages for fuel removal or surveillance inspection, possibly producing shallow surface layers more vulnerable to corrosion and perhaps cracking. This surface concern may outweigh the impact of this phenomenon on bulk mechanical properties. This project will employ irradiated 304 stainless steel derived from EBR-II hex-blocks at 1-28 dpa, steel produced 60 years ago with the same chemical and production specifications used to build currently operating PWRs. The specific focus of this project will be to explore the origins of this late-developing instability phenomenon, following up on clues observed in several earlier studies where the deformation wave front exhibited several very pronounced tendencies associated with irradiation dose, irradiation and test temperature, and especially crystalline orientation of each grain on the front and the irradiation and deformation microstructure within each grain. The irradiation-induced microstructure of these specimens (loops, dislocations, voids, precipitates) have already been determined for these specimens, establishing their relevance to PWR baffle plates at high doses. The specimens currently exist as ~1cm square plates at ~1 mm thickness and will be used to produce mini-tensile specimens, which will be tensile-tested within a TESCAN MIRA3 SEM/EBSD/EDS facility. Periodically the test will be interrupted and EBSD measurements will be performed to measure the evolving deformation and crystallographic orientation. At the end of the test, areas of identified high importance will be FIBed to produce lamella specimens to obtain via TEM microstructural and microchemical information that may illuminate the operating processes in the martensite instability process. The project can be completed in 6 months and will provide guidance concerning the necessity 1) to decide whether follow-on studies involving corrosion of hex-block material, 2) to conduct additional studies using PWR-irradiated material and 3) to consider possibly adding this concern to current surveillance requirements for operating PWRs, especially as the reactors move to ever higher neutron exposures. |
| Award Announced Date | 2020-02-05T14:09:54.63 |
| Awarded Institution | None |
| Facility | None |
| Facility Tech Lead | Kory Linton |
| Irradiation Facility | None |
| PI | Frank Garner |
| PI Email | [email protected] |
| Project Type | RTE |
| RTE Number | 2954 |