(455) Nuclear fission reactions [IB Physics SL/HL]

Nuclear Fission Overview

• Definition of Fission: A process where a heavy nucleus splits into two or more lighter nuclei, occurring when the nucleon number is greater than 230.

Types of Fission

• Spontaneous Fission: Occurs naturally without external cause; examples include

  • Thorium-232

  • Uranium-235

  • Uranium-238

  • Releases neutrons as a byproduct.

• Induced Fission: Result of an artificial trigger, typically by adding a neutron to an unstable nucleus, causing it to split and release energy and additional neutrons.

Chain Reaction Mechanism

• A chain reaction occurs when:

  • Incoming neutrons from fission reactions cause further fission events, dramatically increasing the number of reactions occurring.

• Each reaction generates additional neutrons, leading to exponential growth in energy release.

Enriching Uranium

• Natural Uranium Composition: Mostly uranium-238 (stable); a small amount is uranium-235.

• Purpose of Enrichment: Increase the proportion of uranium-235, essential for sustainable fission reactions in reactors and weapons.

• Enrichment Methods:

  • Use of centrifuges to separate isotopes based on mass differences.

Example of Fission Reaction

• Starting Point: Neutron-induced reaction with Uranium-235.

  • A neutron is absorbed by uranium-235 forming uranium-236, which is unstable.

• Products of Uranium-236 Decay:

  • Produces Barium-144 and Krypton-90 along with energy and two additional neutrons, facilitating further fission reactions.

Energy Calculation in Fission

• Important Equation: Energy (E) = mass defect (m) * c².

• Masses Involved:

  • Left Side: Uranium-235 and neutron.

  • Right Side: Krypton-92, Barium-141, and residual neutrons.

• Energy Released per Reaction:

  • Approximately 173.2 MeV per fission reaction occurs, contributing to the high energy output of nuclear power plants.