It will take something more than a nuclear attack to wipe out taxpayers' obligations to the Internal Revenue Service.

An addition to the Internal Revenue Manual, which is supposed to guide the conduct of all I.R.S. employees, declares that if the bomb is dropped, ''operations will be concentrated on collecting the taxes which will produce the greater revenue yield.''

An I.R.S. spokesman, Johnell Hunter, said today that the new section -titled ''National Emergency Operations'' - had been added to the manual in response to a directive to Government departments from the Federal Emergency Management Agency.

Within 30 days of an attack emergency, the agency would expect to resume assessing and collecting taxes. At that time, the manual states, many employees might find themselves reassigned to carry out essential functions ''regardless of and without any effect on the current positions or grades of the employee.''