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.
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