Energy and Environment The Promise and Problems of Nuclear Energy

Nuclear Energy The nucleus of an atom is the source of nuclear energy.

Nuclear Energy When the nucleus splits (fission), nuclear energy is released in the form of heat energy and light energy. Nuclear energy is also released when nuclei collide at high speeds and join (fuse).

Fission and Fusion Chemical reactions involve outer electrons 30/09/99 Chemical reactions involve outer electrons Nuclear reactions involve changes in nuclei Two types of nuclear reactions are fusion and fission (others are radiation and transmutation) Fission is when a nucleus breaks apart Fusion involves adding nucleons to a nucleus Fission is used in nuclear power plants and powered the first atomic bomb (21 kilotons) Fusion powers stars and may also be used in thermonuclear bombs (60 megatons) Fusion requires that the nucleons be close enough so that the “strong force” can form

Nuclear forces There are two opposing forces in the nucleus: 30/09/99 There are two opposing forces in the nucleus: Electrostatic (+ve proton repels +ve proton) Strong force (nucleons attract each other) The strong force is stronger, but acts over a shorter distance. Adding more nucleons is favored with small nuclei but not with large E.g. adding a proton to a small vs. large nucleus

Fission: When a single nucleus splits to form two smaller nuclei. Large nucleus Two smaller nuclei

Fusion is governed by the nuclear strong force. Fusion: two nuclei stick together (or fuse) to form a new, larger nucleus One new nucleus Two separate nuclei

Suppose we pull the neutron and proton apart: Which situation above has more energy? Why? You must do WORK on the nucleons in order to separate them—thus the separated ones have more energy!

Now imagine putting a hydrogen nucleus together instead of pulling it apart. We normally write this in a special way: 2 H n + p 1 neutron Hydrogen nucleus proton + n p This reads: a neutron and a proton fuse together to form a hydrogen nucleus.

+ H n + p 2 1 n p More energy Less energy Where did the excess energy from the left-hand-side go? There must be energy released in this process!! It comes in the form of Radiant and Thermal energy Nuclear (fusion) Energy Thermal + Radiant Energy

In the case of the fusion of a neutron and a proton, were there any electrostatic forces acting between them? No, the neutron has no charge (it’s neutral). Consider the case where two HYDROGEN nuclei fuse together: + H 1 H 2 1 He 3 2 +

+ H 1 H 2 1 He 3 2 + Are there any electrostatic forces involved in this nuclear fusion reaction? Yes, each one has a proton. What effect should this have on the two nuclei before they fuse? The two nuclei should repel one another.

These get close enough to fuse together!

The result of the situation where they don’t get close enough is just a collision—they bounce off of one another. The result of the situation where they DO get close enough results in fusion: a new nucleus and a release of energy. We can only get the nuclei close enough under extreme heat and pressure. Eg., stars

+ H 1 H 2 1 He 3 2 + 1.6727 x 10-27 kg 5.0066 x 10-27 kg 3.3437 x 10-27 kg Why the difference? Where did the extra mass go?? 1.6727 x 10-27 kg + 3.3437 x 10-27 kg 5.0164 x 10-27kg

E = mc2 Einstein has done it again! The mass was converted into energy: The difference was 0.0098 x 10-27 kg The energy released is then: E = mc2 = (0.0098 x 10-27 kg)(3 x 108 m/s)2 = 8.815 x 10-13 Joules This is the measured energy release.

Nuclear Energy The sun’s energy is produced from a nuclear fusion reaction in which hydrogen nuclei fuse to form helium nuclei.

Nuclear Energy Nuclear energy is the most concentrated form of energy.

Three main types of ionizing radiation 1. Alpha particles - (helium nucleus) can be shielded by a sheet of paper or by human skin. If inhaled or ingested they can be very harmful 2. Beta particles are not stopped by a sheet of paper, but can be stopped by a thicker shield like wood. Beta particles can cause serious damage to your health if ingested

3. Gamma rays are the most penetrating and they penetrate paper, skin, wood, and other substances. A shield at least as thick as a concrete wall is required. This type of radiation causes severe damage to your internal organs

RADIOACTIVE WASTE Materials which are radioactive and for which there is no further use

THREE CATEGORIES OF RADIOACTIVE WASTE Low Level Radioactive Waste • Clothing used by workers• Gasses and liquid emitted by reactor • Disposed of in metal containers on site Intermediate Level Radioactive Waste • Physically or Chemically produced High Level Radioactive Waste • Spent fuel • Neutron bombardment of daughter products produces many nuclides• Re-processing captures fissionable material -- remainder too hot to handle

DISPOSAL OF HIGH LEVEL NUCLEAR WASTE Currently In steel tanks Under water In concrete tanks Alternatives Deep Geological Isolation Subduction zones Outer Space Ice Caps

Environmental Impact Nuclear energy plants produce electricity through the fission of uranium, not the burning of fuels. Consequently, nuclear power plants do not pollute the air with nitrogen oxides, sulfur oxides, dust or greenhouse gases like carbon dioxide. nuclear generated energy has on our environment is that the wastes produced are completely isolated from the environment. 

Advantages - Nuclear fuel is inexpensive Waste is highly compact, unlike carbon dioxide The compacted fuel is easy to transport No greenhouse gases are released by nuclear power plants. Fissile atoms contain vast amounts of energy

Major challenges of nuclear energy - Nuclear radiation accidents Nuclear power requires a large capital cost Long-term storage of nuclear waste is difficult.

Nuclear energy 30/09/99 Define: binding energy, nucleon, fission, nuclear fusion, radioactivity, half-life. What evidence exists that mass and energy are interchangeable? What are the opposing forces that exist within the nucleus?

Binding energy: the energy that holds nucleons together Nucleon: protons and neutrons Nuclear fission: breaking apart nuclei Nuclear fusion: forming or adding to nuclei Radioactivity: releasing small particles or energy from a nucleus Half-life:time taken to lose ½ the radioactivity The existence of binding energy is evidence that mass can be converted to energy 30/09/99

30/09/99 3. The “strong force” attracts adjacent nucleons. Electrostatic repulsion of protons pushes protons away from each other.