Government and utility officials, in separate and sometimes conflicting statements, today began to explain the series of human and mechanical malfunctions that triggered the nation's most serious nuclear accident to date.
At a crowded news conference in nearby Hersehy, Pa., top officials of the Metropolitan Edison co., the utility that operates the billion-dollar Three Mile Island facility that was shut down yesterday, sought to play down the accident.
"We didn't injure anybody through this accident, we didn't seriously contaminate anybody and we certainly didn't kill anybody," said John G. Herbein, vice president for power generation.
Herbein told reporters that the utility's nuclear generating station on the Susquehanna River here was never in any serious danger of a complete "meltdown" of its radioactive core which could have released lethal radioactivity over a wide area.
Nuclear Regulatory Commission officials, however, appeared to contradict some of the assurances from the utility that the problem was neither serious nor caused by human error.
In statements and information delivered during separate press conferences by both groups today, a partial picture began to take shape of the events that occured during the accident.
At about 4 a.m. Wednesday, with the nuclear reactor operatin at about 97 percent of its total power output, two pumps which fed water to the plant's secondary cooling system failed. Federal officials suggested that the pumps may have shut down because of a foul-up in their water cleaning system.
The shutdown of the pumps caused the nuclear reactor's primary water cooling system to build up pressure inside its own enclosed loop. The system flows through the reactor's core, keeping it from building up dangerously high levels of heat.
Within minutes, according to utility officials, the water pressure inside the loop rose to 2,350 pounds per square inch, blowing open a safety valve set to ease unusual pressure in the system. The valve normally closes when the pressure drops 50 pounds below the critical level, officials said, but this time it stuck open.
The malfunctioning valve allowed massive amounts of radioactive cooling water to escape from the primary system and flow down onto the floor of the reactor container.
More serious, the stuck valve caused a drop in the level of the critical coolant for the reactor's radioactive core. Without the coolant, the core rapidly superheats and can go into the castastrophic meltdown stage, officials said.
What happened next is unclear. The plant's Emergency Core Cooling System (ECCCS) was turned on to replace the leaking coolant but someone also turned off a critical pump in the primary loop.
Federal officials speculated today that a worker in the reactor's control room may have mistakenly shut down the pump in the belief that the coolant pressure had stablized. Utility officials said, however, that not enough evidence existed at this point to determine why the pump was deliberately shut down. Herbein said, however, that one reason for the pump shutdown was to preserve the machinery from rising heat in the coolant system.
In any case, the drop in coolant pressure with the pump off was enough to expose part of the reactor's core, a series of 15,000 ziroconium-clad rods contaning the reactor's nuclear fuel. Officials estimated that as many as 1 percent of the rods melted or were damaged, releasing additional radioactivity to the escaping coolant.
An additional error was apparently made when the escaping cooling water-which turned to steam-was discharged from the reactor into the surrounding air between 11 a.m and 1:30 p.m. Wednesday. Company officials acknowledged releasing the steam but said they shut down the discharge immediately after they learned it contained radiation in the form of radioactive iodine, krypton and xenon.
In addition, according to federal officals, radioactive gamma rays penetrated the reactor container's four-foot-thick concrete walls. Utility officals denied today that any gamma rays had escaped through the walls.
According to Metropolitan Edison officials, between 12,000 and 15,000 gallons of escaped, radioactive coolant was pumped from the floor of the reactor container to an adjoining auxiliary building Wednesday morning.
The decision to pump the coolant into the auxiliary building may have been a mistake, utility officials conceded. The ventilation system in the auxiliary building contained no radiation filters and radioactive gas escaped through the system into the open air, officials said.
In addition, they said, additional leaks of radioactivity apparently took place because of faulty seals on several pump shafts in the auxiliary building. The leaking seals were closed today, utility officials said.
While Metropolitan Edison officials denied that serious radioactive contamination occurred outside the reactor, they said that readings inside the dome of the reactor container reached 80 rems, more than 15 times the maximum exposure allowed for a nuclear worker in a year. Federal officials placed the radiation level much higher.
NRC spokesman Jan Strasma said one worker had been exposed to 3.1 rems of radiation, a level he said is not serious, but which requires the company to notify the NRC within 30 days of the incident. Strasma said such an exposure of the employe violates NRC regulations and the company would be cited. The employe was not identified.
Metropolitan Edison president Walter M. Creitz said, however, that the high readings in the dome were questionable but that the plant would remain sealed off for several more days.