Science 30 Unit D Energy and the Environment Outcome 2: Students will describe the sun as Earth s main source of energy and explain the functioning of some conventional and alternative technologies that convert solar, nuclear, tidal and other energy sources into useable forms. Specific Outcome 2.5: Explain the difference between fission and fusion and balance simple nuclear reaction equations to show the conversion of nucleons. Specific Outcome 2.6: Describe the main types and sources of radioactive decay and resulting ionizing radiation. Specific Outcome 2.7: Describe mas-energy changes in fission and fusion reactions, as represented by the formula E = mc 2 Specific Outcome 2.8: Describe, in general terms, the operation of a fission reactor and the current state of fusion research. Textbook reference pages: p. 502 520 in Science 30 HARVESTING NUCLEAR ENERGY What does Nuclear Mean? Our dependence on fossil fuels is based on combustion reactions releasing the chemical potential energy of the fuel. Chemical reactions are based on the rearrangement of electrons. Nuclear energy is all about the nucleus. Nuclear energy involves the rearrangement of the nucleus of the atom, where the protons and neutrons are found Nucleons The nucleus is made up of protons and neutrons, together known as nucleons. Protons Positive charge The number of protons determines the element The atomic number is the number of protons Neutrons Neutral (no charge) Can be any number in a nucleus, although it is often close to the number of protons in smaller elements Although an element must have the same number of protons, isotopes of the element have different numbers of neutron
mass number atomic number Describing an atom - Beryllium Atomic number of 4 o 4 protons o 4 electrons Atomic mass of 9.01 (round to 9) o Most common isotope is beryllium-9 o Most common isotope has 9 nucleons 5 neutrons Practice problem Complete the table below: Isotope Atomic number Mass Number Number of: protons neutrons nucleons carbon-12 6 12 6 6 12 carbon-13 6 13 6 7 13 carbon-14 6 14 6 8 14 nitrogen-14 7 14 7 7 14 The atomic number from the periodic table gives you the number of protons The number after the dash gives you the mass number The number of nucleons is the same as the mass number The number of neutrons = nucleons - protons 2
What's Holding The Nucleus Together??? The nucleus contains a lot of protons, especially in elements with high atomic numbers. o Since protons repel protons (they're all positive), why doesn't the nucleus just fly apart? o The reason is a force called the strong nuclear force This force only works when protons are really close together (like in the nucleus). o It causes protons to pull together. o If the atom is big enough, it's hard for even the strong nuclear force to keep all those protons together. o A large number of neutrons helps to dilute the nucleus so the protons don't have to be quite so close together NUCLEAR REACTIONS Nuclear fusion o When two small nuclides fuse together to make a larger nuclide o Type of reaction occurring in the sun Nuclear fission o When a large nuclide is split into several smaller nuclides o Happens in a nuclear reactor Nuclear decay o When an unstable nuclide emits energy in the form of radiation, in an effort to become more stable o Three categories Alpha, Beta and Gamma Practice Problem Match the following images to the types of reactions described above Fusion Alpha decay Fission Gamma decay RADIATION Ernest Rutherford and others started studying radiation that was emitted by radioactive elements like uranium. Three distinct forms of radiation were discovered, based on their ability to pass through objects and their deflection in magnetic fields o Alpha (α): could barely pass through a single sheet of paper. Deflected as a positive particle in a magnetic field. 3
o o Beta (β): can pass through up to about 3mm of aluminum. Deflected as a negative particle in a magnetic field. Gamma (γ): can pass through several centimetres of LEAD! Not deflected in a magnetic field ALPHA DECAY Atoms can do something that will easily allow them to become more stable. o More stable just means that it will be easier for the strong nuclear force to hold everything together. o The easiest way for this to happen is to eject some nucleons from the nucleus so there is less to hold together Alpha radiation is made up of particles made from two protons and two neutrons. o This alpha particle is exactly the same as a helium nucleus. o Like a helium nucleus, it has a charge of 2+ o o We start with the parent nucleus, uranium-232 (sort of like a reactant in a chemical reaction). On the other side of the reaction we can see it ejects the alpha particle (shown as a helium nucleus). the uranium-232 no longer exists, since the number of protons has changed. in its place we have the daughter nucleus, thorium-228 (this is like a product in a chemical reaction). 4
Practice Problem Many smoke detectors contain the isotope americium-241. Alpha particles emitted during the decay of americium-241 ionize molecules in the air, allowing an electric current to flow between two plates in the smoke detector. During a fire, smoke particles that come between these two plates interfere with the current, setting off the detector s alarm. Write the balanced nuclear equation. Let represent the unknown product. Determine the identity of this product. 241 241 4 = A 95 95 2 = Z A = 241 4 = 237 X = 95 2 = 93 use this as the atomic number The element with the atomic number 93 is neptunium, Np The reaction is therefore: BETA DECAY Atoms can also try another trick to try to become more stable; emitting an electron. o But wait, there aren't any electrons in the nucleus, so how did it come out of there? o In the nucleus, a neutron breaks apart into a proton and an electron. n o p + + e - The electron is ejected, but the proton stays behind in the nucleus. 5
The overall result is that the number of protons in the nucleus increases by one, while the total number of nucleons stays the same Notice that on either side the nuclear numbers still add up to the same amount. o Total number of nucleons (mass number) = 10 o Total number of protons (atomic number) = 4 Because beta radiation releases electrons as the radiation, it is harder to stop with shielding than alpha radiation. At least a few sheets of aluminum is required to slow or stop it. Practice problems If the atom depicted to the right undergoes beta decay, what will be the products? If beta decay produced nitrogen-14, what was the parent isotope? if the product is nitrogen-14, then the parent isotope was carbon-14 GAMMA DECAY As we saw earlier in the course, gamma radiation is actually a type of high energy, high frequency EMR. o There is no actual particle of matter emitted during gamma decays. o Instead, a burst of EMR is released. Gamma radiation often, but not always, happens after an alpha or a beta decay. o This is because after rearranging the nucleus in a decay, the atom often has to let off a little extra energy. o The easiest way to do this is with a burst of EMR. 6
We start with the boron-10 from the previous example of beta decay. The gamma radiation that is emitted is just a burp of energy to help the boron-10 settle itself down. o The Daughter is still boron-10... nothing's changed! o Sometimes gamma decays are shown with the other decay that happened as one line... NUCLEAR FISSION Fission is when a large atom splits apart into smaller atoms. Fission is what happened in the first nuclear bombs, and is still used in nuclear power plants to this day. One of the most popular elements to split is uranium-235. This is because it is stable on its own, but becomes very unstable when only one extra neutron is added 7
CANDU reactor CANadian Deuterium Uranium Reactor o a series of eight nuclear reactors located just east of Toronto, Ontario o produce enough electricity to power a city of about 2 million people o the technology used in the CANDU reactor was developed in Canada and is now in use in six other countries Controlling Nuclear Fission Obviously you can't just let nuclear fission in a nuclear power plant the same way as it happens in a nuclear bomb. There are two main methods used to control the rate at which the nuclear reactor will run: o Moderator: a substance (often water) that slows down neutrons so they can interact with U-235 o Control Rods: rods made of metal (often beryllium) that can absorb neutrons so they never get to interact with the U-235. The goal is to control the chain reaction so that the number of reactions happening stays constant. Otherwise, a meltdown could happen 8
Chernobyl disaster On April 25, 1986 a nuclear meltdown occurred in a plant in Chernobyl, Ukraine It released radioactive materials into the environment, which were carried thousands of kilometers by winds As a result, 237 people suffered from acute radiation sickness, of whom 31 died within the first three months. Children born in the area at the time were found to have low-functioning immune systems due to radiation exposure in utero Radioactivity biomagnified in ecosystems in the area, especially in fish populations, making it unsafe for consumption Fukushima Daiichi nuclear disaster a nuclear disaster at the Fukushima I Nuclear Power Plant in Japan that began on March 11, 2011 and resulted in a nuclear meltdown of three of the plant's six nuclear reactors The failure occurred when the plant was hit by a tsunami that had been triggered by a magnitude 9.0 earthquake. The following day, 12 March, substantial amounts of radioactive material began to be released, creating the largest nuclear incident since Chernobyl Although no fatalities due to short-term radiation exposure were reported, some 300,000 people evacuated the area and deaths did occur as a result The Fukushima Nuclear Accident Independent Investigation Commission found that: o the nuclear disaster was "manmade" o its direct causes were all foreseeable o the plant failed to meet basic safety requirements, such as: assessing the probability of damage, preparing for containing collateral damage from such a disaster, developing evacuation plans 9
So is nuclear power dangerous? Health effects of radiation: o High speed radioactive particles penetrate deep inside the human body where they can damage biological cells and thereby initiate a cancer. If they strike sex cells, they can cause genetic diseases in future offspring. o Radiation occurs naturally in our environment; a typical person is, and always has been struck by 15,000 particles of radiation every second from natural sources, and an average medical X-ray involves being struck by 100 billion. o However, the chance for a particle of radiation entering a human body to cause a cancer or a genetic disease is only one chance in 30 million billion (30 quintillion). Risks from reactor accidents o Estimated by the rapidly developing science of "probabilistic risk analysis" (PRA). o A fuel melt-down might be expected once in 20,000 years of reactor operation. o The average damage for a meltdown would be 400 deaths. To keep the matter in perspective, o the entire release of radioactivity into the sea as a result of the Fukushima meltdown added less than 0.01% to the background radiation of the sea o since air pollution from coal burning is estimated to be causing 10,000 deaths per year, there would have to be 25 melt-downs each year for nuclear power to be as dangerous as coal burning The CANDU reactor has significant differences in design that make a repeat of the Chernobyl incident extremely unlikely However, for many policymakers, events such as Chernobyl and Fukushima make many people nervous about embracing this source of power Mass-Energy Equivalence When a neutron joins U-235 undergo fission, the products that are produced do not have the same mass as the original reactants o This lost mass has been changed into energy according to o E = mc 2 where: E = energy (in J) m =mass (in kg) c = speed of light (3.00 10 8 m/s) Although the change in mass is small, it converts to an enormous amount of energy overall. 10
Practice problem The fission of 1 mol of beryllium-8 produces products that weigh 2.29 x 10-5 kg less than the original reactants. Determine the energy released. Δm = 2.29 x 10-5 kg c = 3.00 x 10 8 m/s ΔE =? ΔE = Δmc 2 = 2.29 x 10-5 kg (3.00 x 10 8 m/s) 2 = 2.06 x 10 12 J The energy released is 2.06 x 10 12 J. Masses of Subatomic Particles & Nuclides o On page 8 of your data sheet you will find the masses of many particles and elements. o By calculating how much mass was on each side, you can figure out how much mass was lost. o Using that info and E = mc 2 you can determine the energy that was released. Practice problem The fission of uranium-235 that occurs in a CANDU reactor involves the following reaction. Calculate the change in mass between the reactants and the products for this reaction and the corresponding energy change. 1. Determine the mass of the reactants, using the table on page 8 of the data booklet. 11
2. Determine the mass of the products, recalling that three neutrons are produced during fission 3. Find the change in mass. 4. Determine the energy change. 12
Nuclear Fusion Recall, fusion is when small atoms are forced together to form a larger atom. o This requires high energy, temperature, and pressure. o This is how the Sun is able to convert hydrogen into helium and other larger elements. This requires high energy, temperature, and pressure. o This is how the Sun is able to convert hydrogen into helium and other larger elements. Current state of fusion technology o Since the 1950s, scientists have examined the possibility of using fusion instead of fission to produce energy The product would be helium instead of plutonium Unlike plutonium, the helium produced would not be radioactive waste o In addition the energy produced from the fusion of hydrogen is roughly 10x greater than the fission of uranium-235 The challenge is getting the two hydrogen nuclei close enough to fuse So far, this has only been accomplished with a very energy intensive combination of lasers and electromagnetic fields o Currently, the energy produced is only marginally greater than the energy required to force the reaction to occur, so it is not a viable option. Practice Questions: Page 503 26 28 13
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