**1. Introduction**

124 Nuclear Power – Practical Aspects

Seed, H.B. and Idriss I.M. (1971) "Simplified Procedure for Evaluating Soil Liquefaction Potential," *Journal of the Soil Mechanics and Foundations Division*, Vol. 97(SM9), pp. 1249-

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In recent years new threats required safety assessment experts to reconsider the internal and external loads of nuclear installations, in particular nuclear power plants, focusing not only on internal hazards but also on the destructive power of external hazards such as aircraft crash, flooding including tsunamis, severe weather conditions and also explosions and blasts and their combination which can cause significant damage on the plant's operability, being potentially conducive to severe accidents. The cumulated effects of such external loads include the destruction of buildings and access ways, the debris build-up, the loss of electrical power supply as well as the loss of cooling capacity of the reactor core and the fuel pools.

International experience has shown that internal hazards such as fire and external hazards can be safety significant contributors to the risk in case of nuclear power plant operation. This is due to the fact that such hazards have the potential to reduce simultaneously the level of redundancy by damaging redundant systems or their supporting systems or even to loose all redundancies at once.

This has been strongly underlined by the nuclear accidents at the Fukushima-Daiichi nuclear power plants in March 2011 resulting from a very strong earthquake and a consequential tsunami.

A challenging prerequisite for any effective protection against external hazards is to accurately assess them systematically regarding the adequacy of their existing protection equipment against hazards, in particular those built to earlier standards.

Therefore, comprehensive safety assessments have to be performed in advance with most actual site-specific data und current knowledge of new research results. Potential methods

to analyse existing nuclear power plants are deterministic, probabilistic or combined methodologies.

Probabilistic Assessment of Nuclear Power Plant Protection Against External Explosions 127

Explosion is used broadly to mean any chemical reaction between solids, liquids, vapours or gases which may cause a substantial rise in pressure, possibly to impulse loads, fire or heat. An explosion can take the form of a deflagration or a detonation. BLEVE (Boiling Liquid Expanding Vapour Explosion) is a physical explosion also resulting in pressure or shock

The most common type of chemical explosion is the heterogeneous explosion. In heterogeneous explosions, a propagating reactive front clearly separates the non-reacted materials from the reaction products. The reaction front, usually called the reaction zone or flame (front), moves through the explosive mixture as the explosion occurs. In this zone the strongly exothermic reactions occur. Heterogeneous explosions are divided into two types:

In deflagrations, the reaction zone travels through the explosive mass at subsonic speed, while the propagation mechanism is heat transfer (by conduction, radiation and convection). Reaction zone propagation velocities (flame speeds) of deflagrations may vary over a wide range and so do the corresponding explosion pressures. One example of a deflagration experiment is shown in Figure 2; in this case the deflagration was very short

In some instances, accelerating deflagrations show a deflagration-to-detonation transition

The major characteristic of a detonation is its extremely high speed: the explosion zone moves at a supersonic speed. While, for deflagrations, the flame speeds are relatively low (typically one to several hundreds of metres per second), detonation flame speeds in air can easily reach one to two kilometres per second. The propagation mechanism of a detonation

**Figure 1.** Types of explosion

deflagrations and detonations.

and lasted less than one second.

(DDT) as shown in Figure 1.

wave.

In the past, most of the engineering work in designing safety features for nuclear power plants has been performed on a deterministic basis. Moreover, the use of deterministic safety analysis is still current practice to review the current safety level of operating nuclear power plants against external hazards.

As an observation from other areas, the probabilistic approach provides different insights into design and availability of systems and components supplementing the results from deterministic safety analyses. A more comprehensive risk assessment including the modeling and assessment of external hazards is usually recommended in the frame of periodic safety reviews which are performed about every ten years to get a global picture of the safety level of the nuclear power plant under consideration and which include a comparison to current safety standards and good practices.

In particular in case of probabilistic safety analyses, such an assessment can be very detailed and time consuming. Therefore, it is necessary to have appropriate procedures to screen out, e.g., buildings of a nuclear installation where no further analysis is required or to have a graded procedure for the respective hazard taking into account plant- and site-specific conditions.

The assessment of external hazards requires detailed knowledge of natural processes, along with plant and site layout. In contrast with almost all internal hazards, external hazards can simultaneously affect the whole facility, including back up safety systems and non-safety systems alike. In addition, the potential for widespread failures and hindrances to human intervention can occur. For multi-facility sites this makes the situation even more complex and it requires appropriate interface arrangements to deal with the potential domino effects.

In contrast to other external hazards (e.g., earthquakes, winds, or floods), an explosion has the following distinguishing features:


An explosion is defined as a rapid and abrupt energy release, which produces a pressure wave and/or shock wave. A pressure wave has a certain pressure rise time, whereas a shock wave has zero pressure rise time. As a result of the pressure and/or shock wave, an explosion is always audible. Explosions can be classified into a number of types as illustrated in Figure 1.

Probabilistic Assessment of Nuclear Power Plant Protection Against External Explosions 127

**Figure 1.** Types of explosion

126 Nuclear Power – Practical Aspects

power plants against external hazards.

comparison to current safety standards and good practices.

methodologies.

conditions.

domino effects.

illustrated in Figure 1.

the following distinguishing features:

magnitude greater than these other hazards.

to analyse existing nuclear power plants are deterministic, probabilistic or combined

In the past, most of the engineering work in designing safety features for nuclear power plants has been performed on a deterministic basis. Moreover, the use of deterministic safety analysis is still current practice to review the current safety level of operating nuclear

As an observation from other areas, the probabilistic approach provides different insights into design and availability of systems and components supplementing the results from deterministic safety analyses. A more comprehensive risk assessment including the modeling and assessment of external hazards is usually recommended in the frame of periodic safety reviews which are performed about every ten years to get a global picture of the safety level of the nuclear power plant under consideration and which include a

In particular in case of probabilistic safety analyses, such an assessment can be very detailed and time consuming. Therefore, it is necessary to have appropriate procedures to screen out, e.g., buildings of a nuclear installation where no further analysis is required or to have a graded procedure for the respective hazard taking into account plant- and site-specific

The assessment of external hazards requires detailed knowledge of natural processes, along with plant and site layout. In contrast with almost all internal hazards, external hazards can simultaneously affect the whole facility, including back up safety systems and non-safety systems alike. In addition, the potential for widespread failures and hindrances to human intervention can occur. For multi-facility sites this makes the situation even more complex and it requires appropriate interface arrangements to deal with the potential

In contrast to other external hazards (e.g., earthquakes, winds, or floods), an explosion has

The intensity of the pressures acting on a targeted building can be several orders of

 The duration of the event is very short, measured in thousands of a second, or milliseconds. This differs from earthquakes and wind gusts, which are measured in

seconds, or sustained wind or flood situations, which may be measured in hours.

An explosion is defined as a rapid and abrupt energy release, which produces a pressure wave and/or shock wave. A pressure wave has a certain pressure rise time, whereas a shock wave has zero pressure rise time. As a result of the pressure and/or shock wave, an explosion is always audible. Explosions can be classified into a number of types as

Explosive pressures decay extremely rapidly with distance from the source.

Explosion is used broadly to mean any chemical reaction between solids, liquids, vapours or gases which may cause a substantial rise in pressure, possibly to impulse loads, fire or heat. An explosion can take the form of a deflagration or a detonation. BLEVE (Boiling Liquid Expanding Vapour Explosion) is a physical explosion also resulting in pressure or shock wave.

The most common type of chemical explosion is the heterogeneous explosion. In heterogeneous explosions, a propagating reactive front clearly separates the non-reacted materials from the reaction products. The reaction front, usually called the reaction zone or flame (front), moves through the explosive mixture as the explosion occurs. In this zone the strongly exothermic reactions occur. Heterogeneous explosions are divided into two types: deflagrations and detonations.

In deflagrations, the reaction zone travels through the explosive mass at subsonic speed, while the propagation mechanism is heat transfer (by conduction, radiation and convection). Reaction zone propagation velocities (flame speeds) of deflagrations may vary over a wide range and so do the corresponding explosion pressures. One example of a deflagration experiment is shown in Figure 2; in this case the deflagration was very short and lasted less than one second.

In some instances, accelerating deflagrations show a deflagration-to-detonation transition (DDT) as shown in Figure 1.

The major characteristic of a detonation is its extremely high speed: the explosion zone moves at a supersonic speed. While, for deflagrations, the flame speeds are relatively low (typically one to several hundreds of metres per second), detonation flame speeds in air can easily reach one to two kilometres per second. The propagation mechanism of a detonation

is an extremely rapid and sharp compression occurring in a shock wave as one can see from Figure 3.

Probabilistic Assessment of Nuclear Power Plant Protection Against External Explosions 129

The first step of the assessment is a screening procedure in order to determine scope and content of the assessment to be performed, the second step is to propose an appropriate approach for those cases where a full scope analysis has to be conducted. In the latter case methods which can be applied to evaluate the probability of occurrence of an external explosion event are, e.g., fault tree analysis, event tree analysis and Monte Carlo simulation. The presented results show that the probability of occurrence of external explosion pressure

Since 2005, a revised guideline for a probabilistic safety assessment [2] as well as revised and extended supporting technical documents [3-4] are issued in Germany which describe the methods and data to be used in performing probabilistic safety assessment in the frame of

In these documents, probabilistic considerations of aircraft crash, external flooding, earthquakes and explosion pressure waves are required. Also on international level, new recommendations regarding external hazards including explosions pressure waves and the

For the site evaluation for nuclear installations which will be built in the future safety requirements have been developed [8-9]. In that context activities in the region that involve the handling, processing, transport and storage of chemicals having a potential for explosions or for the production of gas clouds capable of deflagration or detonation shall be

Hazards associated with chemical explosions shall be expressed in terms of overpressure and toxicity (if applicable), with account taken of the effect of distance. A site shall be considered unsuitable if such activities take place in its vicinity and there are no practicable

The safety assessment should demonstrate that threats from external hazards are either removed, or minimised or tolerated. This may be done by showing that safety related plant buildings and equipment are designed to meet appropriate performance criteria against the postulated external hazard, and by the provision of safety systems which respond to

Explosion pressure waves with relevance to the site can be caused by shipping, fabrication, storage and reloading of explosive materials in closer distances to a nuclear power

These different causes lead to two significant different types of risky situations for the site

1. The explosive material is available as a stationary source in the neighbourhood of the

and the plant which have to be assessed within a probabilistic safety assessment:

plant under consideration (e.g., a storage facility or a fabrication facility).

waves can be successfully assessed by means of the Monte Carlo simulation.

safety assessments to be performed are recently issued (see, e. g., [5-7]).

**2. Guidance on assessing external events** 

comprehensive safety reviews.

identified.

plant.

solutions available.

mitigate the effects of fault sequences.

**Figure 2.** Experiment of a deflagration according to [1]

**Figure 3.** Detonation as the strongest type of explosion according to [1]

In contrast to a reversible adiabatic compression, shock compression occurs irreversibly (non-isotropic), due to the extreme rapidity with which it occurs.

Both types of explosion pressure waves (caused by detonation of liquids or solid explosives or air-gas mixtures and such pressure waves caused by deflagrations of only air-gas mixtures) have to be taken into account in the safety assessment of the plant under consideration.

The first step of the assessment is a screening procedure in order to determine scope and content of the assessment to be performed, the second step is to propose an appropriate approach for those cases where a full scope analysis has to be conducted. In the latter case methods which can be applied to evaluate the probability of occurrence of an external explosion event are, e.g., fault tree analysis, event tree analysis and Monte Carlo simulation.

The presented results show that the probability of occurrence of external explosion pressure waves can be successfully assessed by means of the Monte Carlo simulation.
