After the efforts from hundreds of scientists and more than a few dozens of their autonomous theories, we have been able to harness the energy exploiting the source that’s 175 × 10−10 m in radius. Even further inside the atom; nuclear energy comes from the very heart, the nucleus. Fission of 1 kg of Uranium can produce 24 million kWh of heat energy which is about 3 million times the energy equivalent of oil which produce only 12 KWh per kg upon combustion.How uncanny is it that the smallest known particle to us has been the most efficient and the biggest energy source to date?
Table of Contents.
- Discovery Of Radioactivity
- Harnessing The Nuclear Energy: Nuclear Fission
- Why choose Uranium of all elements?
- Nuclear Bombs
- Power From Nuclear Energy with Working Mechanism
- What might go wrong with nuclear energy!
- Discussing the pros and cons.
- Status Quo and Future of Nuclear Energy
Discovery of Radioactivity
Scientists spent the first half of 20th century trying to decipher the nucleus’ enigma, but its humongous potential was unforeseeable during its infancy: discovery of radioactivity.
On November 8, 1985, William Roentgen was conducting an experiment on Cathode Rays in a vacuum tube connected to the electrode on both ends. He discovered a faint green glow in fluorescent screen painted with barium platocyanide, placed few feet away from the tube. Cathode rays couldn’t penetrate the tube and boards surrounding the experimental setup. He then, concluded it was a different kind of ray and named it ‘X-Ray’.
One year later in 1896, Henry Becquerel was studying the properties of X-ray. He exposed potassium uranyl sulphate-a naturally fluorescent mineral- to sunlight and wrapped it in black paper. He believed that the exposed Uranium absorbed sunlight and emitted X-ray.
On 27th February, the sky was overcast on Paris, but he decided to conduct the experiment anyway. To his surprise, he found that Uranium produced a clear image in photographic plates. The photographic plate was glowing because of something the atom emitted by itself without absorbing external energy.
In 1898, frontiers of their generation, Marie Curie and Pierre Curie isolated two elements that showed the property which Becquerel had discovered: Polonium and Radium. They termed the phenomenon ‘Radioactivity’. Later in 1902, Ernest Rutherford demonstrated a nuclear rearrangement by firing alpha particles emitted from Radium source into Nitrogen atom which converted it into an atom of Oxygen.
After that, a series of theories would be published about the subject but apparently none seemed to contribute to the endeavour until 1932 when the neutron was discovered. Researchers started using it to bombard the atoms to produce new atoms.
Fermi showed that slow neutrons were more effective for this purpose. Fermi bombarded Uranium with a neutron, a different kind of atom appeared along with the release of energy. He believed that he had discovered a new transuranic element.
It was only until 1938 when Otto Han and Fritz Strassman showed that the elements generated were smaller atoms of Barium and Krypton, meaning the atom had split. But the sum of masses after fission was less than of the Uranium. This counter-intuitive phenomenon had the most meticulous of solutions.
Lise Meitner and her nephew Otto Frisch explained this perplexing problem using a theory proposed 33 years ago by Einstein, the special theory of relativity. They proposed that the Uranium captured the neutron which caused vibrations causing the atom to split with the release of energy equivalent to the mass lost during the process.
Using the equation E= m.c2, they calculated the energy released to be 200 million electron volts. Now, the world knew that energy can be generated from the atom itself. But most of the scientists weren’t optimistic for the future of energy from the atoms because the energy released from the reaction is minuscule. (200 million electron volts = 3.2 X 10-11J. It requires roughly 10J energy to lift 1kg of weight by 1 metre.)
In 1933, Leo Szilard came up with a brilliant idea. He pointed out that if a neutron was released during fission, then it could induce another fission which would release more neutrons which would, in turn, cause more reactions. This chain reaction would increase the number of reaction exponentially which would cumulatively release a tremendous amount of energy. Irène Joliot-Curie, daughter of Marie Curie and Frédéric Joliot-Curie, her husband confirmed it experimentally in 1939. This subsequent idea also led to the development of an atomic bomb.
Harnessing the Nuclear Energy
There are two ways to obtain energy from the atom: either by splitting the heavy nucleus into the smaller nucleus or fusing small nuclei to form a heavier nucleus, called fission and fusion respectively. Let’s look at each of them in detail.
Nuclear fission is a nuclear reaction where a heavy atom splits into smaller nuclei releasing energy. It occurs when a heavy atom like Uranium is bombarded by a particle like a neutron or with electromagnetic radiation like gamma-ray. Those particles are chosen because there’s minimal or no chance of interaction due to charge during the bombardment, as neutrons are neutral. Uranium also undergoes fission spontaneously in nature, but it takes place at a very slow pace.
When a Uranium-235 atom bombarded by a slow-moving neutron, the atom absorbs the neutron causing it to become unstable which results in the splitting of the atom. The uranium atom splits in Barium, Krypton along with the release of three neutrons and energy. The released neutron moves with a speed of about 7% of the speed of light. Those neutrons have 15 minutes of mean life before decaying into protons and beta particles. But before that happens, some neutrons strike another Uranium atom in its vicinity causing another fission reaction which in turn release more energy and neutron to induce more fission.
In this way, self-sustaining fission occurs releasing a huge amount of energy. Fission of 1kg of Uranium can produce 24 million kWh of heat energy which is about 3 million times the energy equivalent of oil which produce only 12kWh per kg upon combustion.
Why does fission occur?
The occurrence of fission and amount of energy released in fission is determined by stability of the nucleus. Stability of the nucleus, in turn, is determined by binding energy per nucleon.
Binding energy per nucleon is the energy required to remove a nucleon from the nucleus. A low value of binding energy/ nucleon indicates lesser stability of the atom.
There are two major forces acting on the nuclear level: the strong nuclear force which binds the nucleus together and electrostatic repulsion between protons which tends to pull the nucleus apart. The strong nuclear force is very strong in short-range and decays exponentially. So, in smaller atoms strong nuclear force dominates. But in larger atoms with a diameter greater than 12 nucleons, repulsive force dominates making the atom unstable and prone to fission.
Why Choose Uranium?
Graph of binding energy/nucleon of different element shows that the energy increases till Iron(Fe-56). This is the reason iron is the most stable element.
Then the binding energy per nucleon goes on decreasing as we approach Uranium-238, whose binding energy per nucleon is 7.6 MeV. So, Uranium requires relatively less energy to break the nucleus.
More number of neutrons in Uranium 238 makes it stable. Faster neutron is required to split its atom. So, it might not sustain a chain reaction. Therefore, its isotope Uranium 235 is used. It is relatively less stable and can be split with a slow neutron, creating a self-sustaining chain reaction.
Another Nuclear Fuel : Plutonium
In 1941 scientists at the University of California Berkeley discovered a new element-Plutonium- while bombarding Uranium-238 with deuterium. They found that Plutonium-239 isotope sustains chain reaction and hence can be used for the creation of a bomb. So, the discovery was kept secret because of the ongoing war.
Nuclear Fusion: Fission’s Cousin
Nuclear fusion is the reaction where two small nuclei coalesce to form a heavier nucleus accompanied by the release of energy. It is the mechanism by which the sun produces energy.
More than 620 million metric ton of hydrogen is fused in the Sun every second to produce all the heat and light. Nuclear fusion produces more energy for the same amount of fuel. Further, fusion is a cleaner source of energy than nuclear fission. But achieving fusion has proved to be a very difficult task.
Fusing two atoms requires bringing the atoms together requires overcoming the electrostatic repulsion which is massive on an atomic scale. The temperature of about a 100 million kelvin is required for fusion to occur.
The sun creates such temperature at its core by its huge gravity. Mimicking such conditions on earth are extremely difficult given that extreme amount of pressure and heat is required.
On November 1, 1952, America tested the first fusion bomb. They used energy from fission to create the condition for fusion. But controlled fusion is still a huge challenge. Achieving energy from fusion in an economically feasible way is still a matter of research.
While the scientists were achieving success in nuclear researches, the world was at war. Hitler had started to invade countries of Europe. Development and explanation of fission by German scientists Otto Han, Fritz, Mietner and Frisch raised fear among the scientific community; Hitler might get his hand on a nuclear bomb.
So, in August 1939, Leo Szilard and Eugene Wigner drafted a letter warning about the potential development of a new kind of bomb and requested the United States to acquire Uranium and accelerate the researches. They had it signed by Einstein and sent it to Franklin D. Roosevelt (President of USA at the time.) On 9th October 1941, Roosevelt approved the atomic bomb program and employed armies for site construction. A year later on December 28, 1942 formation of the secret Manhattan Project was authorized.
Research facilities for the project were established in remote places of New Mexico, Tennessee and Washington to maintain secrecy. The project’s weapons research laboratory was established in Los Alamos, New Mexico under the direction of Robert. J Openheimer. Uranium plants were established in Oak Ridge, Tennesse and a full-scale plutonium production plant was established at Hanford, Washington.
Gun-type fission was designed for the uranium bomb. One mass of Uranium was fired at another mass of uranium which reached critical mass (minimum mass required to create sustaining chain reaction) which caused a detonation.
Scientists at Los Alamos tried to design same kind of bomb with Plutonium but they found out that it is extremely difficult to obtain 100% pure Plutonium-239. It was polluted with Plutonium-240. So the two masses in gun-type design would start fission before the mass came together. This would create an explosion without reaching critical mass, failing to meet its potential of a huge explosion.
After trying many calculations and research on the material, scientists came up with the idea of implosion. The idea was to create a spherical configuration and surround it with explosives and place the plutonium core in the centre. The explosion would compress the the core to supercritical density which would trigger fission causing huge explosion.
At 5:30 a.m. on July 16, 1945, the plutonium bomb nicknamed ‘Fat Man’ was tested at a remote site of New Mexico. Oppenheimer named the test “Trinity”. The ‘Little boy’ was detonated over Hiroshima on August 6, 1945. Three days later ‘fat man’ was dropped on Nagasaki which led to the surrender of Japan, the final chapter of World War II.
Power From Nuclear Energy
Once the war ended, all the countries who had advanced on nuclear technology started to research on harnessing usable energy from the nucleus. In the USA, experimental Breeder Reactor I, was completed on December 20, 1951, which was able to produce electricity on small amounts. USSR completed construction of the first nuclear power electricity generator in June 1954. England completed Calder Hall Reactor in 1956 – which was the first reactor to produce nuclear power commercially.
Nuclear Powerplant : Working Mechanism
Different kind of nuclear reactor are currently running in the world. 60% of those reactors are Pressurized Water Reactors (PWRs). So, let’s take a look at how PWRs work.
Pellets of Uranium Oxide (UO2) are packed into fuel rods and arranged in a reactor core. The core is filled with the moderator (water in case of PWR) in order to slow down the neutrons released from fission so that they cause fission in another Uranium atom.
Control rods are dipped into the moderator. They are neutron absorbers like Cadmium, Boron which can be inserted or withdrawn from the core to increase or decrease the rate of fission. The heated product from the fission is transferred to coolant (water) circulating through the core. The water is prevented from boiling by maintaining high pressure through the pressure tubes. Heat is transferred from coolant circuit in the core to secondary coolant circuit via a heat exchanger. The coolant in the secondary circuit gets converted into steam which rotates the steam turbine connected to it. The steam turbine is connected to the generator which generates electricity.
What Can Go Wrong?
If the reaction in core goes out of control and fission starts to occur rapidly, then the core overheats and might explode. The explosion might send cloud of the radioactive substance causing pollution in a huge area. The moderator burns, fission stops and the reactor is destroyed. In the worst-case scenario, there’s a meltdown. The reactor might melt and send the radioactive material penetrating down deep into the ground polluting the land, and water sources.
More than 100 nuclear accidents have occurred. Some of them polluting huge area causing radiation cancer and deaths. Fukushima Daiichi nuclear disaster (2011), Chernobyl disaster (1986), Three Mile Island accident (1979) are some serious nuclear disasters.
Chernobyl disaster (1986)
On 25th April 1986, unit 4 reactor- an RBMK-1000 reactor- was scheduled for maintenance. The reactor was to be shut down. So, the decision was made to take advantage of the shutdown to test whether loss of power and slowing the turbine could provide enough power to operate the cooling water circulating pump. At 1:23 the test began, four instead of six regular pumps were active.
The steam to the turbines was shut down. As the turbine slowed down, the power decreased and the rate of flow of water decreased too. The hot core converted the water into steam creating bubbles (voids) in the core. RBMK reactor has a positive void coefficient, meaning an increase in steam void intensifies fission creating more heat. The neutron which would have been absorbed by heavy water now causes more fission. This causes a further increase in the void which again increases fission and so on. The increased pressure in core ruptured fuel channels and jammed the control rods. This pressure increase caused steam explosion throwing the fission product into the air. 2-3 seconds later second explosion sent out graphite moderator and fuel channel.
UN estimated that 50 people died as a direct result of the explosion. After the explosion, Chernobyl clean up team was sent for cleaning the plant. Russian Academy of sciences shows that there could have been 830,000 people in the cleanup team. They estimated 15% of them had died by 2005. The explosion contaminated 150,000 sq km of the land of Belarus, Russia and Ukraine. More than 4,000 sq km of land was declared an exclusion zone.
Pros and Cons Of Nuclear Energy
Nuclear energy comes with both, the benefit of large amount of energy for long time and grave dangers of radiation always lurking around. Let’s have a look at some pros and cons of using nuclear energy.
- Nuclear energy doesn’t produce greenhouse emissions like CO2 and methane.
- It is a reliable source of energy. Nuclear plants can continuously produce energy for a year or more without interruption.
- Has high fuel to power output ratio: A small power plant can produce up to 1000MW of electricity, enough to fuel a city with half a million population.
- Once a nuclear power plant is built, it is cheaper to get energy from the plant than from other sources like coal, gas.
- Nuclear power plant might explode or meltdown which can have devastating impact on environment and close cities.
- By product of fission are radioactive which takes years to degrade and is very expensive to dispose properly.
- The initial cost of building a reactor is huge, in scale of billions of dollars. So, it might not be feasible to all the countries.
- Uranium and other fuels for reactor are non renewable. So, like fossil fuels we will run out of it in future.
Status Quo and Future
Currently, 440 nuclear reactors are running in 31 countries producing 10% of the world’s electricity. In 1996, nuclear reactors used to produce 17.6% of total electricity. Since then the share of electricity produced from nuclear energy has been decreasing steadily. One of the reasons for this decrease in nuclear power is the Fukushima nuclear disaster in Japan which led to an immediate shutdown of 48 reactors.
Currently, only 9 reactors are functioning in Japan. Further, old power plants will stop functioning in future which will cause a decrease in nuclear power. Though construction of new reactors have begun in Finland, Vietnam, China, they have been delayed by cost overruns and accidents. It takes billions of dollars of investment and decades-long period of time to get a nuclear reactor fully running.
In addition, nuclear fuels for fission are not renewable, meaning it will be all used up at a point. The fusion reactor is still in the development phase. All and all, the popularity of nuclear power is decreasing after a massive spike.
So, countries have shifted to other sources of energy like solar power, wind power. They cost a lot less than nuclear reactors and can be set running in less time. A study by Lazard (an asset management firm) showed that cost per KW for solar energy is less than $1000 whereas the cost per KW for nuclear power is between $6,500 and $12,250.
Needless to say, unless researchers find ways to produce energy from fusion on a commercial scale, the share of nuclear power in global energy production is likely to decrease further with time.