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 304

Maximum defect size, %

9.24 Probability of failure of heat exchanger tubes, depending
on the maximum size of the defect for steam generator No. 4: 1)
realistic estimate; 2) conservative estimate; 3) permissible level
for 10% margin; 4) permissible level for 5% margin; 5) permissible
level in respect of the availability factor.

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Thus, based on the reliability test the permissible depth of the defects of
tubes is defined. The presented approach allows us to estimate the volume
and frequency of eddy current testing to ensure the necessary reliability of
tubes SG heat exchanger tubes. However, it should be noted that to assess
the feasibility of eddy current testing and preventive plugging of tubes, the
effectiveness of this kind of metal inspection should be studied, as well
as the influence of the medium of the second circuit on the origin and
development of defects in metal heat exchanger tubes.