Environmental Engineering Reference
In-Depth Information
exchanger tubes is
n
=
∑
i
P
P
.
[9.16]
n
n
i
=
1
The criterion of reliability of heat exhanger tubes is selected in
accordance with the principles
132
described in section 4.2.
On the basis of the first principle, the PSA data for the given energy
unit and the aggregated IAEA data
134
are used to determine the allowable
probability of 'fracture of the heat exchanger tube'. As part of the PSA,
IAEA publishes periodically summarised data on the probabilities of
initiating events compiled on the basis of the analysis of the reliability
and operating experience with various types of nuclear power plant in
different countries, including Britain, Germany, Russia, France, USA and
Japan. Summarised data
134
from the IAEA include the probability of a
steam generator tube rupture. For nuclear power plants this probability
of the investigated initial event ensures the reactor core safety, i.e., in
accordance with domestic and foreign safety requirements the probability
of core damage is less than 10
-5
for reactor per year, i.e., the probability
of 'steam generator tube rupture' which characterises the reliability of the
steam generator corresponds to the (acceptable) level of reliability at which
operation of the reactor core and nuclear power plant as a whole is safe.
The second principle of design and specification documentation is used
to define the acceptable level of probability of formation of through-wall
cracks and leaks not less than 5 kg/h based on the need to fulfil thermal
engineering function for steam generators (the principle of safe failure).
The third principle is based on the availability factor of the steam
generator. In general, the principle of minimising the number of outages of
a power unit during operation is used. In this case, the availability factor of
equipment in accordance with GOST 24722-81 should be at least 0.972
135
.
If the resulting level of reliability parameters for heat exchanger pipes
does not meet the reliability criterion, the criterion can be satisfied, for
example, by establishing requirements on flaw inspection in the part of
the allowable defect sizes. The development of recommendations on the
admissibility of detected defects is reduced to determining the maximum
admissible sizes for these defects.
The developed approach to performance analysis was used to prepare
recommendations for preventive plugging of the heat exchanger tubes of
steam generators of power units No. 3 and 4 of the Novovoronezh NPP
power units No. 1 and No. 2 of Kola NPP.
For example, the results of eddy current testing, presented by the Kola
nuclear power plant, were used for preliminary analysis and determination
of the size distribution of the detected defects (Table 9.5). The number of
inspected pipes, the number of defects found in the depth of more than 20%
of wall thickness and defect density distribution of in the steam generator
are presented in Table 9.6.
Search WWH ::
Custom Search