1.1. BACKGROUND
The process of safety assessment of a nuclear installation needs to be repeated periodically — in whole or in part, as necessary — in order to take into account changed circumstances with respect to those considered for the design.
Following this principle, the IAEA initiated a major effort in 2001 targeted at the development of guidelines for the assessment of the vulnerability to accidental or postulated human induced external events not foreseen in the design basis. Examples of accidental events include explosions caused by pipeline failures, train crashes and hazardous material leaks from tanks. Examples of postulated external events include station blackouts and loss of ultimate heat sink due to unidentified causes.
There is general agreement among experts that current practice for nuclear power plant design, especially against natural hazards, provides a safety margin and robustness that may enable the plant to withstand some scenarios caused by human induced external events not explicitly considered at the design stage without significant radiological consequences. This is believed to be true for nuclear facilities in general, and for nuclear power plants in particular. However, quantification is needed in order to understand, with a high level of confidence, which events can be screened out in a safety evaluation process and which events require a detailed assessment of the actual plant level performance.
The IAEA has published three Safety Guides that deal with the safety of nuclear power plants against human induced events of accidental origin:
(a) External Human Induced Events in Site Evaluation for Nuclear Power Plants, IAEA Safety Standards Series No. NS-G-3.1 [1];
(b) External Events Excluding Earthquakes in the Design of Nuclear Power Plants, IAEA Safety Standards Series No. NS-G-1.5 [2];
(c) Protection against Internal Fires and Explosions in the Design of Nuclear Power Plants, IAEA Safety Standards Series No. NS-G-1.7 [3].
Reference [1] deals with the examination of a region considered for siting a nuclear power plant, in order to identify hazardous phenomena associated with external human induced events that might occur in the region. It also presents preliminary guidelines for deriving the values of relevant parameters for the design basis. Reference [2] is devoted to the design of protection against the effects of external events, excluding earthquakes. Reference [3] provides design concepts for protection against internal fires and explosions in nuclear power plants.
Structural safety is an important aspect of protection against hazards caused by human induced external events. References [1–3] contain high level guidelines on the characterization of loads arising from hazards caused by human induced external events, such as explosions, impacts or fires. However, they provide almost no guidelines on the structural response analysis for the associated loading effects or on the subsequent evaluation of performance. The loading effects arising out of these hazards are, in general, of an extreme type.
The structural response analysis is more complex than the analysis for other static and dynamic loadings. In addition, the structural performance for this type of extreme loading may not be able to be assessed directly using the available design codes for nuclear safety related structures.
Consequently, there is a need to develop detailed guidelines for load characterization as well as for structural response analysis and performance evaluation using the current state of the art in related areas. This Safety Report is intended to meet this need and to cover the advancement of technology since the publication of Refs [1–3].
This publication is the second in a series of three reports that provide guidelines to support the quantitative evaluation of the safety of facilities subjected to design basis events (DBEs) and beyond design basis events:
(a) Safety Aspects of Nuclear Power Plants in Human Induced External Events:
General Considerations, Safety Reports Series No. 86 [4];
(b) Safety Aspects of Nuclear Power Plants in Human Induced External Events:
Assessment of Structures, Safety Reports Series No. 87;
(c) Safety Aspects of Nuclear Power Plants in Human Induced External Events:
Margin Assessment, Safety Reports Series No. 88 [5].
Reference [4] provides the general framework and includes a road map on how to design and evaluate protection against human induced external hazards.
The report concentrates on an overview of the methodology and on the important conditions in its application to existing and new nuclear power plants. The topics covered include elements of the design/evaluation approach, developed in five phases:
— Phase 1: Event identification;
— Phase 2: Hazard evaluation and load characterization;
— Phase 3: Design and assessment approaches for structures, systems and components (SSCs);
— Phase 4: Plant performance assessment and acceptance criteria;
— Phase 5: Member State response.
The present Safety Report addresses phases 2 and 3 of the general framework. It gives detailed guidelines for the safety assessment of nuclear power plant structures against mechanical impact, explosion and fire hazards caused by human induced external events. The report covers the characterization of loading; the assessment of structural integrity, using both simplified methods and more elaborated methodologies; and the assessment of induced vibration.
Acceptance criteria are given for different failure modes: overall stability, overall bending and shear, local failure modes and induced vibrations. Additionally, since many of the human induced external events may result in a fire, the process of analysing the consequences of a fire is also detailed. Approaches to assessing the barrier fire performance and the fire performance of safety related SSCs are given.
Reference [5] addresses phases 1 and 4 of the general framework. The report describes the procedures for calculating the margins of nuclear power plants in human induced external hazards. Both postulated and accidental hazards are considered. The report focuses on plant and systems performance evaluations.
A graded approach for margin assessment is provided. The first grade consists of a deterministic procedure in which, for each scenario, the existence of at least one undamaged success pathway to complying with the fundamental safety function is investigated. This procedure can be extended later on to calculate probability measures, such as conditional core damage probability and the conditional probability of spent fuel pool loss of cooling and spent fuel damage, for the given scenario. In the most elaborated stage, probabilistic safety assessment techniques are introduced, giving consideration to the probabilistic aspects of the hazards and of the capacity of SSC fragility. Event-tree and fault-tree models are used to compute the usual probabilistic safety assessment metrics, such as core damage frequency, large early release frequency, and frequency of loss of spent fuel pool cooling and spent fuel damage.
In summary, these three implementation reports (Refs [4, 5] and this Safety Report) provide methodologies that can be used in the evaluation of the capacity of SSCs of nuclear power plants subjected to extreme, human induced external events and in the assessment of the resulting safety margin of the facilities. The reports may be useful to nuclear facility owners, operators and regulators who need an understanding of the safety issues in relation to human induced events.
The reports contain a description of internationally accepted methods applied by the engineering community and some examples that may be useful in the evaluation of the need for plant upgrading. Many references are also provided for more detailed guidelines. For technical background, the reports rely on many IAEA safety standards as well as on relevant books.
The three reports share a common thread. Together, they provide an approach to the assessment against extreme, human induced external events that
is fully consistent with the methods used for the evaluation of nuclear facilities subjected to extreme natural events, such as earthquakes and floods.
1.2. OBJECTIVE
In order to ensure the basic safety functions of a nuclear power plant in the event of human induced external events, it is necessary that the structures important to reactor and spent fuel pool safety perform their intended functions, specified in terms of structural integrity, leaktightness and serviceability (to maintain the functionality of systems and components housed by the structures).
The objective of this report is to provide detailed guidelines for characterizing and quantifying loads (static and dynamic) arising from the impact of human induced hazards on the protective features, for evaluating the structural response analysis against these loadings, and for assessing the structural performance against its intended function.
This Safety Report provides an overall framework and outlines the main sequences for an assessment of the designed performance of the protective features in meeting their intended functions. The report cites many technical references on which it is based and describes the procedures for implementing many state of the art practices; the scope of this report does not allow for the inclusion of details and specifics.
1.3. SCOPE
This Safety Report provides a methodology and detailed guidelines for the assessment of nuclear power plant structures against mechanical impact, explosion, fires and missile hazards caused by accidental1 human induced external events. Wilful human induced events, such as military action or industrial sabotage, are not within the scope of the report.
The report covers load characterization, structural response analysis and an assessment of protection of nuclear power plants against human induced external hazards. Detailed guidelines are given about the linear/non-linear analysis of structures for impulsive, impactive and thermal loading; modelling of the loads; material modelling; structural modelling; and consideration of boundary conditions. The report also addresses the approach to structural performance
1 The report distinguishes between purely accidental events (for which a probability of occurrence can be computed) and postulated human induced events (which are defined solely, for instance, for design purposes).
assessment and acceptance criteria for these types of loading scenario. The guidelines provided in this Safety Report are applicable to both new and existing nuclear power plants.
1.4. STRUCTURE
This Safety Report has four sections and three appendices. Section 2 deals with load characterization and its quantification arising from the considered hazards. Material models for both the linear and non-linear behaviour of concrete and steel are described in Section 3. Section 4 describes the different types of structural analysis and simulation techniques that are applicable to these loading conditions. The assessment of performance and acceptance criteria for structural evaluation are addressed in Section 5.
Guidelines for derivation of loading functions caused by military and commercial aircraft crashes are presented in Appendix I. Examples of a simplified method for assessing the structural impact, for the derivation of verified loading functions and for typical dynamic response spectra are given in Appendix II.
Finally, guidelines for design and assessment of concrete elements against explosion are given in Appendix III.