All plant components that assume safety functions to meet the safety goals as described in Section 2.5 must be protected in such a way that they continue to perform their safety

functions even in the event of extreme floods. This calls for a specific plant protection strategy which requires flood protection measures including structural protection measures.

Structural protection measures with their flood protection structures must in principle provide permanent flood protection against design water levels. Alternatively, tempo­rary flood protection measures may be included in the safety strategy if there is sufficient advance warning time. KTA 2207 [23] defines the design water level as the highest water level that can be expected with a probability in excess of 10~4p. a., in front of the protective structure or plant component to be protected.

The main protective measures, especially in coastal areas, include dykes enclosing plant components to be protected against floods. These can be divided into inland and sea dykes, depending on the differing design water levels involved with inland and coastal locations (see also Section 5.3).

Sea dykes are particularly important, as the flood risks involved may be assumed to be relatively high (Figure 4.7) (cf. [24]). For the governing storm flood water level, which consists of the storm flood water level plus wave impact, dykes must be designed to demonstrate a sufficient dyke height, allowing for possible minor wave overflows, and sufficient stability. These proofs are influenced significantly by dyke structure (mate­rial) and cross-section with its internal and external slope angle (geometry).

As well as dyke design verification proofs, there must also be a monitoring programme to review settlements at regular intervals, for example annually, which must be assumed as the settlement forecasts with the proofs. If settlements exceed permitted levels, repairs must be made, and as the storm flood risks are greater in autumn and winter, these can only be carried out during the summer months.

5.2.1 General notes

When considering the partial safety concept, the various ultimate limit states (ULS) and serviceability limit states (SLS) must be verified: that is to say, the actions Ed must not exceed the structural resistance Rd to be considered in each case. For these limit conditions, following DIN 1055-100 [52], we distinguish between a number of effects:

— independent constant actions Gk

— independent actions of pre-stressing Pk

— dominant independent variable actions Qk1

— other independent variable actions Qki (i > 1)

— extraordinary actions Ad

— effects due to earthquakes AEd.

When designing nuclear power plants, the effects designated as external or internal actions (cf. Section 5) can be assigned to the group of ‘extraordinary actions’ or ‘earthquake actions’. These are pre-given as design values (see Section 5), so that implicitly a partial safety factor is given a value of 1.0 and the design basis earthquake to KTA 2201.1 [37] is given a weighting factor g1 to DIN 1055-100 and the importance factor to DIN 4149 [39] is taken as 1.00. All other effects are to be stated as characteristic values.

Design and Construction of Nuclear Power Plants. First Edition.

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To obtain the reference values for these actions, we need to examine different combinations to DIN 1055-100. We distinguish between design situations, as follows:

Ultimate limit state

— Permanent and temporary design situation

— Extraordinary design situation

— Design situation due to earthquake

Serviceability limit states

— Rare (characteristic) combination:

— Frequent combination:

— Quasi-permanent combination:

5.2.2 Partial safety factors and combination factors for actions

Partial safety factors g of effects may be assumed to DIN 1045-1 [21]. Recommended coefficients of combinations factors C to KTA-GS-78 [51] and DIN 25449 [15] are stated in Table 6.1.

The structural geometry around the structural waterproofing must always be defined in the knowledge of the waterproofing structure to be made, involving the specific characteristics of the waterproofing structure.

In this context, we would refer in particular to the rules and regulations for structural waterproofs using bitumen adhesives.

As well as the structural design in principle, there are a number of other factors which play a role:

— designing the concrete base and protective layers (see DIN 18195-10 [95])

— designing the structural joints (movement joints)

— structural joints in common sealed tanks

— structural joints between separate sealed tanks

— design of embedded parts.

In contributing towards covering world energy demand over a forecast period up to 2050 (Figure 1.2), nuclear energy plays a key role in generating electricity, which will mean a large number of newbuild projects worldwide. As the overview in Figure 1.4 shows, as at autumn 2009, as well as the 437 nuclear power plants already in operation, another 53 new nuclear power blocks were under construction, and another 76 new blocks were planned. The new blocks currently being built or planned mostly have (electricity) outputs from 1000 to 1600 MW. (Please note: the figures given in MW below indicate electrical energy, as opposed to thermal energy, which is stated in MWth.)

Nuclear energy now provides around 15% of the electricity generated worldwide. It avoids around 2.5 bn tonnes of CO2 emissions, so it makes a major contribution towards a sustainable electricity supply which achieves the goals in terms of economics, capability and the environment to a large extent.

For Germany, which uses wind energy relatively intensively, direct comparison shows that theoretically more wind energy was installed than nuclear in 2008 (23,300 MW as against 21,497 MW), but nuclear generated much more energy than wind, at

148.8 TWh as against 40.2 TWh. In other words, nuclear energy generates nearly 50% of baseload electricity in Germany.

Just how important nuclear energy is can also be seen from how economical it is in generating electricity. Building new nuclear power plants is relatively expensive in terms of capital costs, but the fuel costs involved (uranium), including disposal, are so low that the total cost (including disposal and end stage planning) of generating electricity is around 3-4 Euro cents per kWh [2]. This means that nuclear power is not affected by volatile fuel prices and guarantees a reliable supply, as the uranium deposits

Autumn 2009:437 nuclear power stations operational (in 32 countries)

53 nuclear power stations under construction (in 14 countries)

USA

France

it Britain Russia Canada

South Korea

Ukraine

Sweden

Spain

Belgium

irlands

Switzer and

Fig. 1.4 Generating energy from nuclear power [1]
that are worth extracting at today’s prices will be enough for more than 200 years, are spread across the world and the countries they originate in are politically stable.

The world, and Europe in particular, has recognised how important nuclear energy is when it comes to generating electricity, as the many newbuild projects show. A number of European countries, including Finland, France and Britain, have actually been building new nuclear power plants or planning them since 2005. These newbuild projects impose different requirements on structural engineering, not just in building them, but in interim and final storage and restoration work. We will look at these tasks, with their specific safety requirements, below.

The average power plant block involves erecting formwork for approx. 500,000 m2 of concrete surface [29].

Precisely in terms of time and costs, it is essential to plan the use of formwork and scaffolding beforehand, as this may affect the performance schedules that the designers produce, such as producing evidence of specific building conditions and additional reinforcement resulting. With complex construction projects, contractors specialising in planning, constructing and providing formwork are involved at an early stage.

The aim in principle is to use formwork and scaffolding elements which are as large as possible and can be used frequently, even if building power plants often involves constructing irregular shapes with variable slab thicknesses and formwork heights.

Making the cupola of a reactor building in a pressurised water reactor presents particular demands. At the Philippsburg 2 nuclear power plant, the safety enclosure of steel plate under the cupola could not withstand any major stresses, so the concreting load had to be borne by projecting formwork construction (Figure 4.17).

When building nuclear power plants, slipforming can be used not only for box — and annular-shaped sections such as chimneys, but also in building large freestanding walls, making consoles without further ado or ‘slipping in’ cutouts. The slipforming method was adopted when making the bioshield at the Kriimmel nuclear power plant. Using heavy concrete and the many cutouts involved had to be included in considerations.

The OL3 construction project used climbing formwork and/or self-climbing formwork for the more standard building structures such as safety containment and aircraft impact structures (Figure 4.18).

The compact layout of the nuclear island components calls for using a special single­headed formwork. The walls enclosing the UKA, UFA, UKS, UJH and UKE construc­tion modules are separated in some cases by as little as 30-40 cm. These narrow spaces must be kept clear at all times.

The confined working space involved rules out double-headed formwork for whichever module comes later in time. Tying and releasing ties on double-headed formwork on the outside of the enclosing wall would be impossible, as the working space required is not available.

The solution adopted in this case was therefore as follows: on the inside of the enclosing wall of the following module, the ‘Trio’ framework formwork section system by the Peri company is used, which transmits its load via Peri SB framework sections to the

lower ceiling or wall-ceiling node point of the lower level. The load is led into the concrete via cast-in tension bars.

The external formwork was made via a special steel formwork section pre-stressed against the outer wall of the preceding building.

The formwork is installed, fixed and removed from the top.

Formwork can be removed once concreting is complete without leaving parts in the join.

The best-known and most used cast-in fastenings are anchor plates of steel to which system components are welded. Where fastening points are already known at the construction stage, the anchor plates used are mainly headed stud anchorings which are built in along with the reinforcement, before pouring concrete (Figure 7.1).

Headed stud anchorings have a general approval issued by the German Institute for Construction Technology (in German; Deutsches Institut fur Bautechnik — DIBt), for general building construction, but not for accidental actions such as earthquakes.

Fig. 7.1 Anchor plate with headed stud anchorings (source: www. halfen. de)

Cast-in fastenings must be planned precisely before construction starts, defining the load to be absorbed and where the fastening points are to be positioned.

Design and Construction of Nuclear Power Plants. First Edition.

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7.1.1 Post-mounted fastenings

Post-mounted fastenings are used to transmit loads wherever detail design has not provided for any cast-in fastening points before construction has started or where new fastenings become necessary in the course of refitting work. However, there are also design challenges here: although loads, dimensions and positions of system compo­nents are known precisely, high density of reinforcement, poor access and closeness to other fastening points or structural section edges may make planning, designing and installing fastening points extremely complex. The options available for post-mounted fastenings are basically as follows:

— run-through anchors

— spreading anchors

— undercut anchors

— composite anchors

— cast-in anchors.

For safety-related fastenings, only fastenings with sufficient mechanical grip should be used [63]. With metallic anchors, this can be achieved very well by using form-locking undercut anchors, so that a general authoritative approval only exists for undercut anchors, although work is underway to achieve an approval for composite anchors in the near future.

Ageing management of nuclear power plants as built structures focuses on the structures that are safety-related. These structures are made of solid reinforced concrete, with the structural component dimensions not being governed by the effects of actions occurring involved in normal operation, but rather in terms of the require­ments of protective effects against radiation and designed to cope with internal events, such as coolant loss, or rare external events, such as earthquakes, explosion pressure waves and aircraft impact.

Design and Construction of Nuclear Power Plants. First Edition.

Rudiger Meiswinkel, Julian Meyer, Jurgen Schnell.

© 2013 Ernst & Sohn GmbH & Co. KG. Published 2013 by Ernst & Sohn GmbH & Co. KG.

Many of the factors that have to be considered as part of ageing management occur in conventional structures just as much as they do in nuclear power plants, so that the knowhow and experience of harmful mechanisms in conventional structures can be applied directly. What is peculiar to nuclear power plants are the radioactivity and increased ambient temperatures which can cause materials to decay if exposed to them.

KTA 1403 [106] divides structural systems into structures/substructures, construction systems and structural components. Structures/substructures are complete buildings or larger sections of buildings to be identified via the power plant coding system (see Section 4.2.1). Construction systems are groups of structural components that perform a common function, such as steel platforms, sealing against water pressure and structural fire protection elements. Construction systems consist of structural compo­nents such as anchor plates, fire doors and fire stop valves.

Structures/substructures, construction systems and structural components need to be classified in accordance with their safety requirements. For ageing management purposes, those that need to be considered are those that are safety-related.

Building structures and buildings need to be checked regularly to see if they have departed from nominal in any way, such as faults due to subsoil settlement or cracking, corrosion or plastics losing their seal and protective functions. Conducted as preventive maintenance, these inspections can be used to detect ageing mechanisms in good time and take appropriate follow-up measures to prevent serious damage.

Many new nuclear power plants are currently being built or are at the planning stage worldwide, especially in China, Japan and the USA; however, there are also numerous newbuild projects at the planning or construction stage in Europe, such as in England, France and Finland. For these European projects, the European nuclear power plant operators have drawn up a catalogue of requirements in the shape of the European Utility Requirements (EUR) [13].

The EUR relates to nuclear power plants as a whole, and includes details of specific nuclear power plant actions to be taken into account when designing them, such as earthquake design spectra for minimum earthquake design requirements and specific load/time functions to protect against aircraft impact. The EUR also includes basic design criteria. In terms of construction design, these criteria assume basically that Eurocode standards will be observed.

The EUR is intended to ensure that the various nuclear power plant providers can rate and offer their products based on them. Corresponding qualifications have already been drafted for system strategies for Europe, such as EPR or AP1000.

5.3.1 Earthquakes

5.3.1.1 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 (cf. Section 3.4.1) and corre­sponding national rules and regulations, such as the German KTA 2201.1 [37], which many other countries also use.

Earthquakes can be defined as shocks to solid rock emanating from an underground source (hypocentre) 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 (cf. [38]). 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 hypocentre, 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 recognised in recorded acceleration time displacements (Figure 5.2). 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 hypocentre 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 hypocentre (Figure 5.3). 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 hypocentre 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 5.2) or the EMS scale 1998 (European Macroseismic Scale). Comparing two earthquakes of the same magnitude but whose hypocentres are at different depths (shallow and deep hypocentres) shows that earthquakes are more intensive the closer their hypocentre 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 5.4).

The safety concept in DIN 1055-100 [52] must be applied for the governing design situation:

Sd < Rd (7.1)

The design actions are to be calculated as per DIN 25449 [15], which defines the specific actions for nuclear power plants and nuclear facilities.

The design resistance is measured the equation:

Rd = Rk= gM (7-2)

The partial safety factor gM of resistance must be considered more closely here, as this has to be determined, not generally but specifically for the failure mode and require­ment category. For anchors, the partial safety factor gMc for concrete failure is obtained from three other partial safety factors gc, g2 and gA:

Y Mc = Vc’V 2’Ya (7-3)

where gc is the partial safety factor for concrete, depending on the requirement category. Partial safety factor g2 reflects the installation safety of the anchor and gA is the specific anchor partial safety factor for attachments in nuclear power plants. In requirement categories A2 and A3, the only anchors that may be used are those with a high installation safety g2 = 1.0. The value gA is set to give a partial safety factor for concrete failure of gMc = 1.5 under all conditions.

Turning to steel failure, the partial safety factor gMs is determined as a function of the load direction and material properties. For tensile loads applies:

For shear loads:

g Ms = !.5

fuk > 800N/mm2 orfyk/fuk > 0.8

7.1.4.1 Installation safety

In various nuclear power plants and nuclear facilities, there are many anchors that were not installed in the correct position, so they were replaced. According to [67], the
anchors used to attach safety-related components must be designed such that they can be checked easily to verify that they have been installed correctly from easily recognisable, objective and doubtless criteria when setting and once installation is completed.

If plans are not followed, the structural engineer must be consulted.

Anchor fastenings must always be installed in accordance with the manufacturers’ instructions, but there are numbers of other conditions which must also be observed:

— Incorrect drillings and damage to existing reinforcement should be avoided by detection.

— The anchoring plate connection area should be even, which can be achieved by applying a thin mortar smoothing layer.

— The distances required from edges causing disturbances must be maintained.

— Bores must be done at right-angles.

— Incorrect drillings must be closed with high strength concrete.

As evidence that anchor fastenings have been properly installed at nuclear power plants and nuclear facilities, an installation report must be produced for each attachment which must be verified by the client/operator and by a licensed structural engineer or the construction inspector. Instructions as to the content of such reports must be taken from those for use in nuclear power plant approvals. The data to be recorded for each location of anchor fastening are:

— application to change/notice of change

— date installed

— client’s/operator’s representative

— installation contractor plus professional construction manager for dowelling

— construction inspector

— fitter (training certificate)

— building

— area

— system

— anchor plate ID no.

— layout drawing

— workshop drawing

— anchor manufacturer

— product designation

— size

— material

— tools used

— borehole checks

— clean

— right-angled

— depth

— diameter

— incorrect drilling present/closed

— reinforcement damaged

— detectable cracks/local damage

— corrosive environment

— check torque

— check anchor plate

— made to workshop drawing

— plate thickness

— axis-edge distances

— through bore diameter

— concrete surface/thickness of smoothing layer at anchor

— check surroundings

— distances from adjacent fastenings

— geometric constraints.