John Preston Structure of the Atom p np n Nucleus

John Preston Isotopes (same types) - Same # of Protons nnp p n p p 2 He nn 4 5

John Preston Isotope Properties  Isotopes of the same element have exactly the same chemical behavior  Isotopes of the same element have different mass  Some are unstable (radioactive) and others are not  For example, 6 C 12 is stable but 6 C 14 is not

John Preston We Are All Made of Isotopes  Most of the carbon in our cells is 6 C 12, but all living things have enough 6 C 14 in them to produce 15.3 beta emissions per minute per gram of carbon  Nothing is more “natural” than being radioactive!

John Preston Archeology  C 14 is produced from C 13 in the upper atmosphere by cosmic rays  Some of the carbon in carbon dioxide is C 14  Living things like trees and plants incorporate the C 14 in wood and fiber while they are alive

John Preston Archeology  C 14 has a half-life of 5,760 years  We can tell how old wood or other plant material is by how much less radioactive it is than when it was alive!

John Preston Types of Radiation Alpha (2p + 2n) Beta (Electron) Gamma (Photon) Neutron Nucleus

John Preston Helium  Alpha radiation is really a helium nucleus - when the particle stops, it acquires two electrons and becomes helium gas  Rocks that contain radioactive elements that emit alpha particles can be dated by how much helium they have in them!  Natural gas contains about 0.4% helium from alpha radiation.

John Preston Enormous Energy Nucleus M1M1 M2M2 M3M3 Energy Mass of M1 is greater than M2 plus M3. E = ((M 1 -(M 2 +M 3 )) C 2 or E=mC 2 When nuclei split, the energy released is millions of times greater than a chemical reaction

John Preston Nuclear Energy Amounts  Chemical energy is measured in electron volts, abbreviated ev.  Even dynamite or TNT yields only a few electron volts of energy per atom during an explosion  Fission of U 235 yields 200 million electron volts of energy per atom!

John Preston Energy  A fuel rod puts out energy for three years before it is replaced in a reactor  Nuclear weapons are rated in thousands or millions of tons of TNT

John Preston Chain Reaction n n n 92 U 235 n n n Moderator N Neutrons need to be slowed down to cause fission efficiently

John Preston Moderators  Two good moderators are water and graphite (carbon)  Reactors in the USA use water as a moderator and as a heat transfer medium  Chernobyl used graphite, which caught fire and help spread the radioactive material

John Preston Few Isotopes Fission Well  Most large atoms can be made to fission  Only two isotopes fission easily enough to maintain useful chain reactions; 92 U 235 and 94 Pu 239

John Preston Nuclear Weapons Proliferation  The only way to separate U 235 from U 238 is to take advantage of the small difference in mass  The main reason more countries do not have nuclear weapons made from concentrated U 235 is that it is very difficult to separate from U 238

John Preston Fissionable Isotopes Are Hard to Concentrate or Make  Only 0.7% (less than 1%) of naturally occurring uranium is U 235  It’s concentration must be brought up to over 3% to be useful in a reactor and about 90% for a bomb

John Preston Fission Products Are Highly Radioactive Nucleus M1M1 M2M2 M3M3 Energy Alpha Beta Gamma Alpha Beta Gamma

John Preston Half Life - Half of Remainder Decay

John Preston Storage of Nuclear Waste  Short half-life means it is very radioactive but not for very long  Long half-life means it is not very radioactive  It will never be zero; nothing is  It is more meaningful to compare it to its naturally radioactive surroundings called “background” radiation

John Preston Radioactive Isotopes Turn into Stable Isotopes 86 Rn Po 218 Alpha (2p+2n) 88 Ra 226 Alpha (2p+2n) Lead RadiumRadonPolonium

John Preston Radon  Radium exists in small amounts in most igneous rocks  It decays into radon, which is a gas that can escape through cracks in the rock and leak into basements  Radon is a heavy gas that will accumulate in low areas unless ventilated; circulate air in basements to remove build-ups

John Preston Radon  If radon is inhaled, it can emit an alpha particle and turn into polonium, which is a solid that stays in the lungs  The particle will continue to emit alpha and beta particles until it decays into a stable isotope of lead

John Preston Neutrons Can Decay 1 p 1 1 n 1 Neutrino (no mass or charge) Beta (an electron) If a neutron in a nucleus decays into a proton, the atom becomes a different element.

John Preston Breeder Reactors Can Produce Fissionable Plutonium from U Pu U n 1 92 U 239 Beta Plutonium can fission as well as 92 U 235

John Preston Reprocessing Fuel Rods  Breeder reactors are designed to transform enough abundant U 238 into fissionable Pu 239 to replace the U 235 that was consumed  Most commercial power reactors are not breeders but do produce small amounts of Pu 239 anyway

John Preston Plutonium  Since Pu 239 is chemically different than uranium, it is much easier to separate and concentrate into bomb material  India, Pakistan, and possibly North Korea have obtained nuclear weapons from reprocessing fuel rods from nuclear power reactors  Plutonium is also a highly toxic chemical