Arab Journal of Nuclear Science and Applications, 48(3), ( ) 2015

Transcription

1 Specific Considerations in the Safety Assessment of Predisposal Radioactive Waste Management Facilities in Light of the Lessons Learned from the Accident at the Fukushima-Daiichi Nuclear Power Plant A. M. Shokr Reactors Department, Nuclear Research Center, Atomic Energy Authority, Cairo, Egypt. Received: 1/2/2015 Accepted: 10/3/2015 ABSTRACT This paper discusses the specific aspects to be considered in performing safety assessment of different types of predisposal radioactive waste management facilities in light of the lessons learned from the Fukushima-Daiichi accident. The assessment methodology, which is developed on a similar approach used for nuclear reactors and fuel cycle facilities, evaluates the robustness of the facility safety systems against the impact of extreme external events, with emphasis on the fulfilment of the basic safety functions. It also considers the existence of spent fuel storage at predisposal radioactive waste management facilities, and the changes of the site characteristics since the time of facilities construction. These elements are presented along with discussions, which also include the associated regulatory considerations. Key words: Safety Assessment/Fukushima-Daiichi Accident/Waste Management Facilities 1. INTRODUCTION The lessons learned from the accident at the Fukushima-Daiichi nuclear power plant are relevant to the safety of predisposal radioactive waste management facilities (RWMFs), when subjected to extreme external hazards. These facilities contain large amounts of radioactive material inventories and, consequently, have high potential hazards. They also have a high potential for an accidental criticality that may be comparable to nuclear power plants in view of the fact that management of radioactive waste may take place in separate dedicated RWMFs, or within larger facilities operated for other purposes such as nuclear power plants or spent fuel reprocessing (1). For some of the RWMFs, the potential for the accumulation and detonation of hydrogen or exothermic chemical reactions following an extreme external event could compromise the containment and increase the spread of contamination. It is also important to note that the majority of these facilities were designed decades ago and their design features are not fully in conformance with the up-to-date safety standards (2). Additionally, in many RWMFs, the combination of static and dynamic containment provides potential pathways to the environment under accident conditions. Furthermore, in some cases the characteristics of the facility s site and the site vicinity may have changed since its construction. Taking the above mentioned factors into account, it is necessary to perform safety assessment of RWMFs (with consideration of spent fuel storage associated with these facilities), for defining and implementing measure for preventing accidents involving large releases of radioactive material at these facilities. In the following sections, the methodology for this safety assessment methodology is developed and specific considerations in its application to different types of RWMFs are discussed. 103

2 2. Safety assessment methodology: The main objective of the safety assessment is to evaluate the robustness of the structures, systems and components (SSCs) important to safety against the impact of extreme external events, with emphasis on the fulfilment of the basic safety functions: Prevention of criticality; Protection against external exposure; confining radioactive materials; Maintaining spent fuel/radioactive waste retrievability; and Removal of decay heat and the dilution of radiolysis gasses, as applicable. The safety aspects of the RWMFs are similar to those of nuclear fuel cycle facilities (NFCFs), and therefore can be subjected to the same assessment methodology. This takes also account for existence of spent fuel storage at RWMFs (3). The methodology for safety assessment for NFCFs was originally developed on the basis of the one for research reactors (4), and takes into account the concept of design extension conditions (DEC). Although it is developed for nuclear reactors, DEC is also valid for RWMFs (5). The concept is based on the feedback from the Fukushima-Daiichi accident and supersedes the concept of beyond design basis. The main technical objective of the analysis of DEC is to provide assurance that the design of the facility is such to prevent accident conditions beyond those considered in the design basis, or to mitigate their consequences as far as reasonably practicable. 2.1 General Considerations in Performing the Safety Assessment: In general, the safety assessment of RWMFs should: Be performed using deterministic methods and by validated and verified computer codes; Refer to the current status of the facility as built and as operated in order to encompass existing and planned operational regimes and modifications. The most up-to-date information on external events and site characteristics should be used. Use the most unfavourable facility conditions, including, radioactive inventory, fuel enrichment and burn-up, and concentration, acidity, and temperature of dissolved fuel; Consider the ageing effects on SSCs important to safety; Take into account occurrence of simultaneous consequential and dependent external events; Consider possible impact of failure or damage to SSCs not important to safety on SSCs important to safety. It should be noted that the site characteristics should also be reviewed to determine whether there are any changes have occurred that may result in an increase in the frequency of occurrence of external events or changes to the radiological consequences. These changes may also have an impact on the emergency response including access to the facility site, availability of on-site and off-site emergency response organizations, and use of the site as an emergency control centre. The site characteristics that need to be reviewed include the distribution of workers within the site or in the surrounding population; site infrastructure and support services; local land use; and hydrology and topology of the site vicinity. Table 1 presents an overview of a two-step process for the safety assessment of RWMFs in light of the feedback from the Fukushima-Daiichi accident. Additional discussions on application of this method to different types of facilities are provided in the following sections. 104

3 Table (1): Overview of the safety assessment methodology. Step Action description Inputs to the analysis Outputs of the analysis 1- Review of Review of the facility - Facility licensing and as-built documentation - Evaluated effect on safety margins of SSCs the design design basis and - Current safety analysis procedures and important to safety basis verification that results - Identified any cliff-edge effect changes are - Operational limits and conditions - Evaluated effect on the fulfilment of basic accommodated by the - New postulated initiating events (PIEs) or safety functions current safety analysis hazards - Identified remedial actions - Ageing effects 2- Assessment of DEC - Results of the latest periodic safety review Identification of DEC - Results of Step 1 - Consideration of consequential and simultaneous failures (including interaction with other facilities in the site) - Records on maintenance and testing Analysis of DEC - Outputs of the previous action - Safety analysis methods and procedures - Existing emergency procedures Evaluation of the consequences associated with DEC Identification of compensatory measures - Outputs of the previous action - Source term - Changes in site characteristics - Identified DEC: Accident conditions more severe than design basis events and that involve additional failure - Evaluated effect on safety margins of SSCs - Identified possible cliff-edge effect and associated timeframe - Evaluated effect on the fulfilment of basic safety functions - Identified possible protection and mitigation measures against DEC - Radiation dose to workers and public - Possible radiological migratory measures - Outputs from the previous actions. - Defined need for strengthening or installation of new SSCs - Defined alternating operating conditions - Defined need for revising emergency plan - Defined need for additional training and qualification of operating personnel 105

4 Step 1- Review of the Design Basis of the Facilities: The objective of this step is to ensure that the design requirements and the underlying data are valid and consistent with the current status of the RWMF and its site. The review should consider all the PIEs originally used in the safety analysis and the continued validity of the data and computer codes used in that analysis; design and administrative provisions; and adequacy of emergency preparedness, including considerations of human factors. Considerations for safety analysis for the predisposal management of radioactive waste are available in the literatures (6). Table 2 shows examples of the PIEs that should be considered in the safety analysis of the RWMFs with discussions of the factors contributing to these events. If the assumptions used for the existing safety analysis or design basis have changed, including any PIEs or new hazards that were not previously addressed, the review needs to verify that these changes are enveloped by the existing safety analysis or that analysis needs to be revised to address them. Consequently, the effect on the safety margins of the SSCs important to safety should be evaluated; any cliff-edge effects identified, and the associated impacts on fulfilment of a basic safety function should be assessed and remediation actions and compensatory measures, as necessary, should be implemented. Step 2 Assessment of the Impact of Conditions More Severe than the Design Basis Accidents: This step compromises assessment of the impact of accident conditions more severe than the design basis accidents or that involve additional failure (i.e. analysis of DEC), including assessment of any consequential loss of the basic safety functions, and the relevance of the mitigatory actions to be taken in order to identify safety improvements. This step of the safety assessment should cover: Events more severe than the design basis events that originate from extreme events and credible combinations of extreme events that could cause damage to SSCs important to safety and challenge the fulfilment of the basic safety functions performed by these SSCs; Progression of events that could lead to significant loss of means of confinement or criticality combined with failure of SSCs important to safety; Interaction between the facility (and the associated facilities) and other facilities in the site, assuming that the extreme external event has affected all of these facilities simultaneously; Adequacy of the emergency arrangements for response to accident conditions. The SSCs that are needed to maintain all or some of the basic safety functions during DEC should be identified. Minimal combinations of these SSCs and human actions that are needed to protect the facility against (or mitigate the consequences of) extreme external events should be identified, as well as any necessary physical improvements to the facility and procedural actions implemented, as appropriate. The analysis should proceed to determine the status of the SSCs (i.e. whether they will continue to perform the intended function or will fail) that support fulfilment of the relevant basic safety functions during the course of accident conditions (including simultaneous or consequential events). A list of these SSCs can then be prepared with information on the contribution of each SSCs to fulfilling one or more main safety functions. This will help verify the robustness of the facility and identify any missing information on important SSCs. 106

5 Table (2): Examples of PIEs to be reconsidered in the safety analysis of RWMFs. PIEs category Contributors Remarks Inadvertent criticality Exposure radiation SSCs failures to Loss of support systems Nonradiological hazards Erroneous handling of fissile material (including erroneous processes), or inadequate administrative control. Discusses criticality safety and more details on contributors to criticality accidents are found in the literatures (2, 7). Inadequate performance of static and dynamic barriers Mechanical or electrical equipment failure Loss of energy or fluids Failure in handling of chemical, toxic, flammable, or explosive substances Applicable only to RWMFs containing fissile materials. Inadvertent criticality may have occurred due to improper change of fissile material properties or distribution of other materials associated with or surrounding fissile materials, as well as improper changes in the mass, geometry, configuration, moderation, reflection, concentration in process solutions, and material, properties of neutron absorbers. Inadvertent criticality may also result due to human errors or failure in use of operating procedures. Examples include loss of shielding, hot cells, improper functioning of ventilation systems, glove boxes, air purification system, leakage from process systems, etc. These include drop of heavy loads, rupture of vessels, or pipes, etc. These include loss of electrical power supply, compressed air, vacuum, chemical reagents and ventilation, and loss of coolant, as applicable (e.g. wet storage spent fuel facilities). Most of RWMFs contain hazardous chemical materials and in some cases materials of chemo-toxicity. In some facilities (e.g. dry fuel storage facilities) radiolysis, if not controlled, may result in exposition due to hydrogen accumulation. External events Earthquake, Flooding, Volcanos, Fire, Extreme weather conditions Flooding includes tsunami, dam failure, etc. Fire includes natural fire and fire (or explosions) from surrounding industrial installations. Extreme weather conditions include heavy snow, tornadoes, hurricanes, cyclones, lightning, extreme high or low temperature, and extreme humidity. Internal events Fire, flooding, and explosion May also include human-induced events Table 3 provides an example of the process of verification of the robustness of SSCs of the facility when subjected extreme external event or combination of events. The subsequent step in the analysis is to evaluate the radiological consequences of loss of the relevant basic safety functions due to failure of the SSCs intended to perform one or more of these safety functions. 107

6 Table (3): Examples of verification of the robustness of SSCs following extreme external events of their combination. SSCs important to safety Basic safety functions Verification results with respect to external events Criticality prevention Prevention of external Confinement Retrievability of waste or Removal of decay heat Earthquake Flooding /Tsunami Tornado Others see Protection /shutdown System Emergency cooling system exposure spent fuel Absorbers X * X NA ** NA NA X... X Barriers X X NA NA NA X X.... X.... Valves NA NA NA NA X.. Pipelines NA NA NA NA X Ventilation Filters NA X X NA NA Fans NA X X NA NA Containment Building Integrity X Electrical systems Batteries Generators.... * SSCs are important to fulfil the basic safety function or during the course of event ** Not Applicable Table 2 108

7 2.2 SPECIFC CONSIDERATIONS IN SAFETY ASSESSMENT OF RWMFs: Spent Fuel Wet Storage Facilities: In addition to the general considerations discussed above, the assessment of the conditions more severe than design basis accidents for spent fuel wet storage facilities should also include evaluation of: The timeframe and potential of water level changes (temperature increase, boiling point, loss of water cover) either due to decay heat effects, or through acute or chronic leakage; Existence of appropriate surveillance systems to provide timely information so as to prevent loss of water cover. These are particularly important since the main goal in such facilities is to preserve the integrity of fuel cladding material, which implies keeping a sufficient level of water in the storage pools with adequate cooling provisions). The considerations mentioned above are also applicable to the safety of spent fuel wet storage associated with research reactors, including the spent fuel storage at ETRR-1 and ETRR-2 research reactors, Egypt Dry Storage of Spent Fuel and Storage of Solid High Radioactive Waste: In addition to the general considerations discussed above, for spent fuel dry storage facilities, the review of the design basis events should cover re-evaluation of the adequacy of SSCs to fulfill the basis safety functions with emphasis on assessment of adequacy of radiation monitoring systems, confinement measures for dry fuel cask storage (loss of confinement is expected to present localized effects), and confinement measures for fuel cladding material storage facilities (in ageing storage oils) where gas dilution systems are required to prevent fire and aerial release of volatile species. With respect to the impact of conditions more severe than the design basis events for spent fuel dry storage (and storage of high solid radioactive waste), the assessment should also include evaluation of: The timeframe available before the decay heat of the dry fuel (stored in casks) would cause fuel failure leading to containment degradation; The timeframe available before the decay heat from the storage of vitrified high radioactive waste would increase more than the limiting value and lead to containment degradation; The timeframe available within which to prevent hydrogen accumulation to unsafe concentrations; The timeframe for exclusion of oxygen from the facility by the effect of the inert gases, and the effect on fire safety; Existence of appropriate surveillance systems for parameters important to safety including, for example, oxygen concentration and ventilation performance. Radioactive Waste Processing and Low and Intermediate Radioactive Waste : These facilities generally contain limited amounts of radioactive materials inventory but may contain significant quantities of chemicals and flammable materials. The following factors should be considered in the safety assessment of these facilities: Kind of waste treatment and storage with their purposes (from treatment facilities for waste incineration, cementation, compaction to storage only facilities), and the associated inventory of radioactive material as determined by the operational limits and conditions; Actual size of the facility which might challenge its retention capability; 109

8 Prosperities of radioactive waste including its physical and chemical form and other hazardous properties such as chemo-toxicity, radio-toxicity, and flammability, and packaging. For low level radioactive waste management facilities, the confinement basic safety function is the most important one and, therefore, review of the design basis events should give increased attention to assessment on possible loss of confinement including evaluation of integrity of structures and radioactive waste packages, and appropriate functioning of the ventilation systems and fire dedication and suppression systems. As confinement of radioactive materials is the primary objective of the safety systems of these facilities, the assessment of the conditions more severe than design basis accidents should cover, in addition to those mentioned above, evaluation of the possibility for reducing potential release of radioactive materials with the available time frame for taking relevant actions (by equipment or through operator s intervention). The considerations mentioned above are applicable to the low and intermediate level liquid waste facility operated by the Hot Laboratory and Waste Management Center (HWLMC), Egypt. 3. REGULATORY CONSIDERATIONS As it was recognized during the early stages of the Fukushima-Daiichi accident, the effective involvement of regulatory body is essential not only during normal operation but also in accident conditions (8). It is also expected that the regulatory bodies will analyze the lessons learned from the Fukushima-Daiichi accident and accordingly may proceed with the revision of existing regulations or with development of new ones. In performing its regulatory functions (9), the regulatory bodies may also consider and perform, as needed, specific inspections aimed at verifying the robustness of SSCs important to safety of RWMFs, operating programmes and procedures (including maintenance, periodic testing and inspection), and emergency arrangements currently in place. The regulatory bodies may also conduct and participate in emergency drills involving severe conditions and transition from emergency operating procedures to severe accident management. In addition, the feedback from the Fukushima-Daiichi accident suggests the need to encourage, commission or direct, by the regulatory bodies, research and development in areas relevant to extreme external events in nuclear installations, including RWMFs. 4. CONCLUSIONS The feedback from the accident at the Fukushima-Daiichi nuclear power plant that is relevant to the safety of RWMFs was discussed, including the need for performing a safety assessment to evaluate the robustness of the facilities safety systems against the impact of extreme external events aiming at identifying adequate engineering or administrative measures to prevent (or mitigate the consequences of) these accidents. Because of similar safety aspects, the assessment methodology for the RWMFs was developed in a similar approach to the safety assessment of NFCFs, and takes account for existence of spent fuel storage at RWMFs. Considerations should also be given to the changes of the facilities site characteristics since the time of their construction as these may have impact on the safety. The discussions of specific features of different kinds of RWMFs showed the necessity to evaluate the timeframe within the accident scenario before a basic safety function is lost and the possibility for interventions to prevent escalation of the accident conditions. In this regard, it is essential to determine the timeframe for losing the ability to remove decay heat in spent fuel storage and high solid radioactive waste facilities, timeframe before accumulating of hydrogen in unsafe 110

9 concentrations in wet spent fuel storage, and before exclusion of oxygen by inert gasses in wet spent fuel storage and storage of high solid waste facilities. Evaluation of the integrity of the containment (or means of confinement) is also important for all kinds of facilities specifically for low and intermediate radioactive waste storage and processing facilities where containment is, in many cases, the primary safety function. Fire analysis (considering temperature rise to an extent affecting criticality safety margins), and consideration of chemical hazards are of equal importance. It is recommended to take account for the relevant considerations discussed in this work by the ETRR-1, ETRR-2 and HLWMC in performing safety assessment of the spent fuel wet storage and low and intermediate level liquid waste facilities in the light of the feedback from the lessons learned from the Fukushima-Daiichi accident. REFERENCES (1) International Atomic Energy Agency, Predisposal Management of Radioactive Waste, IAEA Safety Standards Series No. GSR Part 5, IAEA, Vienna, (2) International Atomic Energy Agency, Safety of Fuel Cycle Facilities, IAEA Safety Standards Series No. NS-R-5 (Rev.1), IAEA, Vienna, (3) International Atomic Energy Agency, Storage of Spent Nuclear Fuel, IAEA Safety Standards Series No SSG-15, IAEA, Vienna, (4) International Atomic Energy Agency, Safety Reassessment for Research Reactors in the Light of the Feedback from the Accident at the Fukushima-Daiichi Nuclear Power Plant, IAEA Safety Report Series No. 80, IAEA, Vienna, (5) International Atomic Energy Agency, Safety of Nuclear Power Plants: Design, Safety Standards SSR-2/1, IAEA, Vienna, (6) International Atomic Energy Agency, The Safety Case and Safety Assessment for the Predisposal Management of Radioactive Waste, IAEA Safety Standards Series No. GSG-3, IAEA, Vienna, (7) International Atomic Energy Agency, Criticality Safety in Handling of Fissile Material, IAEA Safety Standards Series No SSG-27, IAEA, Vienna, (8) International Atomic Energy Agency, Nuclear Safety Review for the Year 2012, GC(56)/NSR/INF2, IAEA, Vienna, (9) International Atomic Energy Agency, Governmental, Legal and Regulatory Framework for Safety, IAEA Safety Standards Series No. GSR Part 1, IAEA, Vienna,