Nuclear Changes

1. Nuclear Changes (This is the stuff that can make you glow in the dark….) 100 trillion watts of fusion power...

2. 7.1 What is radioactivity? • Some elements, particularly those with very large nuclei (note those at the bottom of the Periodic table), tend to have an unstable nucleus. • Occasionally they lose parts or pieces in the form of particles and energy. • The process of an unstable nucleus emitting particles or energy is called radioactivity. • The charged particles or energy released is called nuclear radiation. • There are four types of Nuclear Radiation: Alpha particles, Beta particles, Gamma rays, and Neutrons. These particles fly out of the nucleus and interact with surrounding matter, depending on their charge, energy and mass. (see Table7.1)

3. Alpha particles- consist of two protons and two neutrons (+2 charge). They are essentially helium nuclei. • They are very large for a subatomic particle and don’t penetrate very far into materials. They can travel thru a piece of paper. • Alpha particles ionize matter (take away electrons) as they pass thru it. • Example: emitted by Uranium-238. Small carbon-covered silicon- carbide fuel pellets for use in the Modular Helium Reactor, a power system meant to be safer than present nuclear power plants. A penny provides size comparison.

4. Beta particles- are -1 charged, and essentially a high speed free electron. • Formed by neutrons decaying to form a proton and an electron. Electron is then ejected from the nucleus. • More penetrating…go thru paper easily, but stopped by 3mm of Al or 10mm of wood. • Able to move fast, but ionize other materials and slow down in them.

5. Gamma rays- no mass, no charge. This isn’t made of matter…its energy in an electromagnetic wave, like light, emitted by nucleus. • Very powerful wave, very penetrating (more than alpha and beta). Can go thru 60cm of Al or 7cm of Pb. • Pose risk to health, due to penetrating qualities. • Ex: found in radium, (discovered by Marie Curie).

6. Neutron radioactivity occurs in a neutron-rich nucleus. • A high-energy neutron is emitted from the nucleus in its decay. • Neutrons have no charge, so there’s no charge to slow it down. • Neutron radiation is the most penetrating radiation form…can be very dangerous. It takes about 15cm of solid lead shielding to stop most neutron radiation.

7. When an unstable nucleus emits alpha or beta particles, the number of protons or neutrons changes (see p.223….in beta a neutron turns into a proton). • Not a chemical equation…changing elements. However, notice that the masses still add up equally on each side…mass is still accounted for. • Ra -----> Rn + He • Beta decay equation works same/no mass/-1#. • C -----> N + e • Gamma rays: no change in atomic #, only energy level of the nucleus changes. • Neutron emission discussed later….. Mass# Alpha 226 88 222 86 4 2 226 = 222 + 4 88 = 86 + 2 Atomic# 14 6 14 7 0 -1 14 = 14 + 0 6 = 7 + (-1)

8. It is possible to predict the age of rock by looking at its radioactive decay. Possible to accurately predict time for 1/2 the radioactive part of a rock to decay. This is called its half-life. After its first half-life is over, half of the remaining 50% will decay over the next half-life (down to a quarter of the original). The radioactive mat. decays into new element. Of that quarter, half will decay over the next half-life. In short, a predictable pattern of change results. If one knows how much radioactive material was present to start with, one can predict how old the object is. Radioactive materials can have 1/2lives from part of a second to billions of years…different ones are used for measuring different times. Carbon-14 is used for once-living organisms. (see p.226-227 and Table 7-2)

9. Time out: Why are nuclear issues such strong issues with environmentalists and humanitarians? 8/12/45, Nagasaki after atomic explosion... 8/7/45, Nagasaki before atomic explosion.

10. 7.2 Nuclear Fission and Fusion • Protons and neutrons are tightly packed into nucleus. • Some nuclei are unstable and decay. There may be stable and unstable isotopes of an element. • Stability of a nucleus depends on nuclear force that holds it together…acts between protons and neutrons. • Force of interaction between protons and neutrons that holds nucleus together is called the strong nuclear force. Much stronger than repulsion between protons, but acts over a very short distance…width of 3 protons. • Neutrons help stabilize a nucleus, but too many or too few will cause instability=decay. Anything over 83 protons is always unstable=decay=release energized particles…rest of energy emitted as gamma rays.

11. Nuclear Fission • Fission is the process of a nucleus splitting into 2 or more smaller pieces. • U-235 hit by a neutron breaks into Ba-137 and Kr-84 and releases 15n and energy. (can split to make several combos). • Produces large amounts of energy (HIGH order of magnitude). • When measured, some mass turns out missing. Its exception to the law of consv. of mass/energy, because some of the mass is turned into energy! 1st atomic bomb-Hiroshima “Fat man” bomb -Nagasaki

12. Albert Einstein (1905) actually explained this in the Mass-energy equation. • E = mc2 • Energy=mass X (spd.of light)2 • Mass and energy can be converted into each other. • c is constant and very large, so even a small amount of mass will equal a very large amount of energy. • Mass is usually very stable (thank goodness) except concentrations of large, unstable element nuclei. March 26, 1954, Bikini atoll, 11 megaton explosion.

13. Nuclear detonation energy release is very damaging to environment and life-forms over a large area. Can leave harmful radioactives behind. • Neutrons released by fission can run into other nuclei and cause them to fission, releasing more neutrons….chain reaction. • Nuclear bombs work on a nuclear chain reaction principle. Two or more masses of U-235 are inside. An explosion crushes them close together to make a critical mass that will start and sustain a chain reaction. Pure, weapons-grade U-235 is very dangerous, b/c it easily starts an uncontrolled chain reaction. • Devastating energy in short time.

14. Nagasaki bomb damage, 1945.

15. The more neutrons and the more crowded the nuclei the more chance of a successful chain reaction. • Some materials can absorb neutrons and slow a Rx down. This slower reaction can be used to generate electrical power. • Graphite rods slow reactions in nuclear plants to control the reaction. Three Mile Island Cooling Towers During 1979 Accident Steam blows from the cooling towers of the Three Mile Island nuclear power plant on the night of March 28, 1979, during the most serious accident in U.S. nuclear power history. One of the plant's reactor cores was exposed after a series of equipment failures complicated by human error, resulting in the production of radioactive gases. People in nearby homes (bottom) were evacuated for safety, but fortunately most of the gas was contained.

16. Nuclear Fusion • Energy can also be obtained by combining smaller nuclei to make a larger nucleus. (Fusion) • Stars (sun) use fusion to fuse 4hydrogens to make a helium atom and tremendous amounts of energy as gamma rays. • Large amount of energy needed to overcome repulsion and push them together (stars, heat/pressure).

17. 7.3 Dangers/Benefits of Nuc. Radiation • We are exposed to background radiation every day from sun, soil, water, plants…source: cosmic rays and radioactive materials in ground. This is a normal occurrence. • Skin protects us somewhat, but internal damage from radon gas or penetrating/excessive radiation is dangerous. • Long periods or hi-intensity exposure is worst, can result in radiation sickness:hair loss, sterility, cancer, low WBCs, death of bone, etc. • Penetrating radiation damages DNA. If DNA is badly damaged, its nitrogen bases are repaired wrongly. Cells cannot function, and reproductive cells damage=mutations. • Short-lived isotopes are often used for medical/geology/agriculture as radioactive tracers to locate infections, water with radiation-sensitive detectors.

18. Diagnostic Cobalt 60, a radioactive tracer, glowing in a liquid bath.

19. Small beams of gamma rays can kill tumors. • Nuclear power does not pollute air, is a long-term resource, and is used in dozens of countries. • Nuke power uses U-235, creating radioactive byproducts. These must be handled carefully; closely regulated. Power plants in the U.S. may operate for 40 years before dismantling. Many operate less due to political opposition. This and expense prevent their widespread use. Nuclear fusion experimental chamber.

20. Nuclear fuel waste glowing deep under a liquid protective bath...

21. Nuclear plants produce waste--dangerous long time, must be stored well. • Bury in very deep shafts. Best if: sparsely populated, free from earthquakes, far from groundwater. (Utah, Nevada) Radioactive for 100s or 1000000s of years, very hard to ensure long-term safety. • Nuclear fusion, based on hydrogen, seems best option. Difficult to do...

22. The End ...Of a nuclear fusion fuel pellet imploding under an ultraviolet laser light.