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AMAZON multi-meters discounts AMAZON oscilloscope discounts 1. Overview Compared with designing conventional structures, designing system components and structural systems for nuclear power plants is subject to the maximum safety requirements, which means safety systems for managing incidents must be designed to withstand extraordinary actions at safety levels 3 and 4 (cf. Section 2, Table 2.2 and DIN 25 449). These rare and extremely rare actions are divided into internal and external actions. A summary of internal and external actions appears in Table 1. Typically, internal factors are induced by: - leaks or fractures in Pressurized pipes (e.g. jet loads and differential pressures) - problems and incidents while handling fuel elements (e.g. dropped load scenarios) - internal plant events such as fire, explosion or flood (e.g. temperature or pressure differences). External actions break down into: - natural actions which occur extremely rarely, such as 1 in 100,000 year earthquakes which occur according to KTA 2201.1 and 1 in 10,000 year flood effects to KTA 2207 - man-made actions due to specified airplane crash and explosion pressure wave. 2. Internal factors 2.1 Leaks and ruptures of pipes The impact of leaking/broken pipes must be taken into account in accordance with the underlying safety strategy for a plant. German RSK guidelines, for example, require a leak of 0.1A (where A is the open cross-sectional area of the pipe in question) to be assumed in relevant pipes, such as main coolant pipes, for example, leading to jet loads and differential pressures in combination with increasing temperatures. Jet loads are caused by the impact of the oncoming medium, and act as concentrated loads on the structural member involved. They are expressed as load-time functions or as static equivalent load, stating the impact area, load distribution and impact angle. Figure 1 shows the idealized function of jet load over time. Leaks or ruptures in pressurized pipes induce pressures in the spaces affected which act as loads per unit area over time on the structural members and pressure differentials. What has to be taken into account here is how the differential pressures behave over time, as Figure 1 shows in idealized form. ========== Table 1 Extraordinary actions (internal/external) Internal/External Events Consequences Design incidents (safety level 3) Internal actions Pressurized components leaking or broken Jet loads, differential pressures, support and retention forces, whipping pipes, debris loads, temperatures, water pressure (static) Problems and incidents while handling fuel elements Falling loads Fire or explosion inside plant Pressure and temperature differentials Flooding internally Water pressure (static) External actions Earthquake Mass forces due to self weight of structural components and fittings (components), debris loads, displacements, blast waves due to bursting pressure vessels with high energy content which are not designed against earthquake. Flood Water pressure (static) Beyond design events (safety level 4a) External actions Airplane crash Direct to the surface area hit and induced vibration, secondary impact of falling debris Explosion pressure wave Pressure load affecting the whole building structure, with pre-specified time sequence and induced vibration ========== Combined with the jet loads and pressure forces involved, leaking or broken pipes can increase room temperature and hence structural member temperature. The temperatures in the structural components affected increase subject to a time delay, so that the temperature curves in those structural members must be recorded to obtain a realistic overlap of the jet loads or differential pressures with their associated temperature effects over time. 2.2 Other internal installation events Potential problems and incidents when carrying fuel elements must be considered in the course of the fuel handling process. This mainly involves the consequences of dropping a load, which could happen while handling fuel elements in the fuel element storage pool, including loading fuel element containers or moving them around in the reactor building or interim fuel element storage. In an interim fuel element storage, for example, the possible effects of fuel element containers being dropped must be covered which could occur in the delivery area when lifting fuel element containers off carrier vehicles or in the storage area itself when moving fuel elements by crane. Other internal installation events could include fires, explosions and flooding which could also occur. This calls for specific plant studies to show where such events could occur and what might be the impact of those events in terms of differential temperatures, pressures and flooding heights. Fig. 1 Internal factors (EVI), jet loads and differential pressures 3. External actions 3.1 Earthquakes 3.1.1 General notes For any nuclear installation, the risk of earthquakes at the location concerned must be assessed in principle and it must be designed to deal with seismic effects. Details here can be found in the relevant IAEA Safety Standards and corresponding national rules and regulations, such as the German KTA 2201.1, which many other countries also use. Earthquakes can be defined as shocks to solid rock emanating from an underground source (hypocenter) attributable to natural causes. Earthquakes can be divided into a number of types, depending on what causes them: - Collapse earthquakes When underground cavities suddenly collapse - Volcanic earthquakes Incandescent molten rock rises to the surface from inside the Earth under high pressure - Tectonic earthquakes Sudden violent shifts of rock strata along geological fault lines or faults; with faults, there are three basic kinds of movement: gravity faults, upthrusts and horizontal faults. In what follows, we will concentrate on tectonic earthquakes, as they account for more than 90% of all earthquakes. The effects of such tectonic earthquakes, which induce seismic effects, manifest themselves in considerable amounts of energy being released, due to the rock strata shifting. From the earthquake hypocenter, shock waves spread out at different speeds and amplitudes, referred to as compression or primary waves (P waves) and shear or secondary waves (S waves). These shock waves can also be recognized in recorded acceleration time displacements (Figure 2). The earthquakes themselves which trigger these waves can be defined and/or quantified either by their magnitude or their intensity. Magnitude, which is normally used as local or close earthquake magnitude (ML), measures the energy released at the hypocenter of the earthquake underground. This scale was introduced by C. F. Richter in 1935, and is therefore often referred to as the Richter magnitude, or magnitude on the Richter scale. This magnitude is obtained as the logarithm of the maximum deflection of recorded seismograms, allowing for the distance to the hypocenter (Figure 3). That means each additional unit of magnitude increases the energy released by around approximately 30 times. One of the greatest earthquakes recorded to date occurred in Alaska in 1964, and reached a magnitude of around 8.8. Intensity can be defined as the impact of an earthquake at a given location on the surface of the Earth (normally a land surface) as a function of its magnitude at a given hypocenter depth. Intensity is a measure of the impact of seismic waves and dislocations at the surface of the Earth on people, objects and building structures. The strength of these effects is classified in qualitative terms based on the effects observed in a limited area. Intensity is divided into 12 degrees, which are defined as macro-seismic scales, such as the MSK scale (Medvedev-Sponheuer-Karnik; cf. Table 2) or the EMS scale 1998 (European Macroseismic Scale). Comparing two earthquakes of the same magnitude but whose hypocenters are at different depths (shallow and deep hypocenters) shows that earthquakes are more intensive the closer their hypocenter is to the surface. The level of earthquake governing earthquake design, or design basis earthquake, is given generally by the intensity to be expected for the site. In line with this site-specific intensity, with its associated ground movements (accelerations, velocities, displacements), a ground response spectrum must be defined as the basis for the further design of building structures or components. Such a response spectrum, in the form of an acceleration spectrum, represents the maximum acceleration amplitudes of the vibration of single mass oscillators with different eigenfrequencies and damping in response to a non-stationary excitation (Figure 4). Fig. 2 Earthquake waves spreading out Fig. 3 Classifying earthquakes on the Richter magnitude scale 3.1.2 Defining seismic actions When designing conventional building structures for seismic design, DIN 4149 or DIN EN 1998 gives ground response spectra as a function of rigid body acceleration and the nature of the subsoil. The rigid body acceleration is defined based on the specific German earthquake zone map and represents the intensity at a given location at an exceedance probability of 1/475 a_2_10_3/a. Other European countries have their own national earthquake zone maps. Reference earthquake standards for nuclear installations are necessarily more stringent. As opposed to DIN 4149 or DIN EN 1998, KTA 2201.1 requires a reference earthquake intensity for an exceedance probability of 1_10_5/a to be used. Establishing this calls for highly detailed studies as part of a seismological expertise. KTA 2201.1 requires the design basis earthquake to be defined based on deterministic and probabilistic analyses. The outcome of these analyses, the requirements for which are defined in KTA 2201.1 is a ground response spectrum for both horizontal axes and one for the vertical component. These spectra are taken as free field response spectra for a reference horizon normally defined as the top of the ground. ======== Table 2 Macro-seismic intensity scale MSK 1964 Intensity Observations I Detectable by earthquake recording instruments only II Felt by a few people at rest only III Felt by a few people only IV Widely felt; cutlery and windows shake V Hanging objects swing back and forth; many sleepers wake up. VI Slight damage to buildings, fine cracks in plaster VII Plaster cracks, walls and chimneys split VIII Major cracks in masonry, gables and roof cornices collapse IX Some building walls and roofs collapse; ground tremors X Many buildings collapse; cracks open in ground up to 1 m wide XI Widespread cracks in ground, avalanches XII Major changes to the surface of the Earth ======== 3.1.3 Structural analysis For earthquake design purposes, KTA 2201.1 divides components and building structures into three classes, as follows: Fig. 4 Response spectrum - Class I Components and building structures that are required to fulfill the protective goals (control radioactivity, cool fuel elements and contain radioactive substances) and limiting radiation exposure (safety-related system components and building structures) - Class IIa Components and building structures that do not belong to Class I, but which, due to their own damage and the sequential effects, possibly caused by an earthquake, could detrimentally affect the safety-related functions of Class I components and building structures - Class IIb All other components and building structures The only components and building structures for which seismic safety is required are those in Classes I and IIa. Components and building structures of Class I must be verified in terms of load-carrying capacity, integrity and functional capability, i.e. deformation or crack widths in reinforced concrete must be limited in some cases. For components and building structures of Class IIa, generally verification of load-carrying capacity will be sufficient. To verify earthquake safety, structural analyses are required reflecting the design basis earthquake and its possible consequences. Possible consequences could include the failure of high-energy containers, not designed to withstand earthquakes, such as feed water tanks in the turbine building of a PWR plant. Combined effects of earthquakes and other extraordinary actions are not generally taken into account as they are extremely rare. For structural analysis purposes, earthquake effects are to be set as the ground response spectra for the reference earthquake or compatible recorded acceleration over time curves in each case, recording the simultaneous excitation in both horizontal and the vertical direction. The subsequent superposition of parallel stress variables can be taken either as the root of the sum of the quadratics or the superposition rules as in DIN 4149 or DIN EN 1998. Structural modeling is subject to particular requirements, due to the dynamic effects and to the influence of the subsoil at the site in particular. Precise details of structural modeling, including details of structural damping and subsoil modeling can be found in KTA 2201.1, KTA 2201.2, KTA 2201.3 and KTA 2201.4. In principle, the structural models to be used for the building structure, including the subsoil for the plant components with their support structures are those which record how the structures behave in the governing frequency range of an earthquake. Depending on the purpose of verification involved, it must be decided whether structural modeling requires a level beam model or a spatial beam model or even a spatial surface structure model, allowing for possible decouplings between the building structure as a whole and part structures or decoupling criteria between the building structure as a whole and components. As far as the dynamic behavior of the structure is concerned, the influence of the interaction between structure and subsoil (subsoil-structure interaction) must be taken into account, varying the soil characteristics to give a lower, medium and upper subsoil strength. The results of the calculations at different subsoil strengths must then be included. The structural analyses can be carried out using the usual dynamic calculation methods, including in particular the response spectrum method, frequency range method, time history method and the quasi-static method as a simplified method. These are generally used as linear methods. Non-linear methods such as non-linear time history methods are also used in exceptional cases. The result of the dynamic structural analyses, as well as eigenfrequencies, is to give the internal forces and deformation variables required to assess the strength and deformation behavior of the structure studied. Response spectra can also be calculated at the intersections with other building structures or components to use these to analyze the building structures or components meeting at these nodes. The resulting method to be used in conducting structural analyses of building structures and components with a view to using response spectra is therefore as follows: - Specify the site excitation as ground response spectra or time history (primary response/primary spectra) - Calculate the response over time or response spectra of the structure (secondary response/secondary spectra) - Calculate the response over time or response spectra for system components (tertiary response/spectra) Fig. 5 Response spectrum method (building structure/components) 3.2 Floods 3.2.1 General notes As we saw in section 4.2.6, protecting nuclear power plants against floods as in KTA 2207 involves allowing for a reference flood level with an exceedance probability of 10_4/a, often also referred to as a one in 10,000 year flood. By way of comparison: normal flood protection is based on a flood occurring at a frequency of 10_2/a (100 year flood); one in 10,000 year floods are only considered for high risk potential systems, such as dams. The methods used in calculating the reference flood level with an exceedance probability of 10_4/a for inland and coastal sites, including sites on tidal flows (such as the upper Elbe or Weser rivers) are different. For coastal sites, the reference water levels can be determined directly using storm tidewater levels. For inland sites, on the other hand, we need to calculate the flood runoff from which we can then obtain the design basis water levels using suitable methods. KTA 2207 describes methods both for determining the design basis flood runoff at inland sites and to determine storm tide water levels. 3.2.2 Inland sites For inland waterways, KTA 2207 assumes a flood runoff with an exceedance probability of 10_4/a. This flood runoff can be determined either purely on a basis of probabilities or by extrapolating from statistics available. KTA 2207 uses this extrapolation, which is based on the Kleeberg and Schumann method. This extrapolates from a peak level water runoff with an exceedance probability of 10_2 /a to a peak level water runoff with an exceedance probability of 10_4 /a. This flood runoff value obtained, finally, gives the design basis water level from a corresponding water level runoff relationship for the location concerned. 3.2.3 Coastal sites KTA 2207 defines the reference water level for coastal sites and sites on tidal flows as a storm tide water level with an exceedance probability of 10_4/a. This storm tide water level SFWHð10_4 Þ can be obtained using suitable but highly laborious probabilistic methods, which can also be used to determine flood runoffs. Alternatively, according to the annexe to KTA 2207, a probabilistic based extrapolation method can be used, taking the storm tide water level SFWHð10_4 Þ as the total of a base value BHWð10_2 Þ and an extrapolation difference ED as follows: The design basis water level BHWð10_2Þ with an exceedance probability of 10_2/a is calculated here based on a quantitative statistical extreme value analysis. The spread of the results with the usually long, good-quality time series of water levels on the coasts and in tidal flows is relatively low. Determining the extrapolation difference ED calls for detailed studies of the coastal or estuary levels of the tide flows concerned. At the water gauge sites of Cuxhaven and Brokdorf on the river Elbe, for example, this gives an extrapolation difference ED of the order of 100-150 cm. With dykes, as well as the storm tide water level SFWH(10_4) the wave run-up must also be taken into account (Figure 6) and, having superposed these two variables, the dykes must be designed without waves breaching them or a possible breaching wave putting the stability of the dyke at risk. The wave run-up height at the dyke depends not only on the wave height and wave period, but also on the characteristics of the dyke itself, such as its slope or surface area. When calculating wave heights, it should be borne in mind that these are particularly subject to local wind speed and direction and to the topography of the foreshore. Fig. 6 Sea dyke as flood protection for nuclear power plants 3.3 Airplane crash 3.3.1 General notes Airplane crash must be considered as an exceptional, extremely rare event which, unlike earthquakes or floods, is not rated as an anomaly at safety level 3, but as a beyond design system status condition at safety level 4. An airplane hitting a building has dynamic effects on that building which can be defined as a load over time function. It is appropriate here to distinguish between the different dimensions of military aircraft (small compact) and commercial ones (large). Crashing fast-flying military aircraft was included as a fundamental design event when building new nuclear power plants in Germany, particularly after military aircraft (mainly Starfighter) crashes piled up in the 1970s. In the first instance, therefore, a load over time function was developed for a Starfighter crash and used as the basis for design. Even while designing the Convoy plants and their immediate predecessors, known as pre-Convoy plants, it had been decided to use a more robust design based on a Phantom F-4 crashing at a speed of 215m/s. The requirements involved, including the load over time function, can be found in the RSK guidelines, and became the design standard for German nuclear power plants since the Convoy and pre-Convoy models. Unlike Germany, other countries - with a few exceptions - did not allow for the impact of a fast flying military aircraft when designing and building nuclear power plants. That was evidently because such a scenario was highly unlikely, and the additional construction costs were high. When terrorists flew aircraft into the World Trade Center on 11 September 2001, however, ideas about using airplane crash as a basic design principle changed. Many countries, especially in Europe and the USA, now take airplane crash into account when building new nuclear power plants. It may be assumed that Europeans require new installations to be designed to withstand the impact of both military and commercial aircraft. When designing for airplane crash, it should be borne in mind that redundantly proposed building which are physically separate need not be designed expressly for aircraft impact, as the redundancy means that a aircraft impact can only destroy one of those buildings. 3.3.2 Load over time functions The dynamic effects of an airplane crash give rise to load over time functions which depend on the type of aircraft involved (weight, geometry, impact area) and how fast it is travelling when it hits the building (impact velocity). The load over time function must show in each case that the building affected can withstand the loads, both locally (punch-through) and globally (stability, load bearing to foundations) and that the shock induced by the impact does not damage structural members or components inside the building. We can derive the load over time function by using the RIERA model. This assumes a 'soft impact', that is a rigid wall and the impacting body then deforming. This assumption can be justified by the fact that the buildings concerned are made of solid reinforced concrete with very thick walls (generally_1.50m) and the aircraft body may be taken to be very yielding compared with the building. On a soft impact basis, the reaction force as the ordinate of the load over time function consists of two components: a bursting load component and a component as the product of the aircraft weight and the square of its velocity. The quadratic component shows how important the velocity assumption is. Fig. 7 Aircraft impact, load over time function of a military aircraft (Phantom F-4) Figure 7 shows the load over time function obtained using the RIERA model for a Phantom F-4 hitting at 215m/s as mentioned above. The tests conducted on this in Sandia confirmed this theoretical function: it matches the function specified in the RSK guidelines, and is often used as a design principle when building new nuclear power plants in Europe. The RIERA model can also be used to derive load over time functions for commercial aircraft impacts. Compared with a military aircraft impact, the load over time functions obtained for larger commercial aircraft flying at 100-150m/s give rise to much higher maximum loads and greater pulses accordingly. As a commercial aircraft would have a much larger impact area, on the other hand, the local surface area loads are much less than those of a military aircraft, so that where a military aircraft hits would be much more decisive than a commercial aircraft when conducting the punch-through proof required. It has also been found that the much larger pulse of commercial aircraft in general induces much greater induced vibrations in a building than a military aircraft. 3.4 Explosion pressure wave (chemical explosion) Like an airplane crash, an explosion pressure wave is rated as an extremely rare event (safety level 4), and thus qualifies as beyond design system status. An explosion pressure wave is a chemical explosion in the form of a deflagration (pressure rising relatively quickly, building up reflected pressure). It may be caused by using explosives or if a high-energy container bursts, so that an explosion pressure wave must be accepted as a design basis when carrying hazardous cargos by rail, water or road and when storing containers with high energy content. A chemical explosion causes pressures on the building concerned and induced vibrations in that building. The external explosive loads due to air pressure waves give an explosion pressure which can be expressed in time and place terms as follows: p = ps þ c _ q where ps is the compression pressure, including reflected increase q is the velocity pressure (dynamic pressure) c is a coefficient of form With box-shaped buildings (non-slender structures), the c _ q component may be ignored; with slender structural sections, the explosion pressure can be treated as a static wind load c _ q as defined in DIN 1055-4. For more details of using this function for explosion pressure see DIN 25 449. Fig. 8 Explosion pressure wave to BMI guidelines As a general rule, if no more precise local studies are available, possible explosion pressure waves can be established using the pressure wave in the BMI guidelines. This function, as shown in Figure 8, is specified in the RSK guidelines for PWRs, and represents a conservative assessment of potential explosion pressure waves. This approach assumes that the pressure wave can come from any given direction and that there is a level pressure front. |