tuesday april 8, 2014 (c) mcgraw hill ryerson 2007

•Tuesday April 8, 2014

(c) McGraw Hill Ryerson 2007

Science Journal (10 minutes)

• Initial Response

• What is nuclear fission?

• In what countries in the world would you find a nuclear reactor?

• Can you list one pro and one con of nuclear reactors?

• Why is it easier to crash a neutron into a nucleus than a positive proton into a nucleus to release energy?



7.3 Nuclear Reactions

• Nuclear fission and fusion are processes that involve extremely large amounts of energy. Fission = the splitting of nuclei

Fission is the source of energy for all nuclear power generation used today. The daughter products are often radioactive and are a significant waste

disposal problem. Fusion = the joining of nuclei

Fusion is a nuclear reaction in which small nuclei combine to produce a larger nucleus.

Other subatomic particles as well as energy are released in this process. Fusion is the source of energy in the sun.

See page 312

Nuclear Reactions

• Nuclear power plants can generate large amounts of electricity. Ontario, Quebec and New Brunswick currently generate nuclear power. Canadian-made nuclear reactors are called CANDU reactors.

CANDU reactors are considered safe and effective and are sold

throughout the world We will discuss this in more detail later


The Bruce Nuclear Generating Station on the shore of Lake

Huron, in Ontario


Nuclear Fission

• Nuclear energy used to produce power comes from fission. Nuclear fission is the splitting of one heavy nucleus into two or more smaller

nuclei, some sub-atomic particles, and energy. A heavy nucleus is usually unstable,

due to many positive protons pushing apart. To increase their stability, atoms with heavy nuclei may split into atoms with lighter nuclei.

When fission occurs:1. Energy is produced.

• This energy is used to generate power to support our lifestyle.2. Neutrons are released.3. Video clip

See pages 313 - 314

Albert Einstein’s famous equation E = mc2 illustrates the energy found in even small amounts of matter

Pro and cons associated with Nuclear Reactors


• Nuclear reactors reduce the need for burning fossil fuels (coal and natural gas).

• Nuclear reactors produce wastes that need to be stored safely for hundreds of thousands of years.

• The physical deterioration of nuclear power plants is a problem in Ontario.

• Concern that the nuclear material could be used to make nuclear weapons.

Nuclear Reactions

• Nuclear reactions are different than chemical reactions. In chemical reactions, mass is conserved, and energy changes are

relatively small. There are no changes to the nuclei in chemical reactions.

In nuclear reactions, the actual nucleus of atoms changes. Protons, neutrons, electrons, and/or gamma rays can be lost or gained. Small changes of mass = huge changes in energy


Comparing chemical reactions with nuclear reactions

Chemical Reactions• Atoms are rearranged by breaking

chemical bonds and forming new

bonds.• Only electrons are involved in

bond formation and breaking.• Chemical reactions are

accompanied by the release or absorption of relatively small amounts of energy. Mass is conserved.

Nuclear Reactions• Atoms are changed from one

isotope into another, producing different elements.

• Electrons, protons, neutrons, and other subatomic particles may be involved. Electrons may be produced in the

nucleus.

• Nuclear reactions are accompanied by absorption or release of huge amounts of energy.


Nuclear Equations for Induced Nuclear Reactions

• Are there other kinds of nuclear reactions besides the natural radioactive decay (alpha, beta, and gamma) reactions that we have seen so far?

YES!



Nuclear Equations for Induced Nuclear Reactions

• Natural radioactive decay consists of the release of alpha, beta and gamma radiation. Scientists can also create nuclear reactions immediately by smashing nuclei

with alpha, beta and gamma radiation. This process is called an induced, or forced, nuclear reaction.

Figure 7.19 When a nitrogen-14 nucleus is bombarded by an alpha particle, a flourine-18 nucleus is produced, which decays into oxygen-17 and a proton.

See pages 314 - 315

Nuclear Equation


24 + 7

14 N 817 O + 1

1p

or

24 He + 7

14 N 817 O + 1

1H

The above figure can be written in several possible ways. For example:

Subatomic Particle Symbols


•The proton notation shows that a proton has an atomic number of 1(1 proton) and a mass number of 1(1proton + 0 neutrons).

Subatomic Particle Symbols

A neutron is symbolized meaning that it has an atomic number

(charge) of 0 (0 protons) and a mass number of 1 (0 protons + 1 neutron).


Rules for writing Nuclear Equations

The rules for writing these equations (induced nuclear reactions) are the same as earlier nuclear equations ( radioactive decay).

Mass numbers must equal on both sides of the equation

Charges (represented by atomic numbers) must equal on both sides of the equation

Mass#: 1 + 235 = 236 92 + 141 + (3 x 1) = 236

Charges/atomic#: 0 + 92 = 92 36 + 56 + (3 x 0) = 92



Nuclear Fission of Uranium-235

• It is much easier to crash a neutron into a nucleus than a positive proton into a nucleus to release energy.

Why? Because it takes a tremendous amount of energy for an alpha

particle (with a charge of 2+) to collide with a nitrogen-14 nucleus (with a charge of 7+). The repulsion between the positive charges is very great.

Whereas, a neutron (0 negative charge) colliding with a nucleus takes much lower energy and has 0 positive charge to repel it.

See pages 316 - 317

Neutron colliding with a nucleus


• Most nuclear fission reactors and weapons use this principle.

• A neutron, , crashes into an atom of stable uranium-235, the nucleus absorbs the neutron.

• The mass number of the nucleus increases by one (ie. 235 to 236) creating unstable uranium-236 (number of protons has not changed, this is still an atom of uranium-it is just a different isotope).

• which then undergoes radioactive decay.

01n

• The newly formed and very high-energy uranium-236 is highly unstable and immediately splits apart into

• atoms of krypton and barium• three neutrons• huge quantities of energy.


The induced nuclear fission of uranium-235. This nuclear

reaction is the origin of nuclear power and nuclear bombs.

Lise Meitner- (1878-1968)

• 1939, Austrian physicist Lise Meitner was the first to explain how nuclear fission occurs.

• Meitner was Jewish, and this was not a good thing to be in 1930s Germany! When World War Two began in 1938, Meitner fled just in time to escape Hitler's death camps. She travelled to Sweden and continued her research although she had little equipment or resources.

• Meitner realized that the total mass of the particles produced when the uranium nucleus split was less than that of the original nucleus. She soon realised it had the potential to unleash unimaginable amounts of energy.


Lise Meitner


• Albert Einstein had also been following Meitner's work and in 1939 he wrote a letter to the American president, Franklin D. Roosevelt, explaining what it meant.

• Einstein feared that Germany would make an atomic bomb before the allies. •This led directly to the setting up of the 'Manhattan' project that developed the first atomic bomb only six years later.

•Meitner always regretted her part in the creation of the atomic bomb, also called the 'destroyer of worlds'.

• She refused to work on the project. •When she heard about Hiroshima she was devastated.•She was passed over for the nobel prize-her colleague Otto Hahn received it.•1992 - the element 109, meitnerium was named in her honour

Summary


Practice problem

• Which nuclear symbol correctly completes the following nuclear reaction?


Questions

• What is the indicated daughter nucleus?



• Considered Response

• What is nuclear fission?

• In what countries in the world would you find a nuclear reactor?

• Can you list one pro and one con of nuclear reactors?

• Why is it easier to crash a neutron into a nucleus than a positive proton into a nucleus to release energy?


•Wednesday April 9, 2014



• Initial Response• What does CANDU stand for?

• What is nuclear fusion?

• How do you control a nuclear fission reaction so that it will not go out of control?

• Where does fusion occur and under what conditions?



Chain Reactions

• Once the nuclear fission reaction has started, it can keep going. The neutrons released in the induced reaction can then trigger more

reactions on other uranium-235 atoms. If one fission reaction produces 3 neutrons, these

3 neutrons can cause 3 additional fissions. If those 3 fissions release 9 neutrons, those 9 neutrons

could then produce nine more

fissions and so on.

See page 318

Nuclear Chain Reaction.

The number of fissions and the amount of energy released can increase rapidly and lead to a violent nuclear explosion.

How do you control the Chain reaction so that it will not get out of control?


• Nuclear reactors have complex systems to ensure the chain reaction stays at safe levels.

•Keeping the chain reaction going, while preventing it from getting out of control, requires precise monitoring and continual adjusting.

Fukushima

• Much of the concern about nuclear power plants focuses on the risk of losing control of the nuclear reactor, which could result in -----1. The accidental release of harmful levels of radiation.

2. A violent nuclear explosion For example, the Japanese Fukushima Diichi nuclear disaster.

3min


• video

Other examples

• Three Mile Island in the USA Video 4:30

• Chernobyl (Ukraine) Video 4:30



CANDU Reactors andHazardous Wastes

• Canada’s nuclear research into the safe use of nuclear reactions has resulted in the creation of CANDU reactors. CANDU –” Canadian deuterium uranium” reactor

Deuterium (symbol D or 2H, also known as heavy hydrogen/heavy water) is one of two stable isotopes of hydrogen.

It has a natural abundance in Earth's oceans of about one atom in 6,420 of hydrogen Deuterium is an isotope of hydrogen-1 that is twice as heavy as it has both a proton and a neutron

in its nucleus. They use heavy water as both moderator and coolant

See pages 319 - 320

CANDU Reactors andHazardous Wastes

• CANDU reactors are found in various countries around the world.• Canada, South Korea, China, India, Argentina, Romania and Pakistan

• The design of the CANDU reactor is among the safest in the world The reactors are easy to shut down in an emergency.


Inside a CANDU reactor.

The reactor core produces heat as a result of reactions like the following:

• Nuclear power plants and fossil –fuel burning power plants are similar in that both produce a lot of heat.

• The heat is used to boil water and generate steam, which then drives the turbines that produce electricity.

• A turbine is a large rotating device that can be forced to turn when steam is applied to it. (A turbine drives a generator that produces electricity).


Heat energy produced turns electricity-generating turbines.

You tube clip

Hazardous Wastes

• The fuel used to produce heat in a CANDU reactor is in the form of bundles of rods containing uranium pellets.

• Hazardous wastes produced by nuclear reactions are problematic. Some waste products, like fuel rods, can be re-used.

Some countries reprocess used nuclear fuel to recover material to use in new reactor fuel.


•Each CANDU fuel bundle is about 50cm in length and 10cm in diameter and generates about 1 million kW/h of electricity during its time in the reactor – about 15 months.

Hazardous Wastes

Some products are very radioactive, however, and must be stored away from living things. Most of this waste is buried underground or stored in concrete.

• For example, at the Pickering Ontario, nuclear power plant they have containers that hold nuclear waste.

• About 40 new containers are added each year and each container is designed to last 50 years while a long term storage solution is sought.

Most countries, including Canada, are planning to put used nuclear fuel in metal containers that would be placed deep underground in stable rock formations.


Hazardous Wastes

• The radioactivity of the used fuel bundles decreases significantly with time. However,

It will take 20 half-lives (thousands of years)

before the material is safe. It must be isolated from the natural living environment.



Nuclear Fusion

• Nuclear fusion = joining of two light nuclei into one heavier nucleus. In the core of the Sun, two hydrogen nuclei join

under tremendous heat and pressure to form a

helium nucleus. When the helium atom is formed, huge amounts

of energy are released. The nuclear equation for fusion in the sun and in

fusion reaction experiments is:

See pages 320 - 321

The fusion of hydrogen

nuclei

In the sun, a fusion reaction occurs between hydrogen-2 and hydrogen-3. When these combine a huge about of energy is released.

Nuclear Fusion

• Researchers have worked for over half a century to find technology that will allow us to extract energy from fusion reactions. One of the difficulties is achieving the high temperatures and pressures

needed.

Another is simply finding a way to contain a reaction that is so hot that no vessel can hold it without being destroyed.

• Scientists cannot yet find a safe, manageable method to harness the energy of nuclear fusion. So-called “cold fusion” would occur at

temperatures and pressures that could be

controlled.


Nuclear Fusion-Update

• What’s happening today? Video National Ignition Facility at Lawrence Livermore National Laboratory in California


A cutaway view of a gold cylinder known as a hohlraum, with a spherical fusion fuel capsule gleaming inside. The hohlraum measures about half an inch (1 centimeter) in height, and the capsule is about a tenth of an inch (2mm) in diameter. Beams of light from 192 lasers are directed at the capsule to cause an implosion.

Comparison of Fission and Fusion Reactions


Take the Section 7.3 Quiz


• Considered response

• What does CANDU stand for?

• What is nuclear fusion?

• How do you control a nuclear fission reaction so that it will not go out of control?

• Where does fusion occur and under what conditions?


tuesday april 8, 2014 (c) mcgraw hill ryerson 2007

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