NSUF 21-4248: Microstructural Evolution in Model & Real RPV steel due to Thermal Aging and Low-Dose Irradiation using Atom Probe Tomography
Motivation: Embrittlement due to solute-rich (Cu,Mn) precipitates in reactor pressure vessel (RPV) steels has historically been studied using destructive tests. But with limited test specimens, non-destructive tests need to be explored to in order to monitor RPV health. Transient grating spectroscopy (TGS) can measure the thermal diffusivity of materials non-destructively, making it a viable option for material monitoring if relationships can be constructed showing the changes in precipitate number density and thermal diffusivity. Such robust and easily measurable correlations validated with real RPV specimens, are critical to ensuring light water reactor sustainability (LWRS) to consider life extension to 80-100 years of service.
Preliminary Results: We have used model RPV steels (high Cu) and aged them past an “overaged” condition, meaning the (Cu,Mn) precipitates are too large to be cut by dislocations and further growth only leads to material embrittlement. Using samples aged at different times at 500°C, we have determined the thermal diffusivity of each aging condition and the results closely match literature, showing first a steep decrease in thermal diffusivity, followed by an increase approaching the initial value at overaged conditions. These results will be compared to the results of APT and allow us to construct relationships between thermal diffusivity and precipitate number density and size.
Hypothesized Mechanism: The growth of solute-rich precipitates removes these solute atoms from the matrix of the RPV alloys. The steep decrease in thermal diffusivity at early aging times corresponds to the formation of small precipitates, which scatter electrons better than solute atoms in solution. However, at higher aging times, smaller precipitates coalesce and the resulting precipitates are less effective at scattering electrons. This is indicated in the steady increase of thermal diffusivity after the peak aging time in the model RPV alloys.
Proposed Experiments: To back up the proposed mechanism, atom probe tomography (APT) will be performed in order to observe the microstructure of several model RPV alloys and three samples of a real RPV alloy from Exelon’s Byron Unit 2 PWR, representing up to 20 effective full power years (EFPY) of service. The samples have all experienced different thermal aging and irradiation conditions, which will provide samples of different precipitate size distributions and number densities. This will allow us to see the correlation between thermal diffusivity and precipitate structure, as well as understand how different precipitate formation pathways such as irradiation and thermal aging influence the microstructure. In addition, the correlation between relative changes in thermal diffusivity from the rapidly aged model RPV alloys and the in-service real RPV coupons will help validate studies on model RPV alloys at gauging equivalent aging rates and mechanisms in real RPVs.
Impact: This experiment will remove the bottleneck of destructive testing for RPV alloys. By drawing relationships between the effects of thermal aging and irradiation to the thermal properties of a material, we will be able to better understand the degree of precipitation, and therefore embrittlement, by measuring thermal diffusivity non-destructively. This knowledge and measurement technique could significantly enhance LWRS.
추가 정보
| 필드 | 값 |
|---|---|
| Abstract | Motivation: Embrittlement due to solute-rich (Cu,Mn) precipitates in reactor pressure vessel (RPV) steels has historically been studied using destructive tests. But with limited test specimens, non-destructive tests need to be explored to in order to monitor RPV health. Transient grating spectroscopy (TGS) can measure the thermal diffusivity of materials non-destructively, making it a viable option for material monitoring if relationships can be constructed showing the changes in precipitate number density and thermal diffusivity. Such robust and easily measurable correlations validated with real RPV specimens, are critical to ensuring light water reactor sustainability (LWRS) to consider life extension to 80-100 years of service. Preliminary Results: We have used model RPV steels (high Cu) and aged them past an “overaged” condition, meaning the (Cu,Mn) precipitates are too large to be cut by dislocations and further growth only leads to material embrittlement. Using samples aged at different times at 500°C, we have determined the thermal diffusivity of each aging condition and the results closely match literature, showing first a steep decrease in thermal diffusivity, followed by an increase approaching the initial value at overaged conditions. These results will be compared to the results of APT and allow us to construct relationships between thermal diffusivity and precipitate number density and size. Hypothesized Mechanism: The growth of solute-rich precipitates removes these solute atoms from the matrix of the RPV alloys. The steep decrease in thermal diffusivity at early aging times corresponds to the formation of small precipitates, which scatter electrons better than solute atoms in solution. However, at higher aging times, smaller precipitates coalesce and the resulting precipitates are less effective at scattering electrons. This is indicated in the steady increase of thermal diffusivity after the peak aging time in the model RPV alloys. Proposed Experiments: To back up the proposed mechanism, atom probe tomography (APT) will be performed in order to observe the microstructure of several model RPV alloys and three samples of a real RPV alloy from Exelon’s Byron Unit 2 PWR, representing up to 20 effective full power years (EFPY) of service. The samples have all experienced different thermal aging and irradiation conditions, which will provide samples of different precipitate size distributions and number densities. This will allow us to see the correlation between thermal diffusivity and precipitate structure, as well as understand how different precipitate formation pathways such as irradiation and thermal aging influence the microstructure. In addition, the correlation between relative changes in thermal diffusivity from the rapidly aged model RPV alloys and the in-service real RPV coupons will help validate studies on model RPV alloys at gauging equivalent aging rates and mechanisms in real RPVs. Impact: This experiment will remove the bottleneck of destructive testing for RPV alloys. By drawing relationships between the effects of thermal aging and irradiation to the thermal properties of a material, we will be able to better understand the degree of precipitation, and therefore embrittlement, by measuring thermal diffusivity non-destructively. This knowledge and measurement technique could significantly enhance LWRS. |
| Award Announced Date | 2021-06-07T16:17:41.087 |
| Awarded Institution | Idaho National Laboratory |
| Facility | Advanced Test Reactor |
| Facility Tech Lead | Alina Zackrone, Yaqiao Wu |
| Irradiation Facility | None |
| PI | Benjamin Dacus |
| PI Email | [email protected] |
| Project Type | RTE |
| RTE Number | 4248 |