The basic operation of a nuclear power plant is no different from that of a conventional power plant that burns coal or gas. Both heat water to convert it into pressurised steam, which drives a turbine to generate electricity. The key difference between the two plants lies in the method of heating the water. Conventional power plants burn fossil fuels to heat the water. In a nuclear power plant, this heat is produced by a nuclear fission reaction, wherein energy in the nucleus of an atom is released by splitting the atom.
Everything is made of atoms. Any atom found in nature will be one of 92 types of atoms, also known as elements. (Actually, an element is a pure substance made up of only one type of atoms.) Atoms bind together to form molecules. So, a water molecule is made up of two atoms of hydrogen and one atom of oxygen. Every substance on Earth—metal, plastics, hair, clothing, leaves, glass—is made up of combinations of the 92 atoms that are found in nature.
Atoms are made up of three subatomic particles: the positively charged protons, the neutral neutrons and the negatively charged electrons. Protons and neutrons bind together to form the nucleus of the atom, while the electrons surround and orbit the nucleus.
Every element is characterised by its mass number and atomic number. The mass number is the number of protons and neutrons in its nucleus, while the atomic number is the number of protons. The chemical properties of an atom depend upon the number of protons in it, that is, its atomic number. There are atoms whose nuclei have the same number of protons, but different number of neutrons. The chemical properties of these atoms are identical, since they have the same number of protons. Such atoms are called isotopes. An isotope is designated by its element symbol followed by its mass number. For instance, the three isotopes of uranium are designated as U-234, U-235 and U-238.
Typical fission events release about 200 million eV (electron volts) for each fission event, that is, for the splitting of each atom. In contrast, when a fossil fuel like coal is burnt, it releases only a few eV as energy for each event (that is, for each carbon atom). This is why nuclear fuel contains so much more, millions of times more, energy than fossil fuel: the energy found in one kilogram of uranium is equivalent to the burning of 2000 tons of high-grade coal.
It is this energy released in a nuclear fission reaction that is harnessed to convert water to steam and drive a turbine and generate electricity in a nuclear power plant.
The nuclear fission reaction is accompanied by the emission of several neutrons. Under suitable conditions, the neutrons released in a fission reaction fission at least one more nucleus. This nucleus in turn emits neutrons, and the process repeats. The fission reaction thus becomes self-sustaining, enabling the energy to be released continuously. This self-sustaining fission reaction is known as nuclear chain reaction.
The average number of neutrons from one fission that cause another fission is known as the multiplication factor, k. Nuclear power plants operate at k=1. If k is greater than 1, then the number of fission reactions increases exponentially, which is what happens in an atomic bomb.
The isotopes that can sustain a fission chain reaction are called nuclear fuels. The only isotope that can be used as nuclear fuel and
also occurs naturally in significant quantity is Uranium-235. Other isotopes used as nuclear fuels are artificially produced, plutonium-239 and uranium-233. (Pu-239 occurs naturally only in traces, while U-233 does not occur naturally.)
We discuss the use of U-235 as nuclear fuel here. Uranium has many isotopes. Two, U-238 primarily, and to a lesser extent, U-235, are commonly found in nature. Both U-235 and U-238 undergo spontaneous radioactive decay, but this takes place over periods of millennia: the half-life of U-238 (half-life is the amount of time taken by half the atoms to decay) is about 4.47 billion years and that of U-235 is 704 million years. (For more on radioactivity and half-life, see Chapter 3, Part I.)
While both U-235 and U-238 are fissionable, that is, both undergo fission on capturing a neutron, there is an important difference in their fission properties. U-238 can only be fissioned by fast moving neutrons, it cannot be fissioned by slow moving neutrons; therefore, it cannot sustain a nuclear chain reaction as the neutrons released during its fission inevitably inelastically scatter to lose their energy. However, U-235 has the property that it can be fissioned by slow moving neutrons too. This is what makes it fissile; in other words, it can sustain a nuclear chain reaction and can be used as nuclear fuel.
The concentration of U-235 in naturally occurring uranium ore is just around 0.71%, the remainder being mostly the non-fissile isotope U-238. For most types of reactors, this concentration is insufficient for sustaining a chain reaction and needs to be increased to about 3-5% in order that it can be used as nuclear fuel. This can be done by separating out some U-238 from the uranium mass. This process is called enrichment, and the resulting uranium is called enriched uranium. [Note that not all nuclear reactors need enriched uranium; for example, Heavy Water Reactors use natural (unenriched) uranium.]
As mentioned above, U-235 also undergoes a small amount of spontaneous fission, which releases a few free neutrons into any sample of nuclear fuel. These neutrons collide with other U-235 nuclei in the vicinity, inducing further fissions, releasing yet more neutrons, thus starting a chain reaction.
If exactly one out of the average of roughly 2.5 neutrons released in the fission reaction is captured by another U-235 nucleus to cause another fission, then the chain reaction proceeds in a controlled manner and a steady flow of energy results. However, if on the average, less than one neutron is captured by another U-235 atom, then the chain reaction gradually dies away. And if more than one neutrons are captured, then an uncontrolled chain reaction results, which can cause the nuclear reactor to meltdown; this is also what happens in an atomic bomb. To control the fission reaction in a nuclear reactor, most reactors use control rods that are made of a strongly neutron-absorbent material such as boron or cadmium.
The neutrons released in a fission reaction travel extremely fast, and therefore the possibility of their being captured by another U-235 nucleus is very low. Therefore they need to be slowed down, or moderated. In a nuclear reactor, the fast neutrons are slowed down using a moderator such as heavy water or ordinary water.
Part II: The Nuclear Fuel Cycle
The nuclear fission reaction that we have discussed above is only a small part of the entire complex process of generating electricity from uranium. The entire process is known as the nuclear fuel cycle. We now take a brief look at the various stages of this process (including the phase of uranium enrichment).
Mining: The nuclear fuel cycle starts with mining of uranium. Since 90% of the worldwide uranium ores have uranium content of less than 1%, and more than two-thirds have less than 0.1%, large amounts of ore have to be mined to obtain the amounts of uranium required.
Milling: The mined ore is then trucked to the mill to be processed to extract the uranium. Here, the ore is first ground into fine powder, and then treated with several chemicals to extract the uranium. The coarse powder thus obtained is called yellowcake. It contains 70-90% uranium oxide (U3O8).
Enrichment (not for Heavy Water Reactors): The uranium oxide in the yellowcake contains both the fissile U-235 and non-fissile U-238. The yellow cake is now taken to a processing facility. Here, the uranium oxide is converted to uranium hexafluoride (UF6), as this compound is gaseous at low temperatures and so is easier to work with. The UF6 is now enriched either through diffusion or centrifugation, meaning the proportion of fissile U-235 in it is increased from 0.7 percent to 3-5 percent. The process yields two types of UF6: one is enriched, and the other, which contains primarily U-238, is called depleted, so-called because most of the U-235 has been extracted from it.
Fuel element fabrication: The enriched uranium hexafluoride gas is now converted into solid uranium oxide fuel pellets, each the size of a cigarette filter. These pellets are packed into very thin tubes of an alloy of zirconium, and the tubes are then sealed. These tubes are called fuel rods. Each fuel rod is normally twelve feet long and half-an-inch thick. The finished fuel rods are bundled together to form the fuel assembly (or fuel bundle), which may have as many as 200 fuel rods. Several fuel assemblies are now placed in the reactor core of the nuclear power reactor—the number may go up to several dozen, depending upon the reactor design.
Nuclear reactor: The nuclear reactor is where the nuclear fuel is fissioned and the resulting chain reactions are controlled and sustained at a steady rate.
Decommissioning: Nuclear power plants are designed for an operating life of 30-60 years. When the reactor completes its working life, it is dismantled. Unlike conventional coal and gas power plants, the dismantling of a nuclear power plant is a very long-term, complicated and costly operation, because the entire nuclear power plant, including all its parts, has become radioactively contaminated. The long-term management and clean up of these closed reactors is known as decommissioning, which can take anywhere between 5 to 100 years, depending upon the type of decommissioning plan.
Disposal of radioactive nuclear fuel waste: Every year, one-third of the nuclear fuel rods must be removed from the reactor, because they are so contaminated with fission products that they hinder the efficiency of electricity production. The uranium fuel after being subjected to the fission reaction in the reactor core becomes one billion times more radioactive; a person standing near a single spent fuel rod can acquire a lethal dose within seconds. This spent nuclear fuel is going to be radioactive for tens of thousands of years. Therefore, it needs to be safely stored for centuries to come.
Generally, the spent fuel is first stored for many years in on-site storage ponds and continually cooled by air or water. If it is not continually cooled, the zirconium cladding of the rod could become so hot that it would spontaneously burn, releasing its radioactive inventory. The cooling period can be from a few years to decades. After cooling, there are two options for the waste—either it is reprocessed, or it is moved to dry cask storage.
In the latter case, the spent fuel rods are packed by remote control into highly specialised containers made of metal or concrete designed to shield the radiation. These casks must be stored for centuries to come; however, no country having nuclear plants has succeeded in building such a long-term nuclear waste dump site. Presently, in most countries having nuclear plants, these casks are ‘temporarily’ stored near the spent fuel cooling ponds.
Reprocessing spent fuel: Reprocessing is a chemical process to separate out the uranium and plutonium contained in the spent fuel, which can then be used as fuel for what are known as Fast Breeder Reactors. Reprocessing also segregates the waste into high-level, intermediate-level and low-level wastes.
Part III: The Nuclear Reactor
Most nuclear reactors work on the same basic principles. The basic components common to most types of nuclear reactors are as below:
Reactor core: The part of the nuclear reactor where the nuclear fuel assembly is located.
Moderator: The material in the core which slows down the neutrons released during fission, so that they cause more fission. It is usually ordinary water (used in Light Water Reactors) or heavy water (used in Heavy Water Reactors).
Control rods: These are made with neutron-absorbing material such as cadmium, hafnium or boron, and are inserted or withdrawn from the core to control the rate of reaction, or halt it.
Coolant: A liquid or gas circulating through the core so as to transfer the heat from it. This primary coolant passes through a steam generator (except in Boiling Water Reactors or BWRs), where the heat is transferred to another loop of water (in the so-called secondary circuit) to convert it into steam. This steam drives the turbine. The advantage of this design is that the primary coolant, which has become radioactive, does not come into contact with the turbine.
Pressure vessel: Usually a robust steel vessel containing the reactor core and moderator/coolant.
Steam generator (not in BWRs): Here, the primary coolant bringing heat from the reactor transfers its heat to water in the secondary circuit to convert it into steam.
Containment: This is typically a metre-thick concrete and steel structure around the reactor core. After the zirconium fuel cladding and the reactor pressure vessel, this is the last barrier against a catastrophic release of radioactivity into the atmosphere. Apart from a primary containment, many reactors have a secondary containment too, which is normally a concrete dome enveloping the primary containment as well as the steam systems. This is very common in BWRs, as here most of the steam systems, including the turbine, contain radioactive materials.
Types of Nuclear Reactors
At a basic level, reactors may be classified into two classes: Light Water Reactors (LWRs) and Heavy Water Reactors (HWRs). LWRs are largely of two types, Pressurised Water Reactors (PWRs) and Boiling Water Reactors (BWRs). LWRs, and of them, the PWRs, are the most widespread reactors in operation today. Heavy Water Reactors can also be of different types, one of the most well known being the CANDU reactors developed by Canada, which are a type of Pressurised Heavy Water Reactors (PHWRs). Most of India’s indigenous reactors are CANDU reactors.
Below, we discuss the most well-known type of nuclear power reactor—the PWR, and also the reactor design of most of India’s reactors—the PHWR or CANDU reactor.
Pressurised Water Reactor
A PWR uses ordinary water as both coolant and moderator. It has three water circuits. Water in the primary circuit which flows through the core of the reactor reaches about 325°C; hence it must be kept under about 150 times atmospheric pressure to prevent it from boiling. Water in the primary circuit is also the moderator, and if it starts turning into steam, the fission reaction would slow down. This negative feedback effect is one of the safety features of this type of reactors.
The hot water from the primary cooling circuit heats the water in the secondary circuit, which is under less pressure and therefore gets converted into steam. The steam drives the turbine to produce electricity. The steam is then condensed by water flowing in the tertiary circuit and returned to the steam generator.
Pressurised Heavy Water Reactor (PHWR or CANDU)
A PHWR uses heavy water as the coolant and moderator, instead of ordinary water. Heavy water is a more efficient moderator than ordinary water as it absorbs 600 times fewer neutrons than the latter, implying that the PHWR is more efficient in fissioning U-235 nuclei. Hence, it can sustain a chain reaction with lesser number of U-235 nuclei in uranium as compared to PWRs. Therefore, PHWR uses unenriched uranium, that is, natural uranium (0.7% U-235) oxide, as nuclear fuel, thus saving on enrichment costs. On the other hand, the disadvantage with using heavy water is that it is very costly, costing hundreds of dollars per kilogram.
Conceptually, this reactor is similar to PWRs discussed above. Fission reactions in the reactor core heat the heavy water. This coolant is kept under high pressure to raise its boiling point and avoid significant steam formation in the primary circuit. The hot heavy water generated in this primary circuit is passed through a heat exchanger to heat the ordinary water flowing in the less-pressurised secondary circuit. This water turns to steam and powers the turbine to generate electricity.
The difference in design with PWRs is that the heavy water being used as moderator is kept in a large tank called Calandria and is under low pressure. The heavy water under high pressure that serves as the coolant is kept in small tubes, each 10 cms in diameter, which also contain the fuel bundles. These tubes are then immersed in the moderator tank, the Calandria.
2. Is Nuclear Energy Green?
Prime Minister Manmohan Singh (Aug 21, 2011): “I am convinced that nuclear energy will play an important role in our quest for a clean and environmentally friendly energy mix as a major locomotive to fuel our development process.” [i]
Taking advantage of the growing crisis of global warming, political leaders, administrators and the global nuclear industry have launched a huge propaganda campaign to promote nuclear energy as the panacea for reduction of greenhouse gas emissions.
While it is true that nuclear reactors do not emit greenhouse gases in the same quantity as coal or oil powered generating stations, but to conclude that nuclear energy is “an environment friendly source of power” is a far stretch. Nuclear reactors do not stand alone; the production of nuclear electricity depends upon a vast and complex infrastructure known as the nuclear fuel cycle. And the fact is, the nuclear fuel cycle utilises large quantities of fossil fuel during all its stages, as discussed below.
Carbon Emission and the ‘Nuclear Fuel Cycle’
Uranium mining and milling are very energy intensive processes. The rock is excavated by bulldozers and shovels and then transported in trucks to the milling plant, and all these machines use diesel oil. The ore is ground to powder in electrically powered mills, and fuel is also consumed during conversion of the uranium powder to yellow cake. In fact, mining and milling are so energy intensive that if the concentration of uranium in the ore falls to below 0.01%, then the energy required to extract it from this ore becomes greater than the amount of electricity generated by the nuclear reactor. And most uranium ores are low grade; the high-grade ores are very limited.
The uranium enrichment process is also very energy intensive. For instance, the Paducah enrichment facility in the USA uses the electrical output of two 1,000 MW coal-fired plants for its operation, which emit large quantities of CO2.
The construction of a nuclear reactor is a very high-tech process, requiring an extensive industrial and economic infrastructure. Constructing the reactor also requires a huge amount of concrete and steel. All this consumes huge quantities of fossil fuel. After the reactor’s life is over, its decommissioning is also a very energetic process.[ii]
Finally, constructing the highly specialized containers to store the intensely radioactive waste from the nuclear reactor also consumes huge amounts of energy. This waste has to be stored for a period of time which is beyond our comprehension—hundreds of thousands of years! Its energy costs are unknown.
A study done for the Green parties of the European Parliament by senior scientists Jan Willem Storm van Leeuwen and Philip Smith in 2004 estimated that under the most favourable conditions, the nuclear fuel cycle emits one-third of the carbon dioxide emissions of modern natural gas power stations. They excluded the energy costs of transportation and storage of radioactive waste in their calculations, and also assumed high grade uranium ore is used to make the nuclear fuel. But these high grade ores are finite. Use of the remaining poorer ores in nuclear reactors would produce more CO2 emissions and nuclear energy’s green choga will no longer remain green.[iii]
The concentration of uranium in India’s uranium ores is very low. From the total uranium mined in Jaduguda over the last 40 years, Dr. Surendra Gadekar has estimated that the ore quality at Jaduguda hasn’t been better than 0.03% for many years.[iv] At such meagre concentrations, it is obvious that the total CO2 emissions from the nuclear fuel cycle in India must be fairly high.
Actual Potential: Even Less
However, this represents only half the argument. Burning of fossil fuels is not the only factor responsible for greenhouse gas (GHG) emissions, though it is the largest (see Table 2.1). Obviously, nuclear power cannot help in reducing these other causes of GHG emissions, like use of fertilisers in chemical agriculture, industrial processes that emit GHGs, etc. Then again, fossil fuels are burnt for various uses, and nuclear power can replace fossil fuels only in large scale electricity generation, and not in its other uses, like in the transportation sector.
Worldwide, use of fossil fuels for electricity and heating contributes to only 25% of the total GHG emissions. Therefore, replacing burning of fossil fuels with nuclear energy can only bring about some reduction in this part of the total global GHG emissions. (And that too, assuming that the nuclear energy is generated using high grade uranium ore.)
How much reduction is possible? The International Energy Agency (IEA) has estimated that even if nuclear energy contribution were to quadruple by 2050, it would reduce global CO2 emissions by only 4 percent![v] The crisis of global warming is very acute, and to tackle it, what the world needs is not a marginal reduction in GHG emissions, but deep cuts in them—40 percent by 2020 and 95 percent by 2050. Obviously, nuclear power cannot significantly contribute to bringing about these reductions.
On the other hand, implementation of this scenario would require construction of 32 new 1000 MW nuclear reactors every year from now until 2050. Investment costs for these 1,400 new reactors would exceed $10 trillion at current prices. That is huge! Given the enormous subsidies needed to build just one reactor (discussed in Chapter 5), that would bankrupt even the richest countries!!
What About Renewable Sources of Energy?
The above discussion compared CO2 emissions from the nuclear fuel cycle with that from gas- and coal-fired power plants. The nuclear lobby focuses on this comparison to make an argument for building nuclear power plants. But there is another facet to the whole issue, which the nuclear lobby very conveniently forgets: renewable energy sources emit less greenhouse gases than nuclear plants! In comparison to renewable energy sources, power generated from nuclear reactors releases four to five times more CO2 per unit of energy produced, when taking into account the entire nuclear fuel cycle.[vii]
If the growing crisis of global warming is an argument in support of promoting nuclear energy as compared to electricity from burning fossil fuels, then, by an extension of this same logic, shouldn’t renewable energy be promoted as compared to nuclear energy?
[ii] Helen Caldicott, Nuclear Power is not the answer to Global Warming or anything else, Melbourne University Press, 2006, pp. 7-13.
[iii] Jan Willem Storm van Leeuwen, ‘Nuclear power — the energy balance’, http://www.stormsmith.nl; Helen Caldicott, ibid, p. 6.
[v] Energy Technology Perspectives 2008, IEA/OECD, June 2008, cited in: Nuclear power: a dangerous waste of time, Greenpeace, Jan 2009, http://www.greenpeace.org
[vi] Statistics taken from the flowchart: World Greenhouse Gas Emissions 2005, World Resources Institute, http://www.wri.org