I forgot about Li producing tritium. Scrap lithium as it would require significantly more regulation due to flowing radioactive coolant. Can’t have a PF power source on every corner if it has radioactive coolant. The site license would be impossible to get in a town.
Tin activates as it is used in activation counters. Argon might be an option as it is pretty immune to neutrons but probably has miserable thermal properties otherwise people would use it as a coolant in next gen reactors. Neon is pretty expensive as are other inert gases.
