ROCKVILLE, Md. — The Nuclear Regulatory Commission is approaching completion of an ambitious study that concludes that a meltdown at a typical American reactor would lead to far fewer deaths than previously assumed.

The conclusion, to be published in April after six years of work, is based largely on a radical revision of projections of how much and how quickly cesium 137, a radioactive material that is created when uranium is split, could escape from a nuclear plant after a core meltdown. In past studies, researchers estimated that 60 percent of a reactor core’s cesium inventory could escape; the new estimate is only 1 to 2 percent.

A draft version of the report was provided to The New York Times by the Union of Concerned Scientists, a nuclear watchdog group that has long been critical of the commission’s risk assessments and obtained it through a Freedom of Information Act request. Since the recent triple meltdown at the Fukushima Daiichi nuclear plant in Japan, such groups have been arguing that the commission urgently needs to tighten safeguards for new and aging plants in the United States.

The report is a synthesis of 20 years of computer studies and engineering analyses, stated in complex mathematical terms. In essence, it states that if a prolonged loss of electric power caused a typical American reactor core to melt down, the great bulk of the radioactive material released would remain inside the building even when the reactor’s containment shell was breached.