1 Introduction
1.1 Purpose
This guide aims to guide nuclear facility operators in making safety assessment for decommissioning of nuclear facilities to help them select a strategy, design a scheme, develop a programme and carry out corresponding activities for decommissioning.
1.2 Scope
This guide is applicable to the safety assessment for decommissioning of nuclear power plants and installations, other reactors, nuclear fuel cycle facilities and radioactive waste storage and treatment facilities.
In this guide, the start for the decommissioning of nuclear facilities is that: all spent fuel elements and fissile materials have been completely removed out of the facilities and the operating waste liquid has been treated.
This guide does not cover the demolition of places in nuclear facilities that do not involve radioactivity (such as office buildings outside the nuclear island).
2 Objectives and responsibilities
2.1 Objectives
The purpose of safety assessment for decommissioning of nuclear facilities is to identify possible hazardous factors in decommissioning activities, analyze their hazards and predict their possible consequences, put forward reasonable and feasible safety response actions, and propose technical or management measures and suggestions to eliminate risk and harmful factors or reduce the severity of their consequences to ensure that the decommissioning of nuclear facilities will not result in radiation exposures to working personnel and the public exceeding those specified by relevant national regulations and standards of China and such radiation exposures will remain at a level as low as reasonably achievable (ALARA), thus achieving the ultimate goal of protecting human and environmental safety.
2.2 Responsibility
Nuclear facility operators shall organize and carry out safety assessment for the planned decommissioning activities, and hold responsibility for the results of safety assessment.
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3 Requirements for safety assessment
3.1 General requirements
3.1.1 Systematic assessment shall be carried out on safety-related contents in decommissioning of nuclear facilities, including: (1) to collect necessary information required and be prepared for safety assessment before carrying out such assessment; (2) to identify possible radiation hazards and general industrial hazards; (3) to assess the characteristics of sites and nuclear facilities related to possible radiation hazards; (4) to determine and assess the safety functions required for decommissioning; (5) to assess radioprotective measures and waste management schemes; (6) to assess the engineering scheme of decommissioning to determine whether it meets the safety design requirements related to facilities or decommissioning activities; (7) to assess the long-term safety of nuclear facilities after decommissioning. The key elements of the safety assessment for decommissioning are shown in Figure 1.
Figure 1 Safety assessment for decommissioning of nuclear facilities - Elements and requirements
3.1.2 The safety assessment for decommissioning of nuclear facilities must be thorough and shall ensure that all safety requirements related to decommissioning are met. The scope and complexity of the safety assessment shall match the severity of potential hazards and possible consequences caused by decommissioning nuclear facilities.
3.1.3 For nuclear facilities, the records and information of the operation phase are important data to support the safety assessment for decommissioning, and the operator shall ensure the accuracy and completeness of these data. When a nuclear facility is approved to be shut down, the safety assessment for decommissioning of such facility can be started.
3.2 Factors to be considered in safety assessment
3.2.1 The following factors shall be taken into account in the safety assessment for decommissioning of nuclear facilities:
(1) The scope of safety assessment: the whole or a certain phase of the decommissioning project, the whole or a part of the nuclear facility, and, in a multi-reactor site, one of the nuclear facility or all the nuclear facilities will be decommissioned;
(2) Type, scale and complexity of nuclear facilities;
(3) The physical characteristics of nuclear facilities at the beginning of decommissioning, especially the integrity, availability and aging degree of safety-related buildings (structures), systems and equipment;
(4) The complexity of decommissioning activities;
(5) Uncertain factors, including: accuracy and availability of information related to nuclear facilities (such as design drawings, construction data, completion data, operation logs, maintenance and retrofit records);
(6) Source term characteristics, including: sources of radioactivity (e.g. fission/activation product nuclide), nuclide type and activity concentration or surface pollution level, nuclide half-life, toxicity group and physical and chemical properties (e.g. solid/liquid/gas state, radioactivity, heat release rate or flammability);
(7) Possible hazards and their potential consequences, for which, the natural characteristics of the sites (such as geological characteristics, hydrological characteristics, and impacts from other nearby facilities) need to be taken into account. Initial events and potential consequences, such as human events, fire, flood, falling of heavy objects, collapse of buildings, toxic and harmful chemicals, and extreme temperature;
(8) Characteristics of special materials, such as distribution, inventory, activation or contamination characteristics of special materials including beryllium, chromium, lead, graphite, asbestos, resin or polychlorinated biphenyls;
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(9) Characteristics and reliability of safety-related systems, such as engineered safety systems and control systems, which can prevent accidents or mitigate the consequences caused by accidents;
(10) Safety regulations and standards adopted for safety assessment;
(11) The end state of nuclear facility decommissioning, that is, whether the site reaches unrestricted or restricted use;
(12) Reference the results of safety assessment for decommissioning of other similar nuclear facilities;
(13) Possible adverse effects and scope of decommissioning projects on other nearby nuclear facilities which are still in operation.
3.2.2 If phased decommissioning is adopted, due to the different contents and potential hazards of decommissioning activities in each phase, phased safety assessment shall be considered. In this case, relevant factors shall be considered in the safety assessment according to the requirements of 3.2.1, and the safety assessment shall be adapted to the potential hazards and risks of decommissioning activities.
3.3 Hazard identification and analysis
3.3.1 Hazards and initial events that may have adverse consequences for working personnel, the public and the environment from decommissioning activities shall be identified and analyzed based on information such as radioactive source terms of nuclear facilities.
In the decommissioning of nuclear facilities, general industrial hazards (i.e. non-radioactive hazards) are as important as radiation hazards. During decommissioning, general industrial hazards usually lead to increased radiation risks, for example, fire leads to the failure of radioactive waste packaging, resulting in the leakage and diffusion of radioactive materials. Therefore, during the safety assessment for decommissioning of nuclear facilities, safety assessment personnel shall analyze not only the radiation hazards, but also the general industrial hazards that may cause exacerbate radiation consequences.
The scope of hazard identification and analysis for decommissioning of nuclear facilities shall include:
(1) Radiation hazards, including external exposure and internal exposure (absorption by inhalation, ingestion or through a wound, etc.);
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(2) General industrial hazards, including falling of heavy objects, fire, dust and aerosol inhalation;
(3) Toxic, harmful and other types of hazardous substances, including beryllium, chromium, lead, asbestos, polychlorinated biphenyls, paints, toxic chemical reagents, flammable and explosive substances and suffocating substances.
3.3.2 Consideration shall be given to internal events and external events that may be triggered by decommissioning activities and nuclear facility operators during the decommissioning activities, including off-site accidents (such as fires and explosions caused by combustible gases, and release of toxic and harmful gases from nearby factories), extreme weather conditions (such as hurricanes, snowstorms, rainstorms and extreme temperatures) and natural disasters (floods, tsunamis and earthquakes).
3.3.3 During safety assessment, it is necessary to assess not only the single effect but also the superposition effect of these hazards and events. For example, fire causes radioactive waste packaging to fail, which leads to radiation safety accidents.
3.3.4 A safety assessment shall be conducted for the foreseeable initial events due to the decommissioning of a nuclear facility. If necessary, safety measures shall be proposed to minimize the risk and severity of the consequences of the initial events.
3.3.5 The consequences of identified hazards and initial events shall be quantitatively analyzed and screened to distinguish important hazards and critical initial events, which shall be subjected to the safety assessment.
Contents
1 Introduction
1.1 Purpose
1.2 Scope
2 Objectives and responsibilities
2.1 Objectives
2.2 Responsibility
3 Requirements for safety assessment
3.1 General requirements
3.2 Factors to be considered in safety assessment
3.3 Hazard identification and analysis
3.4 Defense in depth
3.5 Assessment of safety functions
3.6 Optimum analysis
3.7 Assessment of long-term safety
3.8 Engineering analysis
3.9 Waste management assessment
3.10 Uncertainty analysis
3.11 Quality assurance
4 Work process and contents of safety assessment
4.1 General
4.2 Description of nuclear facilities and decommissioning activities
4.3 Identification and screening of hazards and initial events
4.4 Scenario sequence identification
4.5 Hazard analysis
4.6 Assessment of results and determination of safety measures
4.7 Decommissioning safety analysis report
Annex Example of a list of hazards and initial events for decommissioning of nuclear facilities
