Environmental Engineering Reference
In-Depth Information
For thermal stress reduction, a device that inspects the state should be
installed or risk abnormal events. To reduce damage from neutron radia-
tion, the low leakage fuel loading technique can be applied which minimizes
the infl uence of the neutrons on the materials of the pressure vessel through
appropriate arrangement of burned and fresh nuclear fuel. The thermal
annealing technique which returns the hardened pressure vessel materials
to the nature of their raw materials can be carried out near 343°C (650°F)
in water or 430°C in air.
There are two kinds of in-service inspections for vessels: ultrasonic testing
and acoustic emission testing. The ultrasonic testing is described in ASME
Section XI, and it is used to characterize cracks of the HAZ and weld
zone. The uncertainties in this method, especially when it is used on cracks
under cladding, have resulted in conservative regulatory requirements for
use of these fl aw estimates to set the permissible PT limits and evaluate
pressurized thermal shock (PTS) events. ASME Section XI requires four
inspections every ten years, and during this period, it recommends 100%
volumetric inspection on repair welds on all shells, heads and fl anges in the
shell, nozzles in the vessel and beltline parts (Morgan and Livingston, 1995).
This enables closer monitoring at the beginning and growth of potential
fatigue cracks. The sharp cracks found on the surface of the vessel or in the
embrittled beltline are most important to PTS but it is diffi cult to detect
or inspect these cracks. Some studies have developed advanced ultrasonic
techniques for this purpose (Shah and MacDonald, 1993). Acoustic emis-
sion monitoring can be used in online monitoring the growth of cracks if the
surface of a vessel is accessible (Morgan and Livingston, 1995).
7.3.2 Management techniques for reactor internals
Materials used for PWR internals include ferritic steel, wrought austenitic
stainless steel, cast stainless steel (CASS) and Ni alloys. The internals main-
tain the soundness of the geometrical core. The core consists of the upper
core structure, core baffl e/former/barrel, thermal shield and lower core sup-
port structure. The factors which infl uence degradation of these parts are:
thermal plant transient, fl ow-induced vibration, radiation, high temperature,
mechanical and thermal stress, and corrosive coolant. The main degradation
mechanisms are: fatigue; radiation and thermal embrittlement; void swell-
ing; and irradiation assisted stress corrosion cracking (IASCC). IASCC is
a type of SCC indicated by a large quantity of neutrons in a material. The
main objective of degradation management in the case of reactor internals
is to ascertain if the internals support the core and can protect the CRDM
(Morgan and Livingston, 1995).
In-service inspection and surveillance and changing of the materials
are some of the measures used to manage degradation of the internals.
Search WWH ::
Custom Search