Environmental Engineering Reference
In-Depth Information
There are also several design-specifi c TLAAs in the case of the main reactor
building at Paks NPP, for example:
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Fatigue analysis of the containment for increased pressure level during
integral leak-tightness tests.
Analysis of main reactor building settlement.
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The allowable leakage value of VVER-440/213 containment is 14.7% per
day at the design pressure of 2.5 bars. Each of the containments was tested
at this design pressure in the start-up phase. The pressure of the yearly leak-
age tests is 1.2 bar and tests at a pressure of 1.7 bars are also carried out
during the outages. The leakage value for the nominal pressure of 2.5 bars is
calculated via extrapolation from the leak rate results of tests. This practice
has been criticized regarding correctness of the leak rate extrapolated from
the measured ones, and investigations of enhancement of the test pressure
level has been proposed. Nevertheless, the recent reduced pressure test pro-
cedure has obvious advantages compared to the tests at enhanced pressure
level: the time needed for the low-pressure test is short and the load on
containment structures is moderate. According to the results of leak tests
the correct leakage values at the nominal pressure of 2.5 bars can be deter-
mined from the results of tests carried out at considerably lower pressure
values. This statement is based on analyses of numerous tests, including the
results of tests carried out at the design pressure of 2.5 bars at Paks NPP
Unit 2.
Regarding Paks NPP, the analysis of settlement of the main building com-
plex has been identifi ed as a TLAA requirement since an excessive incli-
nation of the main building complex due to differential settlement may
result in non-allowed tilting of the RPV vertical axis, which may in turn
cause problems with the control rods. Additionally, excessive inclinations
can also cause extreme local loading resulting in degradations of the build-
ing. It has to be mentioned that the VVER-440/213 type units at the Paks
NPP have twin-unit-design, that is two main reactor buildings separated by
a dilatation gap are built upon a common base mat. Detailed settlement
control was started during the construction period at Paks. The measured
results are to be evaluated and reported annually. A consolidation process,
prolonged in time, was observed in case of the main reactor buildings, the
settlement of which is still continuing. The phenomenon is related to the
seasonal variation of the water level of the river Danube, which may reach a
value of 9 meters. This variation of river water level infl uences the ground-
water level. According to the data measured in the wells at and around the
plant site, the groundwater level follows the variation of the water level in
the Danube with a certain time delay. The water-table fl uctuations infl u-
ence the stress-deformation conditions in the subsoil. This can explain the
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