Nuclear Energy Chapter 25 6/1/2019

Nuclear binding forces Nucleus: protons+neutrons electrostatic repulsion forces between protons WHAT HOLDS THE NUCLEONS TOGETHER? Nuclear forces: or strong interaction: much stronger than the electrostatic repulsion force between the protons. Energy associated to the strong interaction: Nuclear energy Nuclear energy: a new possible source of energy - rearrangement of the nucleons in the nuclei can liberate this energy studying nuclear forces and energy: - more complicated than studying the electronic shells (smaller, and higher energy required) - radioactive decay - bombarding nucleons with heavy particles: , p, n - in order to reach and penetrate the nucleus high energy required --> achieved with particle accelerators 2 6/1/2019

Accelerators Obtaining high energy electrically charged nuclear “projectiles” Basic principle: using electric field to accelerate charges Two main types of accelerators: - linear accelerators: passing the charged particles through several consecutive accelerating regions (ex. Stanford Linear Accelerator, 3km long, electrons with energies up to: 50 GeV) - circular accelerators (sincrotron) : allows the particles to pass through a single accelerating region many times; particles kept on circular trajectories by strong magnetic fields; maximum energy limited by the overall size; important radiation losses for light particles--> appropriate for heavier charged particles (Fermilab near Chicago, up to 1 TeV!) 3 6/1/2019

The Nuclear Glue Forces between nucleons are revealed by accelerator experiments proton - proton collisions; proton-neutron collisions reveal the nature of the interactions between them n-n interaction obtained indirectly Results: - nuclear forces have a very short range! (3 x 10-15 m) - strongly attractive; 100 times stronger than electrical repulsion - at very-very close distance is repulsive like the electrostatic one - force is a complicated one: depends on relative orientation and some purely quantum mechanical effects - is independent of charge: same: n-n, n-p, p-p - an new additional force in the nucleus, responsible for the  decay process: n--> p + e; weak force (one-billionth the strength of the strong nuclear force) 4 6/1/2019

Nuclear Binding Energy Due to the strong nuclear forces: force is needed to pull them apart--> it takes work to take a stable nucleus apart energy is released when we allow nucleons to combine to form a stable nucleus average binding energy per nucleon: the total amount of energy required to completely disassemble a nucleus divided by the number of nucleons. Total energy of the nucleon related with the total mass: E=mc2 - total energy contains the total nuclear binding energy also - measuring the total mass of a given nucleus and comparing it with the mass of the free nucleons--> information to the binding energy average binding energy per nucleon as a function of nucleon nr. 5 6/1/2019

6 Stability of a nucleus Not all nuclei are stable line of stability: relates the number of protons and neutrons heavier nuclei has more neutrons than protons - explanation: electrostatic interaction between protons (long range forces) - quantum mechanical energy diagrams with or without electrostatic force between the nucleons filling up the available energy levels respecting the minimal energy principle and Pauli’s exclusion principle if the nucleus is above the line of stability: extra neutrons (n-->p+e; decays through  radiation) if the nucleus is below the stability line: extra protons ( or + decay or electron capture) 6 6/1/2019

Nuclear Fission nuclear fission: Heavy elements nuclei (with large number of nucleons) when interacting with projectiles (n, p or  particles) can split in several lighter nuclei --> minimizing the binding energy/nucleon a typical fission fission releases a large amount of energy from the nuclear binding energy; ( ex. above around: 210 MeV) typical energies from chemical reactions: a few eV !!! 7 6/1/2019

Chain Reactions The key to release nuclear energy: a single reaction can trigger additional reactions (neutrons released during the fission of U, can start additional reactions) a single reaction can cause a chain of events--> chain reaction conditions for getting chain reaction: - the produced neutrons should not leave the material before triggering a new reaction (critical mass) - the neutrons should not be captured by other atoms ( ex. isotopes of U-235: U-238…) the reaction can be: - subcritical (less than one new n produces new reaction) - critical (one new n produces in average a new reaction) - supercritical (more than one n from each reaction initiates another reaction)--> nuclear bomb 6/1/2019

Nuclear Reactors To use the nuclear energy--> to control the chain reaction the reaction should not die-out, neither become subcritical! We should work under critical condition--> energy released at steady state controlling and driving the reaction in reactors: - fuel: mixture of U-235 and U-238 (can be enhanced in U-235) bigger amount than the critical mass! - U-238 captures the fast neutrons, and produces no useful reaction - U-235 fission mainly due to slow neutrons - moderator: materials that slow down the neutrons (heavy water, graphite ….) - neutron absorbent rod (boron,…): control mechanism for fne-tuning the criticality - heat-exchanger: remove the heat from the core to drive electric generators (water, steam, etc…) 6/1/2019

Summary nuclei stay together in spite of the electromagnetic repulsion between protons because of a nuclear force. the strong interaction between nucleons has a very short range and is independent of charge accelerated particles from radioactive decay are used as probes to study the structure of stable nuclei the average binding energy per nucleon varies for the stable nuclei. (this can be used to produce nuclear energy). Combining light nuclei or splitting heavier nuclei releases energy. bombarding uranium with neutrons splits the uranium nuclei, releasing large amount of energy and extra neutrons. A single reaction emits two or three neutrons which can trigger additional fission reactions. Nuclear reactors are using fission chain reactions under critical condition 6/1/2019

Home-work assignment 664/ 1-6, 8-18, 22-24; 665/ 25-28, 31, 33-34, 37-40, 43-46 666/ 1-6, 8-14, 17-20 6/1/2019