NSUF 16-771: Investigation of the Thermal Stability of Mn-Ni-Si Precipitates in Ion Irradiated RPV Steels
Accurately predicting embrittlement of reactor pressure vessel (RPV) steels is required to ensure the continued safety of our nation’s light water reactors at extended lifetimes. Current models underpredict embrittlement from high flux test reactor irradiations to extended life fluences. It is generally accepted that the reason for this underprediction is that the current models do not include embrittlement from Mn-Ni-Si precipitates (MNSPs), which do not reach large volume fractions until decades into reactor operation. Though the existence of MNSPs are no longer debated, there is significant disagreement within the RPV community regarding their detailed character and formation mechanism, i.e. radiation enhanced precipitation vs radiation induced segregation. Thus, the aim of this proposal is to elucidate the driving force behind MNSP formation, which will enable more robust predictive models. A Cu-free, 1.6% Ni RPV steel was irradiated to 2.5 dpa at two different irradiation temperatures, 330 and 400°C. While this irradiation dose is ˜ 15 times higher than would ever be seen by an in-service vessel, the purpose of this experiment is not simply to irradiate the material and attempt to compare it to a neutron irradiation. The goal of the irradiation was to generate significant quantities of MNSPs with a wide range of sizes. Atom probe tomography (APT) of the as-irradiated condition showed that this goal was successful in that significant quantities of MNSPs formed at each temperature, with the precipitates in the 400 °C irradiation being ˜ twice the size as those formed at 330°C. The specimens were then annealed in an Ar environment at 425°C for 30 weeks to observe the changes in the precipitates.
We propose to use the TALOS S/TEM in the Low Activation Materials Development and Analysis Laboratory to perform energy dispersive X-ray spectroscopy measurements, which create maps showing the location of different elements. These maps will be complemented with APT measurements of the same conditions. The main purpose of these measurements is the following: 1. Determine if there is a critical size above which precipitates coarsen, while below which they dissolve, and 2. If any precipitates remain stable at long times, measure any changes in their composition during annealing. The first goal will help to elucidate the precipitate formation mechanism, as radiation induced solute clusters should not be stable under annealing and would be expected to dissolve. In addition, if the precipitates are shown to be thermodynamically stable, measuring the critical radius at a given solute content and annealing temperature will help to inform detailed atomistic models of MNSP formation, such as helping to determine the interfacial energy between MNSPs and Fe, which is not known. The second goal is to determine whether the compositions of phases can be altered by radiation and to determine which elements are most affected. This information is relevant not only to RPV steels, but any alloy with applications in reactors where precipitates may form and lead to undesirable property changes.
추가 정보
| 필드 | 값 |
|---|---|
| Abstract | Accurately predicting embrittlement of reactor pressure vessel (RPV) steels is required to ensure the continued safety of our nation’s light water reactors at extended lifetimes. Current models underpredict embrittlement from high flux test reactor irradiations to extended life fluences. It is generally accepted that the reason for this underprediction is that the current models do not include embrittlement from Mn-Ni-Si precipitates (MNSPs), which do not reach large volume fractions until decades into reactor operation. Though the existence of MNSPs are no longer debated, there is significant disagreement within the RPV community regarding their detailed character and formation mechanism, i.e. radiation enhanced precipitation vs radiation induced segregation. Thus, the aim of this proposal is to elucidate the driving force behind MNSP formation, which will enable more robust predictive models. A Cu-free, 1.6% Ni RPV steel was irradiated to 2.5 dpa at two different irradiation temperatures, 330 and 400°C. While this irradiation dose is ˜ 15 times higher than would ever be seen by an in-service vessel, the purpose of this experiment is not simply to irradiate the material and attempt to compare it to a neutron irradiation. The goal of the irradiation was to generate significant quantities of MNSPs with a wide range of sizes. Atom probe tomography (APT) of the as-irradiated condition showed that this goal was successful in that significant quantities of MNSPs formed at each temperature, with the precipitates in the 400 °C irradiation being ˜ twice the size as those formed at 330°C. The specimens were then annealed in an Ar environment at 425°C for 30 weeks to observe the changes in the precipitates. We propose to use the TALOS S/TEM in the Low Activation Materials Development and Analysis Laboratory to perform energy dispersive X-ray spectroscopy measurements, which create maps showing the location of different elements. These maps will be complemented with APT measurements of the same conditions. The main purpose of these measurements is the following: 1. Determine if there is a critical size above which precipitates coarsen, while below which they dissolve, and 2. If any precipitates remain stable at long times, measure any changes in their composition during annealing. The first goal will help to elucidate the precipitate formation mechanism, as radiation induced solute clusters should not be stable under annealing and would be expected to dissolve. In addition, if the precipitates are shown to be thermodynamically stable, measuring the critical radius at a given solute content and annealing temperature will help to inform detailed atomistic models of MNSP formation, such as helping to determine the interfacial energy between MNSPs and Fe, which is not known. The second goal is to determine whether the compositions of phases can be altered by radiation and to determine which elements are most affected. This information is relevant not only to RPV steels, but any alloy with applications in reactors where precipitates may form and lead to undesirable property changes. |
| Award Announced Date | 2016-12-16T07:44:58.687 |
| Awarded Institution | None |
| Facility | None |
| Facility Tech Lead | Kory Linton |
| Irradiation Facility | None |
| PI | Peter Wells |
| PI Email | [email protected] |
| Project Type | RTE |
| RTE Number | 771 |