The Nuclear Regulatory Commission announced its approval yesterday of a drug-testing policy for NRC employes, but said random screening will not begin until details have been negotiated with the agency's union.

The National Treasury Employees Union (NTEU), which represents NRC employes, already has filed a court challenge to President Reagan's 1986 order requiring drug-testing programs for federal employes. Union officials said they would sue the NRC as well if the new policy is put into practice.

"The NRC is a long, long way from implementing this drug-testing proposal," said NTEU President Robert M. Tobias.

In a statement, NRC Chairman Lando W. Zech Jr. said the agency's responsibilities for public safety "warrant strong measures to ensure that NRC establishes itself as an agency with zero tolerance for illegal drug use."

The policy calls for random testing of all employes who have access to vital areas of nuclear power plants or sensitive data and employes who are responsible for responding to nuclear incidents. New employes would be required to pass a drug test before being assigned to jobs subject to random testing. Tobias said the proposal would affect about 1,000 NRC employes, or one-third of the agency's work force.

Under the NRC policy, tests also could be given to any employe suspected of illegal drug use and any employe involved in an accident with government vehicles or equipment.