Page 30 - Nuclear Plant Journal - March April 2013

Old
Reactors,
New Tricks
An excerpt from the original article,
“Old Reactors, New Tricks” by Leonard
J Bond, Pacific Northwest National
Laboratory (PNNL), which originally
appeared in IEEE Spectrum Magazine,
August 2012 issue, pages 30 to 35.
Copyright 2012 IEEE.
My colleagues and I have sought
new types of online monitoring and
nondestructive testing technologies that
can provide early warnings of degrading
materials. Our goal has been to transition
from the current “find and fix” approach
to one we call “model and predict.”
Inside nuclear power station, fierce
forces are at work. In the pressurized
water reactors (PWRs) and boiling water
reactors (BWRs) that generate power
in the United States, the nuclear cores
consist of rods of uranium dioxide. Inside
this radioactive material, a nuclear fission
reaction produces energy and many forms
of radiation, including gamma rays and
neutrons. The extremely high radiation
levels are reduced by about a factor of
20 by the steel walls of a reactor pressure
vessel, and then to safe levels by the
massive reinforced concrete containment
structure that jackets the vessel.
Both reactor types use water as
the coolant. In a PWR, water enters the
reactor core at about 275 °C and is heated
as it flows upward through the core to a
temperature of about 315 °C. The water
remains liquid due to high pressure,
usually around 15.5 megapascals (about
150 times the atmospheric pressure at
sea level). In a BWR, the cooling water
is maintained at about 7.6 MPa so that it
boils in the core at about 285 °C. In both
cases steam is produced to drive turbines
that generate electricity.
High temperature, high pressure,
and radiation all stress a reactor’s
components. Inside a reactor, neutrons
bombard the pressure vessel’s steel walls;
over a period of years, that bombardment
can cause reactions that displace atoms in
the material and produce impurities and
tiny voids. These microscopic phenomena
can reduce the metal’s toughness and its
ability to resist cracking.
The NRC and the nuclear industry,
working with the Electric Power
Research Institute, are now determining
how to measure and monitor the aging of
a reactor’s key components. The major
concerns are embrittlement and cracking
in the reactor pressure vessel and its
piping; degradation of the concrete
containment; aging cables; and corrosion
in buried water pipes. At the moment we
just don’t know which of these problems
will be the most critical in any given plant.
After all, no one has ever before operated
a commercial-scale nuclear reactor for
six or seven decades. We have entered a
new era in the atomic age.
During the past 30 years, many
parts of plants have been replaced or
refurbished, including turbines, some
major piping, and pressure vessel
lids. But the central components of a
nuclear plant—the pressure vessel itself
and its reinforced concrete-and-steel
containment—were never designed for
replacement. The pressure vessel of a
typical 1-gigawatt power plant weighs
about 300 metric tons and is more than
12 meters (39 feet) tall. Most analysts
believe that it would be easier to build a
new plant than to cut into the containment
to extract and replace a pressure vessel.
So how do you determine whether
a vessel or another major component is
robust enough to last another 20 years?
If you want to know what’s
happening in an aging reactor, to really
understand how its thick steel and
tough concrete are faring after years of
relentless bombardment, the best thing
to do may be to listen to it. Nuclear
researchers are now testing acoustic and
ultrasonic monitoring techniques drawn
from the civil and aerospace engineering
communities. The same methods used to
monitor the structural integrity of a bridge
or an airplane may work for a nuclear
pressure vessel as well.
One promising technique was
demonstrated decades ago in an
operational nuclear plant. In 1989,
inspectors at the Limerick Generating
Station, in Pennsylvania, found a tiny
crack in the welding around a pressure
vessel pipe that brought cooling water
into the bottom of the reactor. The
operators concluded that the flaw didn’t
pose a threat, but they wanted to see if
it was possible to monitor crack growth
in an operating plant. They turned to
a technique called acoustic emission
monitoring, which is used to check on
metallic structures like pipelines and
wind-turbine blades.Thismethod relies on
the fact that when a crack grows, acoustic
energy is released in tiny pulses—much
the same way an earthquake sends out
seismic waves. Once the acoustic system
was installed, operators could listen for
the ultrasonic waves that would indicate
a growing fracture.
The acoustic system was kept in
place for three years, during which time
researchers listened in as one part of the
crack grew to a depth of 12 millimeters.
The system also detected the growth of
minuscule cracks that wouldn’t have been
noted by traditional monitoring methods,
and researchers deemed the technology
demonstration a success. In the decades
since, fossil-fueled power plants and
petrochemical facilities have installed
acoustic emission systems to monitor
vessels and pipes. However, nuclear power
stations in the United States have been
slow to adopt this proven technology.
With advances in both computer
hardware and processing software,
acoustic emission systems are now little
larger than a laptop and are capable of
displaying data nearly in real time. At
Pacific Northwest National Laboratory,
(PNNL) Richland, Washington, my
colleagues and I recently tested acoustic
emission monitoring along with another
technique for metal monitoring that
makes use of “guided waves.” In this
technique, transducers generate ultrasonic
waves with specific frequencies, which
propagate through a structure such as
a metal pipe or the walls of a pressure
vessel. Because the ultrasonic waves are
scattered and reflected by discontinuities
in a material, they can provide clear
indications of cracks or corrosion. This
technique could be particularly useful
because it wouldn’t require inspectors
to strip off insulation to inspect pipes,
30 NuclearPlantJournal.comNuclear Plant Journal, March-April 2013