India's largest nuclear power generation complex will soon be alive at Kudankulam, Tamil Nadu. It promises to generate 2,000 MW of energy - with two reactors in place and four more to come: What must be going on inside it?

The Supreme Court on Monday effectively cleared the way for the Kudankulam Nuclear Power Plant (KKNPP) to start functioning by dismissing any fears of far-reaching radioactive effects from the reactors.

Even so, the people of Kudankulam and its surrounding fishing villages protest against the plant and call for its shutdown. Why? Despite accidents being unpredictable by definition, they do occur because the enterprise is immensely complex for anyone to keep track of all its components at the same time.

While the Department of Atomic Energy (DAE) could be addressing this issue well, etc., the complexity increases the opportunities for accidents to occur rather than directly precipitating them. Despite a decent track record for maintenance, the one for compensation in India is dismal, and so the people's fears are bound to persist.

In order to explain how a nuclear power plant works, I've divided it up into short chapters, each explaining the role of an important component. Click on the chapter you wish to read about.

1. The Reactor
2. The Pile
3. The Repository
4. The Refrigeration
5. The Big Picture
6. Points to note

The Reactor

KKNPP houses two VVER-1000 nuclear fission reactors, with four more to come. Each reactor produces 925 MW. As of now, one reactor is being brought to life, and it will supply 460 MW to Tamil Nadu and the remaining 465 MW to other states.

In each reactor - the heart of the plant - a special composition of uranium, called the U-235 isotope, undergoes a nuclear fission reaction, i.e. splits up to release a lot of heat. '235' here is the sum of the number of neutrons and protons in each uranium atom's nucleus. When another neutron is added to it, it becomes a U-236 isotope, i.e. U-236*, in a process called nuclear transmutation.

U-236* is highly unstable because the extra neutron's mass brings some energy with it (i.e., E = mc2) that makes the nucleus restless. It seeks to stabilise itself by losing that extra neutron’s energy, i.e. breaking up into smaller, lighter, less energetic fragments. For example,

U-235 + n --> U-236* --> Rb-93 + Cs-141 + 2n

(Rb = rubidium; Cs = caesium; n = neutron)

Notice how this break-up results in two neutrons while we supplied only one. This occurrence prompted the idea of a nuclear chain reaction, wherein one neutron is used to kickstart a reaction that sustains itself by producing all the subsequent neutrons to bombard U-235. Each reaction releases about 3.2 x 10-11 joules as heat and radiation. As millions upon millions of such reactions happen, the temperature reaches more than 100 million° C.

This is what goes on inside the reactors. Back to top

The Pile

So, a uranium-235 atom can be made to split up and release energy, a process known as fissioning, when a neutron is forced into its nucleus, transmuting it into uranium-236. The energy that this neutron brings is the cause of the restlessness that precipitates the breaking up.

Not all neutrons can cause the U-235 atom to transmute and break up. If the neutron is too energetic by itself, it will cause the U-235 to lose energy in other, non-utilisable ways. If the neutron is too languorous, it will not inspire the U-235 to restlessness. So, it has to be of a specific energy, called the thermal energy, to start a reaction. The two extra neutrons produced with each fission event also have to be at the thermal energy to sustain a chain reaction.

In case a very energetic neutron is inside the reactor, it will be slowed down to thermal energies by what is called the pile. It is a latticed block of alternating graphite and uranium layers, where graphite acts as a moderator. An alternate arrangement is to produce fuel rods of uranium to be erected in the core, interspersed with control rods of boron, all submerged in a liquid moderator.


A schematic representation of the fuel/control rods assemblage inside a VVER-1000 reactor core. Photo: Wikimedia Commons

An allotrope of carbon, graphite is a convenient choice to act as a neutron-moderator for three reasons:

1. The carbon atom has a light nucleus that can absorb a lot of energy from fast neutrons, slowing them down effectively
2. Carbon is easily available as a solid
3. It is light and inexpensive to handle

In the pile's presence, fast neutrons are slowed down and made available for more fission reactions. Since each fission event produces two neutrons while consuming only one, each fission event will precipitate two more events. So, as the reaction progresses, there will be 1, 2, 4, 8, 16, 32, ... fission events, exponentially increasing. This can get easily out of hand and lead to explosive amounts of power being generated.

To avert such crises, the amount of graphite inside the reactor is increased so that all neutrons are absorbed and the reaction is quickly shut down. In many cases, it is preferable to have just one thermal neutron in the mix at any time, so the second neutron is continually absorbed by the control rods. In this scenario, the pile is said to be critical. When there is more than one neutron per fission event, the pile will be supercritical.

Another type of moderator - a liquid one - called heavy water is sometimes used. Like water is H2O, heavy water is D2O, where D (deuterium) is an isotope of H (hydrogen). However, when D2O captures a neutron, deuterium transmutes to tritium, which is nastily radioactive. Cadmium, beryllium and beryllium oxide are also good moderators but are also extremely poisonous. At Kudankulam, the moderator is light water, or H2O. Back to top

The Repository

Here is the typical U-235 fission event:

U-235 + n --> U-236* --> Rb-93 + Cs-141 + 2n

What happens to the rubidium and caesium? They are the radioactive waste products produced after each reaction, and are extremely radioactive.

For example, rubidium-93 becomes strontium-93 over 6 seconds. Strontium-93 subsequently decays to yttrium-93 over 7 minutes; yttrium-93 becomes zirconium-93 over 10 hours; zirconium-93 becomes niobium-93 over 1 million years. Similarly, caesium-141 becomes barium-141 over 25 seconds; barium-141 becomes lanthanum-141 over 18 minutes; lanthanum-141 becomes cerium-141 over 4 hours; cerium-141 becomes praseodymium-141 over 33 days.

Each of these products is radioactive. The shorter decay processes are more radioactively harmful than the longer ones, but that isn't to say the longer ones aren't.

Every step of transmutation involves energetic beta (electrons) and gamma (photons) radiation emission, both very harmful to flora and fauna health. Moreover, these products emerge from a reactor that has experienced temperatures of millions of degrees celsius, so they are incredibly hot and difficult to handle as well. All the same, the products must be stowed somewhere where they bring no harm to their surroundings, and for millions of years, too, because some decay processes last that long.

This calls for the repository, a thick-walled container lined with a tampering material that prevents neutrons and other radiation from escaping, inside which the waste products are placed. The repository is then buried deep in the ground, in the vicinity of no other life form, away from harm. Their longevity, however, is yet to be tested.

Note that in the KKNPP case, the Nuclear Power Corporation of India, Limited (NPCIL) is yet to find a location for setting up repositories to store waste from the plant. As the Supreme Court Bench reportedly said, "... it is of utmost importance that the Union of India, NPCIL, etc., should find out a place for a permanent deep geological (sic) repository [DGR]. Storing of spent nuclear fuel at the site will, in the long run, poses dangerous long-term health and environmental risks. ... Noticeably, the NPCIL does not seem to have a long-term plan, other than stating and hoping that in the near future, it would establish a DGR." Back to top

The Refrigeration

A nuclear reactor produces a large amount of heat which can be converted to electricity by using it to heat water to steam, which will then turn a turbine. If the heat isn't removed quickly and efficiently enough, it will melt the fuel core, resulting in a reactor meltdown. A substance called a coolant is used to absorb the heat from the core and take it away through pipes.

Quoting from ‘Introductory Nuclear Physics’ by Kenneth S. Krane (John Wiley & Sons, Inc., New Delhi, 1988, pp. 511): “Coolant materials can be gases, water or other liquids, or even liquid metals, which have a large heat capacity. Because steam has a small heat capacity, reactors that use water as a coolant may keep it under high pressure, allowing it to remain as a liquid well above its normal boiling point. These are called pressurized-water reactors,” such as the one in Kudankulam.

In fast breeder reactors, where the fuel density is high and the core is often supercritical, liquid sodium is used as a coolant because it can take away heat quickly and from a small area of exposure.

On February 18, I met physicist M.V. Ramana from Princeton University for an interview while he was in Chennai. During the evening, he told me a little-known con of using liquid sodium as coolant. In his words: "Normal sodium is sodium-23, and when it works its way through a reactor, it can absorb a neutron and become sodium-24, which is a gamma-emitter. When there are leaks, for example, the whole area becomes [radioactive]. So you have to actually wait for the sodium to cool, etc."

He also spoke about a more technical concept called void coefficients. If you are interested, an excerpt is provided at the end of this article. Back to top

The Big Picture

The reactor is set up. The fuel and control rods are inserted. A neutron is introduced into the reactor. A fission event occurs that produces heat and more neutrons. These neutrons are trapped or slowed and led to more reactions, producing more heat and more neutrons.

The heat is continuously removed by the coolant circulating the reactor. A pump drives it to a heat-exchanger, where it loses the heat to convert water to steam, cools down itself, and returns to the reactor to remove more heat. The steam is led to a turbine connected to a generator. The steam turns the turbine, and the generator produces power.

After the steam loses its energy, it is turned to water in a condenser, and led back to the heat exchanger.

Once all the U-235 in the fuel rods have been transmuted and spent, the reactor is switched off by inserting the control rods all the way in (i.e., increasing exposure to the pile). The highly radioactive transmuted stuff is then dumped into the repository, buried deep underground. Back to top

Points to note

1. A power plant's design takes into account the type of fuel available, the moderator material, coolants going to be used, and infrastructure available for power transmission. Also, whatever the design, a plethora of safety measures, many of them redundant, are installed around the planet to prevent both foreseeable and unforeseeable adversities.

2. Producing U-235 ready to be used as fuel is very difficult and hardly straightforward. Processes like gaseous diffusion are used to enrich the element such that highly pure U-235 is obtained. Note that highly enriched U-235 is classified as bomb-grade, whereas that used in reactors is 2-3 per cent enriched.

3. The amount of nuclear waste produced by a nuclear reactor is almost equal to, or just a little less, than the amount of fuel used in the reactor itself. Because of this equation, the amount of waste is not revealed lest it betray the amount of uranium used in the reactor. Such care is taken because both the reactants and products are weaponisable. The downside is that even though the uranium used in KKNPP and other such plants is for civic purposes, it is hard to maintain accountability of quantities of nuclear waste and their locations.

4. U-235 is not the only fissionable material around. In fact, India and Australia each host 25 per cent of the world's thorium reserves, i.e. 300,000 tons each. Since it is not fissile by itself, thorium has to first be converted to U-233 using a converter reactor, which is then used as fuel. However, U-235 is still the most common fuel used in fission reactors around the world. For India, 61,000 tons of uranium are available for use in pressurised-water reactors and the same amount for use in fast breeder reactors.

5. In light of NPCIL still having to find a location for the repository, it may be noted that the Fukushima Dai'ichi disaster was compounded by the storage of nuclear waste beneath the reactor core in the nuclear power plant. While a tsunami was necessary to trigger the catastrophe, the consequent contamination would have been far lesser had the repository been in a different location.

6. While a nuclear power plant does produce large quantities of radioactive nuclear waste, a lot is known about their properties to strategise their isolation and disposal. In stark contrast, a coal-burning power plant releases 23 kg of uranium and 46 kg of thorium into the atmosphere for every 1,000 MW produced, and we know very little of their dynamics. This source of uranium and thorium is what is locked up as compounds in the impurities being burnt.

7. U-235 doesn't always decay into isotopes of rubidium and caesium; they are just the most formed products. Less likelier products include xenon-135 and iodine-135, which decay into barium-135 in about 3 million years. Many of these 'waste' products are today capable of being reprocessed to yield more fissile material.

Back to top

Bonus

Schematic graphic (by Satwik Gade)

M.V. Ramana on void coefficients: "Imagine that you have a reactor, using liquid sodium as a coolant, and for whatever reason, there is some local heating that happens.

For example, there may be a blockage of flow inside the pipes, or something like that, so less amount of sodium is coming, and as the sodium is passing through, it’s trying to take away all the heat. What will happen is that the sodium can actually boil off. Let’s imagine that happens.

Then you have a small bubble; in this, sort of, stream of liquid sodium, you have a small bubble of sodium vapor. When the sodium becomes vapor, it’s less effective at scattering neutrons and slowing them down.

What exactly happens is that- There are multiple effects which are happening.

Some neutrons go faster and that changes the probability of their interaction, some of them are scattered out, etc. What essentially happens is that the reactivity of the reactor could increase. When that happens, you call it a positive sodium void coefficient. The opposite is a negative.

The ‘positive’ means that the feedback loop is positive. There’s a small amount of increase in reactivity, the feedback loop is positive, the reactivity becomes more, and so on. If the reactor isn’t quickly shut down, this can actually spiral into a major accident."