Nuclear Power Plants--Waterproofing structures

1. 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, unpressurized 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 the mas 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.

2. Requirements of waterproofing structures

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. 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.

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 localized 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.

3.1 Waterproofing methods and materials

The specifications of DIN 18195-6 'Waterproofing buildings, proofing against outside pressing water and accumulating seepage water, design and execution' largely represents the current state of the art. Continuing intensive technological developments have led to both new waterproofing materials and new waterproofing methods.

Building waterproofs with different kinds of waterproofing strips and adhesives made in suitable combinations may be described as part of the state of the art. The individual components from which building waterproofs are made are defined in the materials tables in DIN 18195-2 'Structural waterproofing'. The state of the art in science and technology is defined in KTA 2501 'Structural waterproofing in nuclear power plants'. Waterproofing structures of various kinds are defined in Table 1.

Table 1 Types of waterproofing structure

High soil pressures from permanent loads cause the bitumen in bituminous waterproofs to 'extrude' laterally. This effect can be counteracted by inserting copper ripple plates in the bitumen strips.

Which waterproofing structure can be used depends primarily on what stresses act on a building waterproof. As well as the verifications required, installation and design issues and particular aspects of execution must also be taken into account.

3.2 Designing structural waterproofing

Structural waterproofing must be designed to withstand the stresses acting immediately on it. These include, on the one hand, the permanent compression, composed of water pressure and ground pressure or soil pressure as the case may be, and on the other hand the cracking in the structures bearing on the structural waterproof. As the manner in which the reinforced concrete structures which carry the structural waterproof crack depends on their design: there is a connection between designing those reinforced concrete structures and the design of the structural waterproof.

When designing structural waterproofing, the behavior must also be considered at pressures which are significantly greater than the limits stated in the rules. More extensive requirements of structural waterproofs in nuclear installations also result from the special load cases mentioned above. The stresses that these cause on structural waterproofs must be determined.

Design rules for structural waterproofs for bridging cracks are defined. They apply to stress cracks not more than 0.5mm at their point of origin and opening gradually over long periods of time up to 5mm. These design rules cannot be used for cracks several millimeters wide that open spontaneously or open and close rapidly due to the risk of the waterproof structure suffering fatigue cracks, even though tests on some waterproofing structures have proven that a structural waterproof is able to bridge such cracks to a limited extent.

3.3 Structural detailing

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)

- designing the structural joints (movement joints)

- structural joints in common sealed tanks

- structural joints between separate sealed tanks

- design of embedded parts

3.4 Designing the structural waterproofing

Structural waterproofing is best designed in two chronologically separate phases:

- inspection and permitting design phase

- execution design phase.

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

- standardized 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)

- details of waterproofing strategy

- details of waterproofing method

- service and special loads

- design rules for service loads

- making penetrations

- verification of suitability

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)

- 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.

3.5 Construction of the structural waterproofing

To provide permanent protection, structural waterproofing not only must it be designed professionally and quality assured, but it must also be built accordingly. When building the structure, the external conditions involved while making the structural waterproof play a governing role.

The issues of importance in connection with building the structure, that make a major contribution to quality assurance (as below), are:

- particular measures to be taken if constructing in bad weather

- temporary protection measures:

- protecting the structural waterproof on horizontal and gently sloping surfaces

- protecting waterproof connections

- protecting against thermal effects

- protecting the structural waterproof on wall surfaces when placing reinforcement

- protecting against penetrating groundwater, accumulating and surface water during construction

- protecting against the penetration of harmful substances

3.6 Quality assurance

For structural waterproofs to work perfectly, their design and execution must be quality assured. Both the processes and the materials involved must be tested and monitored.

There are a number of methods available for testing completed structural waterproofs.

Test results showing that a structural waterproof is as it should be must be recorded in test reports.

Inspecting the structural waterproof should be entrusted to a separate expert as part of the construction law approval process.

4. White tank

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..

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

4.1 System specification

4.1.1 General requirements

Reinforced concrete structures with high resistance to water penetrating prevent water penetrating permanently in liquid form. As well as bearing loads, they also act as structural waterproofing.

Designing and constructing white tanks in Germany is governed by the DAfStb's guidelines on 'Water-impermeable concrete structures', (WU guidelines), as the generally accepted rules of the art. The WU guidelines provide instructions on requirements for fitness for use of water-impermeable reinforced concrete structures.

DAfStb vol. 555 'Explanatory notes to the DAfStb guidelines' contains notes to the WU guidelines. Instructions are contained in DBV bulletin on 'High-grade use of basement floors - building physics and indoor climate'.

There are a number of constraints to be considered when building a white tank:

- nature of the moisture penetration

- design water levels

- type, characteristics and permeability of the subsoil

- chemical characteristics of the water

- establishing the type of use involved, particularly in the light of the stresses and special loads from external and internal events.

Supplementary drainage measures may be taken to protect the structure, particularly against non-pressured water or temporarily accumulating water. For drainage design, dimensioning and execution rules, see DIN 4095.

4.1.2 Engineering principles

Conditions of use

When establishing the basic design concept for a white tank, there are a number of local conditions to be considered:

- height of the reference groundwater level (and/or height of accumulating seepage water)

- height of the minimum groundwater level before starting use

- chemical composition of the groundwater.

If there are any liquid factors to be dealt with from inside, the following must also be established:

- maximum possible height of the liquid level surface

- chemical composition of the liquid

- maximum possible temperature of the liquid

- duration of the liquid load

Particular attention must be paid to limiting any water or other liquids escaping in the event of an incident.

Establishing categories of use under the WU guidelines cannot be applied directly to nuclear power plants, particularly because of what is required in the event of an incident. Instead, unique findings must be made for each individual structure, broken down by structural components, such as floor slab, walls and so on, if possible.

In terms of groundwater loads from outside, we need to define to what extent limited local access of groundwater can be accepted:

- in the phase of structural works, shell and core

- before commissioning

- in use

- during and after incidents

Requirements for liquid stresses from inside must be defined analogously.

Except in the event of an incident, external structures should be designed and dimensioned in accordance with the DAfStb guidelines on concrete structures when dealing with water pollutants.

Design principles

If separating cracks arise in WU structures, these can admit large quantities of water. Even fine cracks usually admit more water than can be displaced by air from the inside. How much water gets in depends mainly on how wide a crack is, how thick a structure is and how high the water level is.

When dealing with separating cracks, the design principles which can be used for nuclear power plant structures exposed to standing groundwater are:

- avoiding separating cracks

- designing to waterproof cracks on schedule prior to commissioning

- cracks waterproofing through self-healing

- designing to run off penetrating water

Whether individual design principles can be implemented must be considered in the light of the construction timetable.

The design principles selected must be justified and recorded, and the findings made must be included in contracts between the parties involved.

4.2 Particular requirements

The inside surfaces of white tanks must be kept permanently free for inspection purposes and to carry out any waterproofing which becomes necessary. Should they only be accessible by dismantling replacement parts, equipment and so on, this must be capable of being done with operations on the run. If access cannot be assured in the area of floor slabs under machinery and so on, structural design measures must be taken to ensure that, if any water does penetrate, it cannot cause damage and can be run off as designed. Wall claddings on the inside of WU structures (liners and tiles) are not allowed.

4.3 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.

4.4 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.

4.5 Penetrations

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.

4.6 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.

4.7 Quality assurance

Erecting and supervising the construction of structures in nuclear power plants are governed largely by laws and enacting provisions. Supervision is enshrined in law in Federal State building regulations.

As well as tests conducted by experts instructed by clients and approval authorities, testing is also conducted extensively based on companies' internal quality assurance systems.

4.8 Repairs

Waterproofing separating cracks and leaking joints and repairing faults is carried out using waterproofing agents to DIN 1504-1 and DAfStb guidelines on 'Protection and repair of reinforced concrete structures'. Such works must be accessible at all times.

Retrospective waterproofing is carried out using supervised and tested crack fillers such as epoxy resins (EP-I), polyurethane resins (PUR-I) and cement-bound systems (cement adhesive, ZL-I, and cement suspensions, ZS-I).

Water-bearing separating cracks must be waterproofed with polyurethane resin.

Polyurethane resin as a filler can be used to:

- close

- waterproof

- limit joint stretching

Specific material conditions must be taken into account when using this filler.

Cement-bound fillers can only be used to a limited extent, as the joint waterproofs they create are rigid and can only stretch to a limited extent.

Where repairs are required -- especially to safety-related structures -- the measures required must follow the latest state of the art of science and technology as far as is necessary at each stage, and carried out in accordance with repair regulations for power plants on site.

5. Waterproofing concept using the example of the OL3 nuclear power plant

We will now look in outline at the waterproofing concept selected at the OL3 nuclear power plant in Finland.

Buildings always conduct their loads via base slabs into the underlying rock. The difference in heights between the rock excavated and the underside of the building floor slab, which may be considerable in some cases, is made up for by using concrete with light constructive reinforcement.

The nuclear island structures below the 0.00m level are made partly as white tanks, restricting crack widths accordingly (_0.2mm in part). A largely stress-free support for the main buildings was achieved, and waterproofing was also provided against seepage and stratum water which can get to the structure via cracks and spaces in the rock in the shape of an external black waterproofing on all vertical wall surfaces from below the top of ground height up to +0.60m, and all horizontal upper structural surfaces, such as upper duct connections below ground height.

The waterproof is built up as follows:

Waterproofing on walls below ground surface level

- plastic-modified bitumen undercoat 0.2-0.3 l/m^2 (cold)

- one layer of plastic-modified bitumen strip.

Additional thermal insulation

- Polystyrol XPS foam, compressive strength _180 kN/m^2

- thickness depending on where applied, between 100 and 160mm.

Waterproofing upper structural surfaces below ground surface level

- sloping concrete at least 2%, at least 100mm thick, with constructive reinforcement (steel mat c/c100, d=6mm),

- plastic-modified bitumen undercoat 0.2-0.3 l/m^2 (cold)

- one layer plastic-modified bitumen coating.

Additional thermal insulation

- Polystyrol XPS foam, compression strength >250 kN/m^2

- thickness 140mm

The equalizing concrete has drains fitted to drain the joint between it and the rock. This drainage was tailored to suit the local rock structure (depths/interfaces): it leads any water which occurs into the drains surrounding the building, which in turn lead it to the pump shafts from which the water is then pumped out.

The drains around the building consist of perforated PP pipes DN 2_110 or 200 in drainage gravel 6/16, enclosed in geotextile wool. In areas concreted against vertical rock surfaces, drainage mats are used to ensure that the rock surface is drained.

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