As the basis for assessing the structural and other technical, personal and adminis — trative/organisational safety measures in nuclear power plants that operators are required to prove, the component Federal and Federal State authorities issued the ‘Guidelines for the protection of nuclear power plants with pressurised water reactors against impacts and other third-party effects’ [25] on 24.11.1987 and analogous guidelines for nuclear power plants with boiling water reactors on 01.12.1994 [26]; these define the protection goals, the buildings and other plant components to be protected and safety measures required.

These guidelines are unpublished, as they are classified.

Based on the scope of these guidelines, specific safety goals are defined with a wide range of system conditions for pressurised water reactors (PWRs) and boiling water reactors (BWRs).

The structural safety measures of the buildings to be protected relate to their outside walls and penetrations (doors, gates and gratings). Different safety areas must be set up to meet safety requirements.

The structural barriers for the widest possible range of safety areas are defined in terms of wall thicknesses and their reinforcement content; the number of access points must be kept to a minimum.

There are specifications defining doors, gates and gratings for different classes of barriers. These are also classified and unpublished.

Verifications of shear resistance of reinforced or pre-stressed concrete members must be conducted to DIN 25449. The verifications are based on the method in accordance
with DIN 1045-1, having regard to the different requirement categories, A1, A2 and A3. Shear force reinforcement will be necessary if, in a cross-section, the design value of the acting shear force VEd is greater than the design value of the shear force VRdct a structural member can withstand without shear force reinforcement, i. e. if:

VEd > VRd, ct (6.7)

The design value VRd, ct considers the different requirement categories A1, A2 and A3 by a factor cd, which is to be taken as 1.0 for A1, 1.15 for A2 and 1.50 for A3. This makes the reference value:

Where

Where

bw is the smallest cross-sectional width within the tension area of the cross-section [mm]

cd is the prefactor reflecting the requirement category

d is the static effective depth of the flexural reinforcement in the cross-section [mm] fck is the characteristic value of the concrete compression strength [N/mm2]

Pi is the longitudinal reinforcement ratio in the tension area

scd is the design value of the axial concrete stress at the height of the centre of gravity

of the cross-section where scd = —— < fctk 0 05

Ac

Shear force design of structural members that are subject to bending stresses must be carried out based on a truss system in which the angle of the strut of the truss must be limited and the shear force reinforcement must be proven as VEd < VRdsy and VEd < VRdmax. The maximum angle of the strut and the design value of the shear force which can be absorbed, limitedby the strength of the shearforce reinforcement VRdsy and the design value of the maximum shear force VRDmax that can be absorbed to DIN 1045-1 must be complied with in accordance with requirement categories A1, A2 and A3.

White tank designs may be considered as structural waterproofing in new building projects, supplemented by black waterproofing if necessary. Additional measures may be required to run off standing water or other liquid media, by way of drainage etc. [97].

Below, we will look at the design principles for building white tanks, focusing on their waterproofing effects against water penetrating from outside.

Radioactivity can be defined as when atomic nuclei of one element turn into nuclei of another element, emitting radiation or particles in the process. Radioactive processes can be divided into decays of different kinds. The most important decay and radiation processes involved with uranium ore are as follows (Figure 2.4):

— Alpha radiation

Has little penetration strength, and can be blocked by just a sheet of paper (1) (discovered by Becquerel 1896).

— Beta radiation

More penetrating than alpha radiation, but can be blocked by thin plate or a few mm of aluminium (2) (discovered by Rutherford 1896).

— Gamma radiation

High-energy short-wave electromagnetic radiation, often created during alpha or beta decay. Can be shielded by plates of varying thickness, depending on how much

energy it contains (3). Nearly all atomic nuclei emit gamma rays (discovered by Villard 1900).

What effects radiation has depends on what kind of radiation it is, what the dosage levels are over time and how sensitive the material being radiated is. Radiation absorbed by the human body is abbreviated to Rad for short (radiation absorbed dose).

At a given energy dose D, the biological effects may vary considerably, depending on the type of radiation involved: so a weighted radiation dose (equivalent dose) is used as the biologically effective dose. This equivalent dose H is expressed in sieverts (Sv), generally quoted as mSv or |mSv, and is calculated from the energy dose D and an assessment factor q which reflects the characteristics of the radiation. This radiation dose over time then gives the radiation load as a dosage level. A number of natural and man-made radiation sources, with their radiation loads, are compared in Figure 2.5.

The radiation load from nuclear power plants is controlled by law, so the limits as stated in the radiation protection regulations must not be exceeded, even where the effects are worst. Nuclear power plants also have retention systems to prevent radioactive substances getting into the environment.

These retention systems include:

— Ventilation systems working at a partial vacuum to ensure that air always flows from less active to more active areas

10 hours flying at 10,000 m: approx. 0.05 mSv p. a.

□

Watching TV regularly: approx. 0.01 mSv p. a.

Fig. 2.5 Natural and man-made radiation sources

— Exhaust systems (microfilters etc.)

— Systems for treating radioactive contaminated water to achieve a high decontamination factor (relative energy levels before and after treatment) and minimise waste.

If we look at the radioactive waste from nuclear power plants more closely, we find that, once it has been used in the nuclear reactor, the high-energy nuclear fuel consists of 95% uranium, 4% fission products and 1% plutonium. This spent nuclear fuel can be reprocessed, recycling its useful component, but the current nuclear consensus in Germany has ruled out reprocessing, so spent fuel elements must be kept in intermediate storage until they are put into final storage at the nuclear power plant sites (see also Section 4.4). As well as this highly active waste, nuclear power plants also produce moderate — and low-activity waste. Putting this more clearly: a 1300MW pressurised water reactor produces around 510m3 of radioactive waste a year in total, of which 1% is highly active and around 92% is low-activity waste (Figure 2.6).

Dismantling nuclear power plants represents a major part of nuclear engineering in Germany today.

Dismantling principles

We need to distinguish here between the systems and structures inside the control area — which could be contaminated or live — and those outside the control area, which are not.

Dismantling the control area, with its contaminated and live sections, breaks down into stages, as follows:

— Shutting the plant down, residual operation, deconstructing the contaminated systems not required for residual operations and making changes to systems and building sections as necessary as takedown proceeds

— Taking down the contaminated, active primary components and the concrete structures which are live from being irradiated for years, and the bioshield in particular

— Demolishing the remaining systems, decontaminating buildings, conducting clear­ing surveys on buildings and external areas

This is aimed at decommissioning the plant as a whole of supervision according to Atomic Energy Act terms.

This is followed by conventional demolition of both the now cleared former control area and the systems and buildings outside the control area.

Weights and costs

For a typical PWR, the demolition and disposal weights estimated in tonnes [t] (internal estimate by HOCHTIEF [31]) are as follows:

— Total power plant: 500,0001

— Control area: 156,5001, of which:

— structural components: 143,0001

— system components: 13,5001

The remaining radioactive waste for final storage is estimated at 4,0001.

Dismantling costs, excluding residual operation, materials handling and packaging costs — may be estimated roughly at €350 m per power plant block [32].

Anchor plates with headed studs are found in power plants, in both safety-related and non-safety-related applications. They are used for example as supporting and load transmission points for platforms as well as for mounting devices for pipe and cable racks, risers etc. Figure 7.2 shows a schematic anchor plate with headed studs.

Anchor plates vary in size from small two-bolt units to groups with nine headed studs; there are even larger sizes, though, such as 4 x 4 or 5 x 5 headed studs up to 2.0 m long anchor strips with up to 25 headed studs. Larger anchor plates often need additional reinforcement to back-anchoring of tensile forces.

Large anchor plates can be used for a great variety of connection arrangements, but their load transmission is always limited to a local area.

In former times headed stud diameters ranged from 3/8" (9.52 mm) to 7/8" (22.22 mm) with a ratio of head to shaft diameter of between 1.60 and 2.0. Headed studs 175 mm

long were preferred: if a greater anchoring length was required, or if it was wished to anchor the headed studs within the background bending compression area, two or even three headed studs would be welded together. This method can still be used today, but it should be noted that, for the extended anchor length to be effective, cushion rings must be fitted at all bolt heads except the final one welded on.

Headed studs were then made in accordance with DIN 17100 of structural steel St37- 3K with a minimum tensile strength of 450 N/mm2. Stainless steel was used in areas where increased corrosion protection was required for plates (1.4571, 1.4401) and headed studs (1.4301, 1.4303). At the time when German nuclear power plants were built, stainless steel bolts were only available up to 16 mm diameter.

For new construction projects, headed studs are available in any length from 50 mm to 525 mm today. Beyond the standard range, special lengths are even available for special applications up to 750 mm, including the welding tools required. The characteristics of the material properties remain unchanged. Under the amended standards, the material designation for the unalloyed bolt steel is S235J2 + C450 according to [69]. The material code for alloy steel is unchanged [70].

Bolt diameters were changed nominally to the metric system, with a new size being added of 25 mm. Stainless bolt heads are currently available in sizes from 10 to 22 mm, and there are plans to add 25 mm diameter bolts to the stainless range also (Table 7.1).

Headed studs are now designated SD for short, and are governed by DIN EN ISO 13918 (formerly DIN 32500-3:1979). As for bolt materials, there is a cross-reference to standard ISO/TR 15608. Under this, material properties are subdivided into groups SD1, SD2 and SD3. The bolt material prescribed under ETA approval is equivalent to group SD1, with a minimum tensile strength of 450 N/mm2. Group SD2 covers materials with a reduced tensile strength of 400 N/mm2 and a yield point of 235 N/mm2. The approvals do not cover group SD2 materials for applications in anchor plates.

for shear oads

Fig. 7.5 Potential failure modes of headed studs according to [72]

Material group SD3 covers stainless steel materials (1.4301, 1.4303) with a yield point of not less than 500N/mm2.

For all three of these groups, the carbon content is limited to 0.20% and they must contain at least 0.02% of relaxing elements.

7.1.3 Load-bearing behaviour of headed studs

Headed stud fastenings are relatively rigid anchor constructions. The welding of headed studs to steel plates and then concreting those steel plates into reinforced concrete components gives a direct interlocking between steel and concrete. Due to forced spreading, the anchor area will not be affected by drilling or splitting forces. Headed studs show a ductile load-bearing behaviour and good deformability at the ultimate limit state.

The structural strength of headed studs depends on many factors, primarily the strength characteristics of the steel and the compressive strength class of concrete. Geometric factors such as diameter and length of the headed studs and distances from free component edges or adjacent anchorings exhibit a major influence, too.

Potential failure modes of anchorings with welded on headed studs as a function of the type of stress are shown in Figure 7.5.

3.2.1 Building structure classification system

Large-scale power plants, whether conventional or nuclear, have many system com­ponents and structures which must be clearly marked and classified. This used to be done earlier using the plant coding system (in German: Anlagen-Kennzeichnungs — System, AKZ), which was replaced by the identification system for power plants (in German: Kraftwerk-Kennzeichensystem, KKS) in the 1980s.

The internationally used KKS system covers 17 digits in different blocks which can be used to designate whole systems, functions, aggregates and operating resources. Viewed as a whole, system components and structures which are universal to all power plants, whether coal-fired, hydroelectric or nuclear, are designated uniformly, such as cooling towers and turbine buildings.

Building structures can be clearly distinguished by three letters of the function code, with the initial letter U being used to designate building structures generally. Building structures for generating heat atomically, for example, are coded UJ, with the third letter such as building structure designation UJA for the inside of reactor buildings and UJB for the annular space of reactor buildings.

Table 4.1 shows some examples of building structures of nuclear power plants designated in accordance with the KKS and AKZ systems. Unlike the more recent Convoy plants (Emsland, Isar 2 and Neckarwestheim 2), older German nuclear power plants still use the AKZ from when they were built. As an example, the layout diagram of the Isar 2 nuclear power plant using the KKS designation system is shown in Figure 4.1.

Of the building structures shown in Figure 4.1, those which are essential in a nuclear power plant are as follows:

— Reactor building

As well as the reactor itself, with a PWR installation, the reactor building also includes all the primary circuit components and, with a BWR one, essential parts of the live steam

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.

system. The reactor building also includes the containment, which must prevent leaks under all prospective problems accidents.

— Auxiliary system building

This building houses various storage, stock and wastewater containers, workshops, barrel stores and filter, ventilation and treatment plants.

— Switchgear building

This building, which is relevant to control and guidance systems, houses all the switchgear and modules which supply the various systems involved with electric power.

— Emergency backup diesel building (emergency generator building)

This building houses the emergency diesel generators that supply electricity to the power plant and hence the residual heat removal systems.

— Emergency feed building

This building houses the emergency feed and residual cooling pumps and their associated systems and the switchgear room. This building also houses the emergency feed system, which in an emergency supplies the boilers with feed water to ablate the residual heat that the reactor generates.

— Vent stack

This chimney releases at a great height the vent air that comes from ventilating buildings and systems. This vent air is monitored for radioactive substances.

— Water supply building

This includes the building works for extracting the cooling water, such as the cooling water extractor building or cooling water pumping station, and the building works for returning the cooling water, such as the outlet structure.

For the purposes of nuclear safety philosophy (cf. Section 2.5), building structures are divided into safety-related and non-safety-related.

Safety-related building structures include such things as the reactor building, auxiliary systems building, switchgear building, emergency backup diesel building or the vent stack as well. In a BWR, as opposed to a PWR, the turbine building is also classified as safety-related, as radioactive live steam is fed directly into the turbine in the turbine building (cf. Section 2). Non-safety-related building structures typically include administrative buildings, workshop buildings, gatehouse and cooling towers.

The layout plan in Figure 4.2 shows the main buildings of a nuclear power plant, using the example of Gundremmingen.

In building design terms, how the buildings are laid out in relation to one another (plant layout) is governed mainly with a view to making safety-related buildings redundant, to protect against external effects (aircraft impact, earthquake, pressure waves etc.) and hence the safety strategy. In this context, the physical separation between redundant buildings and even gap widths between adjacent buildings are also a construction issue.

Redundant buildings, for example, should be spaced so that external events do not stop them being duly redundant. If this cannot be guaranteed, the consequential effects must be studied or the buildings concerned designed to withstand external events.

Another construction aspect of plant layout is the question of optimum building design and/or reducing construction times. With a compact layout, site crane movements tend to overlap, so there are areas which cannot be built at the same time, so the plant takes longer to build. This meant, the buildings before the Convoy stations, were spaced relatively far apart, which also meant that the sites themselves were larger.

This ‘relaxed’ approach has been reversed with more recent plants, like the EPR or KERENA (Figures 2.10 and 2.11); for example, short cable and pipe runs and protecting safety-related building sections under one roof and on a common foundation slab have proved to be more cost-effective.

As we saw in section 4.2.6, protecting nuclear power plants against floods as in KTA 2207 [23] 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.

Anchor fastenings for use in nuclear power plants and nuclear facilities must be designed in accordance with the DIBt Guideline.

3.2.3 Disposal requirements

In Germany, radioactive waste is divided into two kinds:

— radioactive waste producing substantial heat [hot waste]

— radioactive waste producing negligible heat [cool waste]

The latter — minimal thermal radiation waste — can be compared with low radioactive waste, and to some extent with moderately radioactive waste. Radioactive waste producing substantial heat comprises highly radioactive and to some extent moderately radioactive waste.

Waste comes from decommissioning and operating nuclear power plants, from the nuclear industry, nuclear research, and, in very small quantities, from medicine and from the Bundeswehr (German armed forces), and includes contaminated tools, protective clothing, sludges and/or suspensions.

Radioactive waste producing negligible heat accounts for more than 90% of the total volume of waste, but just 0.1% of the total radioactivity of waste to be put into final storage in Germany.