Temporary segments cleared when Database is shutdown.

A temporary segment created in temp tablespace will not be deallocated at the end of the sort. The extents are MARKED as free, are not placed back on the freelist, and are retained for reuse. An element called the Sort Extent Pool (SEP) is created in the SGA to describe them.
Subsequent sort operations will then be allocated extents from this pool via a memory lookup.

Dilip