Serpent forms decay and transmutation paths primarily based on the library id's. When the material compositions contain only transport nuclides, the id's are easily obtained, and daughter nuclides without transport data are assigned with the same id. The material input also allows entries without the library id, which means that the nuclide has no transport data (this happens, for example, if the material composition is written by the burnup routine). Since the decay of these nuclides may lead to the formation of nuclides that actually do have transport data, there must be some id attached to the input value for the formation of the correct path. This problem is solved by assigning each material with a default id. This is set using the "fix" command word in the material card:
where <id> is the library id and <T> is the library temperature. Both values should be the same as those used for the transport nuclides in the material ("09c" and 900, etc).
A word of warning: Setting the fix parameter also fixes the nuclides used in the burnup calculation. This should not be a problem if the file was written by the burnup routine, because it already contains the full set from the previous calculation, but if you want the calculation to include some nuclide that is not in the composition, it should be added in the list with zero density. Also, I haven't tested this feature in a while, so let me know if it doesn't work.