currval is private to a session. currval always returns the last value from YOUR SESSIONS LAST nextval call. It is "session safe".

but, all you need to do is use the RETURNING clause if the trigger *must* be used.

ops$tkyte@ORA9IR2> create table t ( x int, y int );

Table created.

ops$tkyte@ORA9IR2> create sequence s;

Sequence created.

ops$tkyte@ORA9IR2>
ops$tkyte@ORA9IR2> create or replace trigger t_trigger
2 before insert on t for each row
3 begin
4 select s.nextval into :new.x from dual;
5 end;
6 /

Trigger created.

ops$tkyte@ORA9IR2>
ops$tkyte@ORA9IR2>
ops$tkyte@ORA9IR2> declare
2 l_x number;
3 begin
4 insert into t (y) values ( 42 ) returning x into l_x;
5 dbms_output.put_line( 'generated key = ' || l_x );
6 end;
7 /
generated key = 1

PL/SQL procedure successfully completed.