Opinion - Leader Page Articles

A lesson in nuclear reactors

A great deal has been written on nuclear reactors in our media in recent months by commentators, many of whom are unfamiliar with what a nuclear reactor is. To make the whole debate relating to the Indo-U.S. nuclear agreement more meaningful, I propose in this article to describe what a nuclear reactor is and the differences among the many covered by the discussions.

A nuclear reactor is one where a controlled self-sustaining nuclear reaction takes place in which uranium nuclei fission (or break up) releasing energy, manifesting as heat. There are two basic types of reactors, those wherein the neutrons produced in the fission process are slowed down for facilitating further fission of uranium. These reactors are called slow neutron reactors or thermal reactors (the reason is that the neutron velocities are in thermal equilibrium with ambient temperatures). In such reactors, the uranium is dispersed in a slowing down medium (called moderator), which can be graphite, ordinary water (of high purity, called light water in nuclear parlance) or heavy water (present in natural water to the extent of one part in seven thousand).

In a fast reactor, the fission process takes place with high-energy neutrons, not requiring a moderator. But it is necessary to use concentrated fissile materials such as highly enriched uranium or plutonium. In these reactors, large amounts of heat are produced from a small volume thus requiring special materials for taking away the heat.

Removal of heat from thermal reactors is done with coolants such as carbon dioxide gas or light water or heavy water. In fast reactors, it is necessary to employ a coolant such as molten sodium.

Among thermal reactors, there are two basic types, those that can use natural uranium as fuel and those that require enriched uranium as fuel. Naturally occurring uranium has two components U235, present to the extent of one part in one hundred and forty parts, which is fissionable, and U238, which is not fissionable (except with high energy neutrons). But U238 gets converted to artificially created fissionable material plutonium 239, after irradiation in a reactor. Similarly, thorium 232 is not fissionable but gets converted to fissile U233 after irradiation in a reactor. To gain access to Pu239 or U233, it is necessary to “reprocess” the spent fuel consisting of irradiated U238 or Th232.

In the early days of nuclear development, enrichment of uranium was carried out primarily to produce weapon grade U235. The U.S., U.S.S.R., Britain and China built enrichment plants as part of their weapons programmes. France and later India used reactor-produced plutonium for the initial nuclear explosions. The U.S., U.S.S.R., Britain and China also produced reactor made plutonium for their weapons. The U.S. and U.S.S.R. took up development of nuclear propulsion reactors for submarines and these reactors used enriched uranium as fuel and light water as moderator and coolant. These reactor designs were scaled up to provide designs for production of electricity. Such reactors are called Light Water Reactors (LWR) in the West and VVER in the Soviet Union. Typically, these reactors use uranium enriched to between 3 and 5 per cent; submarine reactors use a higher level of enrichment.

The LWRs developed in the U.S. have two variants; those that produce steam in the reactor vessel are called Boiling Water Reactors (BWR) and those where the hot water from the reactor produces steam in external steam generators are called the Pressurised Water Reactors (PWRs). The LWRs have also been adopted in France, Germany, Japan and Korea, after getting the technology from the U.S. The VVER developed in Russia was adopted in East Europe (formerly part of the Soviet Bloc) and also in Finland, India (Kudankulam) and China.

Britain and France which initially did not have large uranium enrichment capability developed a graphite moderated carbon dioxide cooled reactor that could use natural uranium as fuel. These are called GCR or Magnox reactors. Britain had a further variant of the GCR called Advanced Gas Cooled Reactor (AGR) which used slightly enriched uranium. While a number of GCRs were built in Britain and France and some AGRs in Britain, adverse economics and operational complexities resulted in suspending this design.

Canada worked on another reactor design that could use natural uranium as fuel with heavy water as moderator and coolant. The Canadians call it CANDU reactor and the international nuclear community calls it the Pressurised Heavy Water Reactor (PHWR). From the inception of India’s nuclear energy programme, it had zeroed in on reactors that could use natural uranium (available in India, though of low grade and not very extensive) as fuel. It chose the PHWR system developed in Canada and cooperated with the latter on its second nuclear power station located in Rajasthan. India had chosen a two-unit BWR station designed by the U.S. for the first nuclear power station located at Tarapur, based on international competitive bidding.

The PHWR system fitted well into India’s eventual plans to utilise the energy potential of thorium, of which it has a large quantity. The PHWRs are efficient producers of plutonium, needed to fuel Fast Breeder Reactors (FBRs). The FBRs can irradiate thorium in the blanket region and produce U233. U233 can then be used with thorium in thermal or fast reactors. India has designed an Advanced Thermal Reactor (ATR) of 300 MW which will work on U233-Th fuel cycle. Construction work on this may commence in 2008.

Apart from Canada, India is one of the producers of heavy water in industrial quantities and has developed on its own a number of processes for the purpose. India produces all special materials, such as zirconium, and all equipment for PHWRs. It has standardised 220 MW units, of which is has built 10 reactors and four more are in an advanced stage of completion. It also completed two 540 MW reactors at Tarapur recently. It has finalised the design of a 700 MW unit which will be built in a number of locations. India-designed and built PHWRs are the lowest cost reactors in the world.

However, more than 80 per cent of the power reactors operating in the world are LWRs (including VVER which belongs to the same generic type, though there are some important engineering differences). Initially the U.S. and later France, Germany and Japan (in cooperation with the U.S.) and the USSR (now Russia) built strong industrial capabilities to build LWRs. Electric power utilities find it simpler and more convenient to operate LWRs as they have features flowing out of conventional coal-fired steam power technology.

Natural uranium reactors, both Magnox (GCR) and PHWR, require to be fed with fresh fuel regularly (on a daily basis). Some spent fuel has to be taken out when the reactor is operating. The PHWRs, due to their inherent lower reactivity, have a limitation on how quickly they can restart and be loaded, after an interruption due to any fault. The LWRs do not suffer from this disability. They can run for 15-18 months without a fuel change; the latter requires the reactor to be off line for a month or so.

What India is looking for, if it can re-enter international nuclear commerce, is to add some 40,000 MW in the time period 2010-2030. For this to happen, India would like to access LWRs from Russia, France (Franco-German entity) and the U.S. (in cooperation with Japan). There need be no apprehension that large number of LWRs would be imported fully from overseas. Our industry already supplies the whole range of equipment for PHWRs and will certainly participate in supplying components for imported LWRs too. In fact, Korea and China, which are building LWRs, have similar vigorous localisation programmes.

India is a world leader in PHWRs and while currently available uranium can support some 10,000 MW, we may double that using uranium from overseas or from new finds in India. India is also expected to have a lead role globally in development of Fast Breeder Rectors and Thorium-based systems.

Hence the nervousness that India may become a dumping ground for LWRs from the nuclear advanced countries to the detriment of India’s own nuclear industrial capacity building is unwarranted. Re-entering the international civil nuclear energy arena is good for India and good for the world, as it will enhance the development of an important non-carbon source of energy.

Corrections and clarifications

© Copyright 2000 - 2009 The Hindu