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| Fig. 1: Summary of neutron irradiation conditions reported by Hu. [8] (Image source: D. Fenster, after Hu. [8]) |
First wall materials, which directly face plasmas reaching over 100M °C, are one of the foremost mechanical and materials challenges in fusion reactor engineering. In Tokomak and Stellarator fusion reactors, plasma is contained by electromagnetic confinement, but the optimal plasma-facing material to withstand reactor conditions is still an unanswered question of great technological importance. Aside from having to withstand temperatures ten times hotter than the suns core, they must also remain stable in repeated thermal cycling and high neutron flux, resisting sputtering and neutron damage from 14 MeV neutrons that escape the reactors plasma. And, beyond the critical functionality of distributing thermal loads and resisting both swelling and embrittlement, the materials lifespan and resistance to induced radioactivity is of utmost importance for recycling, waste-handling, and cost considerations.
Although some baseline plasma-facing materials, which emerged in the 1970s, have been tested in experimental reactor conditions, these experimental conditions are far less extreme than that of commercial fusion power plant conditions, so we are yet to validate the material properties and lifetimes of these traditional materials. Fig. 1 summarizes the neutron irradiation tests of tungsten thus far, and it is of particular interest that the maximum tested neutron fluence is just 11 dpa. Moreover, the vast majority of experimental testing of plasma-facing first wall materials reaches fluences up to ~1 displacements per atom (dpa), and experimental data for fluences up to ~5-10 dpa are very rare. [1-3] This tested level of neutron bombardment pales in comparison to the neutron fluences of commercial plants, which are expected to reach, conservatively, 10 dpa per year, or 50 dpa over a 5-year first wall lifespan. [4] So, the materials testing conducted thus far is relevant to characterize material behavior during early-stage damage behavior, which does not necessarily indicate whether these materials will hold their properties at >50 dpa over their intended lifetimes and higher neutron fluences and irradiation temperatures.
Though many researchers are treating stable early-stage damage behavior as positive indication of survival in significantly more extreme power plant conditions, it is likely that current first wall materialsincluding leading emerging materials like tungsten and certain high entropy alloyswill not withstand power plant reactor conditions, so the search for first wall materials is ongoing and poses a significant barrier to the actualization of fusion reactors.
When Tokomaks began their materials development in the 1970s, traditional first wall materials like beryllium and carbon-based materials were the primary materials used for the first, plasma-facing walls in electromagnetic confinement reactors. Of the first wave of first wall materials to be used under experimental conditions, beryllium was chosen for its lack of chemical sputtering, its oxygen gettering capabilities, which help maintain plasma purity, and for its low atomic number, which minimizes heat losses when Be atoms are ejected into the plasma, ionized, and emit radiation. Beryllium also has high thermal conductivity, which allows first-wall materials to effectively prevent localized high heat concentrations and move heat to the coolant/blanket which lies behind the first wall. Under testing of fluences up to ~0.74 dpa and ~300 °C, thermal conductivity was within experimental error of the irradiated thermal conductivity. [5] Despite this stable behavior, Be is limited as a first wall material due to its high levels of bubble formation, which increase in growth rate as irradiation temperatures increase. [6] Since commercial reactor temperatures are several orders of magnitude higher than these experimental temperatures, this indicates an important shortcoming in berylliums material behavior as it directly causes swelling and embrittlement of the first wall.
Carbon-based materials, like graphite and carbon fiber composites, have a low atomic number and high thermal conductivity. In addition, their relatively high porosity lends to their ability to absorb strain which results in a high thermal shock resistance since their controlled porosity reduces risk of catastrophic deformation and cracking. However, this high porosity also means that these carbon-based inner wall materials have high transport and retention of hydrogen, causing imbalances in the reactors tritium inventory which is tightly controlled due to its radioactivity. [7] Additionally, carbon-based materials see a fall in thermal conductivity of ~75-90% when exposed to even extremely low neutron fluences of ~0.2 dpa. [5] Since first wall materials are responsible for the distribution of extreme thermal loads from the inside of the reactor to the coolant, this catastrophic decrease in thermal conductivity effectively disqualifies carbon-based materials from usage as a first wall material. Furthermore, the retention of radioactive isotopes in the carbon materials pores also causes radioactivity of the inner wall material itself, making it difficult to recycle or dispose of at its end of life. These pitfalls are pointed to as leading reasons why preeminent programs like ITER (International Thermonuclear Experimental Reactor) use Be instead of carbon-based materials as its first wall material.
Despite berylliums leading performance among traditional materials, it still suffers from limited lifetime under high heat loads due to swelling and embrittlement as well as a low melting point. So, researchers have been developing new inner wall materials that maintain important properties like high thermal conductivity and low surface erosion, but also have optimal thermodynamic stability, fracture and vacancy formation energy, and thermal expansion properties for highest resistance to embrittlement and blistering. Among these are certain high entropy alloys (HEAs), and tungsten-coated or tungsten-based materials, which were a direct successor to beryllium in ITERs first wall. Tungsten possesses an interesting balance of properties: it has excellent thermal properties with a high melting point and relatively high thermal conductivity and relatively low hydrogen retention, but has a very high atomic number, which results in high heat losses when W atoms are sputtered into the plasma. Despite this heat loss problem, Tungsten is still considered a prevailing option for emerging materials since the energy threshold for W atoms to be ejected by hydrogen is quite high, so the material has small amounts of sputtering. Tungsten is also being explored as a thin coating on the surface of less expensive traditional first wall materials in order to combine Tungstens thermal conductivity, high melting point, and low sputtering with traditional materials that have better bulk properties and costs.
Like traditional materials, though, these stable properties of Tungsten have only been tested under experimental reactor conditions, and it is very unclear whether it would withstand commercial reactor conditions. Even under low temperature and low neutron fluences (<0.5 dpa at <400 degrees C), Tungstens atomic displacement causes significant dislocation loops and vacancies which cause irradiation hardening and swelling. [3] This embrittling behavior was even observed at extremely low neutron fluences of .02 dpa, so it is unlikely that it would withstand the neutron fluences expected for a 5-year lifespan in a commercial reactor. [8]
Like Tungsten, HEAs are being looked into for their high temperature capabilities, high radiation tolerance, and low erosion, but HEAs tend to excel over W due to its tunable low induced radioactivity and low hydrogen retention. [9] And, their experimental testing thus far has yielded stable results at experimental reactor conditions. In-situ TEM characterization of W-Ta-Cr-V- Hf shows high thermal stability due to its grain refinement under dual-beam irradiation at up to 1.24 dpa and less phase separation than previously tested HEAs. [10] Once again, however, it is unclear whether these promising early-stage damage behaviors are indicative of the alloys ability to withstand commercial reactor conditions.
Overall, while some HEAs have theoretically more tunable material properties and Tungsten has several years research and characterization, it cannot be ignored that no emerging materials have been tested under commercial plant conditions, and the search for first wall materials is ongoing.
© Daniella Fenster. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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