THE CASE FOR PLUTONIUM fuel and the breeder reactor has been growing steadily more dubious. With each re-examination, the risks become clearer and the benefits dimmer. The lastest analysis, published this week by a research organization called the Mitre Corporation, is remarkable for its intelligence and balance. It deserves to be considered authoritative. It strongly urges the Carter administration to postpone plutonium fuel production "indefinitely" and to relegate the breeder technology to the laboratory as a standby for the middle of the next century. That judgment is unquestionably the right one.

The penalties for a mistake here would be monstrous. Plutonium is one of the most potent poisons ever devised and, in its fuel form, it can be fashioned into nuclear weapons relatively easily.

The technology is within the capacity of terrorist organization as well as national goverments of every kind. Putting the stuff into commercial use would be an irreversible decision.

Yet that is the direction in which U.S. policy has been heading, guided mainly by obsolete assumptions and blind momentum. A reprocessing plant for the manufacture of plutonium now stands, two-thirds completed, near the town of Barnwell, S.C., waiting for a federal policy on subsidies. A breeder reactor is under construction at Clinch River, Tenn., scheduled to go into operation in the early 1980s as a prototype for commercial plutonium-fueled reactors in the 1990s. The Mitre report is addressed primarily to President Carter and his energy advisers. If they follow its clear and forceful logic, they will abandon both of these projects.

The Mitre report is the work of a committee of 21 men who, collectively, know as much about these questions as anyone in the world. One member of the committe, Harold Brown, was president of the California Institute of Technology and is now Secretary of Defense. Another, Kenneth Arrow of Harvard, holds the Nobel Prize in economics. A third, Wolfgang Panofsky, is director of the Stanford Linear Accelerator Center. There's no need to run through the entire list. Their report has been published as a book incidentally, by the Ballinger Co., under the title, "Nuclear Power Issues and Choices." But it covers a great deal more than nuclear issues alone - because to get to the plutonium question, you first have to consider the future American economy, its requirements for energy and the various possible sources.

The nuclear reactors now being built by utilities around the country are fundamentally different from the plutonium breeder - and far less dangerous. The present commercial reactors are fueled with rods of uranium; when the fuel is spent, the fission process has left a residue of plutonium in the rod. But in that form, it's not explosive. It's the next step that takes the country over a new threshhold - the reprocessing procedure that refines the plutonium and mixes it into a new kind of fuel for the breeder, which, in turn, produces more plutonium than it consumes. It's this second fuel, the one that emerges from the reprocessing plant, that contains weaponsgrade plutonium and presents truly extraordinary dangers.

The only conceivable reasons for resorting to the plutonium technology now would be either a looming shortage of uranium or the threat of energy prices so high that they could derail the nation's economic growth.

Plutonium recycling can be postponed indefinitely "at essentially no economic cost," the report finds. "The fears that energy scarcity will force fundamental changes in economic and social structures or the lifestyle of the industrialized world are not well founded."

The Mitre group, in other words, considered both possibilities and concluded that there was nothing to justify risking, in this century, the dire hazards of reliance on plutonium fuels.