Environmental Engineering Reference
In-Depth Information
7. 3
Management techniques: development
and application
In this section, we discuss in further detail management techniques for both
reactor vessels and internals as well as steam generator tubes, pressurizer
nozzles and the CDRM and fi nally, look at applied management practice
around the world.
7.3.1 Management techniques for a reactor vessel
In a pressure vessel and its internals, degradation areas are welds of beltline
regions, inlet-outlet nozzles, CRDM, instrumentation nozzles and fl ange
closure studs. The degradation mechanisms are largely radiation embrittle-
ment, fatigue, IGSCC and boric acid corrosion.
Embrittlement of pressure vessels is a more signifi cant problem in PWR
than in boiling water reactors (BWRs). This is, because in a PWR the layer
of coolant around the core is thinner, so the PWR core generates a 20-100
times greater neutron fl uence. The current design of RPVs does not feature
welded joints in the beltline region, as this is the most radiation-embrittled
zone, but in older vessels there are both circumferential and axial welds in
this area since vessels were manufactured from plates. Current materials
regulations describe the application of low copper materials and low-alloy
steel of SA533B-1 for the fabrication of pressure vessels, so that the parent
metal in the beltline part of the shell is damage resistant. In older type ves-
sels, the most important issue is radiation embrittlement around the weld
zone of the beltline area. The weld zone can easily become more embrittled
than the parent metal not only because copper, nickel and phosphorous
impurities are present, but also because it is the point of connection of var-
ious metals and the heat-affected zone (HAZ). When materials have been
embrittled, the nil ductility transition temperature or the ductile-brittle
transition temperature increases, and the upper shelf energy (USE) value
from the Charpy impact test decreases. As a result, the permissible pres-
sure temperature (PT) of power plants is limited. Damage by fatigue crack-
ing occurs in the beltline weld zone (under normal operating pressure/
heat cycle and abnormal events), closure head studs (during loading cycles
in normal operation and repair), primary coolant entrance and exit noz-
zle (under heat cycling) and penetration and CRD housing (under heat
cycling). Heat cycling can occur in normal operation during the heat-up or
cool-down phases associated with servicing, or may be unexpected (Morgan
and Livingston, 1995).
Degradation management strategies can be categorized as: mitigation,
inspection and surveillance, or repair, as in Table 7.2.
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