Chapter 11: Energy Section 11.2: Nuclear Energy

zIn the 1950’s and 1960’s, nuclear energy was considered to be the energy of the future.

zMany nuclear power plants producing cheap electricity were predicted.

z Then in the 1970’s and 1980’s, plans for 120 nuclear power plants were canceled and about 40 partially build nuclear power plants were abandoned because of safety concerns.

*Nuclear energy – the energy that exists within the nucleus of an atom because of the powerful forces that bind it together.

Nuclear Fission: zIn 1938, two physicists found that when a neutron is shot into the nucleus of a uranium-235 atom, it will split. zThe neutron disturbs and distorts the uranium-235 nucleus so the strong force is no longer strong enough to hold it together.

zIt then splits into two smaller nuclei. *Nuclear fission - the process of splitting a nucleus into two nuclei with smaller masses.

z Only large nuclei can undergo nuclear fission. z It converts a small amount of mass into a huge amount of energy.

z Every nuclear fission reaction produces more free neutrons. z These neutrons can then bombard and split other nuclei in the radioactive sample.

zTo control a nuclear fission reaction, materials are added to absorb the extra neutrons. zThese are called control rods and are often made of cadmium or boron.

z If there is no other material to absorb some of the neutrons, a chain reaction can occur.

*Critical mass - the amount of fissionable material required so that each fission reaction produces approximately one more fission reaction. zIf less than the critical mass of reaction material is available, a chain reaction will not occur and the fission can be controlled.

z Billions of fission reactions can occur each second, producing a tremendous amount of energy. z When carefully controlled, the energy can be used to make electricity.

zA huge amount of heat is generated and used to boil water at very high temperatures, creating high-pressure steam that is used to drive an electric generator.

zWhen nuclear fission reactions are not controlled, they can cause explosions, as in nuclear weapons. zSplitting 1 uranium-235 atom produces 30 million times more energy than reacting 1 molecule of dynamite.

Advantages of Nuclear Energy 1.Nuclear materials are a very rich energy source. zOne gram of Uranium-235 can produce as much energy as 3.5 metric tons of coal.

Advantages of Nuclear Energy 2. Nuclear reactors can run for years without having to be refueled or shut down for repairs. z Many other nations rely heavily on nuclear plants to meet their energy needs.

Advantages of Nuclear Energy 3. Nuclear energy does not produce carbon dioxide or other gases that pollute the atmosphere.

Disadvantages of Nuclear Energy 1.The most serious disadvantage of nuclear energy is that it produces radioactive waste.

Disadvantages of Nuclear Energy z The U.S. does not have a facility for the permanent disposal of nuclear waste. z Each nuclear power plant has a temporary storage facility.

Disadvantages of Nuclear Energy 2. Nuclear fuel is in relatively short supply. zThere is only a 100 – 200 year supply of Uranium-235 left.

Disadvantages of Nuclear Energy 3. Nuclear energy is extremely expensive. z Nuclear power plants are very large and complex and have elaborate safety systems, so they cost a lot to build.

Disadvantages of Nuclear Energy zMinor problems in the nuclear plant can force it to shut down for weeks or months. zThese expenses result in higher utility bills for the consumer.

Disadvantages of Nuclear Energy 4. The radioactive products of fission reactions are very dangerous. z If the enormous heat generated gets out of control, it can destroy the reactor building and spew the radioactive products into the air.

Disadvantages of Nuclear Energy zThe former Soviet Union has had at least 2 major nuclear accidents, one in the Ural Mountains in 1957 and one at Chernobyl in 1986.

Disadvantages of Nuclear Energy z At Chernobyl, the engineers turned off most of the reactor’s safety devices to conduct an unauthorized test. z The test caused massive explosions and contaminated thousands of square kilometers of land.

Disadvantages of Nuclear Energy zHundreds of firefighters and workers died from radiation exposure and thousands more contracted cancer.

Disadvantages of Nuclear Energy zThe nuclear reactor at Chernobyl is an obsolete type no longer used in the United States and the operators there violated basic safety guidelines.

Disadvantages of Nuclear Energy z But the fear of nuclear accidents and the expense caused the worldwide slowdown of nuclear power plant construction.

Nuclear Fusion: z Even more energy can be produced by nuclear fusion than by nuclear fission. *Nuclear fusion - the process of fusing together two atomic nuclei with low masses to form one nucleus with a larger mass.

z The nuclei of atoms are positively charged, so they repel each other. z They have to be moving very fast to have enough kinetic energy to overcome the repelling forces.

zTemperatures of millions of degrees Celsius are required to get the nuclei moving fast enough to get close together and fuse. zTemperatures that high are only found in the center of the Sun and other stars.

zIn the Sun, 4 hydrogen nuclei are converted into 1 helium nucleus. zThis changes a small amount of mass into a huge amount of energy.

z Earth receives some of the energy as heat and light.

zEventually all of the hydrogen in the Sun will be converted into helium and energy and it will die. zThe Sun is expected to live another 5 billion years.

zIn order for nuclear fusion to occur here on Earth, the atomic nuclei must be: yHeated to extremely high temperatures. yMaintained at very high concentrations yBe properly confined.

z Achieving all of these at the same time is extremely difficult, maybe even impossible.