The Three Mile Island accident has forced the Energy Department to revise sharply downward its estimate of how many nuclear power plants will be running in the United States in the year 2000, in part because it believes Congress will declare a moratorium on new nuclear construction.

In a memorandum circulated throughout the department by its Policy Office, the latest estimates of the number of nuclear power plants operating in the United States at the opening of the next century range from 150 to 200, which is 50 to 100 fewer than it figured in estimates made before Three Mile Island.

The memo circulated by Energy's Policy Office states: "Nuclear capacity expansion assumes only those plants under construction are completed by 2000." Does that mean the Policy Office believes that Congress will call at least a temporary halt to nuclear construction? "For planning purposes," one Energy source said, "that is correct."

There is a debate in both houses of Congress on the pros and cons of a moratorium on nuclear construction. The Senate subcommittee on nuclear regulation has said it will demand a moratorium if the Carter administration does not come up with a national plan it can implement by 1985 to dispose of nuclear waste. The House subcommittee on energy and the environment says it will wait until the presidential commission investing Three Mile Island reports its recommendations to the White House next week.

The Kemeny commission, as the presidential commission is called after its chairman, Dartmouth College President John G. Kemeny, will not make any formal recommendations for a nuclear construction moratorium. It is almost sure, however, to call for sweeping revisions in nuclear regulation that, if implemented, would greatly slow nuclear construction.

Just how President Carter might respond to the recommendations of the Kemeny commission is anybody's guess, though he has said he will act on all their recommendations "if they are practical."

White House sources say the president is acutely aware of how countries like in Japan, France and Germany depend on nuclear power and that this dependence is sure to influence the path he takes after the Kemeny commission makes its report.

"These countries are our friends and the president knows they look to us for leadership in the nuclear field," one White House source said. "A moratorium of any serious length of time might not only hurt U.S. relations with those countries, it might put a crimp in their energy plans."

In effect, there is already a moratorium on new nuclear construction in the United States. Only two new orders for nuclear power units have been placed in the United States in the last three years, those by a single electric company, Commonwealth Edison Co. of Chicago.

Seventy-one nuclear power plants now operate in the United States, and another 95 are in various stages of construction. About six of those plants are so far along in construction that they have applied to the NRC for operating licenses. The NRC has said it will not grant any new operating licenses until the Kemeny commission makes its recommendations on nuclear safety.

The electric power industry has told Congress it is impossible to assess the impact of a formal moratorium on new nuclear construction, even a short halt of six months.

It has estimated that a moratorium would force cancellation of about 10 nuclear projects now on the drawing boards and force some switches to oil from nuclear projects that are in the early stages of construction. At the very least, the industry has argued, these changeovers would mean higher electric prices to consumers.

Whether a nuclear moratorium would mean electricity shortages in the future, the Energy Department has said, is also impossible to assess. One reason is that growth in electric power consumption has slowed greatly in the last few years, meaning that industry has not needed as much new electricity as it forecast it did.