3.2.2.1 Raft foundations

As a general rule, as with conventional power plants, nuclear power plant structures are laid on raft foundations, but the demands on the subsoil are extremely high, especially under the reactor building, with its high permanent loads combined with the exceptional actions of aircraft impact or earthquakes. Soil compression from perma­nent loads alone often reaches levels of approx. 500 kN/m2. If an earthquake occurs, or an aircraft hits a station, levels could exceed 1000 kN/m2.

This high level of soil compression often also calls for additional theoretical studies looking for weak points, such as cavities, in the soil; these have a major influence when designing the slab on ground. Such non-constant soil conditions must also be taken into account when considering the soil-structure interaction when designing to resist earthquakes.

For verifications of the serviceability limit states, the partial safety factors for the resistance are generally to be taken as 1.00.

a) 1.00 at favourable effects

b) 1.35 if effect variable can be determined very precisely

c) 1.00 if a linear calculation is used and the rigidity in the structure can be reduced (by cracks forming or relaxation, for example) (see Section 7.2)

d) With category A3 requirements (cf. Section 6.3) the crane load can be ignored as a variable action, i. e. C = 0

The partial safety factors to determine the structural resistance of the ultimate limit states depend on the design situation (permanent and temporary, extraordinary) of the building materials used (concrete, concrete steel, pre-stressed steel, construction steel) and the demands on the structure or structural member in question.

Safety-related structural members are subject to different requirements under these effects. Factors to be taken into account here include:

— Chances of their occurring during working life

— Repair options available

— Limiting the extent of the damage, such that the structural members remain fit for use and system components remain intact and operational.

With these aspects in mind, requirements when designing structural components of nuclear installations are divided into three requirement categories, A1, A2 and A3. These are defined irrespective of the building materials involved as shown in Table 6.1.

Requirement category A1

Those combinations of physical effects corresponding to the permanent and temporary design situations in accordance with DIN 1055-100 will be assigned to requirement category A1. The partial safety factors specified in DIN 1045-1 for the load-bearing capacity regarding permanent and temporary design situations will be assigned to these combinations.

Requirement category A2

Following the method described in DIN 1055-100, those combinations of physical effects that comprise extreme design situations, which must be assumed to occur several times during service life, are assigned to requirement category A2. It must be ensured that the building elements designed accordingly are continuously useable after occurrence of these combinations. In regard to the stability or functional safety of plant components, additional requirements may have to be specified for individual locations (e. g. limit values for deformations and crack widths).

Requirement category A3

Combinations of physical effects comprising extreme design situations with a low probability of occurrence (internal or external events, < 10~4 per year) which must be assumed to occur once during service life will be assigned to requirement category A3. The forming of large cracks and permanent deformations is permitted, provided, these are not prohibited for safety-related reasons. In regard to the stability or functional safety of plant components, additional requirements may have to be specified for individual locations (e. g. limit values for deformations and crack widths) that go beyond the minimum requirements with regard to the load-bearing capacity.

Partial safety factors of structural strength for structural components of concrete, reinforced concrete and pre-stressed concrete in requirement categories A1, A2 and A3 to KTA-GS-78 [51] and DIN 25449 [15] are shown in Table 6.2. Table 6.3

contains the partial safety factors for structural members of steel as recommended in KTA-GS-78.

Structural waterproofing is best designed in two chronologically separate phases [96]:

— inspection and permitting design phase

— execution design phase.

8.3.4.1 Inspection and permitting design phase

Inspection and permitting design is part of the construction and Atomic Energy Act approval procedure. Designs must generally pass inspection before they can be approved.

Inspection and permitting application documents should contain, as a minimum requirement:

— details of the structural waterproof

— layout plan

— overview drawings

— standardised design details

— list of annexes.

The structural waterproofing design should include:

— a list of the structures with structural waterproofing

— foundation depths

— details of ground surface level, power plant zero level, design water levels and flood water levels, design water levels, high water levels (permanent high water level to KTA rule 2207 [23])

— details of waterproofing strategy

— details of waterproofing method

— service and special loads

— design rules for service loads

— making penetrations

— verification of suitability.

8.3.4.2 Execution design phase

As the structural waterproofing execution design and the static load calculations and formwork drawings are dependent on one another, the execution design is carried out at more or less the same time. The structural waterproof must be designed to meet the detail of the stresses acting on it. Any verifications of suitability not to hand must be provided. All the data required for execution must be recorded in overview and detail drawings.

Ground plans, sections, views and even developments, if required should include as a minimum:

— axes, main dimensions, heights

— details of the number and type of layers (designed to meet compression stresses and the flow path of the bitumen adhesive to [92])

— general details, such as the sub-concrete and protective layers

— details of corrosion-protecting steel components

— references to detail drawings and standard details to be used

— references to connecting drawings

— details of settlement differences and other movement processes at structural joints.

1.1 Generating electricity by nuclear power plants

Basically, nuclear power plants work in the same way as coal — and gas-fired plants, converting heat to electricity. Whereas fossil-fuel-fired power plants run on energy media such as oil, lignite or hard coal, nuclear power plants use the heat given off when atomic nuclei split.

Figure 2.1 shows how a nuclear power plant works (in this case, a pressurised water reactor, cf. Section 2.4.2) and shows the whole energy conversion process. Nuclear fission inside the reactor pressure vessel generates heat, which heats water until it vaporises, turning thermal energy into latent energy in steam. This steam, which is under high pressure, then drives the turbines (converting to mechanical energy), which turn the generators connected to them, generating electrical energy, like a bicycle dynamo. Condensing the steam required to drive the turbines is done either by direct flow or seawater cooling or via a cooling system using a cooling tower.

In what follows, we will present some other particular features of construction which are particularly characteristic of building nuclear power plants.

3.2.4.1 Reactor building — containment

The pre-stressed concrete containment in the OL3 reactor building is made of K60 concrete to Finnish standard BY50 (comparable with C50/60) with a steel liner. The cylindrical section has walls 1.3 m thick. The inner steel liner, made of S355J2SN steel, is 6 mm thick. The pressure vessel has an internal radius of 23.40 m and an outer radius of 24.70 m.

To make the cylindrical section of the steel liner, 90° sections 6 m high were delivered to the site. These sections were then assembled to form rings 12 m high and were lifted into place (Figures 4.19-4.21).

Before being lifted into place, segments were coated with epoxy resin based triple-layer paintwork (basecoat, intermediate coat and topcoat).

Once each liner segment was lifted into place, it was welded to the segment below it. Once it was welded, the reinforcement and tendon sleeve tubes were installed. Tensioning blocks for the horizontal tendons of the containment were spaced 120° apart.

Vertical reinforcement joints were made using overlapping or Lenton screwed sockets. The horizontal reinforcement joints were made mostly with overlaps. The connecting reinforcement for the anchor plates integrated in the steel liner (such as polar crane consoles) was made with back closed stirrups, tying the anchor plates to the stirrups with position sockets.

The cylindrical section of the safety containment was shuttered using single-headed self-climbing formwork. The formwork was supported against the structure of the preceding outer containment (APC shell).

One particular structural engineering feature of pressurised water reactors made in Germany is installing the lower steel cap of the steel containment.

The lower section of the spherical steel containment was first made supported on trestles in the spherical concrete segment, so it could be welded on both sides. It was then lowered floating into its final position defined by spacers. Lastly, the cavity remaining was then carefully filled with injection mortar (Figure 4.22).

The load-bearing capacity of fastenings in concrete sufficiently far from edges of structural components can be divided into modes of failure as follows.

— tension failure:

— steel failure

— extrusion

— concrete cone failure

— gaps

— transverse tension failure

— steel failure

— concrete pry-out

— concrete edge fracture.

The design principles can be found in the following bodies of rules:

— ETAG 001(Guideline for European Technical Approval) [64,65]

— CEN/TS (Comite Europeen de Normalisation/Specification Technique) [66]

— Guidelines on assessing anchors fastenings to be used in nuclear power plants and other nuclear installations DIBt [63],

— Guidelines for anchor fastenings in nuclear power plants and other nuclear installa­tions, DIBt, June 2010 [67].

Ageing mechanisms in building materials (physical ageing) involve damage mecha­nisms which cause material characteristics to deteriorate, and which can be caused by the exposures below:

— mechanical attacks, such as stresses imposed by temperatures

— physical attacks (frost, temperature changes and humidity)

— chemical attacks (acids, alkalis etc.)

— biological attacks (bacteria and fungi).

These effects set off ageing mechanisms which, for essential building materials, include:

— concrete: cracking, creep and shrinkage, swelling, secondary curing, carbonisation, damage due to chloride or sulphate attack, alkali reaction, solvent attacks, such as by acids and salts, swelling sulphate attacks, growths (e. g. algae), radiation

— reinforcement steel, pre-stressing steel, construction steel: corrosion

— pre-stressed concrete: loss of tension due to creep and shrinkage

— plastics: fatigue

— coatings: bubbling, cracking, chalking.

Ageing mechanisms can also be caused by changes to the subsoil, such as faults in building structures (cracks), operating problems (skewing turbine foundations etc.) due to the subsoil settlement under load, or deformations or changes of form in dykes through soil consolidation, sagging, settlements or external events.

Nuclear installations must meet stringent safety requirements, and so require design strategies accordingly, but which are not covered by conventional plant and construction rules. In 1972, therefore, the Federal German Ministry of Education and Research (BMBW, now the BMU or Federal Ministry for the Environment, Nature Conservation and Nuclear Safety) set up the Nuclear Safety Standards Commission (KTA), on the model of the German steam boiler committee. The Nuclear Safety Standards Commis­sion has assumed responsibility for drawing up safety rules in nuclear systems and promoting their use, through bringing about consistent opinions amongst specialists from those who build, install and operate nuclear power plants, inspectors and the authorities.

Safety Standards of the Nuclear Safety Commission (KTA safety standard) [14] lay down safety requirements, compliance with which provides the precautions required when building and operating plants in accordance with the state of the art of science and technology. These precautions required under the Atomic Energy Act are necessary to achieve the safety targets as laid down in the Atomic Energy Act and radiological protection ordinance and in more detail in the safety criteria for nuclear power plants and design basis accidents guidelines.

The KTA safety standards cover more than 100 fields which include all the issues relevant to nuclear technology and relevant disciplines. They are reviewed regularly (every five years) to see if they need revising: that means more than 50 proposed rules are currently being considered.

KTA safety standards must be regarded as mandatory overall standards in Germany which must always be complied with. They can be varied, in theory, but that means those involved (nuclear regulators and inspectors) would have to reach a consensus viable in law. KTA safety standards are publicly available, and are not used in Germany alone: there are many countries, especially in Europe, that accept these codes or would even like them to be used for their nuclear installations, so that KTA safety standards are largely available in English also.

3.1.2 DIN Codes

The codes of DIN, the German standards institute, are generally accepted as codes of the art which are reflected in the KTA safety standards. These standards do not normally apply to nuclear facilities, or are even expressly excluded from applying to them; so the DIN has set up a nuclear technology standards committee, now standards committee materials testing (NMP) specialist area 7 — nuclear technology, which is responsible for producing and updating specifically nuclear standards.

As far as construction technology is concerned, there are two of these codes whose status means that they are also applied internationally: DIN 25449 [15] and DIN 25459 [16]. DIN 25449 covers designing reinforced concrete and pre-stressed concrete components to allow for the rare effects from inside (EVI) and outside (EVA) as safety levels 3 and 4 (see Section 2.5). DIN 25459 also lays down rules for designing safety containments using reinforced and pre-stressed concrete. Due to the integrity requirements involved, such containments also require additional claddings such as steel or plastic liners; this standard also deals with their design, including laminate effects with the reinforced or stressed concrete design. DIN 25459 only exists to date as a pre-standard, which is currently being revised and should be published as a fully fledged standard in the near future.

When designing conventional building structures for seismic design, DIN 4149 [39] or DIN EN 1998 [40] 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 [39] or DIN EN 1998, KTA 2201.1 [37] 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.

7.1.4.2 General

In Germany, non-standard construction products are qualified by a so-called General Technical Approval given from the Deutsches Institut fur Bautechnik (DIBt). A General Technical Approval confirms that a non-standard construction product or construction method may be used under German Federal State building regulations.

Since Council Directive 89/106/EEC of 21 December 1988 on the adjustment of laws, regulations and administrative provisions of the European Member States relating to construction products was introduced, the DIBt can also issue European Technical Approvals (ETAs). An ETA for metallic anchors is issued under ETAG 001 [64]. These guidelines for the European Technical Approval for metallic anchors for anchoring in concrete contains documents for assessing anchors and are divided into six parts and three annexes.