Piles
'piles' is the definition.
('atomic reactor' can be a synonym of 'pile')
This is all the clue.
'piles' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for heaps that I've seen before include "Shape (anag) - lots" , "A large number; untidy mounds" , "Mounds, stacks" , "Lots (of piles?)" , "Mounds; lots" .)