Environmental Engineering Reference
In-Depth Information
United States has built no new power plants in the last 30 years and struggles
to operate its existing ones which are over 40 years old. Material degrada-
tion management and maintaining the integrity of plants became impor-
tant in renewal of the operating licenses. Stress corrosion cracking of Alloy
600 steam generator tubing and nozzles, has been a signifi cant mechanism
of degradation since the 1970s, and it is still a poorly understood technical
problem. This chapter examines strategies of degradation management for
PWR reactor pressure vessels, reactor internals, stream generators, pressur-
izers, control rod drive mechanisms (CRDMs) and primary/secondary pip-
ing, and describes some cases of component degradation.
7. 2
Materials management strategies
In order to establish measures for managing materials degradation, the deg-
radation mechanisms must fi rst be fully understood. Inspection techniques,
mitigation methods and repair technologies depend on knowledge grounded
in experimental studies of degradation mechanisms or in fi eld operating
experience within power plants (IAEA, 2011). There are three stages to
managing materials ageing in nuclear power: preventive action; monitoring
and inspection; and repair and replacement. In preventive action, improve-
ment of the materials, reduction of stress and improvement of water chem-
istry can be used as measures to prevent cracks of Ni alloys (IAEA, 2011).
Surveillance of pressure vessels can be carried out through monitoring and
inspection to check the soundness of parts. For example, by checking for
leakage of primary coolant through wall cracks in J-welds of the upper ves-
sel head penetration (VHP) or lower bottom mounted instrumentation
(BMI) nozzles, pressure boundary performance can be maintained. In the
case of coolant leakage, boric acid residues on the outside of the pressure
vessel or carbon steel corrosion products can be detected through visual
inspection. Cracking can occur in operating power plants due to material
properties or residual stress, therefore the timing of cracking can differ from
the experimental result. Regardless of the cause, it is important to detect
cracks as early as possible. Besides visual inspection, methods such as pene-
trant testing or eddy-current testing (ECT), and ultrasonic testing (straight
beam and longitudinal wave angle beam UT) can also be used. In repair and
replacement, the damaged parts should be isolated from the corrosive envi-
ronment, or the tensile stress upon them reduced. In the case that these two
measures are inappropriate, the parts should be replaced with others made
from more corrosion-resistant materials. In order to systematically manage
PWR structural materials, a common objective has been established, and
much research has been done as a collaborative effort between many coun-
tries. The joint research programme for 2011 is as shown in Table 7.1.
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