WITH THE ANNOUNCEMENT on Friday of the president's response to the recommendations of his Commission on the Accident at Three Mile Island, the series of events set in motion by last spring's nuclear accident is now just about completed.Most of the congressional committees have finished their investigatons. The Nuclear Regulatory Commission has instituted some sweeping changes of its own. The industry has announced the creation of a new organization to improve reactor safety. And Congress has considered and rejected several proposals for a moratorium on licensing and building new nuclear plants.

The president's announcement was something of a mixed message. While returning to his earlier characterization of nuclear power as only "an energy source of last resort," he called on the NRC to work "as quickly as possible" and complete "no later than six months from today" its self-imposed pause in issuing new reactor licenses and operating permits. This was far removed from the Kennedy Commission's call for an ultra-cautious, go-slow approach and a lengthy pause just short of a moratorium.

Wisely rejecting the Kemeny group's recommendation that the NRC should be abolished and reconstituted as an executive agency, Mr. Carter announced that the present chairman would be replaced by one of the NRC's four other commissioners. When a vacancy becomes available, he promised, a new chairman will be chosen from outside the agency. The attempt here was apparently to carry out the Kemeny report's chief finding that "fundamental changes will be necessary . . . above all, in the attitudes of the NRC." However, because he did not indicate in any way the reasons for the removal of Chairman Joseph Hendrie or the differences that might be expected between Mr. Hendrie and his successor, the president's action lost much of its potential value.

Finally, while stressing that he supported virtually all of the Kemeny Commission's recommendations, the president stopped short of making specific proposals on several major issues, including remote siting of new power plants away from population centers and changes in the NRC's controversial licensing procedures. Most of his specific proposals addressed programs -- such as the placement of federal inspectors at every reactor -- that are already well under way. In short, the administration's lengthy announcement contains less than meets the eye.

What then has changed as a result of the country's first nuclear accident? There will be some important changes made in every phase of the government's regulatory effort, from reactor design to operator training. Planning for emergencies -- everything frominitial siting of a reactor to pre-drawn evacuation plans -- will become an intergral part of reactor operation for the first time. Possibly -- this is not yet clear -- the utilities will recognize that operating a nuclear reactor is fundamentally different from running a coal- or oil-fired plant, and will adjust accordingly. But the most important change should be the death of the view that prevailed in the nuclear industry and among some of those at the Nuclear Regulatory Commmission that reactors are safe enough, the system is fine, and those who raise safety issues are nothing more than a few anti-nuclear troublemakers and nuts. Three Mile Island gave the lie to that. In the last analysis, the country was very lucky to have learned so much at so little cost.