Four days after the accident at the Three Mile Island atomic power plant in Pennysylvania, the Nuclear Regulatory Commission began yesterday to "blanket the countryside" with devices to monitor and record accumulated radiation.

Until yesterday, monitoring outside the plant site had consisted primarily of spot checks that determined radiation levels at a specific time and place.

Without knowing accumulated dosages, no determination can be made on the levels of exposure of persons in the vicinity of the power plant.

While the plant continues to emit low levels of radiation as its operators attempt to cool down its reactor, the accumulated dose takes on greater importance as a possible long-term health hazard to individuals.

The accident on Wednesday near Harrisburg, Pa., came at a time scientists in and out of government are debating the long-term health effects of low-level radiation.

"The NRC is putting (the devices) out, one every mile, in eight different sectors surrounding the plant," one source said yesterday. Another said some would be located as far as 15 miles from the site.

According to plans, dozens of the devices will be installed and checked every two days, thus providing "the first solid data" on radiation doses that may be absorbed from yesterday on by inhabitants of the area being monitored.

The only cumulative radiation monitoring up to yesterday was done at 17 sites maintained by Metropolitan Edison Co., the operator of the nuclear power plant, it was learned yesterday from NRC sources.

Metropolitan Edison spokesmen who were questioned Friday and yes- terday about such monitoring devices claimed they knew nothing about them.

However, an NRC official said the company had taken data from these sites in for examination on Thursday afternoon, a day after the accident occurred.

According to an NRC official, two of the company's devices showed higher than expected dosages and both were within a mile of the northern perimeter of the plant.

Since the company devices are designed to measure cumulative doses over a three-month period, the readings recorded throught last Thursday do not relate specifically to the accident period.

However, at four-tenths of a mile from the site the monitor showed a radition accumulation of 81 millirems, 65 millirems above the expected level.

Another site, seven-tenths of a mile north of the site, showed 37 millirems, 22 above the normal three-month level.

The federal standard for minimal health risk to the general publis is set at 500 millirems per year. For workers at nuclear facilities, the standard is 5,000 millirems in a year, but only for 12 years.

According to the NRC official, Metropolitan Edison yesterday afternoon rechecked its fixed site monitors but no results from that survey are expected until today.

The devices should show what dosage, if any, accumulated from the two major vents of radioactive gas that have taken place since Wednesday.

The only information to date on radiation after that venting came from a company spokesman who said he had spot recordings of two-tenths of a millirem to five-tenths of a millirem per hour in the direction a radioactive gas plume was traveling.

If the latter rate accumulated in one place for a day, it would reach 12 millirems.

Yesterday afternoon, a company spokesman said one onsite monitor at the plant gate directly downwind from the nuclear core showed a five millirem an hour level.

Late yesterday afternoon it was learned from NRC sources that the state of Pennsylvania was joining NRC in placing new cumulative radiation monitoring devices in the residential areas adjacent to the site.

In addition, the Environment Protection Agency's aircraft with sophisticated radiation monitoring equipment arrived yesterday morning at Harrisburg.