Risk-informed approach

In the performance-based design and operation of modem engineered systems, the accurate assessment of reliability is of paramount importance, particularly for civd, nuclear, aerospace and chemical systems and plants which are safety-critical and must be designed and operated within a risk-informed approach (Pata-lano et ah, 2008). 

Table 6.3 illustrates the various definitions of these features as used in the different calculative approaches. The fracture mechanics approach in the USA is contained in the ASME Code, Section XI (ASME, 2010b), Appendix G. Recently there have been risk-informed probabihstic analyses performed (Gamble et al., 2009) that have been reduced to the same form as shown in Eq. 6.7 with values of = 1, j8 = 61 °C and 7=1. These risk-informed values have been included in the 2011 edition of the ASME Code as an alternative to the traditional deterministic method. The risk-informed approach evolved out of the risk-informed development of the US alternative PTS Rule. 

Most of the methodologies rely on a generic safety factor of 2 applied to pressure stress, with the safety factor on thermal stress set at unity. For leak and hydrostatic tests, the safety factor on pressure generally is reduced to 1.5. The Russian approach and the French Method 2 use a safety factor of unity for pressure stress, but the fracture toughness curve either has a safety factor included (Russian approach) or additional safety factors are applied to the fracture toughness curves (French Method 2). As indicated earlier, the new risk-informed approach which will appear in the 2011 edition of the ASME Code uses a safety factor of 1 on pressure stress. 

Experience on risk-informed approach to Allowed Outage Time changes due planned maintenance... 

The risk-informed approach aims to integrate in a systematic manner quantitative and qualitative, deterministic and probabilistic safety considerations to obtain a balanced decision. In particular, there is explicit consideration of both the chances of events and their potential consequences together with such factors as good engineering practice and sound managerial arrangements. The basic components of risk, chances of occurrence and consequence, are based on sound knowledge or data from experience, or derived from a formal, structured analysis such as a PSA. 

In order to facilitate practical support allowing interpretations and explicit applications of these new requirements, an expert group on PSA in the frame supervisory activities, chaired by the Federal Office for Radiation Protection (BIS), has been established in fall 2012. One topic of discussions is the definition of a significant impact on the PSA results. Moreover, practical examples shall be collected to illustrate a possible risk informed approach. 

Based on today s methods and results of probabilistic safety assessments (PSAs), it is acceptable and considered safe to measure and adjust maintenance strategies according to the frequencies involved and levels of risk (risk informed strategies). The risk informed approach gives better transparency and background for maintenance priorities and for the cost effectiveness of maintenance efforts (personnel, money, dose, waste). The IAEA has discussed this approach in greater depth in Refs [8,9]. 

Cost effective strategies include avoiding excessive maintenance of the plant without compromising plant safety. The identification of the bottom line safety issues that cannot be compromised and of the safety issues that can be re-evaluated (engineering evaluations, PSA based evaluations, etc.) are helpful in this connection. Also, it might be necessary to provide maintenance with support to review maintenance programmes based on the risk informed approach (e.g. simplified reliabihty centred maintenance or the maintenance rule approach). 

Obviously, hazard risk information can be presented eitlier qualitatively or quanlilaiively. This Section provides qualitative risk procedures and information. Several of these approaches are given below. 

The computerized systems, both hardware and software, that form part of the GLP study should comply with the requirements of the principles of GLP. This relates to the development, validation, operation and maintenance of the system. Validation means that tests have been carried out to demonstrate that the system is fit for its intended purpose. Like any other validation, this will be the use of objective evidence to confirm that the pre-set requirements for the system have been met. There will be a number of different types of computer system, ranging from personal computers and programmable analytical instruments to a laboratory information management system (LIMS). The extent of validation depends on the impact the system has on product quality, safety and record integrity. A risk-based approach can be used to assess the extent of validation required, focusing effort on critical areas. A computerized analytical system in a QC laboratory requires full validation (equipment qualification) with clear boundaries set on its range of operation because this has a high... 

Results from the risk assessment are used to inform risk management. The risk manager uses the risk information in conjunction with factors such as the social importance of the risk, the social acceptability of the risk, the economic impacts of risk reduction, engineering, and legislative mandates when deciding on and implementing risk management approaches. 

The committee concludes that descriptive approaches are often important in laying a foundation on which risk-based approaches can build. The interpretative power of risk-based approaches varies widely, depending on the information available. To improve the interpretation of biomonitoring results, an expansion of the scientific database on many chemicals is needed. 

Risk-based approaches try to determine how much risk is associated with a given biomarker result. Those approaches and their interpretive power vary widely with the extent of information available on a chemical and its biomarker. 

It is noteworthy that the styrene reference concentration (RfC) in the Integrated Risk Information System is based on the biomarker-response relationship found in workers (Mutti et al. 1984 EPA 1998). The Environmental Protection Agency (EPA) used the relationship of urinary biomarker to ambient-air concentration of workers to develop an RfC that was adjusted for the difference in exposure time between the workplace and the general population. That is a valid approach because it derives a workplace concentration-toxicity relationship in workers, which can then be adjusted for the general population to account for differences in exposure time and can take uncertainty factors into account. It is different from direct adjustment of the styrene BEI to evaluate human population biomonitoring data on styrene metabolites in urine, which would have the uncertainties described above and in Chapter 5. 

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