Dr. M. R Srinivasan, Member (formerly Chairman) Atomic Energy Commission has treated us, in his own inimitable and lucid style, to a primer, published in The Hindu, on 11/09/2007 under the title "A lesson in nuclear reactors", on various types of nuclear reactors and India's preferences between them. In the article, he has made a comparison between the PHWR line that has been highly evolved by India (from the original CANDU) and the new designs of LWRs that have been evolved mainly by Russia, France, US, and Japan. As of now, in India, further evolution of PHWRs is a continuing and ongoing process. The Canadians are working on advanced CANDUs as well.
He has made the article into a very persuasive wrapper for his own preference, namely, import the LWRs.
After imbibing the main issues in the lesson, I thought I might do a bit of homework, particularly with respect to some of the points he has made. I present below Assignment No 1 of my homework. I hope to present the others in subsequent posts in this blog.
Dr Srinivasan says:
"Electric power utilities find it simpler and more convenient to operate LWRs [than PHWRs] as they have features flowing out of conventional coal-fired steam power technology."
Following could be some important differences between a coal-fired (thermal) power plant and a nuclear power plant, particularly of the PHWR and LWR types:
As a rule, thermal power plants supply superheated steam (higher pressure and temperature) at the turbine inlet. But LWRs and PHWRs, due to inherent properties of various materials used in the reactor core, and other design requirements, cannot supply the turbine with superheated steam. Turbines in these nuclear power plants operate with saturated steam (lower pressure and temperature, but larger mass flow of steam). For the same power generated (usually specified as Megawatts electrical - MWe), a turbine designed to use superheated steam is smaller than one running based on saturated steam. The larger turbine needs more careful handling when it comes to dealing with fluctuations in important parameters (e.g. frequency, voltage) in the grid system into which they are feeding the electricity generated. In a lighter vein one may say that both the turbine and its operators of a nuclear power plant are more stressed-out than in the case of a thermal power plant.
Particularly in the genaration-starved electricity grids in India, large variations in vital parameters (such as voltage and frequency), taking place over both short as well as long duration, are common. Thermal power plants are capable of quickly adapting to and "survive" rapid variations in the electricity grid as a result of mismatch between electric power generation and the demand load. On the other hand, LWR and PHWRs are best suited to "base load" operation.
A second "property" of a nuclear reactor is that even after the chain reactions are stopped following a "trip" (a very fast action, usually in less than 2 seconds, to quench the chain reactions in the core) or a normal shutdown (a somewhat slower process), heat -- called 'decay heat' -- continues to be produced in the fuel and in some of the structural material inside the core. This heat needs to be effectively removed. Ensuring that adequate decay heat removal is taking place is an area of major importance for a nuclear plant operator, unlike in the case of a thermal power plant operator.
The operator of a nuclear power plant, due to both economic and safety reasons, needs to continually monitor and ensure that adequate system integrity exists at all times (that is, there are no unwanted leaks of any of the fluids from the system into the surrounding atmosphere). This may not be as strict an issue in a thermal power plant.
A third important factor in a nuclear power plant is the likely presence of nuclear radiation in the operating areas and the equipment there in. This is not the case in a thermal power plant. All nuclear plant operations and maintenance (O&M) personnel go through rigourous training and certification processes and are well equipped to deal with the environmental conditions in the areas in which they work. Extensive use of special protective clothing and remote tools is a routine feature of in a nuclear power plant.
However, there is a difference between a PHWR and a LWR when we talk about O&M-related considerations. A PHWR plant operator needs to be aware of the possible presence and radiological effects of Tritium [an isotope of Hydrogen, generated through nuclear reactions, from the Heavy Water present in the reactor] in the environment of the areas in the reactor, which he may access for maintenance or operation. He needs to properly utilise techniques and equipment provided for safely carrying out O&M activities according to well laid out procedures. PHWR plant operators undergo appropriate training for this. Tritium is not a major issue in the case of LWRs since Heavy Water is not used in these reactors (although it could be produced, in small quantities, from the Light Water present in the reactor core). Of course, Tritium is not a concern in a thermal power plant.
In summary, from an O&M point of view, an LWR-based nuclear power plant is not much different from a PHWR-based one. Both require high level of operator skills. [This is not to say that a thermal power plant operator does not need to exhibit a high level of skill in his area of work.]
Glossary:
PHWR - - Pressurised Heavy Water Reactor
LWR - - Light Water Reactor
CANDU - - CANada Deuterium Uranium (The original PHWR type reactor developed in Canada)
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