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Nuclear Chemistry is a branch of Chemistry dealing with the reactions involving the change in the nucleus of an atom. In our study of the concepts of chemistry, we have known that the nucleus contains the protons and neutrons of the atom. An atom may have the same number of protons but different number of neutrons, that is same atomic number, but different atomic mass. We call these isotopes. Many elements have isotopes. Hydrogen, for example has three isotopes:

Isotopes of one element have different properties. U-238 undergoes fission reaction, but not U-235. With regards to abundance U-238 is more abundant than U-235, 99.3% and 0.7% respectively.

The number to the upper left of the symbol of the element is the mass number, corresponding to the sum of protons and neutrons; the number to the lower left is the number of protons. Therefore, the number of neutrons is the difference between those two numbers.

In nuclear reaction, the nucleus of an atom is the one that changes. It is a process in which two nuclei or external sub-atomic particle collide creating a change in the identity of the nucleus, or producing a different element. Nuclear reaction is induced by bombarding the atom with energetic particle. That energetic particle can be an alpha particle, beta particle, gamma particle, a neutron, a proton or heavy ion. We will have a separate discussion of the first three particles, alpha, beta and gamma.

Uranium’s isotopes are:

Chemical Reaction vs Nuclear Reaction A traditional chemical reaction involves breaking of chemical bonds of the molecules, the elements rearranges to form a new compound. We have learned that there is energy released or absorbed in breaking bonds of the reactants and in forming products. The electron is the one that participates in chemical reaction. Energy and mass are both conserved in a chemical reaction. The types of atoms on both product and reactant sides are the same. The mass of the reactants and the mass of the products are also the same.

235 U 92 and 238 U 92 There are three types of nuclear reaction: How many neutrons does U-235 has? If you answered 143, you are correct. U-238 has 146 neutrons. Such numerous neutrons. Hydrogen’s isotopes have very few neutrons: Deuterium has one, Tritium has two. Protium has no neutron. When we go into our topic on fission, you would know that it is uranium that undergoes fission. Hydrogen, on the other hand, can undergo fusion, but not fission. Isotopes may be naturally occurring. They are stable isotopes. Other isotopes are products of nuclear reaction or radioactive decay. Nuclear reaction produces unstable isotopes.

A.

Fission – heavy, unstable nucleus splits into two lighter nuclei with energy release

B.

Fusion – two light nuclei combine together releasing vast amount of energy

C.

Decay – unstable atomic nucleus loses energy by radiation

A.

Nuclear Fission

In nuclear fission, energy is released because the total mass of the products is slightly less than the total mass of the reactants. The difference in mass is converted to energy. When matter disappears an equivalent amount of energy appears. Only the nucleus and only heavy elements undergo fission. This energy came from the difference of the mass of the products and the reactants. Consider this nuclear fission:

Chain Reactions On the reaction above, the neutron on the reactant side initiates fission. The three neutrons on the product side, produced from one fission, can again bombard nuclei producing again 3 neutrons each. The reaction becomes self-sustaining and can spread in a fraction of a second. The figure below is a chain reaction of the nuclear reaction above. For each reaction, energy is released. In a chain reaction, tremendous amount of energy is released. How much energy? The mass-energy equation by Albert Einstein enables us to compute for the amount of energy released by an atom.

E = mc2

ΔE = Δmc2

where E = is the energy in Joule m is the mass in kg and A neutron hits U-235 producing an unstable isotope of uranium, U-236. The reaction proceeds immediately to form 2 new elements barium and krypton and three neutrons. Examine the way neutron is written in the reactant. A subscript 0 indicates that it has no positive charge. The superscript 1 indicates the mass number of neutron, which is 1. The neutron bombards the nucleus of U-235 forming a heavier isotope U-236. The brackets on U-236 indicates that it exists momentarily. It splits at once to Ba-141 and Kr-92, releasing three neutrons in the process. Now, let’s see if the above nuclear equation is balanced. Add the subscript and the superscript of all reactants. It must be the same as the sum of all the subscript and superscript. Coefficients are added for balancing. In this case, we add 3 to neutron on the product side. Compare the atomic mass of barium and krypton above with that on the periodic table. You would notice that they are isotopes.

c is the velocity of light which is equal to 3 x 108m/s

Critical mass is the minimum amount of fissionable material that is needed to maintain a nuclear chain reaction. The critical mass depends upon the material’s nuclear properties, density, shape, purity, enrichment, temperature and surroundings. For example, U-235’s critical mass is about 15 kg. If this was brought together in one place with a source of neutrons, fission would occur spontaneously and would continue as long as the critical mass persists. Nuclear weapons work on this principle that’s why nuclear weapons are extremely disastrous. You might think that nuclear power plant is very dangerous. Well, U-235 that is being used as fuel for nuclear power plant is not pure and is not capable of to explode like a nuclear bomb. Besides, there are control rods in the nuclear reactor where reaction occurs. Control rods which are composed of excellent neutron absorber such as cadmium and boron, can slid up or down to absorb fewer or more neutrons. In controlling neutrons, you are also controlling the fission reaction. The reactor is being cooled by the primary and secondary coolants. The primary coolant is passed through the control rods carrying the heat away. It absorbs the heat of the nuclear reaction. By lowering the temperature, the speed of the neutrons decreases. The secondary coolant is the water in the steam generator that does not come in contact with the reactor. Energy is given off during fission. This energy came from the difference of the mass of the products and the reactants. An atom of U-235 weighs exactly 235.043924. If you would keep all the six decimal places, you would find that the mass of the product is less by 0.1%. That difference corresponds to the amount of energy released.

How Nuclear Power Plant Produce Electricity Using that principle of nuclear fission, let’s look at how nuclear power plant produce electricity. Many of Filipinos oppose the opening of our one and only Nuclear Power Plant in Bataan for fear of radiation and explosion. We will see here how baseless are our fears are and how beneficial a nuclear power plant is.

The whole process is similar to any other powerplant, except the source of potential energy. While in other plant, boiler is used to generate steam, in nuclear powerplant, reactor is used. In other powerplant, fossil fuel is used, here U-235 is used to generate the potential energy from steam. The potential energy will be transformed to kinetic energy propelling the turbine. The turbine, attached to the generator will convert mechanical energy to electrical energy. On the other hand, steam will condense, allowing the liquid to pass through cooling tower. Cooled water is directed to a body of water. The body of water supplies the cooling water for the steam generator and for the reactor. The reactor vessel has control rods which control the fission reaction. Without the control rods, the fission reaction will be spontaneous. In that case, explosion may occur. The temperature of the cooling water should be controlled to avoid the destruction of the reactor. The uranium being used as fuel is in the form of uranium (IV) oxide (UO2). It is not pure U235. This is the reason why we should not be afraid that the reactor will explode like an atomic bomb during fission reaction.

The control rods contain at least 200 UO2 pellets. Once started, fission reaction sustains itself in a chain reaction. As we learned, neutron is needed to start the process. This neutron will come from a combination of beryllium-9 and plutonium. Plutonium releases the alpha particle, He:

It means that a gram of that coal when burned will give 30.7 kJ of energy. If we want 9 x 1010 kJ of energy, how much of it are we going to use? Mass of coal = 9 x 1010 kJ / 30.7kJ/g = 2.93 x 109 g or 2.93 x 106 kg What does that mean? About 3 million kilograms of coal will be burned to match the energy that will be given by just 1 kg of U-235. No doubt that using nuclear power plant would lower down the price of electricity. What do you think would be a better choice? On one side, both uranium and fossil fuel are non-renewable source.

There are about 450 nuclear power plants in 30 different countries. The world map below shows the distribution of the nuclear reactors.

Coal Equivalence Nuclear Power Plant is a better source of power than burning fossil fuel. The former does not release carbon dioxide, while burning fossil fuel does. We have clean air with the nuclear power plant. The energy generated by the nuclear power plant is far greater than the energy generated by burning fossil fuel. Let’s prove this. 1. In the above problem solving, we saw that 1 kg of pure U-235 as fuel for nuclear reactor will give us 9 x 1010 kJ of energy. 2.

How much coal are you going to burn to match that energy?

There are several varieties of coal. Anthracite, bituminous, lignite and others. Let us choose one that has the greatest energy content – the bituminous coal. It has 30.7 kJ/g.

See how much nuclear reactors do we have worldwide? In the Philippines, we have our one and only nuclear power plant, The Bataan Nuclear Power Plant (BNPP). The construction began in 1976, with a cost of about US$2.3 billion. It was about to be operated in 1986 when the accident in Ukraine happen. We will see later what really happened to the nuclear power plant in Ukraine. There was a fear that the same thing will happen in BNPP, so the President, Ma. Corazon Aquino, did not allow its operation. Presently, the BNPP is being maintained by the National Power Corporation and there are recommendations to put it in operation. You might be in opposition to the operation of BNPP for fear of nuclear meltdown. Let’s see what happened to two of the most tragic accident involving nuclear power plant, one in Ukraine, the other in Fukushima, Japan.

Ukraine nuclear plant accident, world’s worst nuclear accident

B.

On April 26, 1986, the engineers of that nuclear plant were running safety test when the reactors overheated. This plant has four reactors. Water from the nearby Pripyat River was used to cool the reactors. Although the surrounding region was not heavily populated, approximately 120,000 people lived within a 30-km radius, including the cities of Chernobyl. During an electrical power safety test at Unit 4 reactor, operators deliberately interrupted the flow of cooling water to the core. The temperature of the reactor rose rapidly. In addition, the operators had left an insufficient number of control rods in the reactor and other control rods couldn’t be inserted quickly enough. Furthermore, the steam pressure was too low to provide coolant, due to both operator error and faulty reactor design. An overwhelming power surge produced heat, rupturing the fuel elements and releasing hot reactor fuel particles. These, in turn, exploded on contact with the coolant water, and the reactor core was destroyed in seconds. The heat ignited the graphite used to slow neutrons in the reactor. When water was sprayed on the burning graphite, the water and graphite reacted to produce hydrogen gas. In turn, the hydrogen exploded upon reaction with oxygen in air. The explosion blasted off 4,000-ton steel plate covering the reactor. Fire started in what remained of the building and burned for 20 days. Although a nuclear explosion was not possible, the fire and explosions of hydrogen blew vast quantities of radioactive material. Several people working at the plant were killed outright, and another 31 firefighters died in the cleanup process. More than 6,000 cases of thyroid cancers were reported because of the emission of gamma particle from Iodine- 131, one of the hazardous radioisotopes that was released. (Chemistry in Context, American Chemical Society).

Nuclear fusion is a nuclear reaction where atomic nuclei of smaller atoms (atoms with low atomic number such as hydrogen and helium) fuse to form a heavier nucleus accompanying the release of energy. Fusion is an ordinary reaction in the Sun.

With 450 nuclear plant in the world, less than 5 were recorded as worse nuclear accident. What happened in Ukraine was operator error and faulty design. In Fukushima, a natural disaster, earthquake, triggered the occurrence of tsunami, thus destroying the Fukushima Nuclear Plant.

Nuclear Fusion

In order for nuclei of atoms to fuse, high temperature and pressure are required. Temperature must reach approximately six times those found in the core of the sun. At this heat, hydrogen is no longer a gas but a plasma, an extremely high-energy state of matter where electrons are stripped from their atoms. Fusion is the dominant source of energy for stars in the universe. It can also be a potential source of energy on Earth. When set off in an intentionally uncontrolled chain reaction, it drives the hydrogen bomb. Fusion is also being considered as a possibility to power crafts through space. C.

Radioactive Decay (Radioactivity)

Radioactivity is the decomposition of a nucleus to form a different nucleus. Antonine Henri Becquerel (1852-1908), a French physicist, discovered radioactivity. He was working on his research using photographic plates that were sealed in black paper to keep it away from light. He accidentally left a mineral near one of the sealed plates and found that it became dark, as if exposed to light. There he recognized that the mineral emitted powerful rays that penetrated the sealed paper.

In about 1899, Marie Curie (1867 – 1934) investigated that the rays came from uranium. She was the first to use the term radioactivity describing the spontaneous emission of radiation by some elements. Then Ernest Rutherford (1871-1937) identified alpha and beta particles in his experiment. These series of events and some more personality involved in the discovery of alpha and beta particles are worth reading.

In 1900, Paul Villard, a French chemist and physicist, discovered gamma radiation from radium. Gamma ray or gamma radiation came from radioactive decay of the nucleus of an atom. It is deeply penetrating, can penetrate through skin. Having shortwave, it is the most harmful among the electromagnetic radiations. Below is the spectrum of electromagnetic radiation. Note that gamma ray has the shortest wavelength.

No. of neutrons = atomic mass – atomic number U-235: No. of neutrons = 235 – 92 = 143 U-238: No. of neutrons = 238 – 92 = 146

•

Proton (positive particle)

•

Neutron (neutral particle)

BALANCING NUCLEAR REACTIONS •

A reaction in which one of the products becomes a reactant

•

Making the reaction to become self-sustaining

The uranium being used as fuel is in the form of uranium (IV) oxide (UO2). It is not pure U235. This is the reason why we should not be afraid that the reactor will explode like an atomic bomb during fission reaction. The control rods contain at least 200 UO2 pellets. Once started, fission reaction sustains itself in a chain reaction. As we learned, neutron is needed to start the process.

FUELS Several fuels are available. Natural gas, coal, oil, tar sands are some fuels. Termed as fossil fuels, they are mined and drilled. They are non-renewable and supplies are not easy to replenish. – takes millions of years. In fact, they are depleting. They produce carbon dioxide upon burning because of their high carbon content, and harmful to the environment. But we need them, they are presently the cheapest source of energy compared to renewable sources. Yes, we consider alternate source of energy to, at least, preserve them as well as the environment. This present age, fuels are much needed in industries, machinery, equipment, transportation and others. Let’s take a look at their characteristics and their applications A. Natural gas Nuclear reactor The reactor vessel has control rods which control the fission reaction. Without the control rods, the fission reaction will be spontaneous. In that case, explosion may occur. The temperature of the cooling water should be controlled to avoid the destruction of the reactor.

Composed mostly of methane, natural gas is generally considered either conventional or unconventional, depending on where it’s found underground. Conventional natural gas is located in porous and permeable rock beds or mixed into oil reservoirs and can be accessed via standard drilling. Unconventional natural gas is essentially any form of gas that is too difficult or expensive to extract via regular drilling, requiring a special stimulation technique such as fracking.

Abundance B.

The energy content of a fuel is the amount of potential energy in the fuel that can be converted to heat.

Coal

Coal is a complex mixture of substances, not a single compound. Its chemical formula is C135H96O9NS – carbon being highest in composition (85% by mass) and the elements came from prehistoric plant materials. Some coal samples contain traces of silicon, sodium, calcium, aluminum, nickel, copper, zinc, arsenic, lead and mercury. There are different types of coal. The Table below shows the energy content of different types of coal:

Coal’s extraction is two ways: 1. Underground mining which uses heavy machinery to cut coal from underground deposits 2. Surface mining or strip mining removes entire layer of soil and rock to access coal deposits below. Both methods destroy the environment, but surface mining pollutes the entire ecosystem. C.

Oil

Crude oil or petroleum is a liquid fossil fuel made up mostly of hydrocarbons. It is a partially decomposed organic matter. Being in liquid form, it is easier to extract from underground. It can be transported through pipelines. Regarding its energy content, it is 40 – 60% more concentrated than coal. Petroleum’s main component is gasoline. The components of petroleum can be separated by fractional distillation. Distillation is a separation process in which a solution is heated to its boiling point and vapors are condensed and collected. The diagram below shows the components of petroleum. Crude oil or petroleum is a mixture of hydrocarbons. Hydrocarbons with lesser number of carbon (light components) are first to evaporate and distill. Hydrocarbons with many carbon atoms (heavy hydrocarbons) deposits at the bottom. Asphalt, being the heaviest. Other processes in refining crude oil are thermal cracking, coking and catalytic cracking. Thermal cracking is the process of heating the starting materials to a high temperature. Coking is converting the heaviest tarry crude oil “bottoms” into useful gasoline and diesel fuel and leaving behind a residue of almost pure carbon. Catalytic cracking is the process used to convert high-boiling, high-molecular weight hydrocarbon fractions of petroleum crude oils into more valuable products such as gasoline in the presence of a ca...