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Chapter 5 5.1 – Natural Radioactivity 5.2 – Nuclear Reactions 5.3 – Measurement 5.4 – Half-Life of a Radioisotope 5.5 – Medical Applications using Radioactivity 5.6 – and Fusion Goal: Describe alpha, beta, , and gamma radiation. ▪ Most naturally occurring of elements up to 19 have stable nuclei (plural of nucleus). ▪ In stable nuclei, the nuclear forces balance the repulsions between the positive .

▪ Elements above atomic number 19 generally have one or more isotopes with unstable nuclei. ▪ The nuclear forces cannot offset the repulsions from the greater number of protons. ▪ The nuclei become unstable and spontaneously emits small particles of (radiation) to become more stable.

▪ Unstable nuclei are called radioisotopes and are said to be radioactive. ▪ Radiation can be emitted from a radioisotope in the form of: ▪ Alpha (α) particles ▪ Beta (β) particles ▪ (β+) ▪ Pure energy (example: gamma (γ) rays) ▪ A radioisotope ▪ Is an of an element that emits radiation. ▪ Can be one or more isotopes of an element. ▪ Includes the in its name.

The atomic number of -131 has a mass number of 131 and an atomic number of 53.

By emitting radiation, an unstable nucleus forms a more stable, lower energy nucleus.

Alpha particles are identical to a nucleus (2 protons, 2 ). Mass number = 4 (2 protons + 2 neutrons) Atomic number = 2 (2 protons) Charge = +2

4 or 2훼 By emitting radiation, an unstable nucleus forms a more stable, lower energy nucleus.

Beta particles are high energy Mass number = 0 Atomic number = 0 Charge = -1 (or 1-)

A is formed when a in an unstable nucleus changes into a . By emitting radiation, an unstable nucleus forms a more stable, lower energy nucleus.

Positrons are high energy electrons with a positive charge Mass number = 0 0 Atomic number = 0 +1푒 Charge = +1 (or 1+)

A positron is produced by an unstable nucleus when a proton is transformed into a neutron and a positron. By emitting radiation, an unstable nucleus forms a more stable, lower energy nucleus.

Pure energy called Mass number = 0 Atomic number = 0 Charge = 0

Released when an unstable nucleus undergoes a rearrangement of its particles to give a more stable, lower energy nucleus.

Gamma rays are often emitted along with other types of radiation at the same time.

Identify the type of radiation from the following descriptions:

A. Contains 2 protons and 2 neutrons

B. Has a mass number of 0 and a 1- charge strikes in its path and ▪ Knocks away the electrons in molecules, forming unstable such as H2O+. ▪ Causes undesirable chemical reactions. ▪ Damages the cells which may lose their ability to produce necessary Different types of materials. . radiation penetrate the body to different depths. Most susceptible cells are those that undergo rapid division (regeneration): ▪ adolescent cells ▪ marrow ▪ skin ▪ reproductive organs Fight fire with fire! ▪ intestinal lining ▪ Many professions experience work with radioactive isotopes: ▪ Nuclear technologists, , doctors, and nurses ▪ MRI (magnetic resonance imaging), X-rays, CT scans (computed tomography), PET (positron emission tomography),cancer treatment, etc. ▪ Proper shielding is needed to protect yourself from radiation. ▪ requirements depend on the type of radiation: ▪ Alpha particles are slow and large and only travel a few centimeters before colliding with air molecules, acquire electrons, and become stable helium . ▪ Paper and clothing blocks alpha particles. ▪ Beta particles are very small and move much faster and farther than alpha particles. They can travel several meters. They can pass through paper and penetrate 4-5mm into the body. ▪ Threat: ▪ Heavy clothing (lab coats and gloves) protect against beta particles. ▪ Gamma rays travel great distances through the air and pass through Different types of many materials (including body tissues) radiation penetrate the ▪ Incredibly dangerous. body to different depths. ▪ Only very dense shielding (lead or concrete) will stop gamma rays. ▪ For those working in an environment where radioactive materials are present, limit your exposure by ▪ Minimizing the amount of time spent near a . ▪ Increasing the distance from the source. ▪ Indicate what type of radiation (alpha, beta, and/or gamma) that is protected for each type of shielding.

A. Heavy clothing B. Paper C. Lead D. Thick concrete 5.1 – Natural Radioactivity 5.2 – Nuclear Reactions 5.3 – Radiation Measurement 5.4 – Half-Life of a Radioisotope 5.5 – Medical Applications using Radioactivity 5.6 – Nuclear Fission and Fusion Goal: Write a balanced nuclear equation showing mass numbers for . ▪ In a process called radioactive decay, a nucleus spontaneously breaks down by emitting radiation. ▪ This can be shown with a nuclear equation:

Radioactive nucleus  new nucleus + radiation (α, β, β+, or γ)

251 247 4 98Cf → 96Cm + 2He

The mass number and atomic number may change. The sum of the mass numbers and the atomic numbers must be equal for the reactants (left side) and the products (right side). occurs when a radioactive nucleus emits an , forming a new nucleus with: ▪ The mass number decreased by 4 ▪ The atomic number decreased by 2

Complete the following nuclear equation for the decay of americium-241: ▪ In the nuclear equation for , a beta particle, (an ), is emitted from the nucleus when a neutron in the nucleus breaks down, forming a proton and a beta particle and increasing the atomic number by 1. ▪ Write an equation for the decay of 42K, a beta emitter. ▪ Write the nuclear equation for the beta decay of 60Co. ▪ In positron emission, ▪ A proton is converted to a neutron and a positron

▪ The mass number of the new nucleus is the same, but the atomic number decreases by 1. ▪ Write the nuclear equation for the positron emission of 44K ▪ In gamma radiation ▪ Energy is emitted from an unstable nucleus, indicated by m following the mass number. ▪ The mass number and the atomic number of the new nucleus are the same. When the nuclei of alpha, beta, positron, and gamma emitters emit radiation, new and more stable nuclei are produced. ▪ Radioactive isotopes are produced ▪ When a stable nucleus is converted to a radioactive nucleus by bombarding it with a small particle. ▪ This is called transmutation. ▪ Write the balanced nucleus equation for the bombardment of nickel-58 by 1 a proton 1H, which produces a radioactive isotope and an alpha particle. ▪ What radioactive isotope is produced when a neutron bombards 98Tc, releasing a alpha particle? 5.1 – Natural Radioactivity 5.2 – Nuclear Reactions 5.3 – Radiation Measurement 5.4 – Half-Life of a Radioisotope 5.5 – Medical Applications using Radioactivity 5.6 – Nuclear Fission and Fusion Goal: Describe the detection and measurement of radiation. ▪ A Geiger counter is a common instrument that ▪ Detects beta and gamma radiation ▪ Uses ions produced by radiation to create an electric current.

Ar + radiation  Ar+ + e- ▪ The activity of a radioisotope is defined as the number of disintegrations per second. ▪ The (Ci): the number of disintegrations that occur in 1 s for 1 g of . ▪ Named after Marie and , who discovered radium and . ▪ The SI unit for activity is the becquerel (Bq): 1 disintegration per second. ▪ Another way to measure radiation is by the amount absorbed by a material (such as body tissue). ▪ The (radiation ) is a unit that measures the amount of radiation absorbed by a gram of material. ▪ The SI unit for radiation absorption is the (Gy) – the joules of energy absorbed by 1kg of body tissue. (1 Gy = 100 rad) Convert 2.2 Curie to Becquerel. (1 Ci = 3.7 x 1010 Bq) Convert 320 Gy to rad. (1 Gy = 100 rad) ▪ Another way to measure radiation is by the biological effects of different kinds of radiation. This is called the rem (radiation equivalent in humans). ▪ Alpha particles can’t enter the body through the skin, but if it finds another way (mouth, eyes, injury, etc) it can cause extensive damage with in a short distance in tissue. ▪ High energy radiation (beta, high-energy protons, high-energy neutrons) can travel farther and by doing so cause more damage than alpha. ▪ Gamma rays are damaging because they travel a long way through the body tissue. ▪ To determine the equivalent dose or rem dose, the absorbed dose (rad) is multiplied by a factor that adjust for biological damage cause by a particular form of radiation. Biological damage (rem) = Absorbed dose (rad) x Factor

▪ Beta and gamma radiation, the factor is 1 ▪ High-energy protons and neutrons, the factor is 10 ▪ Alpha particles, the factor is 20 A patient receives 3400mrads of I-131, which emits beta particles. If that factor that adjusts for biological damage is 1 for beta particles, how many rems did the patient receive? ▪ Often the measurement for an equivalent dose will be in millirems (mrem). ▪ 1 rem = 1000 mrem

The SI unit is the (Sv). ▪ 1 Sv = 100 rem ▪ Foodborne illnesses caused by pathogenic bacteria such as Salmonella, Listeria, and E. Coli have become major health concerns in the United States. ▪ The U.S. Food and Drug Administration (FDA) has approved the dose of 0.3 kGy to 1 kGy of radiation produced by -60 or cesium-137 for treatment of foods. ▪ When food passes through a series of racks, gamma rays pass through the food and kill the bacteria without harming the food. ▪ Currently tomatoes, blueberries, strawberries, and mushrooms are being irradiated to allow them to be harvested when they are completely ripe to extend their shelf life.

▪ The FDA requires the symbol (a) to appear on irradiated foods. After two weeks, the irradiated strawberries on ▪ Note that the food never comes into the right show no damage. While contact with the radioactive isotopes those not irradiated grow mold. itself. Only the gamma rays. So the food doesn’t become radioactive. ▪ People who work in radiation laboratories wear attached to their clothing.

▪ Dosimeters detect the amount of from the following: ▪ X-rays ▪ Gamma rays ▪ Beta particles ▪ It is an added level of security to make absolute sure that they are safe. Average annual radiation received by a person in the United States: ▪ The average person in the United States is exposed to 360 mrem of radiation annually. Exposure to radiation occurs every day from naturally occurring radioisotopes in ▪ Buildings where we live and work ▪ Food and water ▪ The air we breathe ▪ Postassium-40 in all potassium-containing foods (bananas!) ▪ Cosmic radiation from the sun ▪ The larger the dose of radiation received at one time, the greater the effect on the body. ▪ Exposure to radiation of 500 rem is expected to cause death in 50% of the people receiving the dose. (called the lethal dose for one-half the population, or LD50.) ▪ Match each property (1-3) with its unit of measurement (A-D).

▪ 1. activity A. mrad ▪ 2. absorbed dose B. mrem ▪ 3. biological dose C. becquerel D. Sv 5.1 – Natural Radioactivity 5.2 – Nuclear Reactions 5.3 – Radiation Measurement 5.4 – Half-Life of a Radioisotope 5.5 – Medical Applications using Radioactivity 5.6 – Nuclear Fission and Fusion Goal: Given the half-life of a radioisotope, calculate the amount of radioisotope remaining after one or more half-lives. ▪ The half life is the amount of time it takes half of a radioactive sample to decay. 131 131 ▪ For example, 53I , has a half-life of 8.0 days. As 53I decays, it 131 produces the non-radioactive isotope , 54Xe and a beta particle.

131 131 0 53I  54Xe + −1e 131 131 0 53I  54Xe + −1e 131 ▪ Suppose we have a sample that initially contains 20. mg of 53I . 131 ▪ In 8.0 days, one-half (10 mg) of all the 53I nuclei in the sample will have decayed, which leaves 10 mg of I-131 left. ▪ You now have 10mg and after another half life of 8 days, 5 mg will have decayed and you will have 5 mg left… etc… A decay curve is a diagram of the decay of a radioactive isotope.

The decay curve for I-131 shows that one-half the sample decays every 8 days.

The radioisotope -90 has a half-life of 38.1 years. If a sample contains 36 mg of Sr-90, how many milligrams will remain after 152.4 years. ▪ Geologist, archaeologists, and historians use knowledge of radioactive isotopes to estimate the age of ancient objects. ▪ The age of an object derived from plants or animals (such as wood, fiber, bone, wool clothing, etc.) is determined by measuring the amount of -14 in the sample. ▪ Carbon-14 is a naturally occurring isotope which is produced in nature in the upper atmosphere. ▪ Living plants continuously absorb carbon-14 through respiration. Once the plant dies, it begins to lose the carbon-14 as it decays. (carbon-14s half-life is 5730 years.) Carbon dating was used to ▪ In carbon-dating, use the level of carbon-14 in determine that the Dead Sea an object to estimate how long it had been decaying and Scrolls are about 2000 years old. guess when it died. ▪ (Living things reach an amount of carbon-14 that stays fairly constant during life.) ▪ A radiological dating method used be geologists uses the same technique as carbon dating, but with -238. ▪ Uranium-238 has a half life of 4.5 billion years and it decays to lead- 206. ▪ By measuring the U-238 and Pb-206 amounts in a rock, the age can be estimated. Over time the amount of U-238 decreases and Pb-206 increases. ▪ This method was used to date moon rocks returned by the Apollo missions. They were found to be about 4 billion years old, approximately the same age of the Earth. Carbon-14 was used to determine the age of the Dead Sea Scrolls. If the Dead Sea Scrolls were determined to be 2000 years old and the half-life of carbon-14 is 5730 years, what fraction of this half-life has passed? The half-life of I-123 is 13 hours. How much of a 64 mg sample of I-123 is left after 26 hours? 5.1 – Natural Radioactivity 5.2 – Nuclear Reactions 5.3 – Radiation Measurement 5.4 – Half-Life of a Radioisotope 5.5 – Medical Applications using Radioactivity 5.6 – Nuclear Fission and Fusion Goal: Describe the use of radioisotopes in medicine. ▪ Radioisotopes with short half-lives are used in because: ▪ The cells in the body do not differentiate between non-radioactive atoms and radioactive atoms (they treat the radioisotopes as they would the regular isotope) ▪ Once incorporated into cells, the radioactive atoms are detected because they emit radiation, giving an image of an organ.

▪ After a radioisotope is ingested by the patient, the body will direct the isotope just as if it was the typical non- radioactive form. ▪ A scanner is then passed over the body above the region where the organ containing the radioisotope is located. ▪ The scanner detects the gamma radiation given off and creates an image based on how much radiation is detected across the organ. ▪ An area of lower or higher radiation can indicate conditions such as a disease of the organ, a tumor, a blood clot, or . ▪ To determine function, doctors use radioactive iodine uptake. ▪ Taken orally, radioactive I-131 mixes with the iodine already in the thyroid. ▪ 24 hours later, the amount of iodine taken up by the thyroid is determined with a scanner. ▪ A person with will have a higher than normal level of radioactive iodine. A person with hypothyroidism will have a lower than normal level. ▪ One treatment for hyperthyroidism is to give a “therapeutic dose” of I-131 (a higher dose than for the scan). The radioactive iodine will destroy some of the cells in the thyroid, lowering the amount of thyroid hormone produced. ▪ Positron emitters with short half-lives: ▪ Can be used to study brain function, , and blood flow. ▪ Might be carbon-11, -15, -13 or -18.

▪ Combine with electrons after emission to produce gamma rays, which are then detected by computers, creating a 3-D image of the organ. These PET scans of the brain show a normal brain on the left and a brain affected by Alzheimer’s disease on the right. ▪ An imaging method used to scan organs such as the brain, , and heart. ▪ A computer monitors the absorption of 30,000 X-ray beams directed at the organ in successive layers. ▪ Differences in absorption based on tissue densities and fluids provide an image of the organ.

▪ The patient does not ingest radioactive material. Rather, radioactive material was used to create the X-rays. ▪ Another name for this is Computerized Axial CT scan of a brain Tomography (CAT scan) shows a tumor (yellow). ▪ An MRI is a powerful imaging technique that is the least invasive imaging method available. ▪ MRI is based on the absorption of energy when protons in atoms are excited then relaxed by radio waves. ▪ When placed within a magnetic field, the protons previously random orientations, align with the field. ▪ While in the magnetic field, radio waves are pulsed at different frequencies (wavelengths). ▪ Protons in different chemical environments resonate at different frequencies (jump to a higher energy state) and then relax back to its original when the radio waves are turned off. ▪ Energy is released, corresponding to particular colors. ▪ MRIs are tuned to hydrogen nuclei because there is such a large abundance of hydrogen in the body (mostly in water). ▪ MRIs are particularly useful for soft body tissue which contains large amounts of water. ▪ Outside of the medical field, this technique is called Nuclear Magnetic Resonance (NMR) and is used extensively in labs. ▪ MRI of heart and lungs. Which of the following radioisotopes are most likely to be used in nuclear medicine?

A. 40K half-life 1.3 x 109 years B. 42K half-life 12 hours C. 131I half-life 8 days ▪ The process of , or seed implantation, is an internal form of radiation for cancer. ▪ The benefit of brachytherapy over traditional external cancer treatments is that the radiation is limited to a very specific area (the direct location of the tumor), where as traditional treatments often effect an entire organ. ▪ Brachytherapy is also quicker than traditional methods. Because the target area is only the cancer, larger doses can be administered at a time, lessening how many treatments are required. Permanent brachytherapy is a treatment option for cancer. ▪ Involves implanting 40+ titanium capsules called seeds in the cancerous area. ▪ Inside the titanium capsules are radioactive isotopes ▪ I-125, Pd-103, or Cs-131 ▪ The isotopes decay by gamma emission which kills the cancer cells’ ability to reproduce. ▪ The “seeds” are inserted once ▪ If the right dosage is used, the isotopes will emit and left inside the body. (Once all its radiation into the cancer then stop before all the radioactive decay has occurred, the seeds are damaging a large amount of body tissue. harmless.) Temporary brachytherapy is also a treatment for . ▪ Long needles are implanted into the tumor containing Ir- 192. ▪ Can be used to deliver a higher dose of radiation over a shorter time compared to permenant and may be repeated in a few days.

The needles are removed after 5-10 minutes. Brachytherapy is also used following cancer (tumor removal). ▪ An Ir-192 isotope is inserted into a implanted in the space left by the removal of the tumor. ▪ The isotope is removed after 5-10 minutes. ▪ The goal is to kill any remaining cancer cells. ▪ The treatment is repeated twice a day for 5 days. Then the catheter is removed and no radioactive material remains in the body. 5.1 – Natural Radioactivity 5.2 – Nuclear Reactions 5.3 – Radiation Measurement 5.4 – Half-Life of a Radioisotope 5.5 – Medical Applications using Radioactivity 5.6 – Nuclear Fission and Fusion Goal: Describe the processes of nuclear fission and fusion. During the 1930s, scientists bombarded uranium-235 with neutrons and discovered that the U- 235 nucleus splits into two smaller nuclei and produces a great amount of energy. Nuclear fission: splitting an into 2+ parts Atomic energy: the energy 1 235 236 91 142 1 generated by fission. 0n + 92U → 92U → 36Kr + 56Ba + 30n + energy ▪ Oddly enough, if you were able to weigh starting materials (neutron and U-235) and then the products (Kr-91, Ba-142, and 3 neutrons) you’d find they didn’t weigh the same. ▪ That is because some of the ‘mass’ was converted to an enormous amount of energy. E = mc2 ▪ Einstein’s famous equation. Energy released (E), mass lost (m), speed of (c) = 3 x 108 m/s. ▪ Even though the mass lost is very very small, when you multiply that by the speed of light squared, it becomes a huge amount of energy. Fission begins when a high-energy neutron collides with a heavy radioactive atom (U-235 for example). ▪ The neutron becomes part of the atom, however the new nucleus is unstable and splits into smaller nuclei. ▪ This also releases several neutrons that can then collide with other radioactive atoms and repeat the process, causing a chain reaction. ▪ An enormous amount of energy is released as well. To sustain a chain reaction, without adding any additional U-235 or neutrons, the original amount of U-235 must equal a critical mass. So much heat and energy build up that an atomic explosion can occur.

Hiroshima, Japan Nagasaki, Japan August 6, 1945 August 9, 1945 During WWII, after Germany’s defeat, Japan refused to surrender. It was clear that Japan would not win in the long run, which appeared to make Japan even more deadly. Japanese forces inflicted the same amount of Allied force casualties in 3 months, as the full war in the previous 1.5 years. Hiroshima, Japan Nagasaki, Japan August 6, 1945 August 9, 1945 President Truman’s war counsel brought him 2 options.

1. To continue current tactics of conventional bombing followed with a massive invasion. Truman was advised that such an invasion would result in U.S. casualties of up to million. 2. Use the atomic bomb in hope of bringing the war to a quick end. This was a heartbreaking and difficult decision to make. President Truman chose to use an atomic bomb regardless of the moral reservations many in his war council voiced as well as scientists who developed it. The first atomic bomb, named Little Boy used Uranium-235. It weighed 9000 pounds. It detonated 2000 feet above Hiroshima and leveled 5 square miles of the city. The blast was the equivalent of 12-15,000 tons of TNT. To America’s dismay, Japanese emperor, Hirohito, refused to surrender. He didn’t believe the U.S. could possibly have a second bomb.

President Truman made the decision that a second bomb was necessary. The second bomb, named Fat Man was bigger than the first. It contained (there was not enough uranium left to make a 2nd bomb with). The bomb was dropped on Nagasaki after thick clouds prevented the first target, Kokura, from being an option. Although the bomb was bigger than the first, Nagasaki was nestled in narrow valleys between mountains that reduced the blast radius to 2.6 square miles. 6 days later, on August 15, 1945, Emperor Hirohito announced Japan’s surrender in a radio broadcast. Ending WWII. In order to reach the critical mass that enabled the bombs to explode, the uranium/plutonium molecules had to be incredibly close together.

In order to achieve this, dynamite encircled the uranium/plutonium and was detonated.

This cannot be achieved in the setting of a . ▪ In a nuclear powerplant, a heavy element undergoes fission and the energy generated from the reaction is converted to electricity. ▪ Uranium-235 and plutonium-239 are common ▪ The amount of energy harnessed from 1 gram of U-235 is the equivalent of burning 6000 pounds of coal. Within the reactor, there is a small box (14” tall, 20” wide) where the fission takes place. Control rods can be inserted into the box or removed as needed. U-235 (or whatever) is added in TINY amounts to the chamber along with high- energy neutrons. The energy due to fission is in the form of heat.

1 235 236 91 142 1 0n + 92U → 92U → 36Kr + 56Ba + 30n + energy A system transfers the heat out of the reactor into a water tank where it heats the water. The steam coming off the water is fed through a turbine which generates electricity and it turns. Nuclear reactors require small amounts of fuel to be added hourly by reactor operators. A reactor never has enough fuel in it to reach critical mass and become explosive. In fact the fuel in reactors are different then those in bombs, and the technology to achieve “bomb grade material” is highly classified. Very few countries in the Reactors are also built with giant world have figured it out. concrete domes with a thickness that blocks radiation, should the worst ever happen. ▪ The “worst that could happen” is called a “reactor meltdown.”

▪ Giant cooling tanks surround the reactor and moderate it’s temperature. ▪ Pipes run from the cooling tanks, through the reactor box, and back into the tanks. ▪ If the reactor is at a power plant, the pipes will run through water tanks before returning to the cooling tanks. ▪ These pipes absorb heat from the reactor, cooling the reactor. ▪ 90,000 gallons of liquid sodium passes through the reactor box PER MINUTE. As the sodium enters the box, it is 700°F and as it leaves it is 820 ° F. A reactor meltdown occurs when the cooling systems fail.

This happened to 3 plants in March 2011 when a 9.0 magnitude earthquake struck off shore of Japan. ▪ The earthquake cut off their power supply. ▪ The 15-meter tsunami flooded the diesel back up generators located in the basements of the plants. Without cooling systems, all three cores (reactor boxes) melted within 3 days.

The danger was the reactor boxes being cracked from the earthquake and radioactive material leaking out and into ground water. ▪ fuel is not explosive. ▪ Nuclear power plant fuel is not explosive. ▪ Nuclear power plant fuel is not explosive. ☺ ▪ The threat from the fuel is leaking and entering the ground.

▪ There were 2-3 explosions at the Fukushima reactors over the week after the earthquake. They were due to operators desperately trying to cool the reactor down. Due to the insane heat (5072 °F ), pressure was building in all the pipe work and to avoid a worse disaster, they let the pipes vent. This released hydrogen gas which is flammable which proceeded to ignite. 100,000 people were evacuated from their homes as a precaution. There have been no deaths or cases of radiation sickness from the accident.

The Fukushima Accident was a wake up call to the nuclear industry world wide. (and a horrifying embarrassment to scientists.) The reactors had been built to withstand tsunamis of 5.7 meters or less based off a tsunami in Chile in 1960. (Japan was 15m) The reactors were also decades out of date. Following Fukushima, nuclear power plants all over the world were shut down to conduct maintenance.

Nuclear power plants are built to withstand every possible disaster. But maintenance is expensive and can slip between the cracks.

The U.S. government has very strict regulations about maintenance of both government and industry owned plants. Twice a year, every reactor is shut down and is thoroughly inspected and maintenance done. (This generally takes 4-8 weeks per reactor.)

Many reactors are also built in a way that even if the power fails, the cooling systems continue to function well enough to prevent melt downs. ▪ In fusion, two small nuclei combine to form a larger nucleus. ▪ Mass is lost, and a tremendous amount of energy is released (even more than from fission.) ▪ A fusion reaction requires 100 million °C to overcome the repulsion of the two nuclei and cause them to undergo fusion. ▪ Fusion reactions occur continuously on the Sun and other stars, providing us with heat and light. ▪ If we could achieve fusion reactors, it would be essentially clean energy. However the temperature to achieve is daunting. ▪ Research groups around the world are attempting to develop the technology needed within our lifetime. 5.1 – Natural Radioactivity 5.2 – Nuclear Reactions 5.3 – Radiation Measurement 5.4 – Half-Life of a Radioisotope 5.5 – Medical Applications using Radioactivity 5.6 – Nuclear Fission and Fusion