WASHINGTON, March 31, 1979 (UPI) - Nuclear engineers face unprecedented decisions on what to do in the next few days at Three Mile Island power plant in Pennsylvania to avert further emergencies.
A large hydrogen gas bubble lodged at the top of the reactor has forced plant operators to halt efforts to bring the unit to a "cold shutdown" for fear the bubble will expand, block vital flows of cooling water and cause fuel melting or - at the unlikely worst - a reactor core meltdown.
Average temperatures in the reactor core are reported at 250-300 degrees Fahrenheit, but there are hot spots at much higher temperatures. With efforts to lower temperatures stopped for perhaps days, officials say, the reactor is in a stable and barely cooling condition, but it continues to leak radiation.
Between one-fourth and one-half of the 36,000 fuel rods in the crippled-reactor core are damaged - bent, split, perhaps partly melted by excessive heat. Damaged rods are restricting the flow or cooling water and emitting high levels of radiation, according to Nuclear Regulatory Commission estimates.
Temperatures at three places in the pressurized reactor core have been measured at between 550 degrees to over 700 degrees. Although pressure in the reactor prevents boiling up to 550 degrees, temperatures above that level can cause cooling water to boil around some fuel elements.
Early today, the NRC reported it was investigating the possibility a hydrogen gas explosion occurred in the reactor containment building a few hours after the initial plant failure Wednesday.
No one knows, according to NRC officials, just how bad conditions really are inside the reactor.
What officials do know is that plant workers managed by late yesterday to stabilize the reactor in a relatively safe "parking" condition, allowing active cooling efforts to stop and offering the hope of a breathing period of several days.
The next step will be to carefully plot how to get rid of the bubble and resume efforts to bring down temperatures in the stricken reactor. A misstep, officials said could lead to a partial or total meltdown - the latter the most serious type of reactor problem.
"We are faced with a decision (on how to proceed) within a few days, rather than hours," said Dudley Thompson of the commission's office of inspection and enforcement. He said the decisions to be made are unprecedented.
"We ultimately face the risk of a meltdown, (depending on) the manner in which we cope with the problem. Depending on which options are taken and what changes are made, we can get the core into trouble. At some point we must cool down the core."
"If there is even a small chance of a meltdown," he said, "we will recommend precautionary evacuation."
Thompson said the term meltdown - as he used it - covered everything from the melting of only a few pellets of uranium within the reactor fuel rods to the total melting of the reactor core. He acknowledged it was a risky word to use, because to many people it means only a total melting of the core.
He said the potential risks range from the "real possibility" of partial melting of the uranium fuel to the "miniscule" chance the reactor could melt. A total meltdown would be the worst type of theoretical atomic plant disaster.
In a partial fuel meltdown, which officials conceded may already have occurred to some degree, radiation would be released within the sealed reactor building and the core might be badly damaged. But there would be no major radiation escape.
The steam explosion would be much like a kitchen pressure cooker blowing up rather than like an atomic bomb blast. Radiation could contaminate an area of 3,000 square miles, and people might have to be evacuated from an area of several hundred square miles, according to NRC reactor risk calculations.
"If we had a total meltdown, it would be pretty close to a catastrophic situation," Thompson said.
The hazard will occur when plant workers try to remove the bubble from the top of the reactor so they can finish cooling the unit down, Thompson said.
Officials described two main options:
--To reduce pressure in the reactor and let the steam bubble expand slowly, maneuvering it bit-by-bit out of the reactor into a pressurizer where it could be vented.
--To do nothing, letting the reactor core cool at its present extremely slow rate for a period of weeks of months.
"It is a ticklish operation," Thompson said.
The closest a U.S. power plant has come to a meltdown catastrophe was in October 1966 when a piece of metal broke loose at the Fermi breeder reactor near Detroit, partially blocking the flow of cooling water and causing a partial melting of reactor fuel.