July 25, 2023
Tokamak Energy is planning to run a pilot fusion plant (ST-E1) in the early 2030s which will be imposing severe neutron damage on the reduced activation ferritic/martensitic (RAFM) steels that will be the main structural materials. The damage modes of particular concern are the atomic displacements leading to vacancy and self-interstitial defect generation (typically quantified in displacements per atom (dpa)), the transmutation reactions generating gaseous products such as helium (typically quantified in atomic parts per million (appm)), and most critically the combination of the two, which will significantly impact hardening, embrittlement and swelling behavior in ways that are hard to predict. There have been various attempts to simulate the combined effect (e.g. using spallation proton neutron irradiations, or multi-ion beam irradiations), but the most promising approach is to expose Ni-doped steels to fission neutrons in a Materials Test Reactor. Doping with 58Ni enhances helium production to the levels observed in the fusion environment, while 60Ni control samples can be used to deconvolute the alloying effects of the Ni.
The Oak Ridge National Laboratory (ORNL) High Flux Isotope Reactor (HFIR) has conducted many long (> 10 year) irradiation campaigns of fusion-relevant materials but the funding has not allowed Post Irradiation Examination (PIE) of some samples of particular interest to Tokamak Energy and the broader fusion community, including Ni-doped RAFM steels irradiated to 40-60 dpa. This is the level of damage predicted after 9 calendar years of operation in the TE pilot plant (with its expected availability of 20%). Samples were irradiated at both 300 degreesC and 500 degreesC, and, when combined with baseline RAFM steel samples irradiated at 300 degreesC, they are a highly valuable resource for illuminating the likely performance of the ST-E1 structural materials over the early years of operation.
It is proposed that these samples be tested at the ORNL Irradiated Materials Examination and Testing (IMET) facility and the Low Activation Materials Development and Analysis (LAMDA) facility. Investigations will include microindentation hardness, fracture toughness (using the Master Curve method), and microstructural characterization (with transmission electron microscopy) to investigate cavity size distribution and density (ii) dislocation line density (iii) dislocation loop size distribution and density (iv) solute clustering or precipitation (v) grain boundary segregation and cavitation. The cavity size distribution will be used to calculate volumetric swelling, while the results on the dislocations and solute clusters/precipitates will be used to inform hardening and embrittlement models. In addition, grain boundaries will be assessed for features that may contribute to intergranular failure. Atom probe tomography (APT) will also be used outside of ORNL to quantify the size and number density of any nano-scale clusters or precipitates formed under irradiation. In particular, data on any Ni-enriched clusters/precipitates are required to deconvolute any dopant alloying effects.
The specific information that this project should produce on samples with combined displacement damage and helium transmutation is: low temperature (300 degreesC) information on hardening and embrittlement (providing an intermediate data point between existing data on F82H (a representative RAFM steel) at ~18 dpa and ~68 dpa (displacements per atom), and high temperature (500 degreesC) information on swelling, extending the existing 8.6 dpa data on F82H up to 45-50 dpa (equivalent to 2 years of full power operation in planned reactors). These latter data will determine whether the swelling will exceed the 5% swelling generally considered unacceptable for structural applications.
These data will be invaluable for informing design and determining structural material lifetimes under fusion neutron exposure for TE and all of the other fusion pilot plant development programs supporting the US decadal vision for accelerating fusion energy.