Analysis of external human induced events during the site evaluation stage of nuclear power plants

Increasing electricity demand and environmental problems such as global warming force the countries to use more clear and economical energy sources. Nuclear energy plays a key role to meet growing electricity demand without contributing to global warming so the importance of the nuclear is increasing.Tukey is also one of the newcomer country to nuclear energy. Turkey has two Nuclear Power Plant Projects. One of these projects is Akkuyu Nuclear Power Plant Project and the other is Sinop Nuclear Power Plant Project. To construct and operate the Nuclear Power Plants (NPPs) some licences and permissions should be taken from the each countries' Regulatory Bodies. Turkish Atomic Energy Authority (TAEK) is the Regulatory Body of Turkey. According to Turkish Atomic Energy Authority Regulations, NPPs' licensing process has three stages. Firstly, Site Licence should be taken for the proposed site and then for construction and operation of the nuclear power plant Construction Licence and Operation Licence should also be taken. During the site licence application, the applicant should prepare and submit Site Report to Turkish Atomic Energy Authority (TAEK) to show the proposed site suitability for the nuclear power plant. Site selection and evaluation of the site are very important because the site should be suitable with the proposed NPPs. After the site evaluation, if some deficiencies related with site cannot be eliminated with engineering solutions or administrative procedures, the site shall be deemed unsuitable. The main objective in site evaluation for NPPs in terms of nuclear safety is to protect the public and the environment from the radiological consequences of radioactive releases due to accidents. One of the specific subject which should be considered and evaluated during the site evaluation stage is External Human Induced Events and this subject should be investigated and assessed for every proposed nuclear power plant site. Evaluation external human induced evets subject is also the subject of this study. The external human-induced events considered in this study are accidental origin. Because it is hard to collect data and evaluate the malicious acts to NPPs. During these evaluations, foreseeable significant changes in land use shall also be considered and evaluated. The content of this study is based on two main documents. One of them is TAEK Regulation on Nuclear Power Plants Sites (Date/Number: 21 March 2009/27176) and the other is IAEA Safety Guide "External Human Induced Events in Site Evaluation For Nuclear Power Plants". To assess these events, firstly the potential sources which may affect the safety of the NPPs shall be defined. Sometimes, the sources can be eliminated with the simple deterministic methods. According to Article 13 of the 27176 numbered TAEK Regulation on Nuclear Power Plant Sites, the defined distance from the site is 10 km. To define potential sources, the site with its surrounding shall be examined for facilities and human activities that may affect the safety of the proposed nuclear facility. Chemical plants, oil refineries, storage depots, road, rail, waterways, air, pipelines and other nuclear facilities at the same site can be the potential sources so during the site investigations they should be considered. Each site has different characteristics so it is not possible to make a general list for potential sources. Site specific data shall be used in the determination of the external events. If enough site data cannot be obtained to evaluate frequency and severity of external human induced events, the regional, national or global datas shall be collected. Potential sources can be classified according to location of the initiating mechanism. If the location of the initiating mechanism is fixed, these sources are categorized as stationary sources. If the location of the initiating mechanism is not totally constrained, these sources are categorized as mobile sources. According to TAEK Regulation 27176 following events shall be considered during site evaluation for external human induced events; • Explosions • Delayed ignition after occurrence of burning gas clouds • Dispersion of toxic and asphyxiant chemicals and radioactive materials • Fires • Collisions to water intake structures of nuclear power plant • Collisions to nuclear power plant • Dispersion of liquid and solid materials • Eddy currents and electromagnetic interference According to these events site related design bases are determined and all these characteristics shall be observed and monitored throughout the lifetime of the NPPs. General procedure to screen and evaluate the hazards can be divided into two steps. First step is the preliminary screening. Events are evaluated in this step according to distance from the site and probability of occurrence. Screening distance value (SDV) for a particular type of source is 10 km according to 27176 numbered TAEK Regulation beyond which the effects of such sources may be ignored. If the source is outside the SDV for the initiating event, no further action is necessary. If the source is in the SDV or any source which is beyond the distance but we think that it may affect the plant safety, the occurrence probability of the event should be determined and compared with the specified Screening Probability Level (SPL). The SPL is identified as in the 27176 numbered TAEK Regulation and IAEA Guide. If the probability of occurrence of the initiating event under consideration is greater than the , a detailed evaluation should be made for this event. To make detailed evaluation, detailed data should be collected. In case of lack of data, the evaluation methods results may not be correct so some mathematical methods (Event Tree, Fault Tree, Monte Carlo Simulation etc.) shall be used during these calculations. During these calculations, meteorological conditions, topographical characteristics and any protective measures related to one of the activities shall be taken into account. In detailed evaluation, the probability of occurrence of each interacting event should be compared with the Design Basis Probability Value (DBPV). A design basis probability value (DBPV) for the interacting event under consideration is determined by dividing the SPL by the Conditional Probability Value (CPV). CPV is the conditional probability that an interacting EHIE will result in a radiological release. It will have a range between near-zero and 1.0 and generally it is taken as 0,1. If the probability is less than the DBPV, no further consideration should be given to that event. If the probability is greater than DBPV, the engineering and administrative measures should be analyzed to prevent or mitigate the effects of events. If the effects cannot be prevented or mitigated, the site shall be deemed unsuitable. Design basis events and parameters are not in the content of this study. In the content of this study firstly external human induced events have been defined and then screening and evaluation processes have also been explained in detail. Related national and international regulations, standards and guides have been reviewed. Example of the studies about the external human induced hazards such as updated Site Report of Akkuyu Nuclear Power Plant and Site Evaluation Report of Darlington Nuclear Power Plant sites have been summarized. To apply the evaluation method, the ship collision accidents in İstanbul Strait and forest fire in İstanbul region have been examined as an example. Most of the ship collisions accidents occur in İstanbul Strait in Turkey so this location has been selected for the calculation of this analysis. Ship collisions in other regions of the Turkey shall be commented depending on İstanbul strait evaluation results. Information Diffusion Method has been used to evaluate the occurrence probability of the ship collisions and forest fire. If enough data cannot be collected for the calculations, Information Diffusion Method is one of the useful method to offset the information deficiency so this method is used for this study. Some example studies about Information Diffusion Method have also been summarized in the content of this study. For the purpose of finding the minimum required data to adopt Information Diffusion Method, calculation has been done with different number of accident data. Also,normal diffusion method and quadratic diffusion method have been used and results have been compared to see the effects of different diffusion functions on the Information Diffusion Method results. Finally, to see the efficiency and effectiveness of Information diffusion method, a comparison has been made between Information Diffusion method results and Alp KÜÇÜKOSMANOĞLU doctoral dissertation results published at 2008 in which Monte Carlo method was used to calculate ship collision risk for İstanbul region . In the conclusion, importance of external human induced events and the evaluation of these events effectively and efficiently with sufficient data and suitable methods is corroborated. Information Diffusion method's importance to offset the information deficiency is also corroborated.