# Why do we not use cold neutrons in nuclear reactors?

Typical nuclear reactors use neutrons in the $10^{-2}$ eV range (thermal neutrons) which corresponds to a fission cross section of a few hundred barns. What stops us from using a super-cold moderator, resulting in $10^{-5}-10^{-3} eV$ neutrons and dramatically increasing the cross section to a few thousand barns (as this would generate more power)? We have experience with handling extremely cold materials (liquid H2 in rockets), so is the intuition wrong or is there simply no reason to do this?