Optimisation of non-destructive testing - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 228

is considered sensitive, in accordance with the Code RCC-M, if the damage
is more than 1. Additional assessment of the state of the element is carried
out for those areas where the fatigue factor is in the range from 0.5 to 1.
Analyzing the permissibility of defects and the risk of rupture, it is
assumed that the area is sensitive if the height of the crack does not
correspond to the accepted safety factor (2 for level A , 1.6 for level C and
1.2 for level D ).
Furthermore, the following zones are regarded as sensitive:
• zones which can operate in a brittle mode: operating temperature is
lower than the ductile-brittle transition temperature of zone;
• zones where the fracture toughness characteristics are low:
J < 50 kJ/m 2 and dJ / da < 20 MPa;
• zones of high mechanical loads: redistribution of load in the zones
with cracks, taking into account the conditional yield strength, must
be greater than 1 ( L r > 1).
The relationship between the volume of the ISI and its worth taking into
account the risk factor is important. The qualitative nature of the approach
does not require determination of failure probability in the absence of
inspection and if inspection is carried out we consider only the reduction
in the probability of failure as a result of effective inspection. Nevertheless,
this qualitative approach can be transformed into a quantitative one, which
will assess the real benefits of carrying out inspection whose volume has
been defined properly.
If we consider the cost as a 'consequence' in a risk matrix, then risk
assessment is the ranking of risks in the values of the potential losses from
unplanned shutdowns. The first assessment should be done without any
inspection. After that, a certain amount of inspection is introduced which
should include some efficiency ISI, in this case we can evaluate how the
risk will change with increased inspection volume. Subtracting the risk
after the introduction of ISI from the risk prior to ISI introduction, we
obtain a measure of risk reduction provided by ISI. If the accumulated risk
is described graphically taking into account the risk reduction provided by
each individual inspection with a given volume, we obtain a curve steadily
decreasing with increasing amount of inspection (see Fig. 7.6). However,
if the cost of each inspection is added, a graph of the total cost for nuclear
power plants is plotted. This curve shows the minimum cost of inspection
in the overall expenditure of the nuclear power plant. The value above
it is the net cost, but the cost of inspection exceeds its benefits and this
increases the total cost for the nuclear power plant in excess of this value
for each additional inspection.
If the effectiveness of ISI is now represented as a parameter, the in
the case of reducing the effectiveness of ISI the probability of failure
increases. Therefore, the total cost of such inefficient inspection will
increase compared to the cost of effective inspection of the same
amount. Consequently, in the case of less effective ISI its lowest total cost

￿ ￿ ￿ ￿ ￿