Optimisation of non-destructive testing - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 220

should not be terminated as a result of changes to the inspection program
(risk ranking, the number and type of inspections);
• programme requirements should be tailored to the specifications, based
on the failure mechanism and the minimal conservatism of the model of
probabilistic risk assessment;
• maintaining or improving plant safety - the use the integrated ISI
programme, which includes the methodology based on risk information,
must demonstrate that plant safety can be maintained or improved.
A comprehensive and consistent assessment of the consequences should
be based not only on the understanding of probabilistic risk assessment and
information specific to each nuclear power plant, but also on the traditional
logic and structural probabilistic risk assessment.
As noted in the documents, it is essential to take into account the
difficulties in assessing the results obtained after inspection based on risk
assessment, as compared with the results obtained after inspection based
on risk information. In this case, the documents note that the use of risk
information in determining the probability of failure must be carefully
coordinated, i.e. to determine how well this information can be entered into
the process of identifying risks, despite the confirmation of the uncertainty
associated with highly reliable components. It is noted that the conservatism
used in the complex process of assessing the probability of failure should
lead only to an increase in the number of inspections, whereas conservatism
in other methodologies/processes may mask other important components
and artificially reduce the number of inspections. It is also noted that when
assessing the probability of failure it is important to take into account
its instability with respect to defence in depth and unknown degradation
mechanisms.
The risk matrix is used to reduce the influence of subjective opinion
(as these documents do not contain methods for estimating the probability
of failure).
The concept of ISI, based on risk information, can be presented as a
scheme (Fig. 7.1). It is recommended to refer to the proposed scheme as
the Main scheme of ISI risk information (the Main scheme herein). The
Main scheme is considered in an IAEA document 108 .
In order to give a short description of each scheme element, its values
and also its impact on the overall process of ISI and improve its efficiency,
implemented through the introduction of assessment methodology based on
risk information, the individual elements of the scheme are given in a large
scale with letter designations,
The basis of this process is the risk. Therefore, the element D (Fig.
7.2) of The Main scheme is the basic element for carrying out risk
assessment. The basic factors are the consequences of failure and the
likelihood of failure. These two factors are not independent of each other,
they form the axis of the scheme of risk assessment. Both axes can be
divided into five sections corresponding to five risk categories:

￿ ￿ ￿ ￿ ￿