Cryogenic plants for Oxygen and Nitrogen produce Argon, Neon, Krypton, Xenon, and Helium, in that order. Argon has a high thermal neutron absorbtion, like Nitrogen, but is widely available. Neon has a low thermal neutron absorbtion, and is available but scarcer. For Fast Neutron Gas Cooled Reactors, Argon is perfectly acceptable and we aren't about to run out.
If we have carbon sequestration rules, then there is also going to be a good deal of byproduct Neon arriving for Pebble Bed Thermal Neutron Gas Cooled Reactors.
Since we have a limited production capability for Pressurised Water Thermal Neutron reactor pressure vessals, the only quick reactor ramp up is going to be Fast Neutron Gas Cooled Reactors. Pebble Bed Thermal Neutron Gas Cooled Reactors don't have to worry so much about pressure surges, so the pressure vessal is simpler and cheaper to build.