In the event of a nuclear war, the Interior Department would try to keep the nation's mines open and the Education Department would seek out teachers to reopen schools, according to a new order signed by President Reagan.

Not surprisingly, the lion's share of the responsibility for responding to nuclear attack is assigned to the Pentagon, according to the recently released directive.The order overhauls a 19-year-old executive directive that was first approved by President Nixon, said Russell Clanahan, a spokesman for the Federal Emergency Management Agency.

The new 27-page document brings up to date the 1969 order by taking into account a number of changes in the structure of the federal bureaucracy since then.

For example, the old Department of Health, Education and Welfare has been split into two Cabinet agencies: the Department of Health and Human Services and the Department of Education.

The 1969 order had been updated periodically but never totally rewritten until now, Clanahan said. The job took several years.

"It had to go around the government several times for coordination," he said. "It did take quite awhile. You'd be amazed at the amount of nit-picking the bureaucracy can do."

The order covers 26 federal agencies, including FEMA, the office charged with coordinating the government's response to a wide variety of disasters, including atomic war.

The new document does not propose spending any more money for civil defense, nor does it propose changing any policies.

When he took office in 1981, Reagan began a large build-up of military forces, and administration officials also proposed increasing the nation's civil defense effort. They said the Soviets have a much larger civil defense system.