Two Riverdale boys playing inside a closed neighborhood Pepco power plant were severely burned last night when a 115,000-volt electrical charge surged through an overhead steel beam on which they were walking, Prince George's County police reported.

The injured youngsters were identified as Andrew Canter, 12, and Shell Estrada, 11, both of 6841 Riverdale Rd.

Canter, who suffered second and third-degree burns over 25 percent of his body, was listed in critical condition.Estrada, who had second - and third-degree burns over 25 percent of his body, was listed as serious. They were taken to the Children's National Medical Center burn unit.

The injured boys were accompanied into the plant by a third boy who escaped unharmed. His identity could not be learned immediately.

Joy Sayre, manager of a 7-Eleven store next to the power plant, said afterward that she "heard the loudest thunderbolt ever" at about 7:45 p.m. A few moments later, a boy she knew only as "Timmy" ran hysterically into the store and jumped over the counter.

"He was yelling, 'Help me. My friends are dead. I've killed my friends,'" Sayre recalled. Sayre said the youth then explained that he had accompanied the two boys into the power station and had watched as the tremendous surge of electricity knocked them from the beam onto the top of a transformer.

"He just grabbed the telephone, dialed 911 and started crying, 'Help me,'" Sayre said.

When members of the Riverdale Volunteer Fire Department arrived on the scene, they found the two boys semi-conscious with burns on their faces, bodies, arms and legs. Canter, the more seriously burned, was flown by helicopter to the Children's burn unit.

The boys' parents, Anthony and Olga Estrada and Walter and Louise Canter, arrived at the hospital within minutes of their sons. The Canters, who are both deaf and recently moved to the area, said in an interview interpreted by their daughter, Roxanne, that they never thought "anything like this" could happen in their neighborhood. "We never knew there would be a power plant next to our home," they said.

Harold Kranz, a spokesman for Children's Hospital, said, "We'll be thinking about skin grafts later, but right now, we're worried about burn shock and infection. We're trying to make some of the fluid replacements as quickly as possible." Maintenance of body fluids in burn victisms is a critical factor in successful treatment, he explained.

The Potomac Electric Power Co. plant located inside a large, 50-foot-high roofless concrete building, is one of many unguarded plants scattered throughout the Washington area. Inside the Riverdale plant are three switch gears and three transformers used to transfer the power of two 115,000-volt feeder lines into nine 13,000-volt lines for residential customers.

The boys had apparently entered the blockhouse-shaped building by shinnying over a low retaining wall in the rear. They then apparently forced themselves through a three-foot-long by nine-inch-wide vent after pulling away a protective mesh screen, police said.

For the Estrada (family, the injured boy's parents said, the day had started festively. Family members spent much of the day in Baltimore where the Estrada children were receiving instructions on procedure for naturalization. The Estradas are from Peru.

Soon after returning from Baltimore, Andrew went to play with friends. The accident occurred a few minutes later, the parents said.

According to police, the boys were walking above the transformers on a metal I-beam that is connected to an electrical feeder line. The I-beam is part of safety release valve system, and when the feeder line overloads, the excess electrical surge is channeled through the beam, Pepco spokesmen said.

An overload occurred just as the boys were walking on the beam, police said.

"This is extremely unfortunate," said Dave Boyce, a Pepco spokesman.

"We do everything we can to see that these facilities are safe, but there is no way to make them absolutely foolproof. These kids seemed pretty determined to get inside. All children should be encouraged not to go near those things."

The accident quickly raised questions among Riverdale community members who have been requesting additional recreational facilities. The injured youths were known by neighbors to be part of a group of children who sought adventure by climbing onto the roof of local supermarkets and into huge storage bins at their apartment building.

"We just don't have anything safe and useful for them to do. They're always getting into trouble because of boredom,' said one man who lived in the apartment complex in which the injured boys resided.

Also contributing to this story were Washington Post staff writers Joseph Contreras and Benjamin Weiser.