Nuclear Fission and Fusion: Power Generation and Fuel Cycle

Posted on Aug 15, 2024 in Chemistry

Nuclear Fusion and Fission

Item 3: Understanding the Basics

Nuclear Fusion

Reactions between nuclei of light atoms lead to the formation of a heavier nucleus, accompanied by the emission of elementary particles and energy.

Nuclear Fission

Reactions in which a heavy nucleus breaks down, usually into two fragments of similar size, emitting neutrons and releasing large amounts of energy.

Fission Reaction and Energy Release

A heavy uranium nucleus absorbs a neutron and splits into two lighter nuclei (fission products), releasing energy, neutrons (1-5), gamma radiation (photons), and other particles like neutrinos. The emitted neutrons can induce further fission reactions, creating a chain reaction.

Maintaining a stable fission chain reaction requires precisely one neutron from each fission to induce a new fission. The average energy released in a fission reaction of Uranium-235 (U-235) is 200 MeV.

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Neutron Types in Fission

Instantaneous Neutrons: Most neutrons are produced within a very short interval (around 10-17 seconds) after fission.

Delayed Neutrons: A small fraction (less than 1%) of neutrons are emitted with a delay (average of 13 seconds) during the decay of fission products.

Types of Nuclear Power Plants

5.1. PWR (Pressurized Water Reactor)

  • Moderator and Coolant: Light water
  • Fuel: Slightly enriched uranium, UO2
  • Fuel Elements: Square matrix zircaloy rods (16×16)
  • Core Pressure: 17 MPa (170 bars)
  • Coolant Temperature: 280 °C (input) to 330 °C (output)
  • Steam Generators: One to four
  • Control Rods: Inserted from the top
  • Unique Feature: Boric acid can be added to the coolant as a neutron absorber

5.2. BWR (Boiling Water Reactor)

  • Principle: Heat from fission boils water directly in the reactor vessel, which also acts as a moderator.
  • Steam Generation: Steam passes through a dryer and then to the turbine.
  • Vessel Size: Twice as large as PWR to accommodate steam dryers.
  • Pressure: 7 MPa (70 bars)
  • Temperature: 300 °C
  • Control Rods: Inserted from the bottom
  • Fuel Elements: 8×8
  • Key Difference: No separate steam generator due to direct steam production in the core.

Nuclear Fuel Cycle

3.1. Procurement of Fuel Material

  1. Mining
  2. Concentration
    • Physics: Removing sterile material
    • Chemistry: Dissolving the ore, solvent extraction, precipitation, filtering, drying, and collection of yellow cake
  3. Purification
    • Dissolving yellow cake in nitric acid and purification processes to obtain UF4
  4. Conversion of UF4 to UF6
  5. Enrichment
    • Enrichment of UF6 gas, conversion back to UF4 and then to UO2 (uranium oxide)
  6. Preparation of Fuel Material

3.2. Fabrication of Fuel Elements

Uranium oxide is converted into ceramic pellets and placed in zirconium alloy rods. These rods are sealed and assembled into fuel elements.

3.3. Management of Fuel in the Reactor

Fuel elements are placed in a metal structure within the pressure vessel. Each element stays in the reactor for three to four years. They are repositioned within the reactor to optimize uranium consumption and compensate for parasitic absorption in fission fragments.

Spent fuel is removed and stored in a cooling pool for decay of radioactivity.

3.4. Spent Fuel Reprocessing

Closed Loop: Spent fuel can be reprocessed to separate usable fuel from fission products for reuse and disposal.

Open Cycle: Spent fuel is treated as waste and stored in underground repositories. Spain currently uses the open cycle approach.