7.3 - Nuclear Reactions Nuclear fission and fusion are processes that involve extremely large amounts of energy. fission = the s p l i t t i n g of nuclei fusion = the joining of nuclei

Nuclear Fission Albert Einstein’s equation E = mc 2 illustrates the energy found in even small amounts of matter

Nuclear Rxns. Nuclear reactions are different from chemical reactions. In chemical reactions… mass is conserved (LOCOM !) energy changes (+ or -) are relatively small there are no changes in the nuclei of atoms the identities of the atoms do not change e -s are involved in formation / breaking of bonds in chem. rxns. rearrangement of the connections between atoms / ions

Nuclear Rxns.

Nuclear Equations for Induced Nuclear Reactions Natural radioactive decay consists of the release of α and β particles, and radiation.  Scientists can also force (‘induce’) nuclear reactions by smashing (‘bombarding’) nuclei with α particles, β particles, or rays.

Nuclear Equations for Induced Nuclear Reactions Note: This is not nuclear decay! The α particle is a reactant in this rxn.

Nuclear Equations for Induced Nuclear Reactions The rules for writing equations for induced nuclear reactions are the same as earlier nuclear equations. The sum of the mass numbers must be = on both sides of the equation The sum of the charges must be = on both sides of the equation

Subatomic Particles in Nuclear Rxns. ProtonNeutron atomic # of 1 mass # of 1 (0 neutrons) atomic # of 0 (0 protons) mass # of 1

Nuclear Fission of Uranium-235 Recall the above induced nuclear rxn …it requires a massive amount of energy to collide the α particle with N-14, because of the repulsion between +ve charges. It requires much less energy to collide a neutral neutron with a nucleus. Most nuclear fission reactors and nuclear weapons use this principle.

Nuclear Fission of Uranium-235 a neutron,, crashes into an atom of stable U-235 the nucleus of U-235 absorbs the neutron and creates the unstable isotope U-236 (recall: the # of protons hasn’t changed!) …

Nuclear Fission of Uranium-235 U-236 immediately undergoes radioactive decay  it splits into 2 smaller nuclei (for example: atoms of Kr-92 and Ba-141)  Kr-92 and Ba-141 are both unstable and will decay into other isotopes, which are radioactive  releases up to 5 neutrons (although 3 is most common)  this triggers more fission rxns!  releases huge amounts of energy!

Nuclear Fission of Uranium-235

Nuclear Fission, in General

Nuclear Fission Practice Problem By bombarding a nucleus of U-235 with a neutron, the nucleus can split in many ways. Find the missing daughter nucleus for each of the following rxns. 1.____ + 2.____ + 3.____ +

Chain Reactions A chain reaction is an ongoing process where one rxn. initiates the next. The neutrons released in the induced reaction can then trigger more fission rxns. on other U- 235 nuclei. A chain reaction can get out of control, quickly releasing a massive amount of energy & resulting in a violent explosion.

Chain Reactions

Enrico Fermi realized that chain rxns. could be controlled by materials that absorb neutrons. Nuclear reactors have complex systems to ensure the chain reaction stays at safe levels. An uncontrolled chain reaction can result in the release of excess energy (an explosion) or harmful levels of radiation. Much of the fear surrounding the use of nuclear power plants is the risk of losing control of the reactor.

Nuclear Power Plants Nuclear power plants can generate large amounts of electricity. ON, QC and New Brunswick currently use nuclear power. Canadian-made nuclear reactors are called CANDU reactors.  CANDU (CANada Deuterium Uranium) reactors use nuclear fission to boil water to generate steam, which drives a conventional turbine to power a generator that produces electrical energy.  the reactor core produces heat as a result of rxns. like…

CANDU Reactors CANDU reactors are used in countries around the world. Canada, South Korea, China, India, Argentina, Romania and Pakistan The reactors are known to be safe and easy to shut down in an emergency. The Bruce Nuclear Generating Station on the shores of Lake Huron, in Ontario

CANDU Reactors and Hazardous Wastes Hazardous wastes produced by nuclear reactions are problematic… many of the waste products are highly radioactive and must be stored away from living things ie. the fuel rods that originally contain nuclear fuel (pellets of uranium) fuel rods stay in the reactor for ≈ 15 months when removed from the reactor, they’re extremely radioactive…stored in water for ≈ 10 years (!!)

CANDU Reactors and Hazardous Wastes …the rods are then put into shielded, dry, above-ground storage containers the fuel rods remain hazardous for many thousands of years and must be kept away from living things during this time some countries that use nuclear power are planning to bury the used fuel rods in stable rock formations in metal containers ( yikes !!!!! )

Nuclear Fusion nuclear fusion = joining of 2 low-mass nuclei into 1 more massive nucleus In the core of the (and other s), there is sufficient heat and pressure to force isotopes of H to collide 2 H nuclei join to form 1 He nucleus when the He atom is formed, enormous amounts of energy are released deuterium (H-2)tritium (H-3)

Problems Extracting Energy from Nuclear Fusion sufficient heat and pressure cannot be achieved problem creating a vessel that could contain a reaction this hot Researchers are working to find a technology that will allow humans to extract energy from nuclear fusion. There is no radioactive waste produced through nuclear fusion!