Cerenkov radiation (light) is produced when charged particles pass through water (for example) faster than the phase velocity of light in water.  The fission and activation products found in spent fuel emit gamma rays, which produce electrons (a charged particle).  The gamma ray induced electrons in turn produce Cerenkov radiation while the spent fuel is in spent fuel pools.  The photo, below, shows the characteristic violet/blue light of Cerenkov radiation.  The Cerenkov radiation will not occur with fresh fuel or non-spent fuel elements in the fuel pool.   

Cerenkov radiation in the core of the Advanced Test Reactor (Photo source: Argonne National Laboratory)