Download - Energy Changes in Nuclear Reactions
Energy Changes in Nuclear Reactions
BY…
E=mc²
• Einstein’s equation that relates mass and energy• E=Energy• m=mass• c=speed of light, 3.00 x 108 m/s• States that mass and energy are proportional• If a system loses mass, it loses energy, and vice
versa• Mass and energy changes are much greater in
nuclear reactions than in chemical reactions
Example of E=mc²• 238 U 234 Th + 4 He• Mass U=238.0003 amu, mass Th=233.9942 amu,
mass He=4.0015 amu• Δm=233.9942g+4.0015g-238.0003=-0.046– Lost mass=exothermic
• Energy change calculated through Einstein’s equation, E=mc²:
• ΔE=Δ(mc2)=c2Δm =(2.9979 x 108 m/s)2(-0.0046 g)(1 kg/100o g) =-4.1 x 1011 kg-m2/s2 = -4.1 x 1011 J
(Note: Δm is converted to kg, SI unit of mass, to get ΔE in joules, SI unit for energy.)
Nuclear Binding Energies– Energy required to separate a nucleus into its individual
nucleons• Masses of nuclei are always less than the masses of
individual nucleons• Mass defect- difference between a nucleus and its
constituent nucleons• Addition of energy to a system must be joined by a
proportional increase in mass• Larger the binding energy is, the more stable the
nucleus is towards decomposition
Nuclear Binding Energies Continued
• Binding energies per nucleon initially increases in magnitude as mass number increases
• Nuclei of intermediate mass numbers are more tightly bonded (more stable) than other nuclei that is smaller or has larger mass numbers
• Trend has two consequences – Heavy nuclei gain stability and give off energy if they divide into
two mid-sized nuclei • Fission
– Greater amounts of energy is release when very light nuclei are combined or fused together to give larger nuclei• Fusion
Biological Effects of Nuclear Radiation
By: Kayla Seider and Hannah Cherry
Radioactivity • We are continually being bombarded by
artificial and natural radiation• Infrared, UV, visible radiation from the sun, radio
waves, microwaves, and x-rays• There is radioactivity in the soil and other
materials
Types of Radiation• If matter absorbs radiation, it can cause either excitation or ionization of the
matter• Excitation occurs when absorbed radiation excites electrons to a higher energy
state or increases the motion of molecules• Causes them to move, vibrate, or rotate
• Ionization occurs when the radiation removes an electron from an atom or molecule• Is more harmful than radiation that doesn’t cause ionization
• Non-ionization is lower in energy or slower moving neurons• Radiofrequency electromagnetic radiation
• Most of the energy is absorbed by water molecules in tissue• Most tissue is 70% water by mass
• Can define ionizing radiation as radiation that can ionize water• X-rays, higher-energy UV, alpha, beta, and gamma rays
What Happens• When ionization radiation passes through living tissue, electrons
are removed from water molecules, forming highly reactive H2O+
• An H2O+ can react with another water molecule to form H3O+ and a neutral OH
• OH becomes a free radical• A free radical is a substance with one or more unpaired electrons• In cells and tissues, these particles can attack a host of
surrounding biomolecules to produce new free radicals• These new free radicals can initiate a large number of chemical
reactions that are able to disrupt the normal operation of cells• Can contribute to cancer, diabetes, stroke, heart attack, Parkinson's,
Alzheimer's, schizophrenia, and hemochromatosis
The Damage• Damage depends on the activity and energy of the radiation, the length
of exposure, and whether the source is inside or outside the body• Gamma rays are harmful outside of the body
• They can penetrate human tissue very easily• Can cause organ damage and genetic damage• Dangerous
• Alpha rays are stopped by skin• In the body, alpha rays are particularly dangerous because they transfer their
energy efficiently to the surrounding tissue causing considerable damage• Beta rays can penetrate about a cm beyond the skin• Tissue that shows the greatest damage are those that reproduce at a
rapid rate• Bone marrow, blood-forming tissues, and lymph nodes
Radiation Doses• Radiation is measured in the gray (Gy) and the rad (radiation absorbed dose)• The gray is equivalent to the absorption of 1 J of energy per kilogram of tissue• The rad is equivalent to the absorption of .01 J of energy per kilogram of tissue• 1 gray= 100 rads
• A rad of alpha radiation causes more damage than a rad of beta radiation• To correct these differences, the radiation dose is multiplied by a factor that
measures the relative biological effectiveness (RBE) of radiation • The exact RBE value varies with dose rate, total dose, and the type of
tissue affected• RBE is approximately 1 for gamma and beta and 10 for alpha
• The product of radiation dose and the RBE gives you the effective dosage in units of rem (roentgen equivalent for man)• Number of rems= (number of rads)(RBE)
• The siervert (Sv) is the unit for effective dosage • 1 Sv= 100 Rem
Radiation DosesDose (rem) Effect of Short-Term exposures to Radiation
0 to 25 No detectable clinical effects
25 to 50 Slight, temporary decrease in white blood cell counts
100 to 200 Nausea; marked decrease in white blood cells
500 Death of half the exposed population within 30 days after exposure
• 600 rem will cause death• Dental x-rays is .5 mrem• The average exposure for a person in one year due to natural sources of ionizing radiation is about 360
mrem
Radiation Therapy• Both healthy and unhealthy cells can be destroyed by radiation
• Can lead to physiological disorders• Cancer is the growth of abnormal cells, that growth produces
malignant tumors• The tumors can be destroyed by exposing them to the same
radiation because rapidly reproducing cells are susceptible to radiation damage • Therefore, cancerous cells are easier to destroy than healthy ones• That’s why radiation is used in cancer treatment
• Side effects• Fatigue, nausea, hair loss, weakened immune system, even death
• Because of these side effects, radiation therapy is a last resort for treatments
True or False• Radiation in Japan is equal to 38,000 bananas• True• About 1,200 radioactive isotopes have been produced in all the known elements• True• You get little amounts of radiation while on a nuclear submarine• True• Burning coal releases more radiation than a nuclear plant does• True
Questions• What is ionizing radiation? • Radiation that can ionize water and it can remove an electron from a molecule
• What is a free radical? Why is it so bad?• A substance with one or more unpaired electrons. They disrupt the normal operations of
cells• Which is smaller the rad or the gray and how are they related to eachother?• The rad is smaller than the gray. 1 gray= 100 rads
• What dose of rems cause death?• 500-600 rems
21.8: Nuclear Fusion
Kyle, Suraj, Brian
Nuclear Fusion
• Energy is produced when light nuclei fuse into heavier ones– Talked about in 21.6 (don’t write this part
down)– Type of reactions responsible for energy
produced by sun
Equal in hottness (write this equation down)
Nuclear fusionIntense workouts
Nuclear Fusion
• Several different types of fusion processes:
Fusion Energy
• Appealing as an energy source– Nonradioactive products– Light isotopes of
hydrogen are easily available
• Currently not used– Extremely high energies
are needed to overcome repulsion of nuclei
Overcoming Nuclei Repulsion• In order to achieve the
required energies, high temperatures must be maintained– Thus, fusion reactions are
known as thermonuclear reactions
• Lowest temperature required for fusion is 40 million Kelvin
• This temperature has only been achieved by hydrogen bombs– Uncontrolled power generation
-Requires 40,000,000 K to initiate
Fusion as energy • Numerous problems must
be overcome before fusion becomes a practical source for energy– High Temperatures to start
reaction– Confining the reaction
• No known material is able to withstand the temperature needed for fusion
• Researches try to use tokomaks to achieve fusion
• Also use lasers
Tokamak
• Uses strong magnetic fields to contain and to hear a fusion reaction
• Have reached 3million degrees Kelvin
• http://www.youtube.com/watch?src_vid=E2-Y8bYtvX4&annotation_id=annotation_49072&feature=iv&v=IU7oMISRS2Y
Nuclear Fission
Jake Wiley, James Haeckel, Sergio Machaca
Fission
• Extremely exothermic• Uranium-233, -235, and Plutonium-239 are
main practical sources• 1 neutron hits a heavy nuclei and causes it to
split• Average of 2.4 neutrons are released• Various and unpredictable products, typically
radioactive
Chain Reactions• Each neutron released can cause another nucleus to split• Critical Mass
– Enough mass of the material is present to sustain the reaction at a constant rate• Uranium ~ 1kg
• Subcritical Mass– Less than critical mass, neutrons escape without hitting any
nuclei• Supercritical Mass
– More than critical mass, reaction proceeds unchecked, typically with violent results
Which one is Subcritical? Supercritical?
Nuclear Arms
• Gun-type– Two subcritical masses
are shot together into a supercritical mass
• Implosion– Subcritical mass of P-239
is compressed by explosives to supercritical mass
Nuclear Reactors
• Fuel rods typically use 3% U-235– Encased in stainless steel or zirconium tubes
• Control rods regulate amount of neutrons– Typically boron or cadmium
• A moderator is used to slow the neutrons as to be more readily absorbed by fuel
• A cooling liquid is used to carry off excess heat– Often the moderator and cooling liquid are one in
the same
Nuclear Reactors (Cont.)
• Excess heat is used to turn water to steam– Used to turn a turbine
• Steam is cooled and condensed– Often cooled with water from
a stream or lake• All incased in reinforced
concrete– Prevents radiation leak– Protects reactor from external
forces
Nuclear Waste
• Estimated at 20 half-lives before safe exposure– Puts used fuel at about 600 years
• Dangerous to handle and transport• Originally stored in pools at reactor and transported to
reprocessing plants– Transportation incredibly unpopular and reprocessing too
hazardous• Spend fuel rods are presently stored on site• Yucca Mountain, Nevada is a possible long term
storage facility
Thorium• Silvery metal• Topic of energy source discussions• Thorium reactors are considered
safer– No chain reaction– Must be bombarded with neutrons to
drive the fission process– Reactor halts process by itself in case of
overheat• No room for mechanical or electrical failure
• Thorium is as abundant as lead
Continued
• Thorium poses fewer environmental hazards– Cleaner than uranium or other radioactive materials
used in other reactors– Can burn up plutonium and toxic waste from old
reactors• Saves money– Does not require isotope separation– High neutron yield, better fission rating, longer fuel
cycles– ~100% of recovered thorium is fit for reactors
Nuclear Reactor Meltdowns
• Three Mile Island, Pennsylvania– Partial Meltdown– March 28, 1979– Fuel rods liquefied
• Chernobyl, Russia– Complete Meltdown– April 26, 1986– Experiment on core failed causing two explosions– Town remains uninhabited
Questions
1. Is Fission an exothermic or endothermic process?
2. What is critical mass and the two types?3. What do the control rods and the moderator
do in a Nuclear reactor?4. Explain this process:
Answers
1. Exothermic: the process releases energy2. The amount of fissionable large enough to
maintain a chain reaction, Sub- less than critical mass and Super- more than
3. Control rods regulate neutrons to keep up chain reaction, while preventing overheating; Moderator slows neutrons to be used more readily by fuel
4. 1 Neutron splits an Uranium nucleus into a Krypton and Barium nucleus and 3 Neutrons
Patterns of Nuclear Stability
Melissa Ross, Lexy Smyles, Kevin Miner
Neutron-to-Proton Ratio
•Strong nuclear force- strong force of attraction that exists between nucleons at close distances• Nucleons= protons and neutrons• Overcomes the repulsive forces of
protons•Nuclei with two or more protons contain neutrons• More protons = more neutrons• Required to bind nucleus together
Neutron-to-Proton Ratio• Atomic number 20 and lower
– 1:1 ratio of protons and neutrons• Higher atomic number
– More neutrons than protons*Neutron to proton ratio of stable nuclei increases with increasing atomic number*
http://www.youtube.com/watch?v=H8Yd2T9MQBU
+
Neutron-to-Proton Ratio• In heavier nuclei, the number of protons increases the proton-proton
repulsions which outweighs sum of: – proton-proton attractions– proton-neutron attractions– neutron-neutron attractions
• THEREFORE… number of neutrons must increase at more rapid rate than number of protons
++0
00
0
0
0
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0
DEMO
Belt of Stability – Area where all stable nuclei lie•Ends at element 83•*All elements with 84 or more protons are radioactive*
Radioactive Decay• Nuclei above belt of stability– High neutron to proton ratio– Move toward belt by emitting beta particle• Decreases number of neutrons and increases
number of protons
• Nuclei below belt of stability– Low neutron to proton ratio– Move toward belt by positron emission or
electron capture • Decrease protons and increase
neutrons • Positron emission more common in
lighter elements• Electron capture more common in
heavier elements
• Nuclei outside belt of stability– Atomic number 84 or higher– Undergo alpha emission
• Decreases both protons and neutrons by two
• Moves diagonally towards belt of stability
http://www.youtube.com/watch?v=VJZuY3_aLnI
Radioactive Series• Some nuclei can’t gain stability through a single emission, so a series of
successive emissions occur • Radioactive series (nuclear disintegration series) – begins with an
unstable nucleus and ends with a stable one• Three exist:– Uranium 238 – lead 206– Uranium 235 – lead 207– Thorium 232 – lead 208
Radioactive series example
Net Reaction:
Further Observations• Magic Numbers– Nucleus with 2, 8, 20, 28, 50, or 82 protons or neutrons (neutrons
include 126) are more stable than those with other numbers• Even number of both protons and neutrons more stable than odd
numbers• Shell Model of the Nucleus – nucleons reside in shells similar to
electron shells
Review
• For the first 20 elements, what is the (approximate) proton to neutron ratio?– 2:1– 1:2– 1:1– 1:3
• Strong nuclear force:– Overcomes the repulsion of protons– Attracts electrons– Charge of neutrons– Repulsive force of protons
• Neutron to proton ratio of stable nuclei:– Is 1:1 for all elements– Is 1:1 for all elements above atomic number
20– Increases with increasing atomic number
above element 20– Decreases with increasing atomic number
above element 20
• All elements with 84 or more protons are:– Stable– Radioactive– Highly electronegative– Good electrical conductors
• Even number of neutrons and protons are:– More stable than odd ones– Less stable than odd ones
Nuclear Reactors
Danielle Gerstman, Bridget Murray, Rachel Santangelo
What’s A Nuclear Reactor? • A device that sustains a controlled nuclear chain
reaction• Take place when a nucleus of an atom gets smacked
by either a subatomic particle or another nucleus– Produces atomic and subatomic products– Fission reaction-the nucleus splits apart
What’s Fission? • The light particle/free neutron collides with the
heavy particle which splits into two or three pieces• Produces energy in the form of both kinetic energy
and electromagnetic radiation• Newly produced free neutrons zoom around and
smack into more uranium or plutonium isotopes • Produces more energy and more free neutrons– Leads to nuclear fission chain reaction
Main Components • Core: contains nuclear fuel and
generates all heat, low-enriched uranium (<5% U-235), control systems, and structural materials
• Coolant: passes through core and transfers the heat from fuel to turbine
• Turbine: transfers heat from coolant to electricity
• Containment: the structure that separates the reactor from the environment. – Usually dome-shaped, made of high-
density, steel-reinforced concrete• Cooling towers: place where some
plants can dump excess heat that cannot be converted to energy
Main Components • Control rods: limit rate of
fission inside fuel rods by absorbing some neutrons
• Fuel pins: smallest unit of reactor (usually uranium-oxide) usually surrounded by metal tube called cladding
• Fuel assemblies: bundles of fuel pins that keep pins close, but far enough away so that coolant can fit between them
• Fuel core: several hundred assemblies
The Pressurized Water Reactor The water is both the coolant and the moderator. Keeps water under pressure so the water heats but does not boil. Heated pressurized water runs through pipes, which heats a separate water line to create steam. The water used to generate steam is never mixed with the pressurized water used to heat it.
High Temperature, Gas Cooled Reactors• Operate at higher
temperatures• Gas is used as primary
coolant• Mostly moderated by
carbon• Can have higher
efficiencies than PWR’s, but is limited by gas coolant (not as effective)
Sodium Cooled Fast Reactor• Cooled by liquid sodium metal
(heavier than hydrogen so neutrons move faster)
• Use metal or oxide fuel• Pros:– Breeds own fuel– Burns own waste– Safer- will shut itself down and
cool decay without working backup system
• Unfortunately, coolant (sodium) reacts with water and air, so leads to sodium fires
Boiling Water ReactorHeats water by generating heat from fission in the reactor vessel to boil water and creates steam, which turns the generator. In both types of plants, the steam is turned back into water and can be used again in the process.
Boiling Water Reactors• Thousands of fuel rods- twelve feet long, straw-like tubes– Fueled is sealed inside them (ceramic pellets of uranium oxide)– Bundled in core of reactor– Heat up during fission chain reaction, so they are submerged in
the coolant (the water- which is kept pressurized so the boiling point is around 550 degrees Fahrenheit)
– Creates high pressure steam that turns turbines to produce electricity
What Is A Meltdown? • If the core gets too hot, the fuel rods can crack
and release radioactive gases• Fuel pellets melt and fall to the reactor floor,
where the hot, radioactive material is able to eat through protective barriers and reach the surrounding environment
• Example meltdown: Nuclear reactors in Japan
What Went Wrong In Japan?
• Nuclear reactors designed to turn off automatically anytime a disaster knocks out the electric grid– The system worked properly
• Even with the plant shutdown, the nuclear fuel still held tremendous heat
• Diesel-powered backup generators are meant to pump water into the plant to cool the fuel,– Systems failed in the tsunami that followed the
earthquake – Emergency batteries provided some power, but not
enough to run the water pumps
Control Rods• Long, thin rods are bundled together into fuel assemblies
and placed in reactor core• Absorb excess neutrons released in fission process
– Reaction will continue unchecked, since one fission will release multiple neutrons but requires only one to begin
– Control rods prevent this, controlling the chain reactions• Rods are slowly lifted until chain reaction can just be
sustained– As the reactions continue, neutron-absorbing material
builds up– Rods will be withdrawn slowly until reactions can’t be
maintained and the fuel must be replenished• If rods do not function properly, reaction will proceed
uncontrolled
Questions• What are cooling towers in reactors for?
– Storage of material holding excess heat that cannot be converted into energy
• How do pressurized water reactors generate energy?– Water is heated under high pressure in pipes, boiling a separate line to
produce steam that runs turbines• Although sodium cooled reactors can be beneficial, why are
they more dangerous than other kinds of reactors?– Sodium, the coolant, reacts with water and air, so it can cause fires
• What causes a nuclear meltdown?– If the core overheats, fuel rods crack and release radioactive material
that passes through protective barriers and into the outside environment
Kwak & Rundle
Radioactivity
General info
• Inside the nucleus– Protons– Neutrons
• All atoms of the same element have equal protons– Which gives them their atomic number– Can have different amounts of neutrons• Isotopes
– Found in many natural abundances
• Stability of an atom depends on the amount of protons and neutrons in the nucleus
Known as Nucleons – location in nucleus
Atomic Number/Number of Protons
Mass Number(Neutrons + Protons)
ChemicalSymbol
• Radioisotopes – atoms containing radioactive nuclei
• Radioactive nuclei are called radionuclides
Radioactive isotopes
Nuclear equations
• Most of the nuclei in nature are stable\• Radionuclides are unstable and
spontaneously give off particles and electromagnetic radiation– Emitting that radiation is one way to make an
atom/radioactive atom become more stable– Radiation comes off as Alpha particles –
identical to Helium-4 nuclei• Consist of two protons and four neutrons• A stream of particles is called Alpha radiation
Equations (cont.)
• Spontaneously decomposes – radioactive decay– Sometimes called alpha decay
• Sum of the mass numbers is the same on both sides– Same deal for atomic numbers
• Radioactive properties of a nucleus are independent from the chemical form of the atom– English – It doesn’t matter what phase it is in
when writing the nuclear equation– Doesn’t matter if it is pure element, or element
in compound
Radioactive Decay• 3 Common Types– Alpha
• Stream of He-4 nuclei– Beta
• Streams of beta particles – high-speed electrons emitted by an unstable nucleus• Causes atomic number to increase
– Converts neutron to a proton
– Gamma• High-energy photons – short length electromagnetic radiation• Does not change atomic number or atomic mass• Almost always accompanies other radioactive emission
– Represents the energy lost when the remaining nucleons reorganize into more stable arrangements
• Capture by the nucleus of an electron from the electron cloud surrounding the nucleus
• Electron is consumed rather than formed; shown on reactant side– Converts proton to
neutron
• Positron – particle that has the same mass as an electron, but positive charge
• Causes atomic number to decrease– Converts proton to
neutron
Other types
Positron Emission Electron Capture
Alpha
Gamma
Beta
Positron Emission
Electron Capture