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Challenges Faced By India In The Design Of Pressurised Heavy Water Nuclear Power Plants
The nuclear power industry has been developing and improving reactor technology for almost five decades and is preparing for the next generations of reactors to fill orders expected in the next five to twenty years. Several generations of reactors are commonly distinguished. Generation I reactors were developed in 1950-60s and outside the UK none are still running today. Generation II reactors are typified by the present US fleet and most in operation elsewhere. Generation III are the Advanced Reactors discussed in this paper. The first are in operation in Japan and others are under construction or ready to be ordered. Generation IV designs are still on the drawing board and will not be operational before 2020 at the earliest.
Reactors derived from designs originally developed for naval use generate about 85% of the worlds nuclear electricity. These and other second-generation nuclear power units have been found to be safe and reliable, but they are being superseded by better designs. A three-stage programme was drawn up to develop nuclear power in India to match with our unique resource position of limited Uranium and large Thorium reserves. For optimum use of our Uranium resources, the first stage of this programme is based on construction of Pressurised Heavy Water Reactors (PHWRs). Indigenously developed Pressurised Heavy Water Reactor technology for 220 MWe units has been a commercial success. Eight such units are in operation and four more are in final stages of construction and commissioning. Total capability for design, construction and operation of these plants has been successfully demonstrated.
Based on this experience, Nuclear Power Corporation has launched construction of two 500 MWe PHWR units at Tarapur - Tarapur Atomic Power Project, Units 3&4 (TAPP 3&4) in October 1998. Some of the technical challenges, which have been successfully overcome in setting up 220 and 500 MWe PHWR plants, are summarized in this article. Electricity is an essential part of an industrialized society, forming one of the pillars of quality of life today. Electricity is the speed for development of a country. Thermal power generation, a major contributor to electricity production in India, turns 100 this year. About 50 years ago, when we gained our independence, India barely had a total installed capacity of 1300 MWe. This has since been enhanced to over 85,000 MWe today. Thermal power stations are the major contributors in this mammoth achievement. Even then, our per capita energy availability remains much below the world average. It is a challenge for all of us in the electricity generation industry to join forces to secure reliable electric supply on a long-term basis.
Visionary architects of science and technology of modern India foresaw the imperative need to develop all the necessary technologies for power generation. It was a result of this recognition that a three-stage programme was drawn up to develop nuclear power in India to match our unique resource position of limited Uranium and large Thorium reserves. For optimum use of the available Uranium resources, the first stage of this programme is based on Pressurised Heavy Water Reactors (PHWRs). These reactors not only use natural Uranium efficiently but also provide Plutonium as a by-product. The Plutonium recovered from the spent fuel will facilitate use of our large Thorium reserves for power production in subsequent stages of the programme. Pressurised Heavy Water Reactor technology developed in our country for 220 MWe units has been a commercial success. Eight such units are in operation and four more are in final stages of construction and g. Total capability for design, construction and operation of these plants has been successfully demonstrated. Based on this experience, Nuclear Power Corporation has launched construction of two 500 MWe PHWR units at Tarapur - Tarapur Power Project, Units 3&4 (TAPP 3&4) in October 1998.
General description of 500 MWe PHWR
A PHWR is fuelled with natural uranium dioxide fuel and is moderated and cooled by heavy water. Separate circuits are used for the moderator and coolant. While the moderator is at low temperature and pressure, the coolant is normally maintained at high temperature and pressure. The reactor vessel, known as calandria, is made of austenitic stainless steel and is located horizontally in a shielded, water filled, concrete vault with stainless steel lining. It houses 392 pressure tubes, also called coolant tubes, made of Zirconium 2.5% Niobium. Zirconium, like heavy water, absorbs only a negligible amount of neutrons. Each pressure tube is surrounded by a thin zircaloy calandria tube. The annular space between the calandria tube and coolant tube is filled with carbon dioxide gas, providing an insulating gap between the coolant and moderator. Each pressure tube contains a string of fuel bundles, each about half metre long. Because a PHWR uses natural uranium, the fuel needs to be replenished on a daily basis. An important feature of the PHWR is that it is refuelled while on power, thus avoiding frequent shutdown of the reactor for refuelling. The fuel bundles are inserted at one end of pressure tube and spent fuel bundles from the channel are discharged at the other end. Bi-directional fuelling in alternate pressure tubes prevents the reactor from having all fresh fuel bundles at one end and irradiated fuel bundles at the other thus resulting in a more symmetrical neutron flux shape. Also, fuelling is normally carried out along the direction of coolant flow.
The closed loop primary coolant circuit (also known as primary heat transport system) has four mushroom type steam generators, four PHT pump motor units, a pressuriser and connected headers and feeders which are further connected to the 392 coolant tubes containing in all 5096 fuel bundles. The PHT system also incorporates a feed and bleeds system and a purification system. Schematic of a typical nuclear power plant of PHWR type is shown in Figure 1. The cylindrical building, shown on left, houses the reactor and other equipment to produce steam which is delivered to the turbine, shown outside this building. The equipment in the steam circuit, also referred to as secondary circuit, are similar to those of a thermal power plant. The difference between the two is that, in the case of a nuclear power plant, the steam produced is not superheated and hence the steam cycle equipment are larger in size. This article briefly deals with the technological aspects of the nuclear equipment mainly located inside the cylindrical building, including the building itself which is known as reactor building or containment building.
Technological challenges in the design of a PHWR
Technological challenges posed in the design and construction of a nuclear power plant are unique in nature. It is a matter of great pride that this technology has been completely mastered by us in India. This article attempts to highlight, in a very brief manner, the summits we have conquered in the following, among many other, fields.
- Materials technology
- Design, theoretical and computational expertise
- Manufacturing technology
- Indigenisation
- Project management
Materials technology
A PHWR needs, in addition to the commercial and conventional industrial materials, many other special materials and alloys manufactured to stringent specifications. For example, in the basic fuel material, which is required in an exceptionally pure form, the neutron absorbing impurities such as Boron, Cadmium, Dysprosium, Gadolinium, etc are controlled to very low ppm levels. Similar is the case for Zirconium alloys used in pressure tube, calandria tube, and fuel cladding. Likewise, the purity and the chemistry of heavy water is also required to be rigidly controlled. Even conventional materials used in the reactor core and associated systems - like stainless steel or carbon steel - have to meet special requirements, for example very low Cobalt impurity. Processing of materials like Uranium, Zirconium and heavy water has been fully developed in India and the technology has been translated to the production plants which are successfully operating as various units of the Development of Atomic Energy. The point to be highlighted here is that for successful design and operation of a NPP, wide variety of high purity materials and alloys needs to be developed not only at the laboratory scale but also on a regular production basis. (Indeed, establishment of a very strong industrial infrastructure in metallurgical and chemical engineering is one of the major challenges to be mastered by any country (particularly a developing country) wishing to embark on self-sufficient long term nuclear power programme). Even the concrete mix used for the reactor building and its internal structures are of special formulations such as heavy concrete, high performance concrete (M60 grade), etc
Design, theoretical and computational expertise
The engineering design of a nuclear power plant is highly complex involving multidisciplinary efforts by reactor physicists and nuclear engineers. Various nuclear and heat transfer processes and structural loads have to be accurately modelled and a large number of usually highly iterative theoretical computations, have to be performed. In order to do this, computational algorithms and computer codes have to be evolved. A host of physical, chemical and engineering properties of all the materials used need to be precisely known in order to satisfactorily evolve the design. Due to the very specialised nature of this field of engineering, these computational techniques are very closely guarded, and are of a proprietary nature.
These design and calculation methodologies have been indigeneously developed and validated. The components of a nuclear reactor have to be designed taking into account various factors such as pressure, temperature, normally applied loading, seismic loading, postulated accident loading, effect of degradation of material properties due to irradiation ... This list could be very long. Designs have to satisfy several national and international engineering design codes. Furthermore, nuclear power plant technology is subjected to very rigorous regulatory oversight by national regulatory authorities and their safety codes and guides. The designer has also to keep in mind many other important factors such as manufacturability, maintainability, provisions to carry in-service inspection, decommissioning aspects as well. Very often, a designer is confronted with a situation when he finds that optimising the design with reference to one particular aspect may result in a sub-optimal design when viewed from another aspect. The design of a NPP is thus both a highly sophisticated science and a highly skilled art.
Safety aspects
Safety of a nuclear power plant is carefully and systematically interwoven in the design of all the systems. A detailed list of accident scenarios is deterministically postulated at the design stage itself. Means are provided in the design of the systems to safely overcome all such postulated situations. Nuclear power plants are designed and built taking into account all postulated external influences. Such influences include seismic and other man made phenomena. The fact that PHWR uses natural uranium fuel, and has a large volume of relatively low temperature moderator water in the core, gives it certain inherently safe characteristics. In addition, the reactor design incorporates built-in safety features for controlling and shutting down the fission chain reaction in the core and ensuring removal of decay heat from the fuel. (Unlike in a thermal power plant, in a nuclear reactor, even after reactor shutdown, the irradiated fuel continues to generate a small amount of heat which must be removed in order to prevent fuel failure). These systems are required to be designed and constructed using proven reliable components in accordance with well established technical concepts. Incorporation of redundancy and diversity right from the conceptualisation stage is a characteristic of a NPP design. PHWR designs rigorously follow these principles.
Safety functions are fully automatic, having priority over manual operator actions. This means that the possibility of human error is minimised. Even so, the power station staff is required to undergo regular on-going training to ensure that they are able to overcome any instances of malfunctioning in the power plant, to bring it to a safety state.
Nuclear radiation
The fission products make the irradiated fuel radioactive. In a NPP, apart from the spent fuel, there are other sources of radioactivity too. Several safety barriers, located one after the other, reliably contain the radioactivity. Even in the severest postulated accident conditions the Uranium dioxide fuel matrix itself retains most of the radioactivity. It is further backed up by the metallic (Zircaloy) cladding used to encapsulate the fuel pellets. The fuel bundles are placed in pressure tubes which are part of the primary coolant system, designed and constructed to withstand high pressures, temperature and material degradation due to irradiation. In addition to all these series safety barriers, an overall containment (reactor building) encloses the entire reactor system.
The containment structure consists of a cylindrical prestressed cement concrete primary containment with a prestressed concrete dome. This inner containment, which is a marvel in the civil engineering design and construction, is surrounded by a secondary containment of reinforced cement concrete. The interspace between the two buildings is maintained below atmospheric pressure. This ensures that radioactive gaseous leaks from the inner containment, if any, under any operating or accident conditions are properly collected, treated and brought to stipulated safety levels before release to the environment.
The inner containment is designed to withstand pressure and temperature conditions created within the building, assuming postulated, double ended rupture of the main steam line or primary coolant system piping. Engineered safety features are further provided in this containment building to quickly bring down the pressure and radioactivity associated with such postulated accidents, to low values, to avoid any potential leak.
Maintainability
The radiation environment, particularly close to the reactor core, poses a unique challenge to the designer to design the equipment with a requirement that no major maintenance shall be required during their operating life and that adequate in service inspection shall be possible. The coolant channel and its associated components are designed in such a manner that they can then remotely be removed from the core and replaced with new parts in a safe manner. One of the most important aspects of our PHWR design is that provision is made in the containment design for easy replacement of the steam generators, should such a necessity arise during the life of the plant (generally taken to be 40 years). In the 500 MWe design, this provision exists in the form of two circular openings, each of about 5.4 M diameter in the domes of the inner and outer containments. Apart from the areas very close to the reactor core, the environment in the reactor building is subject to a low level of radiation during reactor operation. Thus maintainability of equipment inside the reactor building is given special attention during design so as to provide convenient access. Design, manufacture and operation of remote handling tools for inspection, are in themselves, very fascinating hi-tech fields. India is amongst one of the leading PHWR countries in this area.
Special equipment
After prolonged operation, process systems in NPP may contain certain amount of radioactivity. In a PHWR, deuterium in the moderator coverts to tritium, a radioactive isotope of hydrogen. Also, heavy water is a very costly commodity. For both reasons, process equipment such as pumps, valves, instrumentation fitting, pipe joints, etc are all to be designed for zero leak. This is a challenge to all the equipment designers as well as suppliers. A recent feather in the cap of Indian industry is the development of large capacity canned rotor pumps for use in 500 MWe PHWRs. Automatically controlled fuelling machines and associated fuel transfer systems are incorporated in the design. The fuelling machines are hi-tech robots which open the high pressure boundary of the coolant system, insert fresh fuel bundles at the inlet end of the coolant channel and discharge corresponding number of fuel bundles from the other end and close back the pressure boundary again. The highly radioactive fuel is discharged through a fuel transfer system to under water spent fuel storage facility.
Similarly, reactivity control mechanisms and shut-off rods which control the insertion of neutron absorbing materials in a precise manner with desired speed of action, have been successfully developed for PHWRs by Indian industries. For manufacturers, these equipment offer a challenge in precision machining to close tolerances. Needless to say that highest level of Quality Control, Quality Surveillance and Quality Assurance is to be maintained at all and by all agencies - designers, manufacturers, construction and commissioning personnel, and operators.
Indigenisation
Planning of nuclear power in India laid great emphasis on indigenisation of all requisite technologies from the time the first PHWR unit was built in India. To do so, many innovative design alternatives had to be worked out followed by development exercises to demonstrate acceptability of concepts and designs. Design, manufacture and construction have been amply demonstrated in the currently operating PHWR units. There are still a few areas where our indigenisation levels need to be increased, one such example being, computer and electronics hardware. Large investments in hi-tech equipment and proprietary manufacturing processes, with low volume of production required, appear to inhibit indigenous development of these items. Not withstanding the apparently unfavourable short-term economics, conscious decisions should be taken to make additional investments towards indigenisation, from a long-term perspective.
Managerial challenges
"Success" could be measured in many ways. If the mere design and indigenous manufacture of a component were the criterion, then most of what we have achieved so far would be counted as very successful developments. While this may be acceptable during the initial phases of development of nuclear technology in India, at the present time when we have already established a firm-manufacturing base, we need to apply a few more factors in evaluating "success". For a nuclear power to be economical, our present long gestation periods must be shortened. This can be done only through conscious efforts on the part of all of us to meet our commitments to project time schedules and costs.
In terms of overall project costs, typically a twin-unit 500 MWe PHWR would be a "mega project". Mega projects of this nature can no longer be funded or managed by a single entity such as NPCIL. Thus it is essential to forge partnerships between NPCIL and other industrial establishments in India in such a manner that nuclear power projects can be effectively set up, in an economical manner, within acceptable gestation periods. Since international funding is not available for the nuclear power projects set up in India, we must find ways and means to obtain long-term loans.
Conclusion
A few typical examples of design efforts put in for PHWR based power plant are enumerated in this article. The design and development process of these has been quite interesting and experts from various divisions of Bhabha Atomic Research Centre, Nuclear Fuel Complex, Electronic Corporation of India Limited and other DAE units, consultancy organisations and industry have contributed in a large measure in this exercise
To do so, many innovative design alternatives had to be worked out followed by development exercises to demonstrate acceptability of concepts and designs. Design, manufacture and construction have been amply demonstrated in the currently operating PHWR units. There are still a few areas where our indigenisation levels need to be increased, one such example being, computer and electronics hardware. Large investments in hi-tech equipment and proprietary manufacturing processes, with low volume of production required, appear to inhibit indigenous development of these items. Not withstanding the apparently unfavourable short-term economics, conscious decisions should be taken to make additional investments towards indigenisation, from a long-term perspective.