I am bumping this old thread because of a recent post and because it is the top result on google for this very common problem.
What I encoutnered is that RDbias simulates the gamma multiplicity but not the neutrons for fission sources. The most general solution I found was to provide a custom decay file to
/process/had/rdm/setRadioactiveDecayFile Z A <your_file.txt>
The conventions for the file are buried in a readme in a location like …/Geant4/data/RadioactiveDecay5.X as well as the files used specifically for the radioactive decay module. The current version of the readme with 4.11.3 is attached. There are copies of these files elsewhere. You can use the attachment posted in this thread as an example.
README_RDM.txt (2.6 KB)