Environmental Engineering Reference
In-Depth Information
Decreasing the stressors, for example, heating up the water in the emer-
gency core cooling system (ECCS) to lessen thermal shock in a pressur-
ized thermal shock (PTS) situation; steam-line isolation; system solu-
tions interlocks.
Introduction of volumetric non-destructive testing for in-service
inspection.
Annealing of RPV has been implemented at Loviisa NPP and Kola NPP
(also at the shut down plant Bochunice V1). Annealing in the case of
the VVER-440 reactor vessel weld was performed at a temperature of
475
15°C and the holding time was 150 hours. Assessment of annealing
effectiveness (level of properties recovering after annealing), determination
of re-irradiation re-embrittlement rates after annealing, and the behaviour
of VVER-440 weld materials, showed the real possibility of recovering RPV
toughness properties of irradiated VVER-440 RPV materials. Measures
were also taken to improve the knowledge of the vessel material by ves-
sel sampling. A more detailed description of the RPV neutron irradiation
embrittlement issue is provided by Erak
et al
. (2007), for example. Based
on the results of the VVER-440/213 plants, annealing of the RPV has been
implemented at Rivne NPP.
In order to determine the time limit of operation of the RPV, it is neces-
sary to consider and analyse the neutron irradiation damage, thermal age-
ing and low-cycle fatigue in decreasing the fracture toughness of the RPV
materials.
Pressurized thermal shock (PTS) is the most critical lifetime lim-
iting event for the RPV. Since the PTS screening requirement
(pressure-temperature-loading limits) is the lifetime limiting process for the
RPV of VVERs, the methodology of PTS evaluation has to be established
in the national regulations. This will take into account the applicable best
practices, features of the RPV and the thermal-hydraulic peculiarities of the
VVERs. The assumptions of renewed PTS analyses have been confi rmed
with mixing tests. International research projects supported the effort of
VVER plants in the evaluation of PTS for the RPV (IAEA, 2005).
The results of the PTS calculations, based on the analysis of postulated
embedded fl aws, endorse the possibility of 50 years of operation for all of
the units, without annealing of the 5/6 welds. At the Paks NPP, the assump-
tion of the embedded postulated crack (under-cladding semi elliptical type)
was justifi ed by the results of qualifi ed in-service inspections, which followed
the procedure of European Network for Inspection Qualifi cation (ENIQ).
Two types of inspection were applied to the full cladding area: (1) ultra-
sonic inspections from the inner surface and (2) Eddy current inspection,
overlapping the fi rst 5 mm thickness of the RPV inner-wall area. There is
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