**2. Different types of hazards and hazard combinations**

When considering those hazards which may impair the safe operation of an industrial facility, in principle, two types of hazards have to be distinguished: internal and external hazards.

Internal hazards are those occurring under the responsibility of the operator of the industrial facility on the site of the corresponding installations (e.g., one or more industrial plants).

External hazards are those ones occurring independent of the facility being analyzed, off-site, and out of the responsibility of the plant operator. External hazards may result from natural causes—so-called natural hazards—or maybe induced by humans—so-called man-made hazards. Natural hazards can be further subdivided into different classes of hazards corresponding to the types of phenomena covered.

Although this chapter focuses on natural hazards, it is important to list and characterize all types of hazards in order to enable the analyst to perform a complete screening of hazard combinations.

## **2.1. Systematic binning of hazards**

In **Table 1**, an overview of the different classes of internal and external hazards is given. **Tables 2**–**10** provide for all hazard classes mentioned in **Table 1** the binning of individual hazards to the different hazard classes.

#### **I. External hazards**

In case of the electrical power industry, the impact, in particular of seismotectonic, hydrological, and meteorological hazards, is diversified. Examples are the energy production and the transmission and distribution lines of the suppliers where strong winds like tornados result in a disruption of the production (e.g., in case of wind turbines) or of the distribution lines due to trees fallen on overhead lines. An expected low water level may require the shutdown of a

Hazards can arise not only individually but often occur together with other events or hazards. The experience has shown that a variety of combinations of different types are possible. If and how frequently such hazard combinations do occur at a nuclear facility site depends on the site characteristics but also on the facility to be investigated and its design against various

In particular, for combinations of natural hazards with other events, the operating experience of the more recent past has shown that at least some of the huge amount of theoretically possible hazard combinations cannot be excluded to occur in principle. Some of these combinations represent—like individual hazards—low-frequency, high-damage events, others are more frequent, but the damage potential is much lower (so-called high-frequency,

For systematically considering all hazard combinations having the potential to impair the safe operation of an industrial facility, but enabling the analyst to exclude non-negligible combinations as well, the entire set of hazards, which can be anticipated at the site of the facility being analyzed, needs to be identified. In the second step, the individual hazards have to undergo a qualitative and quantitative screening process. In the third step, hazard combinations have to be identified starting from those individual hazards identified and not screened out by qualitative arguments. For these hazard combinations, again the screening

When considering those hazards which may impair the safe operation of an industrial facility, in principle, two types of hazards have to be distinguished: internal and external hazards.

Internal hazards are those occurring under the responsibility of the operator of the industrial facility on the site of the corresponding installations (e.g., one or more industrial

External hazards are those ones occurring independent of the facility being analyzed, off-site, and out of the responsibility of the plant operator. External hazards may result from natural causes—so-called natural hazards—or maybe induced by humans—so-called man-made hazards. Natural hazards can be further subdivided into different classes of hazards correspond-

nuclear power plant because of potential core cooling problems.

**2. Different types of hazards and hazard combinations**

events.

low-damage events).

126 Probabilistic Modeling in System Engineering

has to be performed.

ing to the types of phenomena covered.

plants).

**1.** *Natural hazards* Class A: Seismotectonic hazards Class B: Flooding and other hydrological hazards Class C: Meteorological hazards Class D: Extraterrestrial hazards Class E: Biological hazards Class F: Geological hazards Class H: Natural fires **2.** *Man-made hazards* (Class Z)

### **II.Internal hazards (Class I)**

**Table 1.** Overview of hazard classes, from [1].


**Table 2.** Class A hazards according to [2].


**Hazard Type of individual meteorological hazard**

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C3b Low ground temperature

C6 Extremes of air pressure

C8 Low ground water C9 Low seawater level

C11 White frost, rime

C14 Recurring soil frost

C22 Wind-blown debris C23 Snow avalanche

**Table 4.** Class C hazards according to [2].

C24 Surface ice C25 Frazil ice C26 Ice barriers C27 Mist, fog

C5a High humidity C5b Low humidity

C7 Drought

C10 Icing

C12 Hail

C13 Permafrost

C15 Lightning C16 High wind C17 Tornado C18 Waterspout C19 Snowstorm C20 Sandstorm C21 Salt spray

C4a High cooling water temperature C4b Low cooling water temperature

**Table 3.** Class B hazards according to [2].



**Table 4.** Class C hazards according to [2].

**Hazard Type of individual hydrological hazard**

B2 Flash flood by local extreme precipitation

B4 Flooding by extreme precipitation outside the plant boundary

B6a High water level due to obstructions in the course of the river B6b Low water level due to obstructions in the course of the river B7a High water level by natural changes in the course of the river B7b Low water level by natural changes in the course of the river

B8 Flooding by high fresh water waves due to volcanism, land, or snow slide

B9a High water level with wave formation due to failure of water control or retention systems (e.g., damns,

B9b Low water level with wave formation due to failure of water control or retention systems (e.g., damns,

B16 Instability of coastal areas (of rivers, lakes, oceans) by erosion due to strong water flows or sedimentation

B3 Flooding by melting snow

128 Probabilistic Modeling in System Engineering

B5 Extreme groundwater increase

dykes, etc.)

dykes, etc.)

B11 Tidal bore (running extremely river-up)

B13 Storm-induced waves and monster waves

B15 Corrosion resulting from contact with salt water

**Hazard Type of individual meteorological hazard**

B12 Tidal high water, spring tide

B17 Water flotsam (mud, debris, etc.)

**Table 3.** Class B hazards according to [2].

C1 Precipitation, snow pack C2a High air temperature C2b Low air temperature

C3a High ground temperature

B10 Seiche

B14 Storm surge

B1 Tsunami


**Hazard Type of individual natural fire hazard**

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Z2 Industrial accidents: releases of hazardous substances

Z9 Ship accidents: Releases of solid or liquid substances

Z14 Pipeline accidents: releases of hazardous substances

Z15 Accidental aircraft crash in the airport area Z16 Accidental aircraft crash in air lanes/corridors

Z19 Off-site excavation and construction work

Z23 Underground high-voltage Eddy currents (off-site)

Z22 Electromagnetic interference (EMI)

Z27 Bore by water management activities

Z28b Low water level by building structures

Z29 Man-made ground settlement

**Table 9.** Class Z hazards according to [2].

Z12 Transportation accidents: releases of hazardous substances

Z21 Industrial impurity of high voltage insulations (of switchgears, etc.)

Z24 Flooding due to man-made failure of water control or retention systems

Z28a High water level by building structures (wave breakers, moles, languets)

Z10 Transportation accidents: direct impact Z11 Transportation accidents: explosions

Z13 Pipeline accidents: fire or explosion

Z17 Satellite crash Z18 Drone crash

Z20 External grid stability

Z25 Man-made fire (off-site) Z26 Log jam (e.g., by driftwood)

Z4 Accidental consequences of military facilities Z5 Accidental military releases of hazardous substances Z6 Accidental consequences of military activities

H1 Wildfire

**Table 8.** Class H hazards according to [2].

Z3 Industrial accidents: missiles

Z7 Ship accidents: direct impact Z8 Ship accidents: Collisions with SSC

**Hazard Type of individual man-made hazard** Z1 Industrial accidents: explosions

**Table 5.** Class D hazards according to [2].


**Table 6.** Class E hazards according to [2].


**Table 7.** Class F hazards according to [2].


**Table 8.** Class H hazards according to [2].

**Hazard Type of individual extra-terrestrial hazard**

**Hazard Type of individual biological hazard**

D1 Coronal mass ejection, solar flare

D2 Meteorite fall

E3 Fish, jellyfish

E5 Infestation

E6 Biological flotsam

**Table 5.** Class D hazards according to [2].

130 Probabilistic Modeling in System Engineering

E1 Marine/river/lake growth E2 Crustacean/mollusk growth

E4 Airborne swarms, leaves

E7 Microbiological corrosion

**Hazard Type of individual geological hazard**

F9 Volcanic hazards close to the volcano source F10 Volcanic hazards far away for the volcano source

F3 Debris flow, mud flow (including seismically triggered events)

F6 Karst, leeching of soluble rocks (limestone, gypsum, anhydrite, halite)

**Table 6.** Class E hazards according to [2].

F1 Subaerial slope instability

F4 Natural ground settlement

F5 Ground heave

F11 Methane release F12 Natural radiation

F13 Pole reversal (polar motion)

**Table 7.** Class F hazards according to [2].

F7 Sinkholes F8 Unstable soils

F2 Underwater landslide, and so on


**Table 9.** Class Z hazards according to [2].


Typical examples are electromagnetic interference (EMI) as common cause for a station blackout (SBO) and an internal fire as two correlated events or Tsunami as common cause for exter-

An initial event, for example, a (external or internal) hazard occurs independently from

nal flooding and independent internal fire or explosion, seismic event, and independent

For each category of event combinations with hazards involved those combinations, which can occur site-specifically and according to the design and protection features of the facility or plant to be analyzed, have to be identified and undergo a systematic screening. In the frame of the assessment of the contribution of natural hazards to risk, some hazard classes cannot be combined with natural hazards (hazard classes A to H) depending on the category of combinations. This limits the amount of principally possible combinations

For limiting further detailed analyses only to those hazards and hazard combinations, which can occur at the site and in the facility under investigation, a systematic screening is needed. In Germany, a clearly structured, systematic approach for hazards identification and screening has been developed in the recent past by GRS for probabilistic risk assessment of nuclear power plant sites with respect to hazards [2]. This approach uses for the collection and processing of generic as well as site- and plant-specific information needed for screening and detailed analysis an analytical tool called *Hazards Library*. Based on the information and data available in general and the plant under investigation, a step-wise screening with a qualitative and a quantitative screening step, first for individual hazards and, based on the results of the qualitative screening, afterwards also for hazard combinations, is done. The screening can be performed semi-automatically based on questions to be answered for the qualitative screening and applying preselected quantitative criteria for the quantitative screening. A schematic overview of the screening

The first step is the identification of those hazards, which cannot be directly excluded as practically impossible for the site being analyzed. For the assessment of natural hazards, in this

For a given site, for example a riverine site in central Europe far from any coastal and/or tidal influence in an area with relatively high seismicity and no volcanic history, a variety of natu-

first step, the hazard classes Z and I can be excluded as non-natural hazards.

to a hazard without any common cause. Typical examples are exter-

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nal flooding, internal flooding, and internal fire as three correlated events.

• **Category 3:** Unrelated events:

but simultaneously<sup>1</sup>

**3. Hazards screening**

approach is given in **Figure 1**.

ral hazards can be excluded.

**3.1. Individual hazards screening**

internal fire.

significantly.

**Table 10.** Class I hazards according to [2].

For a systematic assessment of the contribution of natural hazards to risk, only the hazard classes A to H have to be considered as initially occurring individual hazards.
