Category Archives: Rudiger Meiswinkel, Julian Meyer, Jurgen Schnell

Reinforcing steel

One essential characteristic of reinforcing steel in nuclear installations is how ductile it is and hence how readily the internal forces and moments can be redistributed. This characteristic is an essential factor in deciding how robust structures are. These characteristics are particularly important when it comes to extreme extraordinary actions such as aircraft impact or earthquakes to dissipate the energy involved as desired. Bst 1100 (or aircraft steel) was widely used in the past, but its lack of ductility meant that it ceased to be able to meet these requirements; today, normal reinforcing steel B500B is used which meets the ductility requirements.

Outside structural sections to be used in areas to be designed to withstand aircraft impact (APC or airplane crash shells) generally use sleeve joints as it is assumed that the bond will be lost in the immediate vicinity of the impact.

Airplane crash 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 (cf. Section 2.5). 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 215 m/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.

Waterproofing of structures

7.2 Purposes on waterproofing structures

Structures are waterproofed primarily to protect them against penetrating water, which may appear as soil humidity, non-accumulating seepage water, accumulating seepage water, unpressurised surface water and water pressing in from outside. Waterproofing is also used to contain radioactively contaminated liquids arising inside them, particularly in safety-related structures in nuclear power plants.

Structural waterproofing as a ‘black tank’ is applied to the outside of structures — on the side facing the water — and encloses them as a basin or trough with a tightly waterproofed skin. Where a structure is designed as a ‘white tank’, on the other hand, the reinforced concrete structure serves not only to bear the load but also to waterproof the structure.

Design and Construction of Nuclear Power Plants

The “Concrete Yearbook” is a very important source of information for engineers involved in design, analysis, planning and production of concrete structures. It is published on a yearly basis and offers chapters devoted to various subjects with high actuality. Any chapter gives extended information based on the latest state of the art, written by renowned experts in the areas considered. The subjects change every year and may return in later years for an updated treatment. This publication strategy guarantees, that not only the most recent knowledge is involved in the presentation of topics, but that the choice of the topics itself meets the demand of actuality as well.

For decades already the themes chosen are treated in such a way, that on the one hand the reader is informed about the backgrounds and on the other hand gets acquainted with practical experience, methods and rules to bring this knowledge into practice. For practicing engineers, this is an optimum combination. Engineering practice requires knowledge of rules and recommendations, as well as understanding of the theories or assumptions behind them, in order to find adequate solutions for the wide scope of problems of daily or special nature.

During the history of the “Concrete Yearbook” an interesting development was noted. In the early editions themes of interest were chosen on an incidental basis. Meanwhile, however, the building industry has gone through a remarkable development. Where in the past predominantly matters concerning structural safety and serviceability were in the centre of attention, nowadays an increasing awareness develops due to our responsibility with regard to society in a broader sense. This is reflected e. g. by the wish to avoid problems related to limited durability of structures. Expensive repair of structures has been, and unfortunately still is, necessary because of insufficient awareness of deterioration processes of concrete and reinforcing steel in the past. Therefore structural design should focus now on realizing structures with sufficient reliability and serviceability for a specified period of time, without substantial maintenance costs. Moreover we are confronted with a heritage of older structures that should be assessed with regard to their suitability to safely carry the often increased loads applied to them today. Here several aspects of structural engineering have to be considered in an interrelated way, like risk, functionality, serviceability, deterioration processes, strengthening techniques, monitoring, dismantlement, adaptability and recycling of structures and structural materials, and the introduction of modern high performance materials. Also the significance of sustainability is recognized. This added to the awareness that design should not focus only on individual structures and their service life, but as well on their function in a wider context, with regard to harmony with their environment, acceptance by society, the responsible use of resources, low energy consumption and economy. Moreover the construction processes should become cleaner, with less environmental nuisance and pollution.

The editors of the “Concrete Yearbook” have clearly recognized those and other trends and offer now a selection of coherent subjects which resort under a common “umbrella” of a broader societal development of high relevance. In order to be able to cope with the corresponding challenges the reader is informed about progress in technology, theoretical methods, new findings of research, new ideas on design and execution, development in production, assessment and conservation strategies. By the actual selection of topics and the way those are treated, the “Concrete Yearbook” offers a splendid opportunity to get and stay aware of the development of technical knowledge, practical experience and concepts in the field of design of concrete structures on an international level.

Prof. Dr. Ir. Dr.-Ing. h. c. Joost Walraven, TU Delft Honorary president of the international concrete federation fib

Final storage

The Federal Government of Germany has decided to store radioactive waste in final storage facilities in ‘deep geological formations’ to keep them out of the biological cycle for as long as possible. This decision was taken because of Germany’s population density, climatic conditions and the fact that Germany has geological formations that are suitable for this purpose.

Both hot and relatively cool waste will be stored finally in deep geological formations for safety reasons.

Hot radioactive waste (from spent fuel elements) is more active, so the temperatures that radioactive waste generates are correspondingly higher. The Germans are still looking for the most suitable deep geological formations in which to store them finally.

Studies to date have shown that, highly radioactive hot waste can be safely stored finally in deep geological formations even with today’s state of the art science and technology.

Germany has approved the Konrad shaft as the final storage facility for radioactive waste producing negligible heat.

The salt stock Gorleben site is currently the most studied site for a possible final storage facility for radioactive waste producing substantial heat.

Any further investigations have been interrupted by a politically motivated moratorium since 1 January 2000, and have not resumed to date.

The radioactive waste producing substantial heat obtained at present, such as spent fuel rods, is put into storage at the nuclear power plant sites themselves in interim site storage facilities in CASTOR containers.

Other radioactive waste producing substantial heat is prepared and put into storage in glass moulds at the final storage facility in Gorleben, which is also where the waste returned from the reprocessing plants in France and Great Britain is stored.

For radioactive waste whose thermal radiation is negligible, overground interim storage facilities have been set up as collection and buffer stores and as storage facilities, as no final storage facilities are available.

From 1967 to 1978, radioactive waste producing negligible heat — then called low and moderately active waste — was stored at Salzbergwerk Asse II (experimental final storage facility) under the strategy at that time.

Before it was used as a final storage facility, Asse II worked as a salt mine for more than fifty years; the waste is stored in the chambers excavated in the course of extracting the salt. The prevailing geological conditions led to the salt formations moving and loosening, so water penetrated into the mine, which means that Asse does not fully meet the integrity and stability requirements for a final storage facility. The German Federal Office for Radiation Protection, which operated the Asse final storage facility at that time, believes that the site safety conditions at the time only exist to a limited extent today.

As far back as 2007, the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety instructed the Federal Office for Radiation Protection to refit the former Konrad shaft facility at Salzgitter as a final storage facility for radioactive waste producing negligible heat. The Konrad final storage facility has natural barriers which contain the radioactive waste permanently. Above the final storage facility, there is a covering layer of clay up to 400 m thick which prevents surface water penetrating. The storage areas are between the 800 m and 850 m strata.

Nine storage areas have been approved to allow for the storage space originally applied for of 605,000 m3. As matters currently stand, two storage areas capable of holding 280,000 m3 of waste should suffice, as new conservation procedures have reduced the volume of waste involved.

Under the planning approval of 2002, the Konrad final storage facility can hold up to 303,000 m3 of radioactive waste producing negligible heat. By way of comparison: a single CASTOR container with thermally radiant waste contains more radioactivity than the entire 303,000 m3 of radioactive waste Konrad is allowed to hold.

By the time all the nuclear power plants in Germany have reached the end of their working lives, it is estimated that a total of approx. 17,000 t of heavy metals will have accumulated as spent fuel elements, that is thermally radioactive waste producing substantial heat.

Final storage facilities for highly radioactive waste could be any geologically stable ground formations such as salt stocks or rock formations. The structural engineering challenges which final storage facilities present are in particular how to build the tunnels required to access the actual storage facilities and designing the storage facilities at great depths.

image074This model is the most advanced in the world, and was developed at the nuclear power plant site at Olkiluoto, Finland, where the deepest point achieved in the rock formations is approx. 420 m. The highly active waste stored at this depth is fused into glass, and will be enclosed completely in concrete once it has cooled down to some extent.

Reactor containment of steel

Except for two blocks at Gundremmingen nuclear power plant, all containments in Germany are made of steel. The containments of the more recent German PWR plants (Convoy and pre-Convoy plants) consist of a steel sphere 56 m in diameter with walls 30-40 mm thick. These dimensions are based on a design pressure in the range 4-5 bar overpressure at a design temperature of approx. 150 °C. The guideline values for the maximum permitted leakage rate are 0.25-0.50% per day.

Steel containments are designed in accordance with KTA rules. KTA 3401 [59-62] covers materials, design conditions, design, production and testing. The material that KTA 3401.1 [59] requires is 15 MnNi 63 steel, whose mechanical characteristics, with a yield point of 330-370 N/mm2 and a tensile strength of 490-630 N/mm2 are comparable with those of construction steel S355.

Design is governed by KTA 3401.2 [60], and is based on permitted stresses, departing from the partial safety concept. Permitted stresses are defined for four stress levels and the various stress categories, allowing for how steel characteristics change at high temperatures. A loss of coolant accident as the dominant verification demand of the containment is put in the operating stress level and hence not regarded as a failure case.

Stability studies are also required to cover the possibility of a partial vacuum arising in the containment. The pressure tests here assume a partial vacuum of 45 mbar and a partial vacuum of 5-30 mbar in normal operation.

Design and calculation

For optimum design of white tanks in terms of demands and use, the design principles as stated in the WU guidelines must be followed. Other factors to be taken into account in the design concept include in particular when plant is to be commissioned, the water effects when it is in use and the nature of its use.

7.3.2 Joint detailing

Joints in reinforced concrete structures which are highly resistant to water penetration must be permanently water-impermeable with regard to the reference water level and subsoil conditions.

Approval aspects

3.1 Atomic energy and construction law

Building, operating and making major changes to nuclear installations must be approved under atomic energy law. The government approval process used depends on the national law requirements involved in each case, so varies from one country to another.

Before they can be allowed to build and operate nuclear installations, there are a number of basic requirements that applicants and operators have to meet:

— Applicants and operating managers must be reliable and expert.

— Those working at the company must know how to run it safely, what the potential risks are and the safety precautions required.

— Know what precautions are required under the state of the art of science and technology to prevent erection and operating the plant causing damage.

— Know what protection is required against anomalies and other effects by third parties.

— Keep water, air and soil clean in the public interest.

In Germany, the legal framework for using nuclear energy for peaceful purposes is provided by the Atomic Energy Act and regulations issued pursuant to it. The safety goals and measures are laid down in Section 2.5 of the Atomic Energy Act [4]. These are clarified specifically by Atomic Energy Act regulations and internal administrative rules and guidelines, such as the guidelines for PWRs [5] or design basis accidents guidelines [6]. These guidelines cover the design basis accidents to be considered and managed; they also specify the requirements of the radiological protection ordinance [7].

The building structures required to meet the safety targets, which are therefore classified as safety-related, have to meet not only the requirements of construction law but also those of the Atomic Energy Act. That means both planning permission and permission under the Atomic Energy Act are required in Germany. To be approved under Atomic Energy Act, constructors/operators must show that they have taken the necessary precautions against damage in accordance with the state of the art of science and technology. Safety-related building structures must therefore meet not just the conventional requirements of construction law but also additional safety requirements in line with the state of the art of science and technology.

Structural demolition technologies

There are a number of criteria to consider when selecting the right demolition


— Technical criteria: component materials, geometry and accessibility

— Radiation protection criteria: minimising aerosol release, primary and secondary waste, avoiding spreading decontamination, ease of decontamination, high level of recyclability of installations used

— Financial criteria: setup costs, equipment costs, operating costs, cutting services


Fig. 4.26 Making a cut-out in the control area using wire saws and overlapping core drillings

— Strategic criteria: site location, demolition strategy, disposal routes, constraints of materials handling strategy

Demolishing concrete and reinforced concrete structures is often done using wire saws. These have the advantage that large-format blocks can be obtained which can be readily carried away; the drawbacks are that cutting and cooling water may be required and drill holes have to be made to guide the wires in first (Figure 4.26).

Contaminated concrete layers can be demolished as shown below, depending on the nature and depth of the contamination involved (Table 4.2, Figure 4.27).

Table 4.2 Methods for decontaminating reinforced concrete sections

Nature and Depth of Contamination


Loose contamination on surface of concrete which can be wiped off

Vacuum, brush off, wipe off, wash or spray, apply chemicals

Subsurface contamination (has penetrated and attached itself)

Grind off, mill over large areas (Figure 4.27) flame hammer and chisel

Deeper contamination into concrete

Chisel off conventionally, mill over large areas, combine milling and chiselling



Fig. 4.27 HOCHTIEF Decon surface milling system in use [36]


Quality assurance, material quality

General Technical Approvals state that headed studs for anchor plates must only be made of unalloyed steels in materials group SD1 and stainless steels in group SD3 to DIN EN ISO 13918 [73].

For quality assurance purposes, and to certify compliance, materials and production are inspected continuously in the workshop; the production is also monitored by an independent certification body. General Technical Approvals require headed studs to be marked with the appropriate works code on their heads, so that they can be identified easily on site. They must also be stamped with the material used, if using stainless steel. Packs must be CE-marked, stating the products approved.

Steel fastening plates are generally made of non-alloyed steel of strength class S235JR to DIN EN 10025 [69]. Unless improved characteristics are required in the direction of thickness, material quality is certified by a works certificate 2.2 to DIN EN 10204 [78], which must show the as-delivered condition to DIN EN 10025 and the melt analysis and tension test results as a minimum requirement. Additional notched bar impact bending tests must be conducted when using S235J2.

Where plates more than 30 mm thick have structural components welded on, the welding seams of which are subjected to tension, a welded-on bending test must be conducted to SEP 1390 [79] and proven by an acceptance test certificate 3.1B.

Ultrasound testing is not part of the minimum requirements, but to avoid lamination, ultrasound testing is recommended for steel plates 15 mm and over thick on a 200 x 200 mm matrix, even if no quality certificate as per Z-quality is required.

Ultrasound-tested steel plates must be used where mainly live loads apply or where certificates of quality are required under DASt Guideline 014 [80].

Where improved characteristics are required for plates in the direction of thickness, material qualities are proven by acceptance test certificates 3.1B. The certificate of Z-quality required must be testified, stating the reduction of area, by appropriate tension tests in the direction of thickness to DIN EN 10002 [81]. The requirements as laid down in KTA 3205.2 [82], Table 7-1 must be observed.

For plates over 15 mm thick, under tension and bending tension stresses, ultrasound testing must be carried out according to DIN EN 10160 [83] on a 100 x 100 mm matrix.

Material tests also include notched bar impact bending tests to DIN 10045-1 [54].

Anchor plates of S355J2 are subject to basically the same requirements. Materials must have acceptance certificates 3.1B to DIN EN 10204, stating the condition as supplied to DIN EN 10025, and the following test results at as a minimum:

— melt analysis

— tension test

— notched bar impact bending test

— weld-bead bending test.

Where improved properties in the direction of thickness are required, compliance with the carbon equivalent (CEV < 0.45%) must be proven.

Ultrasound testing on a 100 x 100 mm matrix and appropriate tension tests in the direction of thickness are also required in order to verify the certificate of Z-quality.

Anchor plates of non-stainless materials are made of alloyed steel 1.4571 to DIN EN 10088 [70]. The testing required is governed by DIN EN 10088, and must be documented by a certificate of acceptance 3.1 B, including the heat-treated condition.