ok, I got this from alter log

ORA-01555 caused by SQL statement below (Query Duration=11645 sec, SCN: 0x06a4.db3568f0):
Wed Apr 19 01:05:52 2006
SELECT /*+NESTED_TABLE_GET_REFS+*/ "schema_name"."table_name".* FROM "schema_name"."table_name"

Wed Apr 19 01:49:15 2006

What I say:

My Undo Rentention is 2 hrs (7200). The above query stays for 3.23 hrs. Segment usage was maximum around 140MB.

What cause this error to terminate the export? And most importantly DOES Oracle use UNDO while EXPORTING and table segment :(