Is it possible to re-initialize sequence
This thread over on Stack Overflow has the information you are looking for. There are two main options:


Drop the sequence and re-create it, or
Use a trick, courtesy of Tom Kyte:  

either as a one-off:

alter sequence seq increment by {minus_the_current_value} minvalue 0;


or you can create a procedure if you need to do this regularly:

create or replace
procedure reset_seq( p_seq_name in varchar2 )
is
    l_val number;
begin
    execute immediate
    'select ' || p_seq_name || '.nextval from dual' INTO l_val;

    execute immediate
    'alter sequence ' || p_seq_name || ' increment by -' || l_val || 
                                                      ' minvalue 0';

    execute immediate
    'select ' || p_seq_name || '.nextval from dual' INTO l_val;

    execute immediate
    'alter sequence ' || p_seq_name || ' increment by 1 minvalue 0';
end;
/