The fission products. Each fission of U235 or Pu239 produces two fragments of elements of lower atomic number, which are often intensely radioactive.
There are a number of possibilities as regards what happens when a nucleus "disintegrates" as was asked. There are a number of way that a nucleus can disintegrate, or change, so let's look at...
Ernest Rutherford
smaller then the original atom and possibly radioative
Deuterium & 13C Labeled Custom Synthesis Available. for comments and suggestions tweet me at jaffa_tolentino.