Oracle automatically unmount I have a weird issue in my Oracle database, after 10 - 15 minutes of not using it (Not performing any query, insert on SQL Developer tool) then it will say ORA-27101: shared memory realm does not exist so I have to login through sqlplus and do startup. After that I can now use my database again then after 10 - 15 minutes same thing happens.

Below is the alert log but I can't understand it

Sun Jan 24 19:03:29 2016
Errors in file c:\oracle\diag\rdbms\orcl\orcl\trace\orcl_smon_1076.trc  (incident=43416):
ORA-00600: internal error code, arguments: [6856], [0], [102], [], [], [], [], [], [], [], [], []
Incident details in: c:\oracle\diag\rdbms\orcl\orcl\incident\incdir_43416\orcl_smon_1076_i43416.trc
Process debug not enabled via parameter _debug_enable
Sun Jan 24 19:03:34 2016
PMON (ospid: 6464): terminating the instance due to error 474
Sun Jan 24 19:03:54 2016
Instance terminated by PMON, pid = 6464



--
................................
--- End of Call Stack Trace -----
----- Process Map Dump -----
------------------- Memory Map of process ----------------
Start Addr-End Addr   Type     Size        ModuleName
Unab
0x00400000-0x06a4afff Image    0107261952  C:\ORACLE\PRODUCT\11.2.0\DBHOME_1\BIN\ORACLE.EXE
0x575c3a43-0xc6c0a8ab Image    1868852841  C:\WINDOWS\SYSTEM32\USER32.DLL
0x74bf0000-0x74c3afff Image    0000307200  C:\WINDOWS\SYSTEM32\MSWSOCK.DLL
------------------- End of memory map --------------------

----- Guard Frame Metadata Dump -----

internal error ORA-600 seen on the error stack
process appears to be having problems repeatedly, committing suicide
Background_Core_Dump = partial
Ok, that error (ORA-00600 [6856]) is related to corruption.

Normally a DBA would go to My Oracle Support, follow the notes (for example    ORA-600 [6006] ORA-600 [6856] During Startup Instance, Followed by Termination by SMON (Doc ID 549000.1)), identify the affected objects then try to repair them, recover corruption, or restore a valid backup. 

If you can not do that, and this is a sandbox/development database with no important data, in my opinion the easiest and fastest solution for you would be recreating your database with DBCA (Database Configuration Assistant).