Linear is used for most highly enriched (i.e. pure isotope) RBMK fuels, although if you want people to be able to make pure naquadah reactors then you shouldn’t use that (since any linear flux function that is steeper than flat will result in your reactor being overcritical and exploding almost instantly). Stronger medium range fuels and HEU-235 all use the SQRT function because it’s self-balancing and still steeper and therefore more dangerous than LOG fuels. As for the multipliers, all of the fuel values were eyeballed anyway, simply look at comparable fuels and choose whatever value seems reasonable.
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