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 finally, look at applied management practice around the world.

7.1.1 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 flange closure studs. The degradation mechanisms are largely radiation embrittle­ment, fatigue, IGSCC and boric acid corrosion.

Embrittlement of pressure vessels is a more significant 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 fluence. 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 ves­sels, 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 var­ious 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 pres­sure temperature (PT) of power plants is limited. Damage by fatigue crack­ing 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 noz­zle (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.

RPV components	Ageing mechanisms	Management categories	Management techniques Description of techniques

For thermal stress reduction, a device that inspects the state should be installed or risk abnormal events. To reduce damage from neutron radia­tion, the low leakage fuel loading technique can be applied which minimizes the influence 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 flaw 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 flanges 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 difficult to detect or inspect these cracks. Some studies have developed advanced ultrasonic techniques for this purpose (Shah and MacDonald, 1993). Acoustic emis­sion monitoring can be used in online monitoring the growth of cracks if the surface of a vessel is accessible (Morgan and Livingston, 1995).