Physico–statistical models based on the residual defectiveness of structural materials - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 126

nuclear power plant equipment. For example, Fig. 5.6 shows a test sample
of the main circulation line (MCL) of the RBMK-1000 reactor made of
the same grade of steel and electrodes as the piping in nuclear power
plants. The pipe size (diameter and wall thickness) are also consistent
with MCL. This test sample has two welds in which 26 plane and volume
defects were made.
The detectability of inspection was assessed by defect detection
probability (DDP).

Study of detection of defects at the given constant conditions of
inspection

These studies can be illustrated by the example of inspection of a test
sample of the element of the reactor vessel into which incomplete fusion
defects between the cladding and the base metal were introduced.
A crack-like defect was located on the fusion boundary and was
sometimes difficult to detect. Ultrasound testing was the main inspection
method of incomplete fusion defects. However, it is fraught with
considerable difficulties which are caused by various conditions of
propagation of ultrasound in austenitic and pearlitic metal, as well as in
the fusion zone between them. In this regard, detection of such defects is
difficult and associated with many objective and subjective factors such as
qualifications, fatigue, psychological and physical condition of operators,
etc.
The aim of the study was to evaluate the detectability of incomplete
fusion defects between the corrosion-resisting cladding and the base metal of
the casing by different ultrasound operators in conditions close to the actual
conditions. To assess the detectability of incomplete fusion defects between
the corrosion-resisting cladding and the base metal, it is necessary to know
accurately in advance the position, size, shape and nature of the defects. For
this purpose, a special test-sample with the size of 720×960×100 mm,
made of steel 15Kh2NMFA, was produced. The sample surface was clad
with two corrosion-resisting layers in accordance with the manufacturing
technology of the VVER-1000 reactor. The material of the first layer was
steel 07Kh25N13, the second layer was made of 04Kh20N13G2B steel.
The coordinates of the location of discontinuities and their size were
chosen randomly using a random number generator. The area of the
discontinuities was chosen so as to have fewer defects and more acceptable
size equal to 20 mm 2 in accordance with the PK1514-72. 73 Defects with the
area of 12, 20, 29, 63, 113 mm 2 were produced in the test sample. The
number of discontinuities of different standard sizes was: first 12, second
12, third 16, fourth 10, fifth 12. A total of 62 defects were made. Four
defects were located in the zone close to the edge of the plate where
detection is difficult, so they were excluded from consideration.
The test sample was subjected to verification by VNIIAES ultrasonic

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