Getting a nuclear power plant built is a long and costly process - so much so that the N-power industry has almost died. Dozens of licenses, studies, permissions, hearings, and court challenges by environmental opponents must be surmounted. The delays run into years and the expense into billions of dollars.

Yet even when all that is accomplished, legal fights and deliberate inertia by state and local officials can keep the electric power from being switched on. That has been the case with two nuclear plants built years ago in New York and New Hampshire, power plants that have stood silent ever since.The problem is that federal law requires evacuation plans to be drawn up for areas 10 miles around the N-plants. Elected officials have stymied the start of operation of the Seabrook plant in New Hampshire and the Shoreham plant in Long Island simply by refusing to draft such plans.

The Nuclear Regulatory Agency rewrote the rules so that the utilities could draw up evacuation plans. That has been upheld in court, but opponents planned more challenges, saying the rule allowed utilities to assume state and local resources would be available in the event of an emergency.

That seems a safe assumption, but President Reagan solved the argument once and for all this past week by signing an executive order that gave a federal agency - FEMA, or Federal Emergency Management Agency - authority to take over emergency planning when states or local communities refused to do so.

At one stroke, the president eliminated a major obstacle standing in the way of opening nuclear power plants. Perhaps some power plants like Seabrook can finally get into operation.

Critics of the power plants are disappointed by the president's action, saying it is an intrusion of federal power into local authority. They have a point. It always a sad day when federal agencies usurp control over states or local communities.

But the action is understandable. If states or communities refuse to allow an N-plant to be built, that is one thing. But when such a power plant clears all the hurdles after many years and is finally constructed at a cost of billions - for officials to then stand in the way by refusing to draw up emergency plans leaves federal officers little option.

The nation is going to need nuclear-generated electric power in the future. There are problems, certainly; safety concerns, the inevitable waste that must be stored somewhere. But down the road, there may have to be choices between N-power or no electricity. The answer to that choice seems clear.