Introduction
The earth is faced with the challenge of global warming, with most scientific research in agreement that, global warming is resulting from the increase of green house gases especially CO2, generated through burning of fossil fuels. Sun rays irradiate the earth with short wave length making it possible for them to penetrate the atmosphere. On impinging the earth’s surface, they lose some of their energy and are then released back to the atmosphere in form of long wave lengths. On hitting the green house cover in the atmosphere; this long wave radiation is reflected back to the earth leading to trapping of more energy on the earth’s atmosphere. There is an optimal thickness of ozone layer necessary to maintain balanced atmospheric conditions and prevent global warming. This is best achieved through photosynthesis, respiration and farting of living creatures on earth. Our demand for energy has resulted in burning of fossil fuels that leads to increase of green house gases in the atmosphere. The invention of the internal combustion engine brought a revolution in the transport industry until the challenge of emissions arose. Electricity generation from coal and diesel power plants is also significantly contributing to the accumulation of CO2 gas in the atmosphere. In Kenya diesel power plants are used in electricity generation and a coal power plant is already proposed to be constructed in Lamu, implying that Kenya is contributing to global environmental degradation. This challenge can be mitigated through adoption of the electric vehicle (EV), in which the industry has made significant stride and seen brands like Nissan Leaf, Honda and Tesla electric vehicles. There is still a challenge in these products on the aspect of range; making internal combustion engine still more competitive than the EV. Another solution would be the use of electric train in rail transport which was not implemented in Kenya during the construction of the standard gauge rail way. One solution to this range issue is the application of the Fuel cell in which there has been significant progress and commercial products are already in the market like the Toyota Mirai which is an electrochemical solution to global warming. The problem with EV and electric train solutions is the source of electric power which will determine, if there is any gain in carbon credit in adoption of these technologies. For any gain in carbon credit, the electricity for charging the electric vehicles and running of railway transport should be generated through technologies that don’t result in emission of CO2, such technologies include Solar, Hydro, Geothermal, Wind and Ocean tides. Likewise the source of hydrogen for fuel cells should be from clean sources such as electrolysis of water with green source of electricity. Nuclear energy has been considered green until concerns associated with safety arose as a result of accidents such as the Three Mile Island, Chernobyl and Fukushima accidents. Kenya has considered Atomic energy as a solution to our increasing need for electric power and a conversation on the safety of this mode of electricity is necessary to ensure that as a country we make an informed decision on whether to go on with this plan and more so, on the location of a nuclear power plant. In this article an account of the electromagnetic wave and their interaction with matter is given and how nuclear power plant works including types of nuclear reactors.
Electro-magnetic Radiation.
Electromagnetic radiation consists of electromagnetic waves, which are synchronized oscillations of electric and magnetic fields (as shown in fig. 2) that propagate at the speed of light, which, in a vacuum. Fig 1 show the range of electromagnetic waves that consists of radio waves, microwave, infrared, visible light, ultraviolet, x-rays, γ-rays. X-rays and γ-rays are ionizing radiation and for Kenya to invest in atomic energy more awareness on these radiations in the general population is required.
The quantum energy of microwave photon is in the range of 1x 10-6 eV to 1 x 10-3 eV which is in the range of energies separating the quantum states of molecular rotation and torsion. Rotation produces heat from which microwave heating is achieved. Microwaves do not posses enough energy to induce ionization and they do not produce radiation damage.
The quantum energy of infrared photon is in the range of 1 x 10-3 eV to 1.7 eV which is in the range of energies separating the quantum states of molecular vibrations the human body absorbs infrared more strongly than microwaves, but less strongly than visible light. The result of infrared absorption is heating of tissue.
The quantum energy of visible photon is in the range of 2 eV to 3 eV which is the range of energy that can promote electrons to higher energy level (valence electrons). Visible light is absorbed strongly by human body, especially light towards the blue.
The quantum energy of ultraviolet (UV) phonon is in the range of 3 eV to 103 eV. UV photo is categorized as UVA with λ (wave length) 315 nm to 400 nm (Not absorbed by ozone) and UVB with λ 280 nm to 315 nm absorbed by ozone, The latter have higher energy and can induce photo ionization and hence the need to protect the ozone layer and use of sun screen (by susceptible members of society such as people with albinism) as this photon has the potential of inducing skin cancer.
X-rays are produced when an electron is ejected from the inner shell of an atom another electron moves from the outer shell to replace the ejected electron and in the process an x-ray is produced. The quantum energy of x-ray photon is in the range of 103 eV to 105 eV, much too higher to be absorbed in electron transitions between states for most atoms. They can interact with electrons by completely knocking it out of the atom and hence classified as ionizing radiation. This can occur by giving all the energy to an electron (photo ionization) or by giving part of the energy to the electron and the reminder to a lower energy photon (Compton scattering).
The γ-rays are produced when the nucleus of an atom is broken apart during radioactive decay. Iron element has the highest binding energy per nucleon and this binding energy drops off as the mass of the atom increases after iron. Of all the thermodynamic determinants (fundamental forces of nature) i.e. electric field, magnetic field, gravity, strong nucleus force and weak nucleus force, The strong nuclear force is the strongest owing to the fact that net positive charge of the nucleus does not repel each other due to electromagnetic force. Weak nucleus force causes a neutron to change to a proton and vice versa (explained through other subatomic particles to be explained another day). This weak nuclear force brings about release of binding energy during fusion and fission. It is therefore responsible for radioactive decay (Fission) of certain nucleus. The quantum energy of γ-ray photon is in the range > 105 eV. γ-rays interacts with matter by completely knocking electrons off their orbital’s and diving the nuclear to subatomic particles of protons and neutrons resulting to release of more energy from the splitting atom hence the term chain reaction.
The ionizing radiations have short wave lengths, large wave numbers and high energy as depicted in fig. 3. Ionizing radiations penetrates biological tissue materials with no immediate pain or other sensation. Their main hazard is caused by ionization with reactive ions hydroxyl ions OH– which interfere with chemical operation of biological cell resulting to damage and with potential of genetic damage or mutation. If Kenya is to invest in nuclear energy the general public need to be well informed of the potential danger associated with ionizing radiation.
Chain reaction
In a nuclear reactor chain reaction is triggered by splitting an atom and sending neutrons flying in all directions. If one of these neutrons hit an atom of a nuclear fuel material say uranium (U-235) it splits and set the chain reaction in the reactor, though this is a matter of probability. The number of atoms that split sends out several neutrons where some are lost, some leak out of the reactor and not unless pure uranium (U-235) is used with a resultant of nuclear bomb, others gets absorbed in to other elements making the fuel such uranium U-238 that make about 146 parts out of 147 parts of naturally occurring uranium. This explains why a big pile of uranium in one spot cannot trigger a chain reaction. If the nuclear reaction would be started on such a pile it would fizzle out immediately because very few neutrons would find U-235 atoms to split. To start chain reactions, triggered neutrons must be conserved until they achieve some useful work. One way of conserving neutrons is slowing them down until they are moving within a reactor as fast as phonon vibration of uranium U-235 atoms. Neutrons in this speed are called ‘thermalized’ neutrons and are likely to split U-235 atoms than fast moving neutrons. To slow neutrons down in a reactor, a moderator is required. Fuel material cannot be used as a moderator owing to large size of its atoms when compared to neutrons; neutrons would just bounce off without slowing down. This makes it necessary for small atom to be used as a moderator and hydrogen has been found to be the smallest atom for use as a moderator.
Hydrogen gas is not suited for this purpose as atoms are usually far apart decreasing the probability of slowing a neutron. Water works well because it packs a lot of hydrogen atoms together in a small space and neutrons have to travel a short distance before hitting a hydrogen atom and getting thermalized to split the U-235 atoms. There is a probability that a neutron might get thermalized or be absorbed by the hydrogen atom making a Deuterium atom (H+n =D), thus converting the water to heavy water 2H2O (D2O) and this won’t be noticed until the chain reaction fizzles out. This can be fixed by enriching naturally occurring uranium from 0.7% U-235, 99.3% U-238 to 5% U-235, 95% U-238 and in this case light (ordinary) water (H2O) can be used as a moderator otherwise heavy water (2H2O) should be used as a moderator with natural uranium fuel. It is expensive to enrich uranium and at the same time it is expensive to separate deuterium water from ordinary water. It is the choice of modulator that determines the type and design of a nuclear reactor.
Nuclear reactor types.
The type of reactor that a country like Kenya might adopt is largely dependent on the nuclear set up economy. The nuclear arms race during the cold war made the United States of America to acquire large quantities of enriched uranium for manufacturing of bombs though most of its bombs are made of plutonium. This means that most of the nuclear reactors designed and manufactured in the United States use ordinary water as a neutrons moderator. H2O moderator reactors are more compact than 2H2O reactors and are preferable for naval air craft carries and sub-marines. On the other hand Canada has cheap supply of 2H2O, as in 1997 Atomic energy of Canada Limited was the world’s leading supplier of deuterium. Deuterium water is used in CANDU(Canada Deuterium Uranium) reactors design as neutrons moderator depicted in fig.4. The CANDU happens to be the only 2H2O moderated reactor in commercial use.
Another major producer of 2H2O is India which has most of its nuclear power plants using 2H2O as neutron moderator and exports the rest to other countries. Fig. 5 show a scheme of Magnox reactors that uses graphite as a moderator and it is associated with the defunct BNFL (British Nuclear Fuel Limited). These reactors have been replaced with AGR (Advanced Gas cooled reactors), that still use graphite as neutrons moderator.
There are many more designs of reactors including the RBMK reactor associated with the Russians and was applied in the ill-fated Chernobyl power plant, boiling water reactors (BWR), pressurized water reactors (PWR) and currently developed advanced pressurized water AP1000 and the new generation Canada Deuterium Uranium with added safety features.
Discussion.
Despite the many improvement in design and safety of nuclear reactors, there is danger in generating of electric energy through these means. The proponents of the Three Mile Island (See fig. 6) had promised the society of electric power source that was too cheap to meter and assured the public of their safety, but a disaster did happen and one of the reactor building still stands as monument that reminds us of technological disaster. In our quest for nuclear energy as a country we need to be very careful about location and security of a nuclear power plant. Imagine of a “China syndrome” in a densely populated area like Nairobi city and its environs. We need to be more cautious and avoid technological bravado.
From the Chernobyl disaster (See fig.7 and fig. 8), it emerged that the most dangerous time of any system is when it is changing state during which mistakes can be catastrophic. Many lives were lost immediately and many more were lost thereafter and the effects of ionizing radiation on living thing are still being experienced today. Kenya needs to educate its masses on the dangers of these radiations and a serious public participation is required before adoption of such technology in electric power production. The pro and cons associated with this technology needs to be well explained to majority of Kenyans, avoiding the secrecy that has been associated with the nuclear industry. The financial muscle of the leading players in this industry should not be used to dupe Kenyans to accepting dangerous technology.
Challenges towards decommissioning of Fukushima Daiichi Nuclear Power station of the Tokyo electric Power Company should encourage Kenya to make informed choices before use of atomic energy in electric power generation. With virtually all building in Kenya constructed without taking into account seismic activities, this would be catastrophic if replicated in a nuclear reactor building. The Japanese achieved complete cold shutdown within nine months after the accident in 11th March 2011, within two years removal of fuel from the spent fuel pool was started, by the year 2021 removal of fuel debris is expected to commence and full decommissioning is scheduled to be completed by the year 2051. The challenges to quick decommissioning include lack of robotic technology that can walk up and down stairs with a load, robotic technology whose communication and materials composition cannot be affected by ionizing electromagnetic radiation. A lesson was learnt such that future reactor buildings should be constructed with rams instead of stairs. Most of the developed world is shutting down its nuclear reactors, is it the time for Kenya to invest in this technology?
The world is faced with a serious challenge of spent nuclear fuel disposal. The current scenario is that most of the spent fuel is stored in pools in the basements of nuclear power plants as depicted in fig. 9. Fig 10 shows a BNFL recycling facility which resulted in accumulation of nuclear waste; making Britain the second country with largest amount of the most toxic poison known to human kind after the United States of America. There have been attempts to come up with an international repository in Australia, Argentina, China and Namibia, where Pangea rock exists. This lead to the creation of Pangea Resources Australia Pty Ltd which promises to make the world a safer place by burying 20% of the world nuclear waste (Including plutonium from decommissioned war heads) in Australia. Despite the companies big money and influence they have never got the necessary approvals, bring to question the thinking in the nuclear industry, that it can solve its problems by burying them in a remote location. The Manhattan program in the United stated saw the creation of 1400 km2 of highly radioactive land which will take about 50 years to clean at a daily cost of 5.5 million dollars. If Kenya invests in nuclear power generation how will it handle its nuclear waste? The nuclear industry is highly regulated by the International Atomic Energy Agency (IAEA) , will Kenya be in a position to make decisions on the safest and cost effective technology in this field?
The security situation in the great lakes region is of concern; will Kenya not became part of the “loose nuke problem”? Will there be no concerns on modifications of reactors with blankets to produce weapon grade Plutonium? Kenya has a large geothermal capacity which is safer, renewable and easier to handle than nuclear energy. This should be fully exploited before investing in nuclear power generation.
Conclusion.
Global warming is serious threat to human existence which needs to be combated by switching to renewable energy sources. Nuclear energy is considered green, but has the potential of ending the world if handled irresponsibly. If Kenya wishes to develop this industry the masses should be well educated on the implications of ionizing radiation. The sitting of the power plant should be in such a way that there is a low population density within the plant evacuation radius. We should otherwise fully develop all the other renewable energy sources before adopting atomic energy and that is why our next article will be on solar panels.
Bibliography.
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- Robert J. Naumann, Physics and Chemistry of Materials, CRC press Taylor and Francis Group, 2009 ISBN: 978-1-4200-6133-8.
- M. Sze and M.K. Lee, Semiconductor Devices Physics and Technology, John Wiley & Sons, Inc. (USA 2012). ISBN: 978-0470-53794-7.
- The IEE, Nuclear reactor types, An environmental & Energy Fact File, The Institution of Electrical Engineers, Savoy place London 2005: ISBN: 0-85296-581-8.
- International Exchange Center study Tour in Fukushima notice 3rd and 4th March 2016.
- Fukushima Research Infrastructural Creation Center Sector of Fukushima Research and Development Japan Atomic Energy Agency, Future from Fukushima, October 2015.
By Eng. Benedict Mutunga PE, MIEK, Meng (Materials Science and Engineering), Mechanical Engineer, Ministry of Transport, Infrastructure, Housing, Urban Development and Public Works.
