Hi Rebecca, welcome to the forum.
I think the idea of a high-entropy alloy is an interesting one. In some ways it mirrors the idea of the ionically-bonded salts, in that they exist at a state of maximum entropy, and this means that all that banging around from neutrons and gammas does not change their bulk properties. Could something like that be done with metallic alloy? Perhaps, but one would still have to contend with the neutron absorption problems that are going to exist in any nickel-based alloy. There are going to be helium-forming reactions in nickel from the neutron flux:
58Ni + n --> 59Ni
59Ni + n --> 4He + 56Fe
It has always seemed to me most prudent, and actually achievable, to keep metallic alloys out of the neutron flux as much as possible. With the two-fluid molten-salt reactor design this appears to be possible, since the reactor vessel can be protected from the neutron flux of the core by the neutronically-absorptive blanket fluid. Through proper design, the only materials that have to be in the really intense part of the neutron flux are the fuel and blanket salts (which are impervious to radiation damage) and the graphite (which unfortunately isn't, but we need it anyway). The reactor vessel, which will be metallic, can be kept "shielded" by the blanket in a good two-fluid design.
Of course, the ORNL MSRP departed from the two-fluid design in about 1968 and was on a one-fluid design for the remainder of time that the project ran, until about 1976, so contending with more significant neutron damage to the metallic alloy was something that they were thinking about.
In this 1978 report, Herb McCoy goes over the materials challenges for MSRs and the current state of the art. Since this document was finished about two years after the program was really finally cancelled, it is probably about as good a resource as anywhere to commit to understanding before attempting an effort in materials development.
ORNL-TM-5920: Status of Materials Development for Molten-Salt-ReactorsQuote:
Iron- and nickel-base alloys can be embrittled in a thermal neutron flux by the transmutation of tramp 10B to helium and lithium. This process generally results in the transmutation of most of the 10B by fluxes of thermal neutrons on the order of 10^20 n/cm^2, and usually yields from 1 to 10 atomic ppm of He. With Ni there is a further thermal two-step transmutation involving these reactions:
58Ni + n --> 59Ni
59Ni + n --> 4He + 56Fe
This sequence of reactions does not saturate, and although the cross sections are still in question, it would produce a maximum of 40 atomic ppm of He in the vessel over a 30-year MSBR lifetime. Helium from both sources collects in the grain boundaries and causes degradation of the mechanical properties at elevated temperatures. The effect manifests itself in reduced rupture life and reduced fracture strain.
I don't know if you've already reviewed this document, let me know. I think it's a very good place to start.