WASHINGTON (AP) _ The operator of Pennsylvania's Limerick 2 nuclear plant was granted a full- power license Friday after agreeing to make nearly $2 million in modifications to bolster the plant's ability to withstand a severe accident.

The $2.9 billion reactor had been in the planning and construction stages for 20 years. It is the nation's 111th licensed nuclear plant and the fourth to receive a full-power license this year.

The operator, Philadelphia Electric Co., said it could begin the steady rise to full-power at the plant as soon as Saturday.

Before the 4-0 vote by the Nuclear Regulatory Commission, Philadelphia Electric and the citizens' group Limerick Ecology Action reached an unusual agreement in a dispute that threatened to send the parties to court.

LEA had been pressing the utility and the commission to consider possible design changes at the suburban Philadelphia plant to lessen the risk and environmental impact of a severe accident.

But Philadelphia Electric agreed to make two plant modifications, evaluate the possibility of several others, and allow an LEA safety expert on a review team to study risk reduction efforts at Limerick.

The modifications were not required by the commission, which said the design question would not have obstructed the licensing process.

''This agreement is precedent-setting,'' said LEA attorney Charles Elliott. ''Never before has a citizens' group obtained specific severe accident mitigation design changes at a nuclear facility and an ongoing formal role through its experts in further risk reduction measures by a nuclear utility.''

In one modification, Philadelphia Electric said it would bolster its ability to solidify molten material in the reactor core in the event of a severe accident. That measure would prevent the molten material from spreading.

The utility has also agreed to enable the fire protection system to be used as a backstop to keep pressure from building up in the containment structure in case of a large steam leak or core melt accident.

Construction of Limerick 2 was halted for four years in the mid-1980s by the Pennsylvania Public Utility Commission, which said the plant was not in the public interest. Construction was permitted to resume in 1986 on the condition that costs be kept below $3.2 billion.