Hi,

SMON does not de-allocate Temporary extents whislt the system is up even when a user process dies. This space will be re-used if a user process needs it.

SMON will de-allocate these extents after the instance has been shutdown and re-started. How long has your instance been up ?
18GB is big and it might take a long time to de-allocate the extents.

Run this query to see if the extents are being de-allocated

SELECT COUNT(*) FROM DBA_EXTENTS WHERE SEGMENT_TYPE='TEMPORARY';

if the count is decreasing then it is likely that SMON is cleaning up the temporary extents.