**4.2. Assessment**

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 wave event


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 of the classes A and C fail.

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 using another appropriate method).

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.

The occurrence frequency of a detonation is several orders of magnitude lower compared with a deflagration [20]. As far as the distance of the area where the deflagration started has a distance larger than 100 m from the plant under consideration (see safety margins in accordance with [18]), no endangerment of the plant buildings has to be assumed.

In case of accidents with materials with the potential of a detonation (in particular explosives, ammunition, gases exothermically disintegrating) the detonation is expected to occur at the accident location, i.e. at a transport route or a fixed industrial installation. Here the approach as provided in formula (2) is applied:

$$H\_{E,SMZ} = H\_{\text{UL,SMZ}} \cdot W\_Z \tag{2}$$

Probabilistic Assessment of Nuclear Power Plant Protection Against External Explosions 135

In the case of a deviation from the BMI guideline [18] partial results of the total occurrence frequency of the event arise which contribute directly to the frequency of the hazard states. These contributions are to be determined by a differentiated view of the assigned explosion

**5. Occurrence frequency of accidents during the transport of explosive** 

One important input for the calculations is the occurrence frequency of accidents during the transport of explosive material with different transportation means. Information has to be gathered from the competent institutions in the respective country. As an example the

According to the accident statistics of the German Railways there was in Germany in the time frame of 10 years no accident of dangerous goods transports with explosive materials. From the zero-error statistics, there is the expectation value for the current admission rate of accidents in Germany in dangerous goods transports by rail with explosive materials

,

,

*UEG B*

explosive materials by rail in the vicinity of the nuclear power plant, LE,B train transport kilometers per year with explosive materials,

a minimum distance of the railway line to the nuclear power plant,

hazardous situation for the nuclear power plant.

The section length l can be calculated from

2 10 *UEG B <sup>h</sup>*

*h H n l*

HUEG,B yearly frequency of accidents in case of transports of dangerous goods with

n number of transports (trains) per year with an explosive good passing the nuclear

l section length l along the nuclear power plant (e.g. l = 2 km) which could lead to a

1

,

, *UEG B*

*E B*

*<sup>a</sup>* (4)

*<sup>L</sup>* (5)

2 2 *l ra* 2 (6)

pressure waves and their effects.

approach in Germany is shortly described.

**5.1. Train accident statistic** 

Thus, HUEG,B is defined as

**materials** 

(hUEG,B):

with

with

power plant,

with

HE,SMZ Annual frequency of a explosion pressure wave by explosives, ammunition or gases exothermically disintegrating in the surroundings of the nuclear power plant,

HU,SMZ Annual frequency of accidents with explosives, ammunition or gases exothermically disintegrating in the surroundings of the nuclear power plant,

WZ Conditional probability of the ignition in case of an accident.

The deflagration pressure of maximal 10 bar drops over 100 m around a factor 1E04, so that within the power station pressure values within the range of the wind pressures are reached.

In case of explosive gas air mixtures (combustible gases with air; inflammable steams, e.g. also of liquid gas, with air) clouds can appear and a drifting of these clouds from the place where the accident happened into the direction of the plant is possible.

In this situation the deflagration can take place in the area of the plant buildings. The approach applied for this case is described in the following equation [20]:

$$H\_{E,GLG} = H\_{\rm{ul,GLG}} \cdot \mathcal{W}\_{\rm{M}} \cdot \mathcal{W}\_{\rm{D}} \cdot \mathcal{W}\_{\rm{Z}} \tag{3}$$

with

HE,GLG Annual frequency of an explosion pressure wave by gas air mixtures in the surroundings of the nuclear power plant,

HU,GLG Annual frequency of accidents with combustible gas in the surroundings of the nuclear power plant,

WM Conditional probability for the development of an explosive gas air mixture in case of an accident with combustible gas,

WD Conditional probability for drifting of the gas air mixture to the nuclear power plant (as a result of temporal averaging of the arising wind directions),

WZ Conditional probability of the ignition at the area of the plant.

In a more detailed verification the assumptions introduced can be replaced by plant-specific proofs, considering the different effects of the determined explosion pressure waves.

In the case of a deviation from the BMI guideline [18] partial results of the total occurrence frequency of the event arise which contribute directly to the frequency of the hazard states. These contributions are to be determined by a differentiated view of the assigned explosion pressure waves and their effects.
