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Divisione Nucleare Progetto Project PDS-XADS <strong>AnsaldoEnergia</strong> Identificativo Document no. XADS 20 TRIX 009 Rev. Rev. 1 Cl. ris. class Pagina Page 7 2. INTRODUCTION The XADS (eXperimental Accelerator Driven System) [1] is designed to contain and limit radioactivity releases to the external atmosphere in order to guarantee that no undue risk occurs for the population surrounding the plant (the design of the XADS shall ensure that an evacuation plan for the population is not necessary). Similarly to nuclear power plants this is achieved by providing physical barriers between the radioactive isotopes and the external environment (namely the fuel cladding, the primary system boundary and the containment for fission products and the latter two for activation and spallation products). The XADS design must be able to fulfill the above goal both during Design Basis Conditions (DBC) and Design Extension Conditions (DEC). The Master Logic Diagram (MLD), systematically describing all the abnormal or accident conditions resulting in potential challenges to the physical barriers is described. Based on it the list of accident initiating events is worked out and each event is attributed to the appropriate category. The representative events of each category (which will have to be analyzed in order to determine their consequences) will be identified. It is pointed out that, at the current status of development of the XADS design, limited technical information, or even almost no information, is available for some systems, in terms of operating modes and parameters as well as, sometimes, operating process. While this is inherent to the early design phase of a largely innovative concept, it implies that the depth of the presented analyses is therefore necessarily commensurate to the amount of available information. It will be possible to work out more details as soon as the design development will make available additional pertinent information. DNU 020/1
Divisione Nucleare Progetto Project PDS-XADS <strong>AnsaldoEnergia</strong> Identificativo Document no. XADS 20 TRIX 009 Rev. Rev. 1 Cl. ris. class Pagina Page 8 3. IDENTIFICATION OF ACCIDENT INITIATING EVENTS 3.1 GENERAL The objective of this section is to define a comprehensive set of design basis, abnormal and accident events that encompasses both the anticipated occurrences and the conceivable malfunctions and failures that pose challenges to the integrity of the physical barriers (fuel cladding, reactor primary system, containment) and/or may result in radioactivity releases to the environment. These are the so-called "Design Basis Conditions", which are deterministically factored in the design of a nuclear power plant and will consistently be factored in the design of the XADS. In addition to the Design Basis Conditions, the so called "Design Extension Conditions" are also normally addressed in the design of a nuclear power plan, to show that probabilistic safety objectives for core damage and for large releases of radioactivity to the external atmosphere are met. Design Extension Conditions for the XADS include, in principle, Anticipated Transients Without Proton Beam Trip, Complex Sequences with multiple independent malfunctions and Sequences of Events involving Severe Core Damage. A comprehensive approach to Design Extension Conditions is not part of this document and will not therefore be dealt with in this section. Two approaches can be taken in identifying the accident initiating events. One is a comprehensive engineering evaluation, taking into consideration information from previous risk assessments, documentation reflecting operating histories and plant-specific design data. The information is evaluated and a list of initiating events is compiled, based on the engineering judgment derived from the evaluation. Another approach is to more formally organize the search for initiating events by constructing a top-level logic model (called Master Logic Diagram) and then deducing the appropriate set of initiating events. Because of the peculiar characteristics of the XADS (and the resulting lack of plant specific data as well as, even more so, of operating data) the second approach has been adopted to define the initiating events set. 3.2 BRIEF DESCRIPTION OF THE MAIN COMPONENTS/SYSTEMS OF THE XADS The main components/systems of the XADS, relevant for safety analyses, are listed below with a brief description of their functions. Moreover a brief description of some relevant DNU 020/1
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