To above guy
"...UNTIL THE END of the damage step..."
Winged Kuriboh effect activates after it is destroyed and sent to the graveyard, which happens AT THE END of the damage step. So Reactor Dragon can not prevent Winged Kuriboh effect from activating in the graveyard.
"until the end of the Damage Step" in this context means "until the Damage Step ends". You can't use cards like Winged Kuriboh or Grave Squirmer if Reactor Dragon attacks it. (The end of the Damage Step is still part of the Damage Step, hence why you can't chain a card like Silent Graveyard to Winged Kuriboh.)