Feeding humans from dead, dying or living biomass looks trivial.Cellulose
makes much vegetable material and is a polymer of D-glucose Cellulose
we know already the enzyme that humans lack to break cellulose to D-glucose, one fundamental source of energy Glucose
so we can just use the enzyme, or some chemical processing instead, to produce D-glucose that we eat
So would we compete for the unpolluted biomass after a nuclear war? Perhaps not, because the radionuclides with significant life are inorganic: I, Cs, Sr and the others. So we might remove the radionuclides by chemical processing apparatus
, perhaps some sort of resin or electrolysis or just precipitation.
Again, providing the calories amount to humans is easy. The vitamins, essential elements and others are more difficult.