Using probabilistic methods for solving the problem of ensuring leak tightness of heat exchanger tubes of nuclear power plant steam generators - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 301

Table 9.7 Results of the statistical analysis of 'lack of material' for heat
exchanger tubes of steam generator unit No. 1 Kola NPP

Straight section of pipeline

Sealing
area

Parameter

SF1 SG2 SG3 SG4 SG5, 6

q 

0.949 0.875 0.968 0.968 0.940 0.987

 0.211 0.226 0.247 0.232 0.227 0.227



2
Dw =

50 80 119 43 43 17

D ( x ) 0.054 0.06 0.069 0.067 0.064 0.073

w Dx = () 0.233 0.245 0.264 0.258 0.253 0.270

The values of critical stress intensity factor K 1SCC in a corrosive
medium for the analysis of stress corrosion cracking were estimated
using the data on crack resistance of austenitic steels under the effect of
corrosion 136 . Statistical analysis was performed using the normal law of
distribution of K 1SCC with the mathematical expectation of 12,00 MPa·m 0.5
and a standard deviation of 3.03 MPa·m 0.5 in conditions of a corrosive boilin
medium in the secondary circuit.
The criterion which provides the necessary level of reliability of heat
exchanger tubes was selected as follows. On the basis of the first principle
of reliability according to the PSA data for the Kola NOO the frequency of
the initiating event 'rupture of heat exchanger tubes' was adopted on average
equal to 2·10 -3 for reactor per year. This does not pose any significant
danger to the reactor core. Given that unit No. 1 of the Kola NPP consists
of six steam generators, the permissible level of probability of large-scale
destruction of at least one heat exchanger tube is assumed to be 3.3·10 -4 per
steam generator per year. This value agrees well with the IAEA data 134 . Then
the condition that ensures the implementation of the first principle of
reliability of heat exchanger tubes for safe operation of the reactor core,
can be written as

￿ ￿ ￿ ￿ ￿

5136

∑

i − 4

P < 3.3·10 .

[9.17]

cr n

i = 1

On the basis of the second principle of reliability it is assumed that in
case of leaks up to 10% of the total number of HET can be plugged so that
the reactor can reach 100% of its capacity. At the same time, for each of
the six steam generators tested it is necessary to determine the individual
reliability criterion due to different levels of damage and, consequently, the
difference between the numbers of heat exchanger tubes plugged in the 30
years of operation. According to these data, the average number of failed
tubes to be plugged per year should not exceed the value determined from
the condition of available reserves of the heat transfer surface and of extend