FILE NAMING IN RUNTIME in scheduler/plsql code in 102g. please help, thankU

Question

Hi,
we have a scheduler job which calls/executes a stored procedure that generates a report file. however we need to give file name run at run time and need to copy it to shared folder. we do not want use any shell scripts here and like to complete this work with either PLSQL block or scheduler job. please suggest us how we can specify file name at each run time that differs hour wise.
the procedure code for example is: failure: not getting file name correctly
DECLARE
dtime VARCHAR2(11):=to_Char(sysdate,'YYYYMMDD_HH24');
F1 UTL_FILE.FILE_TYPE;
BEGIN
F1 := UTL_FILE.FOPEN('DIR_REPORTS','REP_''dtime''.txt','W',32767);
UTL_FILE.PUT_LINE(F1,' Data');
UTL_FILE.FFLUSH (F1);
UTL_FILE.NEW_LINE(F1);
end;
please give us your help on nameing file at runtime .. filename_yyyymmdd_hh24.txt.
thanks in advance.
/mr

Answer 1

F1 := UTL_FILE.FOPEN('DIR_REPORTS','REP_'||dtime||'.txt','W',32767);
line giving
ORA-29283: invalid file operation
ORA-06512: at "SYS.UTL_FILE", line 475
ORA-29283: invalid file operation
ORA-06512: at line 5
errors. earlier i was able to generate files earlier but not with suggested change.. anything to be done more???
thank you .. please assist..