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
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). Indias 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
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 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.
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
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.
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.
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
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
Seismic Category 3 1) Turbine Building2) Other pump houses
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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 pr...