Before the building of the pre-stressed concrete pressure vessel at Schmehausen and the pre­stressed concrete containment at Gundremmingen, pre-stressing was only used as a general rule in Germany in wide-spanned precast girders in the turbine buildings and other special support structures, such as the instrument room inside the reactor building at Kriimmel.

3.2.2 Reactor building

In terms of structural particularities, it is the reactor building that poses the highest requirements. In what follows, we will limit ourselves to looking at reactor buildings in light water reactors, PWRs and BWRs. The different functional requirements involved here also mean that the shapes of the buildings themselves differ, rectangular buildings being preferred for BWRs and curved building structures with circular footprints (cylindrical or spherical) for PWRs.

A Convoy type reactor building is shown in Figure 4.3. This consists of the spherical reinforced concrete shell typical of many PWRs, with very thick walls (h = 1.80 m)

System components

1. Reactor pressure vessel

2. Steam generator

3. Circulation pump

4. Main coolant lines

5. Pressuriser

Structural components

6. Fuel pool

7. Containment

8. Outer reinforced concrete shell

9. Annular space

^9.60

Fig. 4.4 Reactor building with fuel element storage and safety building of a PWR-EPR type [22]

designed to withstand an aircraft crash. This encloses a steel safety container as containment, which maintains integrity even in an anomaly.

The further development of the Convoy power plant model as part of the Franco — German partnership led to the EPR, a Generation Ш+ reactor, as is currently being built in Finland and France. The main features of the EPR reactor building are as follows (Figure 4.4):

— There is a clear structural separation between the building complexes of the nuclear island (reactor building, fuel element storage building, safety systems building etc.) and those of the conventional island (turbine building, etc., which is why it is also often called the ‘turbine island’).

— There is a common baseplate for the relevant buildings on the nuclear island, to make it easier in the event of an earthquake to manage the induced shocks acting on the building structures and mechanical components and avoid individual buildings shifting in relation to one another.

— The double-shelled outer wall structure of the reactor building consists of an outer reinforced concrete wall 1.80m thick, an air gap of 1.30 m and an inner pre-stressed concrete wall 1.30 m thick. The inner wall is of pre-stressed concrete design, with an additional steel liner 6 mm thick on the inside to ensure that the containment does not lose its integrity even in an extreme accident (internal pressure approx. 0.5 MPa at temperatures of approx. 150 °C)

The so-called ‘double containment concept’ described above has established itself worldwide as far as the layout of the reactor building is concerned. What this means is that external influences, such as earthquakes, aircraft impact, pressure waves etc., can be absorbed by a reinforced concrete structure of a suitable thickness (APC shell). The
integrity of the reactor building to contain radioactive substances is maintained by a separate integrity barrier, which constitutes the actual containment or safety enclosure of the reactor building. As well as maintaining integrity, however, the containment must also contain the internal pressures resulting from operations and accidents, plus high thermal stresses.

There are a number of containment concepts, depending on what kind of reactor is involved:

— Reactor containment of steel (e. g. Convoy PWR models)

— Pre-stressed concrete containments without liners (in French N4 reactors, for example, but note that this concept has not proved itself, as integrity requirements cannot be met long term)

— Pre-stressed concrete containment with steel liner (e. g. EPR, approx. 6 mm thick)

— Non-pre-stressed steel containment with steel liner (e. g. KERENA, approx. 10 mm thick)

This list in itself makes it clear that a combination of pre-stressed and reinforced concrete and its associated steel liner is extremely important as an integrity barrier.

Regulatory authorities worldwide are demanding increasingly that plant technology provides passive safety systems and robust design. Whether this should also apply to structural engineering, even using pre-stressing with composite construction, raises ‘argumentation problems’ as far as this robustness requirement is concerned. The pre­stressing, which is usually extremely high, must be maintained over the very long period of more than 80 years. Monitoring pre-stressing with composite construction is difficult, and pre-stressed members cannot be replaced in practice.

The very high pre-stressing also has other drawbacks, as it devolves creep and shrinkage onto the steel liner and other steel components, such as pipe mountings and locks.

More recent containment developments, like AREVA’s KERENA containment, thus omit the pre-stressing, preferring instead to use thicker steel liners and suitable composite construction elements as structural elements in a composite construction with the concrete.

In terms of building construction, making the reactor building roof structure is particularly important, as it has to be extremely thick to withstand the impact of an aircraft. Once the reactor pressure vessel is installed, the interior work begins soon afterwards, so that supporting the shell inwardly is no longer possible in most cases. So the hemispherical roof structure with cylindrical reactor buildings, which can be precast separately, was developed — this was done also because curved structures are much better at withstanding the stresses of an aircraft impact once the membrane strength cuts in than flat surfaces.

On the other hand, doubly curved load-bearing structures are more expensive and take longer to construct, which is why the roof structures of Gundremmingen B and C reactor buildings were made with precast, wedge-shaped laid precast segments with locally cast concrete added (Figure 4.5).

Fig. 4.5 Gundremmingen B and C reactor building roof structure, with precast wedge-shaped laid precast segments with cast concrete added locally [17]

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 [47,48]. 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 > 1.50 m) 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.

Figure 5.7 shows the load over time function obtained using the RIERA model for a Phantom F-4 hitting at 215 m/s as mentioned above. The tests conducted on this in Sandia confirmed this theoretical function: it matches the function specified in the RSK guidelines [5], 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 100-150 m/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.

With nuclear power plants, system components must be protected against effects so that they can do their job in operating conditions as intended and if accidents arise [92]. This puts additional requirements on waterproofing structures. Including external effects (earthquakes, aircraft impact, explosion pressure waves) and internal effects from accidents as the case may be, structure waterproofing is subject not only to static, but also to higher transient dynamic loads.

7.3 Black tank

As a general rule, waterproofing structures to protect against water penetration are carried out in accordance with the DIN 18195 series of standards. How this structural waterproofing behaves under what are normally long-term static loads is sufficiently known; but the design constraints developed from the series of standards above are not always sufficient for nuclear structures. Special load cases which act on the structures which carry the structural waterproofing can cause deformation and displacement which affect the structural waterproofing.

In these special load cases, as well as the localised high levels of pressure from the working load, a number of other types of stress can also arise:

— transient higher transverse compression stress

— transient intermittent transverse tension stress (gaping gap opening/nominal fracture point between structures and their environments)

— transient intermittent shear stress at the waterproofing level.

How structural waterproofing behaves in terms of bridging cracks is also important.

Despite all the efforts being put into expanding renewable energy sources, large-scale power plants will be essential as part of a reliable energy supply strategy for as long as we can see. Given that nuclear power is low on CO2 emissions and has no competitors when it comes to being operated cheaply, many countries are moving into or expanding nuclear energy to cover their baseload supply. Germany will need its existing nuclear power plants to supply it with cost-effective, reliable energy for many years to come, and the financial power of German utility companies like E. ON and RWE and German design and construction knowhow is helping realise new building projects in neigh­bouring countries. At home, there are many challenges to be met when it comes to continuously updating existing plant. The authors are extensively involved in design­ing, operating and inspecting existing plant, designing newbuilds, doing retrofits and conversions and updating specific nuclear power rules.

We would like to thank Christina Busse and Bjorn Elsche of E. ON-Kernkraft GmbH, Hanover, Frau Jelena Trubnikova and Alexander Fischer, Stephan Fromknecht, Wolf­gang Fuchs, Andreas Garg, Thomas Griinzig, Heribert Hansen, Peter Kretzschmar, Mark Kritzmann, Hamid Sadegh-Azar, Thomas Springsguth and Marco Tschotschel of HOCHTIEF Consult IKS Energy, Frankfurt am Main for their assistance in writing this work. Some text modules were supplied by Soren Miiller and Martin Schafer of the staff of the Technical University Kaiserslautern and Ralf Schliwa of BORAPA Ingenieurgesellschaft. Final editing was by Frau Tanja Volk.

Rudiger Meiswinkel Julian Meyer Jurgen Schnell

1 Introduction

This section deals with aspects of building design execution which are specific to nuclear power plants, first looking back at the building of more recent nuclear power plants in Germany, which were built in the 1980s. We will also look at experience and current developments in constructing the Olkiluoto 3 (OL3) power plant in Finland.

3.2.4 Site installations

Construction sites for nuclear power plants are some of the largest construction sites there are, employing several thousand people.

A section from the site installation plan for KRB II Gundremmingen can be seen in Figure 4.13. Apart from the site management and workshop buildings, the infra­structure is particularly important: barracks, a canteen to cater for the workers, utility and disposal lines and parking places must be designed and installed.

With the OL3 project, building the nuclear islands took 13 tower cranes at times, a stationary Demag PC 9600 crane with a capacity of 1000 t to install the steel components of the safety containment, plus mobile cranes to lift in the equipment

© Site management © Accommodation (D Sanitary facilities (4) Workshops © Stores

© Subcontractors’ workshops (7) Canteen © Parking spaces © Barracks

© Caravan park with sanitary facilities and parking spaces

IB Staff buildings [ZD Site management CZD Workshops and stores IB subcontractors Bi Power plant buildings

Fig. 4.13 Section of site installations plan KRB II Gundremmingen [17]

to be used. Three of the tower cranes, two of them inside the reactor building, could not be supplied directly, but had to be served by other cranes (Figure 4.14).

An anti-crane collision system was used at OL3 which analysed where crabs, out­riggers and counterweights were and, if need be, restricted adjacent crane movements to prevent them colliding.

The crane layout selected allows all cranes to rotate freely with the crabs run in, at times when they were not in use, such as on rest days or in strong winds.

Not all structural sections are pre-stressed, even in pre-stressed concrete containments. Pressurised water reactors have cylindrical containments with cupolas on top. The cylinder walls and cupola are pre-stressed, the base slab is not. Boiling water reactors, on the other hand, have flat cylindrical covers; only the cylinder walls being pre­stressed. Pre-stressing increases the containment’s serviceability, i. e. it keeps defor­mation and cracking low; but the cross-section of the concrete cannot be overpressed completely, to ensure integrity in problem areas such as transitions between structural

sections or around openings, so today’s pre-stressed concrete containments are fitted with steel liners to guarantee their integrity. One example of a pre-stressed concrete containment with a steel liner is EPR containment.

The steel liner is anchored to the concrete structure via headed studs and/or steel profiles to give a composite steel-concrete structure. To avoid affecting the pre­stressing, the steel liner is made of thin plate, t = 6 mm, for example. When verifying the structural strength of a containment, the steel liner is only taken into account if its effects are adverse.

Pre-stressing the concrete structure induces a compressive strain in the steel liner. Pre­stressing also induces a time — and stress-based concrete creep which devolves the stresses involved and puts an additional compressive strain on the steel liner. The liner also expands under the influence of dissipation of the heat of hydration and as the concrete shrinks and also under the effects of operating conditions and in incident cases. The verification of liner integrity is obtained by limiting the liner strains and the action effects of the connectors.

Pre-stressed concrete containments with steel liners can be designed using DIN 25459 [16].

Penetrations must be waterproofed against water pressure as a matter of course, by using standard systems and/or certified products, for example with sleeve tubes and annular waterproofs, flange pipes with rigid pipe connections or core drills with medium pipes and annular waterproofs.

7.3.3 Responsibilities

When deciding on a white tank, everyone involved, and in particular those involved in the design, plus the client and any contractors already involved when the design stage starts, must be familiar with the design principles involved in a white tank. All findings and decisions required under the WU guidelines in terms of design, implementation and quality assurance must be recorded. All those involved must work together. Competences and responsibilities must be laid down and recorded in writing, clearly and unambiguously, before starting work.

The inspection required to be approved under the Atomic Energy Act includes a holistic examination of the safety precautions of the building structures. This involves defining the interface between the building structures (structural engineering) and plant components (plant engineering) and hence the distinction between construction and atomic energy law. Generally speaking, plant components such as pipes and containers are part of the building, so that the fastenings in each case (anchor plate) constitute the

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.

interface. In exceptional cases, this interface will have to be defined in the official planning process.

Each interface must have an interface document which, amongst other things, specifies the loads calculated from the plant technology and the structural member or structure in each case. Such documents are generally called structural design requirements. They are first considered as part of the atomic energy law terms, covering systems engineer­ing aspects, and then used as the basis for the construction assessment.

5.1 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 [15]). These rare and extremely rare actions are divided into internal and external actions.

A summary of internal and external actions appears in Table 5.1. Typically, internal factors are induced by:

— leaks or fractures in pressurised 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 [37] and 1 in 10,000 year flood effects to KTA 2207 [23]

— man-made actions due to specified airplane crash and explosion pressure wave.

7.1.3.2 Manufacturing of anchor plates with headed studs

Headed studs are welded onto steel plates by stud welding with arc stud welding to DIN EN ISO 14555 [84] using barrier gas or ceramic ferrules.

The welding contractors concerned must hold appropriate welding certificates to DIN 18800-7 [85] extended for bolt welding to DIN EN 14555.

Ensuring that welded joints meet quality requirements is carried out in accordance with the provisions of DIN EN ISO 14555 in conjunction with DIN EN 3834 [86].

For anchor plates which are stressed in the direction of thickness, the requirements of KTA 3205-2 [82], Table 7-1 on pre-heating welding areas must be observed.

7.1.3.3 Installing anchor plates on site

Anchor plates must be installed by skilled personnel in accordance with formwork or specific installation drawings. Suitable steps must be taken to prevent them shifting during concreting, such as being bolted or nailed to the formwork.

The tack welds are often observed being used between headed studs and reinforcement but this is not permitted. Spot welding may cause local brittleness and softening of the material the studs are made of, and this may also lead to unwanted notch effects.

When placing anchor plates within the formwork and the reinforcement cage, care must be taken to ensure that the headed studs and the reinforcement are in the right position. In particular, back-tying reinforcement must be installed as specified in drawings, observing carefully the anchoring lengths shown. Adequate spacing is required to avoid cavities or shrink holes in the load induction area.

With horizontally fixed embedded parts, there is a risk of air penetrating during pouring, so the General Technical Approvals require ventilation bores to be made from an edge length of 400 x 400 mm. As feeding in fresh concrete under a horizontal anchor plate from one side is not a reliable method of avoiding air inclusions, it is advisable to provide ventilation bores from edge lengths as little as 200 x 200 mm.

Checks must be made to ensure that anchor plates are correctly installed and that reinforcement is in the right position, with installation records as evidence.