Environmental Engineering Reference
In-Depth Information
AISI 347). Components of the primary circuit in contact with the primary
coolant, other than the RPV, are also made of austenitic stainless steel, that
is the piping of the primary loop, the main circulating pumps, gate valves
and the emergency and auxiliary systems pipework.
From the point of view of longer-term operation, the main defi ciency of
VVER-440/230 was the high irradiation exposure of the reactor pressure
vessel wall by fast neutrons, and the relatively quick embrittlement of the
RPV material. The issue had been aggravated by the lack of a proper RPV
surveillance programme at these plants. Several attempts have been made to
assess the embrittlement of the base and weld material of those RPVs. For
the fi rst generation RPVs, essential data for RPV materials were absent, for
example transition temperature, concentration of copper and phosphorus;
the archive metal of the RPVs was not available. The phosphorus and cop-
per contents in the welds of VVER-440/230 are in the range 0.030-0.048%
and 0.10-0.18%, respectively. In the case of VVER-440/213, the same con-
centrations are in the range 0.010-0.028% for P and 0.03-0.18% for Cu
(Brumovsky
et al
. 2005; Vasiliev and Kopiev, 2007).
Reactor pressure vessel surveillance programmes became obligatory
in all VVER plants that had been commissioned after Units 1 and 2 at
Loviisa. Proper RPV surveillance programmes have been implemented at
VVER-440/213 plants outside of the former Soviet Union from the com-
mencement of plant operation.
An 'Extended Surveillance Specimen Programme' was prepared with
the objective of validating the results of the standard programme (Kupca,
2006). It aimed to increase the accuracy of the neutron fl uence measure-
ment, make a substantial improvement in the determination of the actual
temperature of irradiation, fi x the orientation of RPV samples to the centre
of the reactor core, minimize the differences in neutron dose between the
Charpy-V notch and crack-opening-displacement specimens and evaluate
any dose-rate effects. For Units 1 and 2 of the Mochovce NPP, a completely
new surveillance programme was prepared, based on the philosophy that
the results of the programme must be available during the entire service
life of the NPP. The new, advanced surveillance programme deals with the
irradiation embrittlement of both the weld area heat affected zone and the
austenitic stainless steel cladding of the RPV, which were not previously
evaluated in surveillance programmes.
Several measures were implemented for the resolution of the RPV
embrittlement issue:
Reducing neutron fl ux on the RPV, low leakage core design, dummy
shielding assemblies.
Annealing, that is effecting a change of material properties.
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