In Antonov An-225, why there are radioactive / nuclear signs?
"Radiation" is a broad term and does not always imply radio active isotope material. High power radar units can have adverse effects on the human body. 


  The power that radar systems emit varies from a few milliwatts (police
  traffic control radar) to many kilowatts (large space tracking
  radars). However, a number of factors significantly reduce human
  exposure to RF generated by radar systems, often by a factor of at
  least 100:
  
  
  Radar systems send electromagnetic waves in pulses and not continuously. This makes the average power emitted much lower than the
  peak pulse power.
  Radars are directional and the RF energy they generate is contained in beams that are very narrow and resemble the beam of a spotlight. RF
  levels away from the main beam fall off rapidly. In most cases, these
  levels are thousands of times lower than in the main beam.
  Many radars have antennas which are continuously rotating or varying their elevation by a nodding motion, thus constantly changing the
  direction of the beam.
  Areas, where dangerous human exposure may occur are normally inaccessible to unauthorized personnel.
  


The warnings are likely due to the emissions of the radar units in the dome.