The use case that inclusion of files in a branch that poison a branch. Nuclear waste is typically considered unusable, bulk data that snuck into a branch. Good examples of nuclear waste include log files, temporary files and any data resulting from test cases.
The problem with nuclear waste is that even after a standard remove command the data is still present in the RCS data for a branch. This results in reduced performance -- particularly in the cases of pushing and pulling branches.
This type of waste is unlike real nuclear waste because no known half-life exists. Worse yet, branch nuclear waste can grow over time!