Environmental Engineering Reference
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corrosion of the heat-exchange tubes
formation of deposit
diffi culties in measuring and regulating the SG water level.
A study performed by the International Atomic Energy Agency summa-
rizes the status of knowledge on steam generator ageing: TECDOC-1577
(IAEA, 2007b).
8.3.3 Ageing of the structures
VVER-440/213 containments
The reduced pressure containment of VVER-440/213 is made of reinforced
concrete and the steel liner ensures its leak tightness. Therefore, the basic
concern is the effect of ageing on the containment leak-tightness. The leak
rates of the VVER-440/213 containment, allowed by the design and justifi ed
by the regular integral tests, is equal to 14.7%/day at the post large-break
LOCA, when the design internal containment pressure equals 2.4 MPa. It
is clearly higher at some plants than what is allowable for Western NPP
containments. Therefore, the goal of the VVER operators is to improve
the leak tightness. (It should be noted that comparison with Western NPP
containments is not straightforward. This is because, in connection with the
design basis accidents, the pressure suppression system tends to cause pres-
sures below atmospheric, rather than overpressure, at the time period when
the atmosphere of the containment has its highest contents of radioactive
aerosols, and when the potential for radioactive releases would thus be the
highest.)
Containment leakage has a complex origin. Investigations carried out at
the Paks and Bochunice NPPs, almost from the time of start-up tests, show
that the poor sealing of doors and hatches mainly cause the containment
leakage and thus the leakage is a maintenance problem rather than an age-
ing issue.
Some VVER plants are built on relatively soft soil. Geodetic control of the
settlement of the main building of these plants was started during construc-
tion and it is periodically performed. The phenomenon might be a concern
when there is uneven settlement, that is the differential movement causes
unacceptable additional deformation of the structures. Experience shows
that the differential movement may cause cracks in non-structural masonry
walls. Another concern might be if the non-uniform settlement results in
non-allowed tilting of the RPV vertical axis, which would cause problems
for control rod drive mechanisms (CRDMs). The operating experience and
analysis of settlement with extrapolation to extended operational lifetime is
discussed for the Paks NPP (Katona
et al
., 2009a).
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