Nuclear Reactor Introduction and Fuel Cycle

Neutron Interaction Probabilities and Isotopic Behavior

• Cross Sections and Energy Dependence:

  • The probability of a neutron interaction is formally referred to as a cross section.
  • Interaction probabilities are not constant; they are highly dependent on the energy level of the neutron.
  • For the isotope $^{235}U$, the interaction cross section decreases precipitously at energy levels around $1\,MeV$. Neutrons at this energy level are considered "fast."

• Uranium Isotope Roles ($^{235}U$ vs. $^{238}U$):

  • In a standard reactor fuel (often contained in ceramic form), $^{238}U$ does not typically undergo fission.
  • Instead, $^{238}U$ interacts with thermal neutrons through a process known as activation.

• The Activation Process:

  • When $^{238}U$ absorbs a neutron, it begins a decay chain to create plutonium.
  • The progression follows: $^{238}U$ absorbs a neutron, decays into Neptunium, and finally decays into Plutonium.

Nuclear Reactor Design and Thermodynamic Cycles

• Primary Function:

  • The fundamental purpose of a nuclear reactor (similar to fossil fuel plants and theoretical fusion plants) is to generate heat to boil water.
  • The energy released by nuclear fission is approximately $200\,MeV$ per atom.
  • This heat creates steam, which turns a turbine, which then turns a generator to produce electricity.

• Societal Dependence on Electricity:

  • Electricity is the foundational utility for modern survival; without it, there would be no food distribution, transportation systems, water treatment, and limited access to housing and clothing.

• Pressurized Water Reactors (PWR):

  • A PWR utilizes two distinct loops of water.
  • Primary Loop: This water circulates through the reactor core and stays at extremely high pressure and high temperature. Because of the high pressure, this water never boils.
  • Heat Exchanger: The primary loop passes through a heat exchanger where it transfers thermal energy to a secondary loop.
  • Secondary Loop (Boiler): The water in this loop boils to create steam, which drives the turbine and generator.
  • Condensation: The steam must be cooled back into liquid form to be recycled. This heat removal is achieved using cooling towers, lakes, or oceans.

• Boiling Water Reactors (BWR):

  • A BWR utilizes a single loop system.
  • Water is boiled directly inside the reactor core.
  • The resulting steam is sent directly to the turbine to generate power.
  • Like the PWR, the steam must eventually be condensed back into liquid via a cooling tower, river, lake, or ocean to complete the cycle.

Workforce Requirements and Global Nuclear Expansion

• Workforce Composition:

  • Running a nuclear power plant requires a diverse set of skills, and only about $5\%$ of the workforce requires advanced, specialized nuclear training.
  • The remaining $95\%$ of the staff consists of roles common to any thermal power plant, such as:
    • Pipefitters
    • Welders
    • Electricians
    • Craftspersons familiar with condensers, pumps, and valves.

• Growth Projections and Policy:

  • Currently, nuclear energy provides approximately $20\%$ of the electricity in the United States.
  • Expansion Goals: There is a significant governmental and international push to expand nuclear capacity by the year 2050.
    • A former administration committed to tripling nuclear energy output.
    • The current administration has set a goal to quintuple nuclear energy output by 2050.
  • International Commitment: At COP 26, an international consensus was reached to triple nuclear energy by 2050 as part of a global effort to move away from coal and achieve greenhouse gas-free electricity.

• Economic Opportunity:

  • Given the goal to significantly increase nuclear output within the next 20 years, there is a projected shortage of nuclear engineers and skilled workers, leading to high demand for these skill sets.

Questions & Discussion

• Question from Josiah: Does the $^{238}U$ undergo fission, or is it just the $^{235}U$ inside of the ceramic?
• Response: $^{238}U$ does not undergo fission in this context. At specific energy levels, particularly with thermal neutrons, the cross section for $^{235}U$ drops, and the $^{238}U$ instead absorbs the neutron to undergo activation, eventually becoming plutonium.