atw - International Journal for Nuclear Power | 08/09.2019

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atw Vol. 64 (2019) | Issue 8/9 ı August/September DECOMMISSIONING AND WASTE MANAGEMENT 412 | | Fig. 3. LOPA Approach. Bowtie Diagrams The use of ‘Bowtie’ diagrams (Figure 4) in the process industry is so-called because it visualises the management of risk in the shape of a bow-tie graphical representation of the relationship between the initiating event and consequences with emphasis on the link between the safety measures and the management system in one, easy to understand picture. This can be used in conjunction with other analytical techniques. Based on NNL’s experience, chemotoxic assessments often require consideration of complex engineered systems which demand detailed analysis to determine faults sequence and safety measures. The use of Bowtie diagrams as an analytical tool alone does have limitations for some risk management problems, in particular if there is a | | Fig. 4. Bowtie Diagram Approach. | | Fig. 5. NNL Tool box Graded Approach. requirement to quantify the level of risk in absolute terms. It is considered that there are better ways of modelling complex systems and their relationship between the hazard, safety measures and overall risk, if this is required. NNL Graded Approach The NNL toolbox of chemotoxic technical guides and methodologies recommends the use of a proportionate graded approach to safety designations (Figure 5). In general, the risk from chemotoxic, asphyxiation or explosive accidents must: be demonstrated to be limited by robust qualitative arguments using engineering judgement; be the subject of a probabilistic safety assessment to meet the risk target defined by R2P2; or be demonstrated to be in alignment with best practice. If a sufficiently robust deterministic argument can be made, these are used in preference to a probabilistic safety assessment. If a probabilistic assessment is required, the NNL chemotoxic toolbox includes the provision of fault tree analysis codes to support frequency calculations and the modelling of inter-relationships between the initiating event and safety measures themselves. The challenge is to identify a proportionate number of safety designations for implementation. Over-designation can have a negative effect on safety. If too many items of equipment or procedures are safety-designated, their importance to safety can be ‘ diluted’. This is in addition to the separate effect of over-designation resulting in significant cost implications. There is a clear link between the NNL chemotoxic toolbox graded approach to safety which underpins the R2P2 broadly acceptable risk criteria. It is argued that this approach therefore demonstrates that the risks are ALARP under the HSE regulations and this understanding has been used to produce numerous chemotoxic safety cases in the nuclear sector, which meet Regulatory expectations at a number of different sites within the UK. The general approach applied to significant faults, which have the potential for fatality, is that they require the designation of a passive safety measure such as a vessel, or two independent engineered safety measures such as Control, Electrical and Instrumentation Equipment (CE&I) or mechanical devices. The engineered safety measures require a predefined action on outage or substitution arrangement. Alternatively, it is possible for Operational Safety Measures to be claimed in line with the ERIC PD principle. The number of independent measures can be reduced, such as in the case of low initiating event frequencies, or highly reliable safety measures, or where it can be demonstrated to be in alignment with good practice guides. It should be noted that good practice guides, should be used carefully as these are often generic and may not reflect the exact fault scenario or environment. These should therefore be interpreted in alignment with a degree of risk assurance. The general approach for lesser significant faults, which have the potential for serious harm, is that they require the designation of a single safety measure, which can either be passive, engineering or an operational Decommissioning and Waste Management A Pragmatic Approach to Chemotoxic Safety in the Nuclear Industry ı Howard Chapman, Marc Thoma and Stephen Lawton
atw Vol. 64 (2019) | Issue 8/9 ı August/September | | Fig. 6. Factors Influencing Overall Risk. safety in line with the ERIC PD principle. Chemotoxic hazards which fall below the serious harm chemotoxic thresholds are adequately assessed by the basic (tabular style) COSHH risk assessment. One key difference between the NNL chemotoxic graded approach and the LOPA process is in the use of Conditional Modifiers. The graded approach takes a conservative stance to the assessment of the initiating event frequency, with no allowance being made for Conditional Modifiers. Although it is a cautious stance, it is argued it minimises the potential for expensive retro-engineering work. It is NNL’s view that chemotoxic risks are shown to be ALARP through the qualitative graded approach to safety, probabilistic safety assessment, or, demonstration of assurance with good practice. Implementation All key engineered and operation safety measures that provide protection against chemotoxic hazards are identified with suitable and appropriate nomenclature to highlight their importance to safety within the facility. This information usually feeds into a category management process to ensure engineered measures are substantiated (i.e. can perform what is required) and are maintained throughout the life cycle of the facility and operational measures can be performed (i.e. through underpinning instructions and training). It also enables suitable and where appropriate, substitution arrangements to be put in place and safe operating methods to allow operations to proceed safely. The Safety Assessment Equilibrium The NNL proportionate graded approach provides a robust assessment method to ensure the appro priate amount of risk reduction is provided to achieve the risk target. The overall risk frequency is a function of the magnitude of the Initiating Event Frequency, Integrity ( reliability) of the safety measure and the Layers of Safety (number of independent safety measures) as illustrated in Figure 6. The chemotoxic safety assessment has very little influence on the Initiating Event Frequency which drives the risk reduction required from the safety measure(s). For example, frequent initiating events will demand a greater degree of risk reduction than less frequent initiating events to achieve the R2P2 broadly acceptable risk target. For a typical initiating event, the dilemma is often a choice of placing reliance upon a single but complex safety measure, versus multiple layers of safety measures. Complex systems typically demand significant effort and therefore cost to substantiate and maintain, compared with multiple simpler systems. All of the various analytical techniques described above enable a demonstration of managing risk. More specifically, the NNL graded approach is based on an appreciation of understandings about the practicalities of providing and substantiating one single very high level of integrity safety measure versus the provision of multiple lower level integrity measures. For the majority of faults which have the potential to result in fatality, the general approach of identifying two independent safety measures would ensure that the risks are broadly acceptable. In summary, the NNL approach adopted involves finding the equilibrium between the required number of protection layer(s) of safety measures to achieve the most cost effective and pragmatic degree of risk reduction. In addition, the NNL approach has the flexibility to accommodate unique scenarios and demonstrate an acceptable risk through probabilistic safety assessment, or assurance with good practice guides. References [1] United Kingdom Government, “Health and Safety at Work Act,” 1974. [2] United Kingdom Government, “Energy Act,” 2013. [3] United Kingdom Government, “Nuclear Installations Act,” 1965. [4] Health & Safety Executive, “Reducing Risks, Protecting People,” 2001. [5] Office for Nuclear Regulation, “Risk informed regulatory decision making,” 2017. [6] Statutory Instruments, “The Classification, Labelling and Packaging of Chemicals Regulations 2015,” 2015. [7] European Council, “Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH), Regulation No. 1907/2006,” 2006. [8] Statutory Instruments, “The Control of Substances Hazardous to Health, 2002, No.2677,” 2002. [9] Statutory Instruments, “The Dangerous Substances and Explosive Atmospheres Regulations 2002, No. 2776,” 2002. [10] Statutory Instruments, “Confined Space Regulations 1997, No 1713,” 1997. [11] Statutory Instruments, “The Control of Major Accident Hazards Regulations 2015, No. 483,” 2015. [12] HSE, “Toxicity Levels of Chemicals,” [Online]. Available: http://www.hse.gov.uk/chemicals/haztox.htm. [13] National Institute for Occupational Safety and Health, “ NIOSH Pocket Guide to Chemical Hazards, No. 2005-149,” 2005. [14] HSE, “Workplace Exposure Limits, EH40/2005,” Third Edition 2018. [15] Health & Safety Executive, “Toxicity Levels of Chemicals,” [Online]. Available: http://www.hse.gov.uk/chemicals/ haztox.htm. [16] Health & Safety Executive, “Workplace Exposure Limits, EH40/2005,” Third Edition 2018. Authors Howard Chapman Marc Thoma Stephen Lawton Chadwick House Birchwood Park Warrington, Cheshire WA3 6AE United Kingdom DECOMMISSIONING AND WASTE MANAGEMENT 413 Decommissioning and Waste Management A Pragmatic Approach to Chemotoxic Safety in the Nuclear Industry ı Howard Chapman, Marc Thoma and Stephen Lawton
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