Internal flooding

A characteristic of "external" events is their propensity for causing multiple failures. This is simplified somewhat for internal flooding by assuming equipment fails when it is submerged, hence, failure is determined by the water level and the location of equipment. 

A flooding analysis for major plant systems uses a qualitative fault tree that takes into account the elevation of system components. This procedure, illustrated in Figure 5.3-1, uses a fault tree captioned for the top event, "core melt due to internal flood." The fault tree is developed under the assumption that a flood causes a transient, small, or large LOCA, or causes the failure of... 

The internal flood analysis was conventional and consistent with the NUREG 1150 and IPE studies. The steps were ... 

On August 8, 1985, the U.S. Nuclear Regulatory Commission (NRCf requested the operators of nuclear power plants in the U.S. to perform Individual Plant Examinations (IPE) on their plants. IPEs are probabilistic analyses that estimate the core damage frequency (CDF) and containment performance for accidents initiated by internal events (including internal flooding, but excluding internal fire). Generic Letter (GL) 88-20 was issued to implement the IPE request to identify any plant-specific vulnerabilities to severe accidents and report the results to the Commission. ... 

Chu, T. L., Z. Musicki, and P. Kohut, 1995, Results and Insights of Internal Fire and Internal Flood Analysis of the Surry Unit 1 Nuclear Power Plant during Mid-loop Operations, PSA 95 Seoul Korea, pp 967-973, November. 

Accidents originating inside the plant but which affect the entire plant area are termed area accidents. In particular, these maybe fires and internal floods, typically started by breaks in the service water system. 

In particular, as well as a break, a leak with a limited area should be considered to be a PIE that could lead to an internal flooding hazard. For flange connections and for different types of sealing, the possible leak areas should be analysed case by case. 

Internal flooding and spraying due to leaks or breaks of pipes, pumps, valves —Internal missiles ... 

Dominant accident sequences (internal events and internal flooding [IF])... 

Internal flooding sources E-3 Loss of confinement—Ground/surface water runoff E-5 Criticality—Increased moderation... 

IH 5 Need for systematic internal flooding assessment including backflow through floor drains (Bulgaria, Czech Republic, France, Germany, India, Republic of Korea, Ukraine, USA)... 

In some countries, further plant modifications resulted from enhanced regulatory requirements according to the development of the state of the art in science and technology of precautionary measures in safety related areas as e.g. fire protection, internal flooding and earthquake. 

Need for systematic internal flooding assessment including backflow through floor drains (IH 5)... 

If the safety and support systems are not adequately protected against internal flooding hazards, the safe shutdown of the plant can be impaired or an accident initiated. 

The Kozloduy NPP plans to perform an analysis of internal flooding. 

For the latest generation of German PWRs and BWRs, a physical separation of the safety trains is applied. To prevent effects of internal flooding on more than one redundancy, a specific detection of leaks followed by isolating the affected train is implemented. 

Specific review of internal flooding aspect carried out for few such report incidents and corrective measures taken. 

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