Study Urged for National ID System Friday, April 12, 2002    WASHINGTON - While a national identity card has been widely discussed following the terrorist attacks, a panel of the National Academy of Sciences says any such system must carefully balance security needs with privacy concerns.    A well-run national system would make it more difficult for a person to have multiple identities and would help in finding people such as potential terrorists, the committee concluded. But serious questions must be addressed about how to protect privacy, who would use the system, whether participation would be mandatory, the type of information to be collected and how to deal with any failure or misuse of the system.