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�APRIL 2011 The IndIan ConCreTe Journal

Concrete structures of nuclear power plants

Shylamoni P. and Prabir C. Basu

In a thermal power plant, water is converted to steam by applying heat energy. The steam produced with high pressure and temperature drives the turbine/generator producing electricity. Burning coal or oil or gas produces heat in case of fossil fuel fired thermal plant, while in nuclear power plant heat is generated by fission reaction of nuclear fuel in a reactor. Safety in design, construction and maintenance of concrete structures play important role in safe operation of a nuclear power plant.

Keywords: Nuclear power plant, safety classification, seismic classification, design classification, concrete structures, safety design bases, safety requirements of NPP concrete structures, safety and security against malevolent acts.

Three basic circuits of nuclear power plants are:1

Primary circuit: This system is housed within reactor building, generally consists of reactor to produce heat from nuclear energy; and primary heat transfer (PHT) system that transfer the heat from reactor through coolant to boiler commonly known as steam generator.

Secondary circuit: This system consists of steam generator, a heat exchanger within which water is converted to steam from the heat input carried by coolant; and turbo generator that generate electricity from the steam.

Third circuit: This is principally for transporting the residual heat from the condenser attached

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to turbo generator to the heat sink, which is generally a cooling tower.

Figure 1 shows the schematic diagram delineating these three basic systems of pressurised water reactor (PWR) based nuclear power plants (NPP). In this type of reactor, coolant is normal water or light water (H2O). India’s indigenous reactor is pressurised heavy water reactor (PHWR), Figure 2. PHWR based NPP uses heavy water (D2O) as coolant.

Civil engineering structure is part and parcel of a nuclear power plant irrespective of its type. Concrete is the commonly used civil engineering construction material in nuclear industries. Concrete, as a construction material of civil engineering structures, enjoys a wide range of acceptability because of a number of advantageous properties it has; mould-ability, easy manufacturing process, usage of mainly locally available ingredients, relatively less production cost, good strength in compression, etc. In addition, concrete has very good shielding property against the radiation effect, especially gamma radiation. Concrete mix can be tailor made depending on the functional requirements of structures using admixtures. For example, concrete can be manufactured for different values of density (low, medium and high), strength (normal, moderate, high and ultra high), permeability, wear resistance, shielding, heat resistance capability. Various types of concrete, such as normal concrete, heavy concrete, borated concrete are used in nuclear reactors.

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Concrete structure houses, protect and provide desired operation conditions of the systems, components and equipment of an NPP. Figure 3 depicts typical layout of 220 MWe PHWR based Indian NPP. Major concrete structures of commercial nuclear power plants are reactor building including support vault (vault supporting reactor pressure vessel in case of PWR and calandria vault in case of PHWR) and containment structures, spent fuel bay, stack, cooling tower, intake and out fall structure etc.

Nuclear fission produces radioactivity, which is injurious to the health of operators and public as well as for environment. Principal objective of engineering and operation of an NPP is to contain the activity within the envelope of designated buildings and ensure that the release remain within acceptable limit. This results in requirements of high safety level in design of concrete structure, stringent quality assurance in construction, and safe operational practice of concrete structures. In addition security is another safety concern for nuclear power plants. The present paper provides a brief account

of these safety aspects considered in engineering of NPP concrete structures.

Safety design basesAny industrial activity includes certain risks to human beings and the environment and requires an endeavour to keep the risks low. The typical risk of an NPP is connected with the potential hazard of ionising radiation. The goal of nuclear safety, basis for safe design, is therefore, to protect site personnel, the public and the environment by establishing and maintaining effective safeguard against the radiological hazard.

Objective of nuclear safety is to be achieved both in normal as well as abnormal conditions. The abnormal conditions could be of:

Accidental origin, or

Malafied origin

Accidental origin is unintentional adverse condition generated by malfunctioning of SSCs of NPP, or by

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natural phenomena. Malafied origins are those adverse conditions generated by the act of human being with an intention to cause harm to the plant.

Normal operation means operating of a plant or equipment within specified operational limits and conditions. In case of a nuclear power plant, this includes, start-up, power operation, shutting down, shutdown state, maintenance, testing and refueling. The accident condition is substantial deviations from operational states, which could lead to release of unacceptable quantities of radioactive materials. Figure 4 summarises major accident conditions considered in the design of an NPP. The accident conditions important for nuclear safety are principally of two types, conditions originated due to internal events and those due to external events. Examples of internal events that cause accident conditions are internal missiles, loss of coolant accident (LOCA), main steam line break (MSLB), failure of pipes causing internal floods, etc. Earthquakes, floods/tsunami, extreme wind, aircraft crash, etc. are the example of external events. Terrorist attack, sabotage, etc. causes abnormal conditions of malafied origin.

Ensuring the radiation exposure of the plant personnel, public and environment within appropriate prescribed limits under all operational states and within acceptable limits under all postulated accident conditions is the principal objective of safe design and operation practice of NPP. To ensure safety, means are provided for:2

Safe shutdown of reactor, and maintain and monitoring it in the safe shutdown condition in operational states, and during and after accident conditions,

Remove residual heat from the core after reactor shutdown, including accident conditions.

Reduce the potential for the release of radioactive materials and to ensure that any releases are below prescribed limits during operational states and below acceptable limits during accident conditions.

Design conditions like strength and serviceability arising out of the above safety functions is key to the safe design

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of concrete structures of an NPP. To achieve the desired level of safety, concept of as low as reasonably achievable (ALARA) is practised. In addition the design process incorporates defence in depth concept, i.e. multiple levels of protection are provided.

Safety requirements in design of concrete structures and buildings depend on the consequence of its failure. All buildings need not be designed for same level of safety. Stringency in design of a structure depends on the safety functions it needs to perform. Like other structures, systems and components (SSCs) of NPP, concrete structures are classified (safety, seismic, quality, and design) for this purpose, depending on their safety functions.3

Safety classification3

Safety classification of concrete structures are made based on the safety functions they performs. Safety functions with similar degree of importance can be put under one class. The highest ranked safety class has the most stringent design requirements. The following safety classification is adopted in the design of an NPP:

1) Safety Class-1The SSCs required to perform the safety functions necessary to prevent the release of a substantial fraction of core fission product inventory to the containment or environment are classified as Safety Class 1.

2) Safety Class-2This incorporates those safety functions necessary to mitigate the consequences of an accident. Safety Class-2 also includes those safety functions necessary to prevent anticipated operational occurrences from leading to accident conditions, and those safety functions, which helps in minimisation of propagation of accident.

3) Safety Class-3This incorporates those safety functions, which perform a support role to safety functions in safety classes 1, 2 and 3. It also includes those safety functions associated with decay heat removal from spent fuel outside the reactor coolant system and those associated with maintaining sub-critically of fuel stored outside the reactor coolant system.

4) Safety Class-4This incorporates those safety functions, which do not fall within Safety Classes 1, 2 or 3.

SSC falling under the category of Safety Classes 1, 2 and 3, known as SSC important to nuclear safety or nuclear safety class SSCs. Concrete structures associated with primary circuit and support systems fall under the category of nuclear safety class structure. The structures associated with secondary and third circuit are non-nuclear safety (NNS) class structures.

Seismic classification3

Two levels of earthquake are considered in design of NPP: “Safe Shutdown Earthquake (SSE)” and “Operating Basis Earthquake (OBE)”. SSE is that earthquake which produces the maximum vibratory ground motion for which certain structures, systems and components are designed to remain functional. Return period for SSE is considered as 104 years. It is based upon an evaluation of the maximum earthquake potential considering the regional and local geology and seismology and specific characteristics of local sub-surface material. OBE is that earthquake which produces the vibratory ground motion for which the features of NPP necessary for continued operation without undue risk to health and safety of the public are designed to remain functional. It is that earthquake which, considering the regional and local geology and seismology and specific characteristics of local sub-surface material, could reasonably be expected to affect the plant site during the operating life of the plant. The return period for OBE is considered as 100 years.

Seismic classification of all the plant buildings and structures, and other non-plant structures are classified into three seismic categories in terms of their importance to safety in the event of an earthquake.

1) Seismic Category-1Items coming under this category are designed for SSE and OBE. This includes:

Items whose failure could directly or indirectly cause accident conditions.

Items required for shutting down the reactor, monitoring critical parameters, maintaining the reactor in a safe shutdown condition and removing residual heat for a long term.

Items that are required to prevent radioactive releases.

Items that are required to maintain releases below limits established by the Regulatory Body during design basis accident conditions (e.g. containment system).

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2) Seismic category-2This includes:

Items required preventing the escape of radioactivity beyond limits prescribed for normal operation.

Items required mitigating those accident conditions, which may last for such long periods

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that there is a reasonable likelihood that an earthquake of the defined severity may occur during this period.

An item coming under this category is designed for OBE only.

3) Seismic Category-3This includes all items, which are not safety related and not covered in Seismic Category-1 or 2. Design of these items may be carried out following IS 1893.

Requirements of seismic classification of NPP concrete structure are summarised in Table 1.

Quality classification3

The quality requirements of structures, systems and components are commensurate with their Safety Classification. Accordingly, Quality Classification is assigned. The concrete structures which are of Safety Class and which fall under Seismic Categories 1 and 2 shall meet the quality requirements laid down in Safety Codes/Standards of nuclear components.

Design classification3

The civil engineering structures are categorised into four Design Classes (DC) depending on the design approach, requirements, and criteria. These classes are:

DC1 : Pressurised Concrete Reactor Vessels (PCRVs)

DC2 : Containment Structures.

DC3 : Internal structures of Reactor Building, Auxiliary and safety related balance of plant

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Table 1. Seismic classification requirements of NPP concrete structuresSeismic classification Buildings / structures Earthquake levels

Seismic Category – 1 1) Reactor Building including containment, calandria vault and internal structures2) Reactor Auxiliary Building3) Service Building4) Spent Fuel Building, Spent Fuel Bay, Spent Fuel Transfer Duct5) Control Building6) Diesel Generator / Station Auxiliary Building 7) Induced Draft Cooling Tower8) Stack9) Safety related pump house10) Fire water pump house11) Safety Related Tunnels & Trenches

SSE and OBE

Seismic Category – 2 1) Waste Management Building2) D2O Upgrading Plant

OBE

Seismic Category – 3 1) Turbine Building2) Other pump houses

IS 1893


buildings and structures of an NPP; and Civil engineering structures of other nuclear facilities.

DC4 : Non-Safety Class Structures.

Concrete structures of nPP4

Most of the civil engineering structures of NPP are concrete structures. Important concrete structures of a 220 MWe PHWR based NPP are described briefly.

Reactor building (RB)Reactor building is the most important building of an NPP. Typical cross section of reactor building is given in Figure 5. It consists of inner and outer containment and internal structures. Details of these buildings are given below:

1) Inner containment structure (ICS)Inner containment structure is generally a prestressed concrete structure. It consists of inner containment wall (ICW), inner containment dome and base raft. ICW is vertical cylindrical prestressed shell with small and large openings for entry of people and equipment. It is capped with prestressed concrete segmented hemispherical dome with four large opening for erection of equipment. Base raft is RCC thick circular slab. ICS is designed for Safety Class – 2 and Seismic Category – 1. Important functional roles of ICS are; providing operating environment for reactor, to mitigate the effects of accident conditions like LOCA and main steam line break (MSLB) by providing primary barrier to contain core fission product and radioactivity within acceptable limit, providing adequate radiation shielding along with secondary containment structure during normal and accident conditions, providing passages for movement of personnel and equipment and for services through penetrations and to withstand all postulated loading conditions.

2) Secondary containment structure (SCS)SCS contains of secondary containment wall (SCW) and dome. Its configuration is RCC vertical cylindrical shell with large and small openings and capped with RCC segmented hemispherical shell with four number of large openings. Important functional roles of SCS are protecting PCS from natural external events, providing operating environments of primary containment systems, and SSCs housed and supported within annular space, adequate radiation shielding along with PCS, provide passages for movement of personnel and equipment and for services through penetrations, resisting earth pressure and ingress of subsurface water, and withstanding all postulated loading conditions.

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SCS is designed for Safety Class – 3 and Seismic Category – 1.

3) Base raft (foundation)Base raft of reactor building is circular reinforced concrete plate structural element, thickness varies from 3.5 m to 5.0 m, with annular opening of size 2.0 m X 2.0 m embedded within it below prestressed concrete wall to provide space for stressing of cables traversing through ICW and IC dome. Important functional roles of base raft are; common foundation of IS, ICW and SCW, providing primary containment envelop along with ICW and IC Dome, water retention capability, mitigating the effect of earthquakes on ICS, SCS, IS, reactor, and associated SSCs housed inside the reactor building and to withstand all postulated loading effect. Raft is designed for Safety Class – 3 and Seismic Category – 1.

4) Internal structures (IS)Calandria vault, floors, walls and other vaults make the internal structures of RB. These are multistoried RCC structure built-up primarily with walls and slabs; total height is about 50 m out of which 13.0 m embedded in ground. Important functions of IS are; housing and supporting of reactor & other associated SSCs, providing operational environment, radiation shielding, mitigating the effect of external events such as earthquakes, mitigating the effect of accident conditions due to internal events and to withstand all postulated loading conditions. IS is designed for Safety Class – 3 and Seismic Category – 1.

Spent fuel storage bay (SFSB)Spent fuel storage bay consists of spent fuel building (SFB) and spent fuel pool (SFP). Configuration of SFB is multistoried RCC frame structure with basement. SFP is a water leak tight pool type structure mostly embedded in the ground with RCC wall and raft. SFSB is designed for Safety Class – 3 and Seismic Category – 1. Important functions of SFB is to house, support and mitigate the effect of earthquakes on SSCs, to house spent fuel bundles inside the pool water, to protect radiological hazards and withstanding all postulated loading conditions including earth and water retention capability.

Control building (CB)Configuration of CB is multistoreyed RC framed structure with basement. Important functional roles are housing and supporting SSCs, mitigating the effects of external events like earthquake, rain, wind, solar radiation etc, providing operational environment for SSCs, earth and water retention capability and to withstand all


postulated loading conditions. It is designed for Safety Class – 3 and Seismic Category – 1.

Service building (SB)Configuration of SB is also a multistoried RC framed structure with basement, Figure 6. Important functional roles are housing and supporting SSCs, mitigating the effects of external events like earthquake, rain, wind, solar radiation etc, providing operating environment for SSCs, earth and water retention capability and to withstand all postulated loading conditions. It is designed for Safety Class – 3 and Seismic Category – 1.

StackStack is a self-supporting vertical cantilever tapered slender structure with circular cross section. Important functional role is to provide suitable conditions of ventilation system, to withstand the effects of external events like earthquake, rain, wind, solar radiation etc, and to withstand all postulated loading conditions. It is designed for Safety Class – 3 and Seismic Category – 1.

Induced drought cooling towers (IDCT)Configuration of IDCT is RCC framed structure with peripheral and baffle wall and basement. Important functional roles are supporting SSCs for water cooling, mitigating the effects of external events like earthquake, rain, wind, solar radiation etc, basement to have earth and water retention capability and to withstand all postulated loading conditions. It is designed for Safety Class – 3 and Seismic Category – 1.

Diesel generator building (DGB)Configuration of DGB is multistoreyed RC framed structure with basement. Important functional roles are housing and supporting SSCs including diesel generators, mitigating the effects of external events like earthquake, rain, wind, solar radiation etc, providing operational environment for SSCs, earth and water retention capability and to withstand all postulated loading conditions. It is designed for Safety Class – 3 and Seismic Category – 1.

Reactor auxiliary building (RAB)RAB is multistoried RC framed structure with basement. Important functional roles are housing and supporting


SSCs, radiation shielding, mitigating the effects of external events like earthquake, rain, wind, solar radiation etc, providing operational environment for SSCs, earth and water retention capability and to withstand all postulated loading conditions. It is designed for Safety Class – 3 and Seismic Category – 1.

D2O upgrading structureD2O upgrading building (UB) and D2O upgrading tower (UT) are part of this structure. UB is multistoried RC framed structure with basement. UT is self-supporting cantilever steel structure cladded with GI sheets and supported on RCC foundation. Important functional roles are housing and supporting SSCs, mitigating the effects of external events like earthquake, rain, wind, solar radiation etc, providing operational environment for SSCs, and to withstand all postulated loading conditions. UT will be supporting D2O distillation column also. It is designed for Safety Class – 3 and Seismic Category – 2.

Turbine building (TB)TB consists of building and turbo generator foundation (TGB). Building is multistoreyed RC framed structure with basement and TGB is multistoreyed framed structure. Important functional roles of TGB are to support turbo generator (TG). TB to house SSCs including TG for power generation, mitigating the effects of external events like earthquake, rain, wind, solar radiation etc, providing operational environment for SSCs, earth and water retention capability and to withstand all postulated loading conditions. It is designed for Safety Class – NNS and Seismic Category – 3.

Safety requirements of nPP concrete structures3

Concrete structures serve the purpose of nuclear safety in two ways, it supports or houses the safety related SSCs such that no fault can occur due to the effects of certain postulated initiating events (PIEs), which might otherwise have caused release of activity. Secondly, given a condition, the release of activity beyond the structural boundary of building is kept within permissible limit. Adequate provisions are made in design for conducting tests as required during commissioning, operation and in-service inspection to achieve the above purposes.

Ageing degradation of SSCs, often caused or accelerated by factors related to exposure to hostile environment or inadequate measures for quality assurance or deficiency in engineering or their combination, could impair their performance related to intended safety functions and

thus results a potential risk to public health and safety. In coastal site, risk of corrosion of outdoor structures is high particularly for the structures made of reinforced concrete, steel and other embedded parts. Measures against ageing degradation are considered in the design and construction of the structures. In addition, effective ageing management of these structures are planned and implemented to ensure their fitness-for-service throughout the service life.

Nuclear safety related NPP concrete structures like any other structures are engineered for industrial safety. In addition, the requirements of radiological safety are adhered to in design. Security against malafied activities is also a concern. Non-nuclear safety related structures are engineered in similar way that followed in normal industrial facilities.

Industrial safetyDesign for industrial safety is mainly carried out by adhering to atomic energy factory rules. Job hazard analysis for most of the construction activities is carried out. Steps to mitigate hazards associated with construction activities are taken into account. Measures including monitoring of fitness of equipment and machinery are strictly implemented.

Protection against fire hazard is important for NPP structures. This consists of direct measures (i.e. fire detection and fighting) and passive or inbuilt measures. The provision of passive measures includes choice of fire resistant materials, barriers, etc. in construction of building structures.

For fire resistance design of structural elements, a concept of fire rating is adopted. The fire rating indicates the time for which the element, when exposed to fire, retains its structural and functional integrity. The fire rating depends on factors like fire resistance properties of the exposed material, source and type of fire, fire inventory, provision for direct measures and availability of area for direct measures. The design value of fire rating is assigned on the basis of safety functions and requirements of performance of the safety functions. Buildings important to safety are designed for fire ratings, generally, as given below:

Roof and external cladding : 3 hours

Internal walls, slabs and any fire barrier : 3 hours

Any load bearing structural component : ≥ 2 hours

When a structural element of a building or structure passes through more than one compartment or room,


the design fire rating of the element is taken the highest value of the fire rating of the rooms or compartments through which it is passing.

Radiological safetyFor radiological protection, structural members having shielding requirements are designed satisfying the following:

Required shielding properties (high density concrete; about 3600 kg/m).

Cross sectional dimensions of a structural element are determined satisfying both structural strength and shielding requirements.

No linear through crack across the thickness is allowed.

Construction joints, expansion joints etc. are staggered/stepped.

Safety & security against malafied activitiesBest way to ensure safety and security of NPP against human induced abnormal condition of malafied origin, like terrorist attack, is engineering of appropriate safety and security system, their proper operation and implementation of emergency plan in the event of the disaster. For safeguard, a comprehensive and integrated safety and security system (S&SS) backed up by strong administrative and operational measure is required. The design and operation of S&SS are based on technological, physical and administrative factors.

Safety and security system (S&SS)5

S&SS provides mitigation measures by means of deterring, delaying and denying terrorist attack. Major component of S&SS is physical protection system consisting of the following,

Fencing/barrier

Access control for personal, vehicle and material

Fire/blast resistance compartment for vital areas

Blast resistant design of building and structure

Surveillance system

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Surveillance system principally includes monitoring of the building areas in order to detect any activities that could lead to terrorist attack or sabotage. For systematic implementation and operation of S&SS in the event of an attack, a number of procedures are required to be developed.

Audit, and training of both security personnel as well as operator are important for effective implementation of S&SS.

Security consideration in design of NPP concrete structure5,6

Engineering of S&SS system covers establishment of design basis threat, design and execution of S&SS.

1) design basis threatIt is credible characteristic and attributes of a potential adversary (internal and external). The design basis threat (DBT) is characterised from the knowledge on who are the adversaries (their number, sophistication, knowledge, skill, and perseverance); intention (political, economical, publicity); and weapon, tools and tactics. The attributes of DBT are derived from the above characteristics and are specified in terms of the following parameters:

potential to kill/injure people and deactivation of core function of the structure,

loading effect on structures due to missiles and blast, and

duration.

2) BlastPrincipal objective of security consideration in design of NPP concrete structure is to mitigate the hazardous effect of missiles and blast. The hazards are blast wave, fire, missiles and flood.

Blast resistant design6

Blast resistance design of concrete structures constitutes an important component of physical protection system.6 Major features of blast resistant design are:

Identification of vital areas

Appropriate layout of building structure to make access difficult and access to vital area more difficult adopting the layout as passive defense concept.

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1�APRIL 2011 The IndIan ConCreTe Journal

Fire rating of the critical components of structure

General blast resistant design criteria

Special blast resistance design criteria

Qualification of designer

design requirements of nPP concrete structureAll concrete structures are safely designed against the loading effects caused during construction, normal operation conditions and accident conditions. Important requirements for safe design are grouped in the following three,

Integrity

Serviceability (including stability)

Durability

These design requirements are satisfied by three basic characteristics; strength (integrity), stiffness (for serviceability) and ability for continuous performance of intended functions in desired way over the service life (durability). Deterioration of these properties degrades concrete structure.

StrengthThe design for strength of concrete structures generally results in finalising cross sectional area and reinforcement quantity in structural element. The design is influenced by plant layout, structural configuration, and the assignment of stiffness to the structural elements. Plant layout as well as building layout are developed with proper consideration of system performance and functional requirements. Effort is made to minimise uncertainties in design at the conceptual stage. The plant layout and configuration planning of individual buildings and structures are made in such a manner that well established methodology can be applied in analysis and with established assumptions. Conceptual development is made such that the design problem could be solved with the help of present state of art. Relevant requirements of industrial safety are need to be satisfied in layout.

ServiceabilityAll serviceability requirements such as deflection, crack width, etc. are determined from the safety as well as functional requirements of structures, systems

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and components. Structures are designed for all these serviceability requirements.

Structures are designed to satisfy the following stability requirements, as applicable:

Elastic stability

Foundation stability against overturning, sliding and floatation

Stability against aerodynamic effects

DurabilityIn earlier days, it was believed that concrete is a durable material, which was found not to be correct with experience. Further, concrete is not a stable material. It undergoes volume changes due to secondary effects like shrinkage, creep, etc. Concrete structure is generally a passive system; its durability is endangered by the effect of operating environment of the plant as well as ambient and surrounding environmental condition. Factors related to such undesirable environment, both operating and natural, are stress, temperature, erosion, corrosion, radiation effect, hydrogen embrittlement, vibration, fatigue and fretting. Formation and/or growth of algae, barnacle etc. also causes defects in structure. The structural defects such as dimensional imperfections, discontinuity, irregularity or fault in the structure such as crack, crevice, spalling, delamination, cavity, porosity, etc. aggravates the situation.

engineering of concrete structures3

Concrete structures of NPP are engineered to meet their safety as well as security requirements, as per their classifications, in the following stages:

Layout

Design

Construction

Pre-commissioning Tests

Operation

Decommissioning

LayoutDevelopment of plant layout and structural layout are the important activities coming under this.

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Plant layoutThe conceptual plant layout development is carried out considering the following:

The requirements arising out of system performance and safety functions are satisfied.

Requirement of radiation zoning is fulfilled.

Proper segregation of plant areas are achieved and are consistent with plant safety requirements.

Buildings and roads are so laid out that unobstructed access is always available.

Proper turning radii at road curves and gradients are provided for the movement of heavy crane and other vehicles.

Provision is made for space around buildings for erection facility, cranes, etc. during construction.

Buildings and structures important to safety are placed outside the area prone to low trajectory turbine missiles.

Sufficient gap for seismic isolation or shake space between adjacent structural parts or buildings is provided.

Requirements arising from other site specific conditions are accounted for.

Proper access control measures are to be provided.

Security consideration.

Structural layoutThe structural configuration is developed with following considerations.

Plant and system safety requirements are satisfied.

All emergency requirements arising out of industrial and nuclear safety are satisfied.

The safety related systems and components of similar safety class/ seismic category are located and placed suitably in buildings/ structures of appropriate classification as far as possible.

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Structural connections between different safety class structures and seismic category structures are avoided as far as possible.

The structural system of individual building is as simple, symmetrical and regular as possible.

Avoiding protruding sections (lack of symmetry) as far as practicable.

Locating the centre of gravity of structure as low as possible.

Making the centre of rigidity at various elevations as close to the centre of mass at that elevation as practical.

Internal arrangement of structures should be such that less important structural elements would protect the more important ones to a good extent.

Materials are so selected that the safety of the building is enhanced.

Avoiding use of different grade of concrete for primary structural elements of the same structure as far as practicable.

Direct and easy emergency escape routes with reliable lighting and other building services for the use of the plant personnel are provided.

Access planning to ensure effective control of personnel movement for preventing spread of radioactivity within the plant and outside to be made. For this purpose, adequate monitoring, washing and change facilities are provided with clear demarcation or barricades between the various radiation zones.

Personnel and equipment accesses to the reactor building through air locks should ensure that separation of the containment environment from the outside environment is achieved at all times.

Provision of fire protection.

Placement of foundation of all adjacent buildings and structures to be done in order to reduce differential settlement between the adjacent buildings and structures as much as practicable.

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Overlapping of foundation of different structures are avoided as far as possible.

Easy maintenance and surveillance.

Security consideration.

DesignMethodologyDesign methodology followed for concrete structures depends on the design classification. Both working stress and limit state method is adopted depending on design classification of the structures; Table 2 summarises the methodology. Acceptable design standard of pressurised concrete reactor vessel is ASME Section III, Div-2.7 The containment structure (DC2) is designed following ASME Section III, Div-2 (CC 3000).7 All other safety related concrete structures are designed using AERB/SS/CSE-1.8 IS 456 is applicable for NNS class structure.9

loads and load combinationsCharacterisation of individual loads is given in Annexure – I. The following load combinations are generally considered in the design of buildings/structures:

LC1 : Normal Load combinations – The normal load combinations involve only normal loads.

LC2 : Severe Environmental Load Combinations – These load combinations include normal and severe environmental loads.

LC3 : Extreme Environmental Load Combinations – Normal and extreme environmental loadings are included in these load combinations.

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LC4 : Abnormal Load Combinations – These load combinations include normal and abnormal loads.

LC5 : Abnormal-Severe Environmental Load Combinations – These load combinations include normal, severe environmental and abnormal loads.

LC6 : Abnormal-Extreme Environmental Load Combinations – These load combinations include normal, abnormal and extreme environmental loads.

Safety factorsDepending upon the load combinations to be considered in the design, two types of design conditions are specified:

Normal Design Condition: This includes the Load Combinations LC1 and LC2.

Abnormal Design Condition: This includes the Load Combinations LC3, LC4, LC5 and LC6.

In limit state design method total safety is divided into two parts; partial safety factors to load or load factors (gf), and partial safety factors to material (gm). Load factors for the load combinations, described in proceeding section, for DC3 class structures are given in Table 3. Since the abnormal design condition involves loads, which are less frequent, lower material safety factors are specified so that the design is not dominated by the abnormal condition alone i.e. risk involved with each of these design conditions is optimised. Partial safety factor (gm) to materials considered for normal and accident conditions is given in Table 4.

In the case of safety for foundation stability, no increase in soil bearing capacity is allowed for earthquake condition. Loss of contact for foundation is limited to one third of the foundation area. Factor of safety (FOS) against overturning and sliding considered in design is given in Table 5.

analysisStatic analysis is carried out for all types of static loadings. Objective of dynamic analysis of structures is to determine structural response under seismic or other design basis dynamic loadings. Linear structural analysis is used to evaluate structural response for the design of new plants. However, nonlinear analysis may be required in certain cases like raft lift-off analysis due to seismic excitation. With the advent of high speed computer, detailed finite element model is now common.

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Table 2. Design methodology for concrete structures of NPP

Structures / Buildings Design classification

Design method

Pressurised concrete reactor vessels (PCRVs)

DC1 WSM

Containment structures DC2 WSM & LSM

Internal structures of reactor buildings, auxiliary and safety related balance of plant buildings and structures of NPP; and civil engineering structures of other nuclear facility

DC3 LSM

Non-safety class structures. DC4 LSM

WSM = Working stress methodLSM = Limit state method


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t

F h

Nor

mal

des

ign

cond

ition

LC1:

Nor

mal

load

co

mbi

natio

ns1

1.0

1.4

1.6

1.6

1.6

--

--

--

--

--

--

-

20.

751.

41.

61.

61.

61.

4-

--

--

--

--

--

-

31.

01.

41.

61.

61.

6-

-1.

6-

--

--

--

--

-

40.

751.

41.

61.

61.

61.

4-

1.6

--

--

--

--

--

51.

01.

41.

61.

6-

-1.

6-

--

--

--

--

--

LC2:

Sev

ere

envi

ronm

enta

l loa

d co

mbi

natio

ns

61.

00.

9-

--

--

-1.

6-

--

--

--

--

71.

01.

41.

61.

61.

6-

--

1.6

--

--

--

--

-

80.

751.

41.

61.

61.

61.

4-

-1.

6-

--

--

--

--

91.

01.

41.

61.

61.

6-

-1.

61.

6-

--

--

--

--

100.

751.

41.

61.

61.

61.

4-

1.6

1.6

--

--

--

--

-

Abn

orm

al d

esig

n co

nditi

on

LC3:

Ext

rem

e en

viro

nmen

tal l

oad

com

bina

tions

111.

00.

9-

--

--

--

1.0

--

--

--

--

121.

01.

01.

01.

01.

0-

--

-1.

0-

--

--

--

-

131.

01.

01.

01.

01.

01.

0-

--

1.0

--

--

--

--

LC4:

Abn

orm

al lo

ad

com

bina

tions

141.

01.

01.

0-

--

--

--

1.25

1.0

1.0

--

--

1.0

151.

01.

01.

01.

01.

01.

0-

1.0

--

--

--

--

1.0

-

LC5:

Abn

orm

al

seve

re

envi

ronm

enta

l loa

d co

mbi

natio

ns

161.

01.

01.

0-

--

--

1.15

-1.

151.

01.

01.

01.

01.

0-

-

171.

01.

01.

01.

01.

01.

0-

1.0

1.0

-1.

0-

--

--

1.0

-

LC6:

Abn

orm

al

extr

eme

envi

ronm

enta

l loa

d co

mbi

natio

ns

181.

01.

01.

01.

0-

--

--

1.0

1.0

1.0

1.0

1.0

1.0

1.0

--

Not

e: 1

. All

load

com

bina

tions

sha

ll be

che

cked

for f

ull a

nd z

ero

live

load

con

ditio

ns2.

Effe

ct o

f lat

eral

ear

th p

ress

ure

shal

l be

cons

ider

ed in

des

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whe

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ritic

al


Table 5. Factor of safety considered for various design conditions

Loading Condition

Overturning Floatation Sliding

OBE 1.5 - 1.5

SSE 1.1 1.1 1.1

Figure 7 depicts typical models for static analysis of containment structure and seismic response analysis of complete reactor building.

Construction3

General requirements for construction of concrete structures of an NPP are:

The construction methodology is so adopted that the design intents of the buildings/structures are satisfied.

The sequence of construction is such that the construction activities of any building do not jeopardise the safety of the adjacent or nearby buildings/structures or part of it which has already been constructed.

1.

2.

Requirements of industrial safety as per Atomic Energy (Factories) Rules to be satisfied.

3.

Table 4. Partial safety factor to materials Material gm for normal condition gm for accident condition

Concrete 1.5 1.3

Steel 1.15 1.0


T h e c o n s t r u c t i o n m e t h o d o l o g y a n d sequences should have due considerations of decommissioning.

Adequate experiments are made by way of mock-up simulation or by way of laboratory experiments whenever difficult construction is foreseen or new equipment and methods are employed.

Strict quality control is followed.

Emphasis is made on the following activities for concreting; shuttering, concrete production, concrete transportation, concrete placements, compaction (especially the need for vibrator qualification), curing and post concrete inspection including the documentation of findings.

Pre-Commissioning Tests3,12

Prior to commissioning of the plant, tests on certain civil engineering structures are performed. This includes structural integrity test and leakage rate test of containment structure, hydro test of spent fuel pool and water tanks and shielding adequacy including radiation streaming of civil engineering structures, if identified during engineering stage. Leakage control test of chimney, if specified, during design stage is also conducted. Such tests are carried out following acceptance criteria as specified in AERB safety documents.

Operation3

MonitoringProvision, wherever necessary, is made for monitoring of the structures. Adequate instrumentation to collect data on parameters such as temperature, strain, deformation, settlement, vibration, deterioration and leakage for monitoring and assessment of structural behaviour and ageing management as well as for life extension studies are provided. Requirements for strong motion seismic instrumentation of NPPs are also met.

Maintenance during operationMaintenance during operation is carried out as preventive measure of wear and tear of the equipment and structures. Maintenance activities are of two types; preventive and breakdown. Objective of preventive maintenance is to prevent failure by means of regular checking and inspection. Breakdown maintenance is carried out after failure of equipment for repair. Concrete

4.

5.

6.

structures being passive system, preventive maintenance is the major aspect. Preventive maintenance has strong bearing on the life of structure.

ageing managementLike any created element, a concrete structure has a life. This life is often shortened under the effect of degradation. Appropriate management of concrete structures not only ensures life against such degradation but also extent its life. One important task of concrete structure management during the service period is inspection. In view of this, it is necessary to implement appropriate ageing management program of the civil engineering concrete structures for continued safe operation of nuclear facilities.

decommissioning13

Decommissioning of a nuclear facility are the actions taken at the end of its operating life to retire it from service in a manner that provides adequate protection for health and safety of workers and the general public and the protection of the environment. Concrete structures important to safety are required to perform safety functions during entire operating life of the plant and some of them are required to be serviceable depending upon the stage of decommissioning or part of the plant even after decommissioning of the plant. Following are the considerations at the design stage with regard to decommissioning:

Identification of buildings/structures which are to be kept under surveillance for a long time after decommissioning of the facility and development of suitable design criteria for these buildings, including consideration of PIEs during decommissioned period.

Structural layout to facilitate removal of structures, systems and components prior to dismantling of the building.

Design the structure such that it would facilitate dismantling

Suitable measures, such as appropriate surface finish, surface hardness, painting etc. for easy decontamination. The painting shall withstand requisite radiation field.

Limiting the consequences of degraded structural elements of buildings.

1.

2.

3.

4.

5.


Inspection10,11

All the tasks concerning the management of concrete structures during its life cycle involve with either inspection or results of inspection. Inspection of concrete structure is important in accomplishing the safety. Inspection is examination, observation, measurement or test undertaken to assess, structures, systems and components and materials, as well as operational activities, technical processing, organisational process, procedure and personal competence.

Audit is a systematic independent and documented process for obtaining evidence and evaluating it objectively to determine the extent to which identified criteria are fulfilled. Difference between audit and inspection is that in the first one verification of findings/observations are done with respect to established criteria, while in second one aims for making observations.

Inspection, with respect to concrete structures, is generally an organised and formal exercise involving with physical examination, measurement, test and gauges applied to certain characteristics of the structure with defined objective. Inspection of concrete structure is conducted in different stages of its life cycle – construction, pre-service and during operation. Inspections for maintenance and in-service inspections are two common types of inspection conducted for the concrete structures during operation period of the plant.

Inspection during constructionInspection during construction of concrete structures is carried out to assess compliance of various procedures during pre-construction, construction and post construction stages. The important factor to be considered in determining the extent of this inspection is the potential of the construction and construction methodology to incur errors, omissions which might result in distress of the structure and non-compliance of other design intents such as shielding requirements in appropriate implementation of quality assurance plan etc.

Pre-service inspectionA pre-service inspection is performed before the commencement of operation to provide data in initial conditions supplementing construction data as a basis for comparison with subsequent in-service inspection. In this inspection, similar in method, techniques and use of equipment as those which are planned to be used later on, as far as practicable is adopted.

Maintenance inspection during operationInspection for preventive maintenance is to find out normal wear and tear during operation. It is conducted in shorter frequency following laid down procedure. It is different from breakdown inspection, which is carried out following accidental condition. Objective of preventive maintenance is to detect whether any damage occurred in the structure, if so the extent of damage. Intention of this type of inspection is to assess the extent of damage for working out the repairing measure, if needed.

In-service inspectionThe in-service inspection program forms part of measures to be undertaken by the operating organisation in ensuring the safe operation of the plant. Principal objective of in-service inspection is to evaluate the status of structures against the effects of deterioration factors and with respect to continued safe performance. Specification for in-service inspection and its requirement may be different for different structures depending on the type of concrete used and functional requirements. Various methods and tools are used for the in-service inspection. One of the important aspects of the in-service inspection is to draw conclusion on the cause of deterioration, effect of deterioration, and also to identify the probable measures for rectification.

Quality assurance program14

An overall Quality Assurance Program (QAP) in respect of civil engineering structures covering all phases of a Nuclear Power Plant viz. design, construction, commissioning, operation and decommissioning is developed and implemented in each phase so as to achieve adequate assurance on quality and safety. The detailed QAP for each constituent phase forms part of this overall QAP. Requirements of both overall QAP and the relevant detailed QAP for constituent phase meet the requirements of applicable AERB Safety Codes and Guides. The program requires comprehensive planning, organisation, implementation (task performance), verification and certifications appropriate to task necessary to assure the requisite quality.

The QAP contains details in respect of the following; Quality policy, management, performance functions, quality control which broadly includes verification and corrective functions, documentation and audit. Manual for quality assurance programme is prepared for each project. Systematic and documented internal and external audit are carried out to verify compliance with, and to determine the effectiveness of the various


Annexure I. Characterisation of individual loadsIndividual Load Class Category

Name Symbol

Dead Load1 DL Static Normal

Prestressing Force F Static Normal

Lateral Earth Pressure H Static Normal

Loading effect due to Support settlement SST Static Normal

Equipment load (numbered) EQ Static Normal

Live Load2 LL Static Normal

Hydrostatic load HS Static Normal

Test Pressure Pt Impulsive3 Normal

Pressure load resulting from pressure variation either inside or outside the containment Pv Impulsive3 Normal

Operating Temperature To Static Normal

Thermal Load during pressure testing Tt Static Normal

Loading effect due to the solar radiation SR Static Severe environmental

Reaction due to pipe, etc. Ro Static Normal

Operating basis earthquake (OBE) Eo Impulsive3 Severe environmental

Loading effect due to external flooding (design basis flood) FF Impulsive3 Severe environmental

Severe wind load Wc Impulsive3 Severe environmental

Maximum differential pressure generated from postulated accident used as design basis accident Pa Impulsive3 Abnormal

Pipe and equipment reactions generated by postulated accident used as design basis and including Ro

Ra

Impulsive3 Abnormal

Hydro-dynamic loading- Due to OBE- Due to SSE- Due to Design Basis Accident

HM Impulsive3 Severe

environmentalExtreme

environmentalAbnormal

Maximum attainable temperature due to postulated accident used as design basis Ta Impulsive3 Abnormal

Loading effect due to internal flooding F2 Impulsive3 Abnormal

Loading due to pipe rupture, jet impingment pipe whip and pressure transient- Jet impingment- Pipe whip- Reaction due to pipe whip

YjYmYr

Impulsive Impulsive Impactive

AbnormalAbnormalAbnormal

Missiles- Due to land, water and air transport- Missiles due to external events like turbine and other rotary machinery disintegration- Internal missiles

MTMEMI

ImpactiveImpactiveImpactive

AbnormalAbnormalAbnormal

Loading effect due to aircraft impact MA Impactive Abnormal

Drop loading LD Impactive Abnormal

Safe shutdown earthquake Ess Impulsive3 Extreme environmental

Extreme wind load (wind induced missiles only)

Wt Impactive Extreme environmental

Notes: 1. Effect due to shrinkage, heat of hydration etc. pertaining to concrete structure will fall into the category of dead load. 2. For convenience live load may be subdivided into Live load during normal condition (LLn) and Live load during shutdown

condition (LLs). 3. The element sections could be designed for these loadings considering the effect as static type though the structural response may

be determined by dynamic analysis.


elements of QAP. Adequate QA documents are prepared and maintained in a systematic manner for easy retrieval to provide objective evidence of quality to meet the requirements of applicable Standards, Codes, Guides and Specifications. During all stages of NPP, QAP need to be implemented stringently to achieve required quality in design, construction and operation.

SummaryThe paper deals with the safety in different aspects of concrete structures of nuclear power plants. Brief description of major concrete structures of an NPP is presented. The paper covers the safety design bases, and requirements for safety, design and security. Various requirements in engineering of NPP concrete structures, i.e. design, construction and operation are also addressed.

Plant layout and structural layout; design for integrity, serviceability and durability, construction and operational aspects are important for accomplishing the safety of nuclear power plants. Inspection of concrete structures at various stages plays important role in achieving as well as sustaining safety during the operational life of nuclear power plants. All life cycle activities are performed following stringent quality assurance plan.

Safety requirements in engineering of nuclear power plant concrete structures are not same for all. Stringency in design of a structure depends on the safety function it needs to perform. For this purpose structures are classified from safety considerations as well as seismic design point of view. Based on these two classifications, design classification of structure is made. Requirements for engineering of nuclear power plant concrete structure are formulated on the basis of these classifications.

From security considerations, main concern of nuclear power plant structures is its resistance against missile and blast loading. The loading effect due to blast and missile are derived from the design basis threat.

AcknowledgmentThis paper was previously published at the fib days 2009, an International conference during January 29-30, 2010 at Kolkata and is reproduced here for wider dissemination.

referencesSamuel Glasstone, Alexander Sesonske, Nuclear Reactor Engineering (1994).Atomic Energy Regulatory Board, Design for Safety in Pressurised Heavy Water Based Nuclear Power Plants, AERB Safety Code No. SC/D, Mumbai, India (1989).Atomic Energy Regulatory Board, Civil Engineering Structures for the Safety of Nuclear Facilities, AERB Safety Standard No. SS/CSE, Mumbai, India (1998).Nuclear Power Corporation Of India Limited, Preliminary Safety Analysis Report of Kaiga Atomic Power Project-1&2Federal Emergency Management Agency, Risk Assessment: How to Guide to Mitigate Potential Terrorist Attacks Against Buildings, FEMA 452, USA (2005).Indian Institute Of Technology, Kanpur And Gujarat State Disaster Mitigation Authority, IITK-GSDMA Guidelines on Measures to Mitigate Effects of Terrorist Attacks on Buildings, India (2007).American Society Of Mechanical Engineers, Boiler and Pressure Vessel Code, Section III, Division 2, Code for Concrete Reactor Vessels and Containments (1998).Atomic Energy Regulatory Board, Design of Concrete Structures Important to Safety of Nuclear Facilities, AERB Safety Standard No. SS/CSE-1, Mumbai, India (2002).Bureau Of Indian Standards, Plain and Reinforced Concrete - Code of Practice, IS456, New Delhi, India (2000).Atomic Energy Regulatory Board, Regulatory Inspection and Enforcement in Nuclear Power Plants and Research Reactors, AERB Safety Manual No. SM/G-1, Mumbai, India (2007).Atomic Energy Regulatory Board, In-Service Inspection of Civil Engineering Structures Important to Safety of Nuclear Power Plants, AERB Safety Manual No. SM/CSE-2, Mumbai, India (2004).Atomic Energy Regulatory Board, Proof and Leakage Rate Testing of Reactor Containments, AERB Safety Guide No. SG/O15, Mumbai, India (1998).Atomic Energy Regulatory Board, Decommissioning of Nuclear Facilities, AERB Safety Manual No. SM/ DECOM-1, Mumbai, India (1998).Atomic Energy Regulatory Board, Quality Assurance for Safety in Nuclear Power Plants, AERB Safety Code No. SC/QA, Mumbai, India (2009).

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14.

Ms. Shylamoni P. holds a B.Tech in Civil Engineering from T.K.M. College of Engineering, Kollam, Kerala. She is a Scientific Officer (E) in the Information & Technical Services Division of AERB, Mumbai. Her expertise and interests include analysis and design of steel and concrete and multi-storeyed industrial structures, review

of civil engineering and siting aspects of nuclear facilities including nuclear power plants.

Dr. P.C. Basu holds BE (civil) from Bengal Engineering and Science University, West Bengal. He also holds an M.Tech degree from Indian Institute of Technology (IIT), Kanpur and PhD from Liverpool University, UK. He is former Director, Civil and Structural Engineering Division of Atomic Energy Regulatory Board (AERB), Mumbai. His research interests are in

the fields of high performance concrete (HPC) composites and earthquake engineering. Presently, Dr. Basu has joined The International Seismic Safety Center (ISSC), International Atomic Energy Agency (IAEA), Vienna as a Consultant.