  We study the recombination of two neutrons and deuteron into neutron and
${}^3$H using realistic nucleon-nucleon potential models. Exact Alt,
Grassberger, and Sandhas equations for the four-nucleon transition operators
are solved in the momentum-space framework using the complex-energy method with
special integration weights. We find that at astrophysical or laboratory
neutron densities the production of ${}^3$H via the neutron-neutron-deuteron
recombination is much slower as compared to the radiative neutron-deuteron
capture. We also calculate neutron-${}^3$H elastic and total cross sections.
