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 302

the life of the reactor for a further 15 years. The conservative variant in
which the number of plugged tubes is 5% was also considered.
Enforcement of the second principle of reliability of heat exchanger
tubes of the steam genarator is restricted to the fact that the number of heat
exchanger tubes which can be plugged for every steam generator annually
cannot exceed the permissible number. Consequently, the condition that
ensures the implementation of the second principle of reliability of steam
generator tubes can be written as a sum of the probabilities of a leak in the
heat exchanger tubes, exceeding the permissible number:

5136

∑

i
Tn

− 7

P < 10 ,

[9.18]

ik =

0

where n is the number of working heat exchanger tubes, k 0 is the number
of heat exchanger tubes with unacceptable leaks.
The probability level of 10 -7 per steam generator per year, corresponding
to the probability of 10 -5 per reactor for the operating life of 15 years is
conservative, 'rigid' and ensures SG operation at 100%. Equation [9.18]
gives the condition which the probability of a leak in heat exchanger
tubes must satisfy and which should be used to determine the admissible
probability values for the leakage of each steam generator (Table 9.8).
Consideration of the third principle of reliability comes down to
determining the conditions under which the availability factor of equipment
is equal to 0.972 135 at the recovery time of the steam generator in case
of leaks in heat exchanger tubes over a period of 10 days. Then, the
permissible probability of formation of a leak is 4·10 -4 per year for a normal
operating period resulting in the availability factor of 0.972.
In conducting performance tests of heat exchanger tubes of the steam
generator on unit No. 1 of the Kola NPP, approximately 500 calculation
variants were produced and processed on the basis of the conservative and
realistic approaches. This provided a large amount of information. The
main dependence of the probabilities of occurrence of leaks and large-
scale destruction on the maximum defect depth was determined for every
steam generator and then used to fulfil three principles of reliability

￿ ￿ ￿ ￿ ￿

Table 9.8 Permitted levels of failure probability of heat exchanger tubes for
steam generator on unit 1 of the Kola NPP

Steam generator number

Parameter

1 2 3 4 5 6

Failure probability for
10% margin

2.31·10 -3 2.30·10 -3 2.13·10 -3 2.4·10 -3 2.34·10 -3 2.36·10 -3

Failure probability for
5% margin

6.18·10 -4 6.13·10 -4 4.89·10 -4 6.86·10 -4 6.38·10 -4 6.52·10 -4