**3. Screening process**

130 Nuclear Power – Practical Aspects

more detail.

2. The explosive material is mobile, i.e. it is shipped in close distance to the plant on the

In the latter case, the situation is not stable and changes with the varying distances. Moreover, the transport way could be a straight line or a bent which has to be addressed in

Usually, a uniformly distributed accident probability is assumed along the transport way. However, in reality the accident probability may increase in junctions or confluences and – in case of rivers and roads – in curves or strictures. Such an example is explained later on in

Accidents with explosive material are not only theoretical considerations but happen in

Data for traffic accidents on rail or road involving explosions are provided in reference [13]. From the total number of accidents (1932) in this database 37% occurred on railways and 63% on roads. The accidents are classified into four different types: release, explosion, fire and gas cloud. The analysis has shown that in the majority of accident gas was released, followed by fires. Explosions appeared in 14% and gas clouds in only 6%. The most frequent

One extremely severe transportation accident took place in June 2009 in Viareggio which resulted in comprehensive safety evaluations [14-15]. Although no industrial plant was damaged in this accident, the potential explosion severity is visible. The accident followed the derailment of a train carrying 14 tank cars of liquefied petroleum gas. The first tank car was punctured after the derailment releasing its entire content that ignited causing an

A more recent accident happened in January 2011 on the river Rhine in Germany, fortunately without any environmental consequences. However, a ship capsized and blocked for many weeks the river for other transportation but, in particular, had the potential to lead to an explosion because – in addition to 2400 tons mainly of sulphuric acid

A further event happened on Mach 11, 2011 on the river Elbe where a transport ship had a damage of an engine and, thus, needed to be anchored outside the regular waterway. One of the questions which arise from this event was if the boundary conditions usually applied and discussed below could be violated because the ship leaves the determined waterway

For the respective nuclear power plant comprehensive investigations regarding explosions pressure waves have been performed within the periodic safety review. This includes the identification of the types of ships which are running on the river, the TNT equivalent, the real distance between the ships and the nuclear power plant. Information is in particular based on the information of the Water and Shipping Office Hamburg. This information shows that the biggest tanker ever transported gas on the river Elbe required a maximal

extended and severe flash-fire that set on fire several houses and lead to 31 fatalities.

road, by train or on ships along a river or the sea nearby.

initiating event with 73.5% of the accidents result from collisions.

reality, sometimes with catastrophic consequences.

– one tank also contained water and hydrogen.

and was, therefore, nearer to the nuclear power plant.

the calculations - see [10] for a straight road and [11-12] for a bent river.

In a first step, the important areas of the plant are divided into the three classes A, B and C for the analysis of explosion pressure waves to reflect the degree of protection against the impact by the explosion pressure waves. These classes are the same as for the consideration of aircraft crashes [16].

Class A contains systems, where in case of their damages a hazard state directly arises or where an initiating event may occur which cannot be controlled by the emergency cooling system.

Class B contains systems where in case of their damages a hazard state not directly arises, but where an initiating event may occur which is controlled by the emergency cooling system.

Class C contains the safety systems needed for core cooling.

Typical examples of these different classes are [17]:

Examples for class A are the primary coolant circuit, the main steam safety and shut-off valve equipment in case of PWR or pressure relief valves in case of BWR.

Examples for class B are the network connection with the machine transformers and auxiliary power systems (emergency case), the turbine building (main steam line break, loss of the main heat sink, loss of the main feed water) and the switchgear building. The possibility of false signals in the damage control plants with the consequence of a loss of coolant accident has to be considered.

Class C (separated emergency building) consists of buildings that are structurally designed to withstand external influences, including those buildings which are designed against external events. In general a destruction of these systems does not lead to an occurrence of an initiating event. If - in addition to emergency cooling system functions - also further system functions are located in the same building, these assumptions have to be reviewed. If necessary, the results of this review are to be considered in the analysis.

Basic idea in case of explosion pressure waves is a prescribed check if the frequency of core damage states is less than 1E-07 per year for the plant under consideration. This is the case when


 the safety distances according to the BMI guideline [18] are fulfilled, based on the formula (1):

$$R = 8 \cdot L^{1/3} \tag{1}$$

Probabilistic Assessment of Nuclear Power Plant Protection Against External Explosions 133

**4. Methods as recommended in the German PSA document for nuclear** 

The German PSA document on methods [3] describes the approaches to be used in the probabilistic safety assessment which have to be performed in the frame of comprehensive

One part of this approach is dedicated to the screening process already explained in section 2, the further parts of this document deal in more detail with the occurrence frequency of explosion pressure waves taking into account the site-specific situation, sources of possible explosion pressure waves in the surrounding of the plant, and the procedure for the calculation of occurrence frequencies of accidents during transportation of explosive material by ships, trains or trucks and of accidents of stationary plants near the plant under

In case that the plant buildings classified as A and C are designed according to the BMI guideline [18] and the safety margins regarding distance and mass of the explosive material are kept, it can be assumed that in the most unfavourable case of an explosion pressure

 due to the event explosion pressure wave a system failure occurs in the class B and an initiating event is initiated which can be controlled by the emergency cooling system as

the emergency cooling system is protected against the effects of the event explosion

In the most unfavourable case, a loss of offsite power with destruction of the secondary plant parts (main heat sink, feed water supply) can be assumed, which occurs with the total occurrence frequency of the event explosion pressure wave. It is assumed for simplifying the analysis that together with the occurrence of this event those systems which are outside

For the calculation of the frequency of the hazard state, resulting from explosion pressure waves, this initiating event and the incident-controlling functions of the emergency cooling system (stochastic non-availabilities) are to be modelled and quantified in an event tree (or

The frequency of the event explosion pressure wave to be chosen is the sum of all contributions of the events detonation and deflagration, as far as they can lead to hazardous states of the plant, resulting from accidents during transportation procedures or the

operation of stationary plants in the surrounding of the plant under consideration.

no event is initiated which directly leads to a hazard state,

**power plants** 

consideration.

wave event

**4.2. Assessment** 

designed,

pressure wave.

of the classes A and C fail.

using another appropriate method).

**4.1. Introduction** 

safety reviews of nuclear power plants.

with

R = safety distance (in m) of the place where the explosive gas is handled from to the respective plant which should be larger than 100 m, and

L = assumed mass of the explosive material (in kg).

It should be noticed that the total mass to be assumed depends on the type of explosive material.

For the case that the prerequisites of this prescribed check are met, no further probabilistic considerations are necessary.

**Figure 4.** Pressure behaviour at the building for a single pressure wave according to [18]

Otherwise the procedure has to be in accordance with the graded process of evidence regarding explosion pressure waves as presented in Table 1 (see [19]).


**Table 1.** The graded process of analysing explosion pressure waves
