EXTERNAL HAZARDS – SCREENING
PROCEDURES AND PROBABILISTIC
ASSESSMENT
Berg H.P., Görtz R., Schimetschka E.
Bundesamt für Strahlenschutz (BfS), Willy-Brandt-Str. 5, D-38226 Salzgitter, Germany
Abstract: In particular in case of probabilistic analyses of external hazards, the as
-sessment can be very detailed and time consuming. Therefore, it is necessary to have procedures to screen out, e.g., rooms or buildings of a complex plant where no fur -ther detailed analysis is required or to have a graded procedure for the respective hazard taking into account plant- and site-specific conditions. Examples for such screening procedures are provided for rooms and buildings of a nuclear power plant for the external hazards aircraft crash, external flooding, explosion pressure waves and seismic hazards.
1. Introduction
International experience has shown that internal hazards such as fires and internal flooding and external hazards (e.g. aircraft crash, floodflooding) can be safety significant contribu tors to the risk in case of nuclear plants` operation. This is due to the fact that such haz -ards have the potential to reduce simultaneously the level of redundancy by damaging redundant systems or their supporting systems. Thus, the regulators expect the licensees to justify their arrangements for assessing the vulnerability of plant and structures, deter mining how the safe operation of a plant is affected, and introducing measures to pre -vent a hazard developing and mitigate against its effects if it should nevertheless de-velop.
Methods to analyse existing plants systematically regarding the adequacy of their exist-ing protection equipment against internal and external hazards can be deterministic as well as probabilistic.
The German PSA Guide, issued in 1997, contained reference listings of initiating events for NPP with PWR and BWR respectively, which have to be checked plant specifically with respect to applicability and completeness. Plant internal fires and plant internal flooding were included in these listings, but not explicitly external hazards.
Detailed instructions have been provided in technical documents on PSA methods which have been developed by a working group of technical experts from nuclear industry, au-thorities and technical safety organizations chaired by BfS, also provided in 1997. In October 2002, the Commission on Reactor Safety of the States Committee for Atomic Nuclear Energy has agreed to a new draft of the PSA Guide, in December 2002 the li-censees have explained their comments on this guide. An updated draft has been com-pleted in September 2004. The corresponding technical documents have been revised. The further steps have been the discussion of the whole sets of documents in the respec -tive committees including the German Reactor Safety Commission.
Regarding external hazards, the updated probabilistic safety assessment guidance docu-ments require probabilistic considerations of aircraft crash, external flooding, earthquake and explosions pressure waves. The respective documents have been issued in autumn 2005 [1 – 3].
Appropriate screening procedures are those which on the one hand allow to constrain the complexity of the analysis and, on the other hand, ensure that relevant information are not lost during the screening process and all safety significant parts of the plant are taken into account. These screening procedures are described in [2] in more details.
2. Approaches to screening criteria for external hazards
As already indicated, also for the external hazards the first step is a screening process in order to determine scope and content of the assessment to be performed. The approach for these screening processes is different for each type of external hazard.
The German updated probabilistic safety assessment guidance documents require proba-bilistic considerations of aircraft crash, external flooding, earthquake and explosions pressure waves. The proposed screening criteria for these four external hazards are shortly indicated taking into account the German regulatory framework for nuclear power plants.
2.1. Aircraft crash
The consideration regarding this hazard does as not cover an intended aircraft crash. Both, crashes of military aircrafts and of commercial aircrafts contribute to the plant risk. The location of the plants is important, both with respect to the distance from a nearby airport ant to a close-by airlane and also, whether it is situated in an area of land-ing and take-off traffic. Because of the central position of Germany within Europe, there is a close-meshed net of civil airlanes with a high density of flights. Moreover, German and Allied Air Force units are stationed in Germany, thus there might be a nonnegligi -ble hazard due to aircrafts crashing on nuclear sites. However, the statistics on crashes of military aircrafts shows a significant decrease in the last decade (cf. Fig. 1).
Fig. 1. Number of crashes of military aircrafts in Germany
German nuclear power plants can be divided in three generations with respect to aircraft crash depending on the load assumptions which had been the basis for the structural de -sign of the building structures to protect against hints of aircrafts or wracked plane parts. The consequences of hints in case of buildings which are not protected depend on the plant specific layout of buildings and systems.
The latest requirements with respect to aircraft crash are laid down in a document of the German Reactor Safety Commission issued in 1981 [4]. A load function for buildings to be protected (reactor building etc.) has been defined mainly based on theoretical calcu -lations assuming an impact of the military aircraft “Phantom F4” (Fig. 2).
Fig. 2. Load-time diagram (RSK-Guideline)
The graded process for the extent of the analyses to be performed in the case of aircraft crashes is described in Table 1.
Table 1. The graded process of evidence regarding aircraft crash impact
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 198419851986198719881989199019911992199319941995199619971998199920002001200220032004 year n u m b er
Criterion Extent of analysis
Structures designed according to the state of the art (RSK-Guideline) and not located in a military zone for fly maneuver drills
Analysis is not necessary Contribution is negligible compared to the
other contributions in the PSA A conservative rough-analysis regard-ing the consequences of impact on im-portant areas A, B, C where
A: e.g. primary circuit B: e.g. turbine building
C: separated emergency building
Not negligible Detailed probabilistic analysis on
plant areas (A, B, C) e.g. by Monte-Carlo-methods
2.2. External flooding
External flooding belongs to a class of external events, which are caused independently of human influence, but the subsequent possible damage is determined to a large extent by human factors, such as dam design, plant site selection, plant design etc.
With respect to the phenomena leading to a flooding event, the German nuclear power plants can be divided into two basic categories: “River-Site NPPs” and “Tidal-River NPPs”. In the first case a high water-level situation arises from an unfavourable ratio of water inflow to outflow, in the second case the coincidence of storm, flooding and high tide is the determining factor. In the proposed method, the frequency of reaching ex -tremely high water levels is determined by an extrapolation of actually measured water-level data according to various established methods [5, 6]. The underlying probabilistic considerations and the procedures how to calculate the exceedance frequencies has re-cently been developed and issued in November 2004 as part of the German Nuclear Safety Standard KTA 2207 “Flood Protection for Nuclear Power Plants”.
Two of the substantial modifications and innovations are listed as follows:
The design of the protection of nuclear power plants against flooding emanates from a rare flooding event with an exceeding frequency of 10-4/a, but it is underlined that the methods used to determine the design water level must be different for river sites with-out and for sites with tidal influences. For river sites withwith-out tidal influence, the design water level can be assessed using the flood flow of the river with the given exceeding frequency as basis. For river sites with tidal influences, an extreme tide caused by storm (storm-tide) must be assumed. Therefore, it is prescribed to determine statistically the storm-tide water level with an exceeding frequency of 10-2/a plus a site-specific addend. In conclusions, a storm-tide must be covered with an exceeding frequency of 10-4. The loads due to the design flooding must be combined with other loads:
external loads of normal usage (e.g. dead load, live load, operational loads, earth thrust, wind load),
level, streaming water, waves, upswing, flotsam, ice pressure),
loads of events as a consequence of the design flooding (e.g. undermining, erosion). The graded approach for external flooding can be summarized as given in Table 2.
Table 2. The graded process of evidence regarding external flooding
Criterion Extent of analysis
Flooding of plant site can be practicable ex-cluded due to the NPP grounded level com-pared with surroundings level
No analysis necessary 1. The plant is designed against design
flooding with an exceedance probability of 10-4 per year
2. Design with permanent protection mea-sures
3. Shut down of the plant according to the instructions of the operation manual at a specified water level which is signifi-cantly below the level
4. Conditional probability for water impact in case of design flooding less than 10-2
Determination of possible water paths in relevant structures and estimation of the conditional probability for water impact in case of the design flooding
Other design Determination of the exceedance for
the design flooding of the plant up to a value of > 10-4 per year, detailed event sequence considerations including the quantification of core damage fre-quency
2.3. Explosion pressure wave
Pressure waves come from external events, caused by shipping, fabrication, storage and reloading of explosive materials in closer distances to the NPP. 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, are distinguished.
As in the case of the consideration of aircraft crashes, the important areas of the plant is divided into the same there classes A, B and C for the analysis of the explosion pressure waves.
Basic idea in case of explosion pressure waves is a prescribed check if the frequency of care damage states is less than 10-7 per year for the plant under consideration. This is the case when:
the total occurrence frequency of the event “explosion pressure wave” (i.e. the sum of all contributions from detonation and deflagration) is determined to be less than 10-5 per year,
Fig. 3,
the safety distances according to [7] are fulfilled, based on the formula 3 kg L m 8 R (1) where:
R - safety distance which should be larger than 100 m, L - assumed mass of the explosive material.
0 1 0 0 2 0 0 0 , 3 0 0 , 4 5 Z e i t n a c h B e g i n n d e s D r u c k a n s t i e g s ( m s ) Ü b er d ru ck a m G eb ä u d e ( b ar )
Fig. 3. Pressure behaviour at the building for a single pressure wave
For the case that the prerequisites of this prescribed check are met, no further probabilis -tic considerations are necessary, otherwise the procedure is in accordance with the graded process of evidence regarding explosion pressure waves given in Table 3.
Table 3. The graded process of explosion pressure waves
Criterion Extent of analysis
1. Occurrence frequency <10-5/a
2. Design of areas A and C according to load assumptions and safety distances deter-mined in [7]
Verification using the prescribed check
1.Not fulfilled
2.Fulfilled Conservative estimation of occurrence fre-quency
1.Not fulfilled Detailed probabilistic analysis
Time after start of pressure increase
O ve rp re ss ur e at th e bu ild in g
2.Not fulfilled 2.4. Seismic hazard
The German NPPs are located in regions of different seismic activities. Compared with other regions, for instance, California and Japan, the seismic activity in Germany is about 2 orders of magnitude lower, but still not negligible and has therefore to be as-sessed in site specific safety considerations.
Table 4. The graded process of evidence regarding the event “earthquake” according the actually determined intensity I of the earthquake at the plant site
Criterion Extent of analysis
I < 6 Not analysis necessary
6 < I < 7 As a first step, a plant walk down has to be conducted. Indication of unsufficient margins to cope with earthquake loads have to be evalu-ated on the basis of existing proofs. Eventually, further investigations or measures for safety improvement are necessary
I > 7 Analysis using the fragility curves approach The proposed method for a probabilistic earthquake analysis for an intensity > 7 follows corresponding analyses already performed in the USA and other countries. The analysis is based on calculations that have been performed within the deterministic earthquake analysis of the buildings and components of the plant for design and licensing purposes. For all buildings and components of the highest class in the classification systems of structures, systems and components, corresponding to the German KTA rule 2201.1, a fragility analysis is done, determining the relationship between the peak ground acceler -ation and the probability of failure.
3. Concluding remarks
The approach for a probabilistic assessment of external hazards to be applied within comprehensive safety reviews of NPP in Germany starts with a screening process which should not be too conservative so that an unmanageable number of scenarios and build -ings remains for the detailed quantitative analysis. However, it has to be ensured that all relevant areas are investigated within the quantitative analysis. This screening procedure is different for the different types of hazards.
However, for those structures, systems or components which have not been screened out or where a coarse meshed analysis is not sufficient the second step is to perform a quan
titative analysis. In a final step, the frequency of initiating events induced by the respec -tive hazard, the main contributors and the calculated core damage frequency are deter-mined.
However, it should be underlined that the probabilistic assessment of external hazards, as an important part of PSA, has not yet achieved the same level of methodological ma-turity as being typical for other disciplines of PSA. Therefore, it is intended to conduct a kind of a pilot study to get feedback from these analyses for an improvement of the guidance documents.
References
1. Bundesministerium für Umwelt, Naturschutz und Reaktorsicherheit (BMU): Sicherheitsüberprüfung für Kernkraftwerke gemäß §19a des Atomgesetzes – Leifaden Probabilistische Sicherheitsanalyse, 31. Januar 2005, Bekanntmachung vom 30. August 2005, Bundesanzeiger Nr. 207a vom 03. November 2005
2. Facharbeitskreis Probabilistische Sicherheitsanalyse für Kernkraftwerke: Methoden zur probabilistischen Sicherheitsanalyse für Kernkraftwerke, Stand: August 2005, BfS-SCHR – 37/05, Salzgitter, Oktober 2005
3. Facharbeitskreis Probabilistische Sicherheitsanalyse für Kernkraftwerke: Daten zur probabilistischen Sicherheitsanalyse für Kernkraftwerke, Stand: August 2005, BfS-SCHR – 38/05, Salzgitter, Oktober 2005
4. RSK-Guidelines for Pressurized Water Reactors 1981. 3rd Edition, October 18, 1981, amended 1982 and 1984
5. Kleeberg H.-B., Schumann A. H.: Ableitung von Bemessungsabflüssen kleiner Überschreitungswahrscheinlichkeiten, Wasserwirtschaft, V.21, Nr. 2, p.90 -95, 2001 6. Jensen J. et. Al.: Neue Verfahren zur Abschätzung von seltenen
Sturmflutwasser-ständen, HANSA, Hamburg 11/2003
7. Federal Minister of the Interior: Richtlinie für den Schutz von Kernkraftwerken gegen Druckwellen aus chemischen Reaktionen durch Auslegung der Kernkraftwerke hinsichtlich ihrer Festigkeit und induzierter Schwingungen sowie durch Sicherheitsabstände, BAnz Nr. 179, Sept. 1976
