Energy What Is a Nuclear Reaction?

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Option C: Energy

• A nuclear reaction is any reaction that involves the nucleus. • These reactions change the identity of an atom, as opposed to chemical reactions which only involve valence electrons.

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The nucleus

• The nucleus is made up of protons and neutrons. • We know what the protons do – they provide an electrostatic attraction to the electrons close… but what about the neutrons?

The Neutrons

• The major function of the neutrons is to hold the nucleus together. • The neutrons provide a strong nuclear force of attraction within the nucleus, counteracting the repulsion between the positively charged protons.

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How is the nucleus held together?

• In the 1930’s it was first observed that the mass of an atoms nucleus is less than the sum of the masses of the protons + neutrons…? • Some of the mass of the nucleus is converted into energy to hold the nucleus together.

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Mass Defect

• The difference in mass of the nucleus and it’s parts is referred to as the mass defect, and the energy (e=mc 2) it provided is called the nuclear binding energy.

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Nuclear vs. Chemical Reactions

• This nuclear binding energy is released during nuclear reactions (fission & fusion), and is ~1,000,000X greater than the chemical bond energy released during chemical reactions.

What makes an isotope radioactive?

• Elements are radioactive when their nucleus is unstable. • The stabilizing force of the neutrons is effective for smaller elements, though all elements above lead are radioactive.

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Band of Stability

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All Radiation is not the same

• Radiation, the particles and/or energy given off by nuclei undergoing radioactive decay, comes in a number of forms. • The forms we need to know are: – Alpha emission – Beta emission – Gamma emission

Alpha Emission (too many protons)

Emits an alpha particle • Made up of 2 protons and 2 neutrons • Effect on parent: – Mass lost, resulting in a new element

Danger Level Extremely ionizing but least penetrating

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Beta Emission (too many neutrons)

• A neutron is converted into a proton and an electron. The electron is emitted as a β particle • Result: – parent changes slightly in mass (new element produced) Danger Level Less ionizing but more penetrating than alpha

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Gamma Rays

• A form of EMR given off when nucleons rearrange themselves inside a nucleus • Don’t result in a change in the identity of the atom • Gamma rays typically come from the nucleus, Danger Level while x-rays come from Least ionizing but the electrons. much penetrating than alpha

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How long will radioactive isotopes decay?

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It depends

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Types of Nuclear Reactions

Fission & Fusion

Nuclear Fission

• Involves the splitting of a heavy nucleus into smaller elements, releasing the nuclear binding energy.

• This can be done for isotopes larger than Fe-56, though is primarily done with U-235 & Pu-239.

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Artificially Induced Fission

• First discovered by German scientists in 1938 when they discovered barium after bombarding uranium with neutrons.

A chain reaction

• Assuming a critical mass of fissionable material is present, a chain reaction can occur as each fission releases additional neutrons.

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Controlling the Reaction

• The fission reaction has to occur at an acceptable speed in power plants, or else the energy would be lost. • Moderators are used to slow down the neutrons, while control rods (often graphite) can absorb the neutrons

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Fuel Rods

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Control Rods

Uranium Enrichment

• U-235 is the most fissionable isotope, but… • Naturally occurring uranium is <1% U-235. • Uranium enrichment increases the percentage U-235, increasing the likelihood of fission.

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Breeder Reactors

• As the supply of U-235 is limited, “breeder reactors” are used to convert the more common U-238 into fissionable Pu-239.

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The problem with breeders…

• Plutonium is highly toxic and these reactors are more susceptible to accidents. • Plutonium can be concentrated from reactor grade to weapons grade very easily.

Issues with nuclear energy

• Nuclear waste is radioactive for a very long time. – High Level Waste (ex. Control rod) • high activity; long half-life – Low-level Waste (ex. Fuel containers, clothing) • low activity; short half-life • And of course there is always the potential for a major accident.

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Three Mile Island (1979)

Chernobyl (1986)

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Fukushima (2011)

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Fukushima (2011)

Disposal of high-level waste

• Requires a geologically-stable area with impervious rock and away from water supplies.

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Uses of Nuclear Power

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Nuclear Fusion

• Occurs when light nuclei are brought together to form heavier elements. • This typically involves the fusion of two hydrogens to form helium.

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The up-side of fusion

• It releases 3-4X the energy of a fission reaction. • It produces almost no radioactive waste. • The supply of deuterium (H-2) is cheap and almost unlimited (from the ocean)

The down-side of fusion

• It requires way more energy than fission to begin (reactants must be in a plasma state). • It is much harder to control.

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Cold Fusion

• Many attempts have been made to design a controllable fusion reactor (pg. 533) and will certainly continue, as success would mean electricity would cost us pennies per day.

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Fusion reactions power stars!

• Fusion reactions generate the energy released from stars, which explains why absorption data shows us that the Sun is made up of primarily hydrogen and helium.

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The Electromagnetic Spectrum

• Every object above 0 K emits EMR of different wavelengths. • The Sun is hot enough to emit all wavelengths of visible light (white) • Atmospheric gases absorb some wavelengths of this light, which allows us to identify them.

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The End

No HW except reading C.4 (pages 542-550)

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BONUS COVERAGE!!!

Natural Sources

Terrestrial Cosmic

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Terrestrial

• Consists of radioactive elements found here on Earth • By far the major contributor to terrestrial radiation is Radon-222, which is a naturally occurring decay product of Uranium-238, which is found in the soil

Radon Gas

• A colorless, odorless gas • The EPA estimates that whose concentration it causes 21,000 deaths can build up in poorly from lung cancer each ventilated homes. Not year. uncommon in our area.

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Stanley Watras

• The build-up of radon gas in homes was made famous when an engineer repeatedly set off radiation detectors at a new nuclear power plant – that didn’t yet have radioactive material. Testing in his home revealed that his families cancer risk would be the same as if they smoked 135 packs of cigarettes daily.

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Radon Abatement Systems

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Natural Sources

Cosmic

Cosmic Radiation

• Consists of a variety of high energy photons primarily from the sun and supernovas

• Though mostly blocked by the earth’s atmosphere and magnetic field, it still increases our cancer risk

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Poses a serious stumbling block to any thoughts of space travel