Safety Case Assessment Research Articles

Contribution of the results of the CEBAMA project to decrease uncertainties in the Safety Case and Performance Assessment of radioactive waste repositories

Cement-based materials are key materials used in repositories of low, intermediate and high-level radioactive waste in any host-rock concept. These materials are used for waste conditioning, liners, seals as well as structural components. The CEBAMA (CEment-BAsed MAterials, properties, evolution, barrier functions) project (2015–2019) has been an initiative granted by the European Commission under the Horizon 2020 Research and Training Programme of the European Atomic Energy Community (EURATOM), focused on the study of cementitious materials. The project has addressed key issues of relevance for long-term safety and key scientific questions related to the use of cement-based materials in nuclear waste disposal applications. This publication presents the advances resulting from the research undertaken in the CEBAMA project, with special emphasis on their contribution to decrease uncertainties in the Safety Case and the Performance Assessment of radioactive waste repositories. The analysis is presented in three different axes, in coincidence with the lines of research of the project: degradation of cement-based materials and its impact on cement/clay interfaces; retention of safety relevant radionuclides and toxicants on cement-based materials and modelling advances in cementitious related systems. The research investigations have provided important new insights on process understanding and model developments which have significantly decreased the level of uncertainty.

A systematic approach and tool support for GSN-based safety case assessment

Context. In safety-critical domains, safety cases are widely used to demonstrate the safety of systems. A safety case is an argumentation for showing confidence in the claimed safety assurance of a system, which should be comprehensible and well-structured. Typically, safety cases can be represented in plain text or graphic way, such as Goal Structuring Notation (GSN). After safety cases are developed, assessment of safety cases needs to be performed to check the quality of them. Besides, different roles are involved during this process: safety case developers and safety case assessors. Objective. During the safety case assessment process, safety case assessors are required to evaluate the validity of a safety case and discuss their judgement with safety case developers. Currently, the outcome of a safety case assessment and the way of providing judgement are not systematically supported, which may cause inconsistent outcomes and wrong judgements. Therefore a systematic process of safety case assessment is required. Moreover, to support safety case assessment in an efficient and effective way, tool support is needed. Recently, a number of safety case editors are developed to support safety case development with the GSN. These editors support the development and management of safety cases. However, only a few editors offer limited functionalities for safety case assessment which is one of the crucial phases of the safety assurance process. This motivates us to develop a tool to support safety case assessment. Method. In this paper, we first identify two research questions. Resulting in two directions for further study have been identified: formalising the safety case assessment process and developing safety case tooling. First, we carried out a study on the state of art on safety case assessment and safety case tooling. Based on our findings, we formalize the safety assessment process by identifying the typical steps in safety case assessment. This assessment process can guide assessors to assess a safety case from a general level to a detailed level and provide reliable and understandable feedback to developers. Finally two industrial case studies are carried out to validate the proposed assessment process. Results. To support the proposed process, a prototype tool for safety case assessment was developed. A number of required features are implemented in the prototype tooling, among other it provides a complete and self-contained evaluation system to measure the quality of the safety case. Moreover, the case study validations show potential for facilitating safety assessment in practice. Conclusions. In this paper, two research questions are identified and the solutions of them are discussed. Then we propose a systematic approach for safety case assessment. For demonstration, a tool support is also developed. For validation two industrial case studies have been carried out to show the effectiveness of the proposed process.

Safety assessment driving radioactive waste management solutions (sadrwms methodology) implemented in a software tool (safran)

In 2004, the International Atomic Energy Agency (IAEA) organized the International Project on Safety Assessment Driving Radioactive Waste Management Solutions (SADRWMS) to examine international approaches to safety assessment for predisposal management of radioactive waste. The initial outcome of the SADRWMS Project was achieved through the development of flowcharts which could be used to improve the mechanisms for applying safety assessment methodologies to predisposal management of radioactive waste. These flowcharts have since been incorporated into DS284 ( General Safety Guide on the Safety Case and Safety Assessment for Predisposal Management of Radioactive Waste), and were also considered during the early development stages of the Safety Assessment Framework (SAFRAN) Tool. In 2009 the IAEA presented DS284 to the IAEA Waste Safety Standards Committee, during which it was proposed that the graded approach to safety case and safety assessment be illustrated through the development of Safety Reports for representative predisposal radioactive waste management facilities and activities. To oversee the development of these reports, it was agreed to establish the International Project on Complementary Safety Reports: Development and Application to Waste Management Facilities (CRAFT). The goal of the CRAFT project is to develop complementary reports by 2014, which the IAEA could then publish as IAEA Safety Reports. The present work describes how the DS284 methodology and SAFRAN Tool can be applied in the development and review of the safety case and safety assessment to a range of predisposal waste management facilities or activities within the Region.

Flammable gas cloud build up in a ventilated enclosure

Ventilation is frequently used as a means for preventing the build up of flammable or toxic gases in enclosed spaces. The effectiveness of the ventilation often has to be considered as part of a safety case or risk assessment. In this paper methods for assessing ventilation effectiveness for hazardous area classification are examined. The analysis uses data produced from Computational Fluid Dynamics (CFD) simulations of low-pressure jet releases of flammable gas in a ventilated enclosure. The CFD model is validated against experimental measurements of gas releases in a ventilation-controlled test chamber. Good agreement is found between the model predictions and the experimental data. Analysis of the CFD results shows that the flammable gas cloud volume resulting from a leak is largely dependent on the mass release rate of flammable gas and the ventilation rate of the enclosure. The effectiveness of the ventilation for preventing the build up of flammable gas can therefore be assessed by considering the average gas concentration at the enclosure outlet(s). It is found that the ventilation rate of the enclosure provides a more useful measure of ventilation effectiveness than considering the enclosure air change rate.

Use of risk assessment in the nuclear industry with specific reference to the Australian situation

The use of risk assessment in the nuclear industry began in the 1970s as a complementary approach to the deterministic methods used to assess the safety of nuclear facilities. As experience with the theory and application of probabilistic methods has grown, so too has its application. In the last decade, the use of probabilistic safety assessment has become commonplace for all phases of the life of a plant, including siting, design, construction, operation and decommissioning. In the particular case of operation of plant, the use of a ‘living’ safety case or probabilistic safety assessment, building upon operational experience, is becoming more widespread, both as an operational tool and as a basis for communication with the regulator. In the case of deciding upon a site for a proposed reactor, use is also being made of probabilistic methods in defining the effect of design parameters. Going hand in hand with this increased use of risk based methods has been the development of assessment criteria against which to judge the results being obtained from the risk analyses. This paper reviews the use of risk assessment in the light of the need for acceptability criteria and shows how these tools are applied in the Australian nuclear industry, with specific reference to the probabilistic safety assessment (PSA) performed of HIFAR.

Mitigation of dense gas releases within buildings: validation of CFD modelling

When an accidental release of a hazardous material is considered within a safety case or risk assessment, its off-site effects are generally assessed by calculating the dispersion of vapour from the site. Although most installations handling flammable materials will be in the open air, many types of plant, particularly those handling toxics, are enclosed, partly to provide some form of containment and hence, to mitigate the effects of any release. When such a release occurs within a building, the gas or vapour will undergo some mixing before emerging from any opening. The degree of mixing will depend upon the building geometry and the nature of the ventilation, which in turn may be modified by the leak. This situation is considered in this paper, with specific application to calculating the rate of release of a dense vapour from a building. The paper describes the application of computational fluid dynamics (CFD) techniques to modelling the release and mixing processes within buildings. Examples of validation calculations for simple geometric arrangements, as well as more complex geometries representative of an industrial site, are described. The results demonstrate the capabilities of CFD for this application but highlight the need for careful modelling of the near-wall flows and heat transfer, and need for an accurate fluid dynamics and thermodynamic representation of the release source.

Mitigation of dense gas releases in buildings: use of simple models

When an accidental release of a hazardous material is considered within a safety case or risk assessment, its off-site effects are generally assessed by calculating the dispersion of vapour from the site. Although most installations handling flammables will be in the open air, many types of plant, particularly those handling toxics, are enclosed, partly to provide some form of containment and hence to mitigate the effects of any release. When such a release occurs within a building, the gas or vapour will undergo some mixing before emerging from any openings. The degree of mixing will depend upon the building geometry and the nature of the ventilation, which in turn may be modified by the leak. This situation is considered in this paper, with specific application to calculating the rate of release of a dense vapour from a building. All the calculations presented are based upon simple zone modelling, such that the region occupied by the vapour is assumed to be well mixed, and, in the isothermal case, either its concentration or its depth increases as it is fed by the gas leak. Transfer of air or gas/air mixture through the building openings is estimated by use of standard ventilation calculation methods. For the non-isothermal case, a preliminary model is presented in which it is assumed that there is complete mixing throughout the building and no wind-driven ventilation effects. A moderate release of chlorine is used as an example, and results are shown of the effects of various ventilation possibilities on the release rate to the atmosphere. In addition, comparisons are given between model results and experimental data, demonstrating the level of confidence which can be placed in the models, and also identifying areas where there is scope for further improvement.

Assessment of jack-up boat impact analysis methodology

In 1991 the Health and Safety Executive (HSE) commissioned a study to review the resistance of jack-ups to boat impact. A resulting conclusion was that jack-ups offer only limited resistance to severe damage in such events. Subsequent safety case assessment in the period 1993–1995 highlighted a significant range in the predicted impact resistance of jack-ups, which assessors believe to be more dependent on the approach followed rather than on the basic design. As a result further work was considered necessary to identify key factors that duty holders should be considering as part of the analysis undertaken to demonstrate the case for safety. This paper has been produced in order to disseminate the findings of such a study carried out on behalf of the HSE. The study carried out by Det Norske Veritas (DNV) presents the results of the appropriateness and applicability of a range of typical methodologies used by the offshore industry to analyse the response and impact resistance of jack-up designs to boat impact. The collision between offshore platforms and boats is generally a very complex problem and it is often not practical to perform rigorous finite element analyses to include all effects and sequences during the collision. A significant part of the collision energy is dissipated as strain energy and except for global deformation modes, the contribution from elastic straining can normally be neglected. Hence, many times it is necessary to apply simplified techniques. However, key aspects such as dynamic effects are often ignored, sometimes to the detriment of the final calculated energy absorption capacity of the jack-up. A number of approaches were received and identified by the HSE as typical approaches for design justification of Jack-Ups in the North Sea within safety case submissions. In order to assess and compare the approaches a case study based on a typical jack-up design with three legs was used, each leg consisting of triangular sections and K-bracing. Impact resistance calculations were performed, ranging from simple elastic hand calculations to non-linear dynamic FE analyses. In one approach where an assessment by calculation would have proved too time consuming, a qualitative assessment has been carried out instead. The results of the assessments based on the different approaches are presented, compared and discussed in this paper.

Some aspects of the safety of jack-ups on location and during transit

Abstract The paper reviews HSE's overall approach to safety case assessment, and considers in some detail certain aspects of the assessment of jack-ups in relation to: • • Risk Assessment • • Emergency Evacuation and Rescue • • Maritime Integrity • • Structural Integrity. Indications are given of some issues which have arisen through assessment, and interactions which have taken place between HSE and duty holders in an effort to resolve these issues.

SAFETY IN THE OFFSHORE PETROLEUM INDUSTRY: THE NEW REGIME

Following the recommendations of the UK Cullen and COSOP reports, a new regulatory and operational safety regime is being introduced across Australian territorial waters for the offshore petroleum industry. This paper details the concepts behind the new regime including the development and implementation strategies employed to introduce it and the implications to industry and government instrumentalities. The recognition that safety in the work place is an outcome of managerial commitment and a function of work system control is resulting in legislative reforms that will reduce the focus on physical compliance, in favour of a goal-setting legislative approach that places emphasis on systems of managing safety at the work place. By focussing upon ends rather than means, objective based regulation requires organisations to manage the design, construction and operation of facilities to reduce risk to a 'reasonably practicable level*. The details of the management arrangements and risk assessment studies once submitted in a 'safety case' and acknowledged by the regulator, form a co-regulatory guidance document that sets both the standards to be achieved and the mechanism for achieving them. Auditing of the operator against an accepted safety case will occur on a regular basis. Driven by measurement of an operator's safety performance, selected systems will be targeted and audited against the safety case by a team of auditors composed of both government inspectors and operator personnel. Performance measures will be both proactive and reactive in nature and include among other measures, results obtained through the newly created national data base for incidents and accidents. Preparation and assessment of safety cases provides regulators and operators alike with difficulties. Consistency of assessment and objective determination that acceptable minimum standards have been met provide a challenge to government agencies and to the petroleum industry. Recognition of the nature of this challenge has led to the adoption of Quality Management principles among government utilities. A key feature of the Quality Management System will be the introduction of systems that drive change through cyclic updates of the safety case assessment procedures.The safety case concept institutionalises a dialogue between regulator and operator that will facilitate the preparation, submission and assessment of the safety case. This initiative provides for a staged submission process to correspond to the development phase of the facility. It is considered that this methodology will ensure minimal delays in assessment and provide certainty in the business planning process. The new safety case regime provides for a greater level of employee participation both in the preparation of safety cases and the maintenance of safety at the work place. Transparency of approach by both operators and regulators will be needed if public and workforce confidence are to be maintained. Co-operation and continual improvement will hopefully be the hall mark of the Australian safety case regime.