The degree of NDT system perfection is characterised by its detectability of flaws which are considered impermissible in accordance with the normative and technical documentation. By inspection with several techniques the tolerance of products is estimated by using various parameters. Thus, by UST of weldments the measurable parameter will be the equivalent flaw area, and by X-ray inspection - its projection on film. Both parameters define indirectly the true flaw size, but their correlation with real sizes is different. Besides, two NDT techniques are used for welded joints, only because they have various detectability of flaws of various types. The calculation of the reliability of each method and NDT system as a whole, conducted for specific welds, permits to solve the problem of the optimal combination of these methods regarding their application.The methodology of probability indexes calculation of continuous NDT system reliability has been developed. It is suggested to estimate the reliability of these systems with the following indexes:
- Probability of impermissible flaws detection in real defectoscopy systems for products of nuclear power; this varies from 0.8 to 0.99;
- Probability of impermissible defects absence in tolerated products, intended for operation; this index varies from 0.98 to 0.999.
The suggested approach gives an important information and may be applied for the service life assessment of NPP equipment.
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