External events, hazards flooding

The Level 1 LPS PSA of Mochovce Unit 1 includes as internal events LOCAs, transients and internal hazards, i.e. fires and floods. External events included seismic events, air craft crashes, influences of external industrial facilities, and extreme meteorological conditions. Operational experience and results obtained from PSA for Bohunice V-2 NPP and Dukovany NPP as Mochovce NPP are used as much as possible. 

The hazards associated with external events (and combinations of events) that are to be considered in the design of the reactor shall be determined. The combination of external events with anticipated operational occurrences or DBA conditions shall be considered for those cases in which an anticipated operational occurrence or a DBA condition is caused by the external event and where there is a need to consider long lasting external events (such as flooding) or long post-event recovery times. 

The potential for internal hazards such as fire, flooding, missile generation, pipe whip, jet impact or the release of fluid from failed systems or from other installations on the site shall be taken into account in the design of the research reactor facility. Appropriate preventive and mitigatory measures shall be taken to ensure that nuclear safety is not compromised. Some external events could initiate internal fires or floods or lead to the generation of missiles. Such interrelation of external and internal events shall also be considered in the design, where appropriate. 

External event Event caused by (I) a namral hazard—earthquake, flood, tornado, extreme temperature, lightning, etc or (2) man-induced events— aircraft crash, missile, nearby industrial activity, sabotage, etc. 

Recommendations on design features relating to all external events, excluding earthquakes, have been incorporated into this publication, which makes reference to the Safety Guides on external human induced events, flood hazards and extreme meteorological events [2-4],... 

The external events considered in this report include both natural hazards and human induced hazards from sources external to the site or external to the safety related buildings. Explicit reference is made to the most common external event scenarios considered in the design of research reactors (earthquake, wind, precipitation (snow, rain, hail), flood, explosions and aircraft crash, external fire), for which special recommendations are provided. However, the approach to the safety evaluation discussed in the present publication can be applied to any scenario included in the facility s safety analysis report. 

The probability of failure of structures, systems or components as the result of external events is computed as the product of the full range hazard curve of the external events convoluted with the derivative of the fragility of the structure, system or component under consideration, as shown in Section 4. The fragility of structures, systems and components is defined as the cumulative conditional Pp (unacceptable performance) versus the selected hazard parameter. The hazard parameter is typically represented by factors such as the peak ground acceleration (PGA) for earthquakes, the water depth for floods and the maximum wind speed for winds. 

Protection against external events [4, 5] and internal hazards [6]. The system should be so designed and laid out that no external event or internal hazard considered in the design (such as a pipe break or a flood) has the potential to prevent it from performing its intended safety functions. In particular the capability of the system or its components should be maintained under the most severe seismic conditions considered in the design. 

The purpose of the present Safety Guide is to provide recommendations relating to the evaluation of the flood hazard for a nuclear power plant on a coastal or river site so as to enable the identification of hazardous phenomena associated with flooding events initiated by natural and human induced events external to the site. 

The availability of communication routes external to the site during and after a flooding event involves facilities that are not always under the direct control of the site administrators. Since the availability of such communication routes is a key part of the emergency planning, a dedicated analysis of the flooding scenario should be performed together with the competent authorities as part of the hazard evaluation for the site. 

For probabilistic risk assessment of the APIOOO, the list of postulated faults encompasses at-power operations, shutdown operations and those internal and external hazards to which plant risk is most sensitive (internal fire, internal flood and seismic events). The list of initiating events has been developed by the following process (which is discussed further in Chapter 19 of Reference 5.6 ... 

The potable water day tank is above the main control room area. It is filled as and when required but is otherwise isolated from external sources thus any leakage is limited to the day tank volume. The potable water piping from the tank is sized so that even in the event of a pipe rapture the leak rate would be modest. The pipes are routed within the maimed spaces, so that any leakage would be detected quickly. The potable water is thus not a flooding hazard to the safety signifieant equipment within the clean Auxiliary Building. 

The external hazards considered are seismic events, external floods, extreme weather, lightning, external electromagnetic interference, and aircraft crash analysis demonstrates that the design features of the API000 provide appropriate protection against these hazards (External Hazard Report, Reference 14.30). Therefore, it is demonstrated that external hazards do not present a significant probability of a major accident to the environment. Hence, the risk of acute environmental pollution as a result of external hazards is not considered further in this section. Plant faults and internal hazards are discussed further below. 

Originally, a more comprehensive update of the section on external hazards was planned. To date, the supplementary document, however, will focus on seismic events and external flooding. 

Like in a Full-Power PSA, hazards can be divided into two groups, internal hazards and external hazards. Internal events include fire, floods and events like drop of heavy loads. These events in comparison to power state are differently treated in a SLP PSA due to their specific attributes. 

In a similar way to the first report in this series. Ref. [3], this PHWR specific volume also deals only with internal events originating in the reactor or in its associated process systems. It does not cover originating events affecting broad areas of the plant (often called internal and external hazards), such as fires, floods (internal and external), earthquakes and aircraft crashes. However, analysis of the consequences of these events from a thermohydraulic point of view is partially covered by the present guidance. The emphasis in this guidance is on the transient behaviour of the reactor and its systems, including the containment and/or confinement. 

The layout of safety systems should be such that the minimum required capability is maintained in the event of a failure in one train of protection or in the event of needing to survive any internal and/or external hazards (e.g. earthquake, fire and flooding). 

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