This stubborn problem threatens the main radiation barrier at the plants:
the garage-size steel vessels that cradle tons of radioactive fuel.
In certain emergencies, these vessels would flood with cooling water. If the
vessel walls are too brittle, they could shatter and spew their radioactive
contents into the environment.
This kind of accident is most likely to occur at pressurized water reactors,
the superheated, high-pressure cookers that make up two-thirds of the U.S.
commercial nuclear industry.
In the early days, scientists knew that these vessels could grow brittle
from years of exposure to the neutrons darting from the core. But they had
only vague ideas about how long that would take.
The U.S. Nuclear Regulatory Commission decided to set a 200-degree
Fahrenheit benchmark known as “reference temperature” — a calculated
measurement that predicts the threshold at which the vessel could break
apart. The higher the reference temperature, the more likely the coolant
will crack the vessel.
By 1982, 14 nuclear plants had violated the standard. An NRC staff report
offered the faint reassurance that no shutdowns would be needed “in the next
The agency went to work — not figuring out how to fix vessels, but
justifying a higher standard.
In 1985, the NRC raised reference temperatures limits to 270 degrees for
vessel plates and up to 300 degrees for some welds.
This still wasn’t enough. In 1994, the staff again issued a warning: Vessel
welds with high copper content were turning brittle faster than expected.
Nine plants might need to close early.
Plants set about rearranging fuel rods to minimize radiation damage to the
vessel walls. But reactors kept creeping toward violating the standards.
So regulations again needed to be loosened. In 2007, a top NRC official
declared outright that the old safety margins were “overly conservative.”
Within two years, the NRC proposed even looser standards for reference
temperatures: up to 356 degrees.
These limits were adopted last year, again helping a handful of reactors
stay within the rules.
Industry and regulators say these rollbacks are safe. However defended,
though, they provide less of a margin for error.
“They do it to protect the acceptable lifetime of a plant and to squeeze
more lifetime out of it,” said retired NRC engineer Demetrios Basdekas, who
tried to challenge embrittlement standards from within the agency.
The repeated relaxation of the standards might not be enough in the long
Per Peterson, who studies reactor safety at the University of California,
Berkeley, predicted that the life span of some reactors will still be
limited by embrittlement.
And predicting future embrittlement may prove difficult.
When reactors were built beginning in the 1960s, officials placed metal test
samples inside the vessels to monitor brittle conditions. Industry experts
have warned that the supply of samples is dwindling.