**7. Countermeasures to avoid or mitigate the adverse effects of external explosions**

Knowledge of the explosion characteristics and the structural impact on buildings of the respective plant is necessary to determine the appropriate countermeasures in order to ensure a safe operation of the nuclear power plant. However, fundamental changes of the plant under consideration are mainly possible only during the design and construction phase.

Basic features of the loads induced on structures by air blasts are described in IAEA Design Guide [5] in terms of a normalized distance that takes into account the amount and type of the explosive charge. The guide presents charts that allow the determination of the peak value of the incident pressure, the total impulse of the positive phase and other relevant design parameters, which are generally used for design or verification purposes of sensitive nuclear structures. For the determination of the resulting actions on structures subjected to a specified blast event, the load-time functions induced by the incident pressure wave must be evaluated in the next step.

In general, it is impossible to protect structures from all man-made and natural hazards. However, assessing the possible damages caused by a defined hazard enables risk-informed decisions about the kinds and number of design changes needed to effectively protect the relevant structures of the nuclear power plants.

Probabilistic Assessment of Nuclear Power Plant Protection Against External Explosions 149

One organizational possibility is to interdict the transport of explosive material, e.g. on a road, in the neighbourhood of the plant. Another solution is to close the road for transit

One technical countermeasure to reduce the explosion frequency on site is the installation of an automatic ignition system placed at a save distance from the site. An assessment has been performed for such an installation which showed that – if the igniters are correctly designed and installed – the shock wave impact after an ignition on the buildings will be limited and

The evaluation of external hazards in relation to nuclear power plant design is traditionally considered as a two-step process. The detailed evaluation is preceded by a screening phase where potential scenarios are identified. Many scenarios are screened out on the basis of different criteria, such as distance from the site, probability of occurrence, expected consequence on the plant, or because their effects on the plant are expected to be enveloped by some others. Typically, explosion pressure waves are part of the probabilistic safety assessment as in case of comprehensive periodic safety

In the German safety guidance document on methods [3] the screening process for the explosion events is explicitly described. The classes of buildings with respect to their protection are the same as for the aircraft crash assessments. Since the updated PSA guideline has been issued in 2005 also requiring the assessment of external events, first practical experience in performing and reviewing the external probabilistic safety assessments are available. One topic is the assessment of the conditional probability of the occurrence of external explosion pressure wave and the discussion of appropriate methods

The presented case study and its results (provided in Figures 10, 12, 14 and Tables 4, 5, 6) in the second part of this paper indicate that the conditional probability of occurrence of external explosion pressure waves in consideration of realistic conditions (accident frequency depending on environmental conditions, wind direction and wind speed) can be

As a next step the assessment of explosion events should be extended to include much more

Different ignition models are discussed in [32]. The applied model should be more realistic like the applied exponentially-distributed ignition model; moreover the applicability to

successfully assessed by means of the Monte Carlo simulation.

the extent of the hazard and the explosive gas mixture,

the ignition probability that depends on environmental conditions [31].

integrate the new ignition model into the Monte Carlo algorithm should be given.

traffic such that the road is only leading to the nuclear power plant.

will not cause any structural damage.

**8. Concluding remarks** 

according to the state of the art.

realistic boundary conditions regarding

reviews.

This is, in particular, required for the designs of nuclear power plants which are currently under construction or even in the planning phase. Such a hazard assessment has been recently performed [29-30] and for this case a detonation at the highway close to the nuclear power plant has been postulated. For the scenario, the maximum overpressure caused by the explosion has been determined to check if the plant could survive the detonation without damage.

Figure 15 shows the particle velocity field in the pressure wave just before the wave front arrives at the plant under consideration.

**Figure 15.** Velocity field at the pressure wave front just before shock wave arrival at the nuclear power plant according to [30]

In case of a plant already operating since several years, the implementation of effective countermeasures is much more difficult or even not possible.

On the one hand, comprehensive calculations can be performed to show that existing assumptions in the calculation provided for the licensing of the plant have been very conservative.

On the other hand, organizational and technical provisions can be taken to reduce the occurrence of an external explosion pressure wave at the plant.

One organizational possibility is to interdict the transport of explosive material, e.g. on a road, in the neighbourhood of the plant. Another solution is to close the road for transit traffic such that the road is only leading to the nuclear power plant.

One technical countermeasure to reduce the explosion frequency on site is the installation of an automatic ignition system placed at a save distance from the site. An assessment has been performed for such an installation which showed that – if the igniters are correctly designed and installed – the shock wave impact after an ignition on the buildings will be limited and will not cause any structural damage.
