Nuclear Power Plant Operation

The primary function of a nuclear power plant is to generate electricity.
Electricity generation involves using a heat source to boil water, creating steam that drives a turbine. The turbine then turns an electrical generator, producing electricity.
Fossil fuel plants use burning coal, oil, or gas as a heat source, while nuclear plants use a nuclear reactor.

Nuclear reactors use uranium as fuel, specifically enriched uranium.
Natural uranium consists mainly of two isotopes: uranium-238 (U-238) and uranium-235 (U-235).
U-238: Approximately 99.3% of natural uranium. Its nucleus contains 146 neutrons and 92 protons, totaling 238 nucleons.
U-235: Approximately 0.7% of natural uranium. It has 143 neutrons and 92 protons in its nucleus.
U-235 is fissile, meaning it can undergo spontaneous fission, splitting into two smaller nuclei (fission products) and releasing two to five neutrons. This process also releases energy in the form of heat.
To enhance fission efficiency, uranium fuel is enriched to increase the U-235 concentration from 0.7% to 3-4%.
Fission products are radioactive and decay by emitting beta particles (high-speed electrons) and gamma rays. This radiation contributes approximately 7% of the heat in a reactor that has been operating for several months.
While spontaneous fission of U-235 occurs slowly, it can be induced by a neutron hitting the nucleus. The released neutrons can then trigger fission in other U-235 nuclei, leading to a chain reaction.
If enough U-235 nuclei are close together, the chain reaction can escalate rapidly, releasing significant energy in a short time. This principle is utilized in nuclear weapons, where uranium is enriched to over 50% U-235 to create a large explosion. Nuclear reactors, using fuel enriched to only 3-4% U-235, cannot explode like a nuclear bomb.

Ideally, a nuclear reactor maintains a constant rate of fission to sustain a stable high temperature.
To achieve this, the fission rate must be controlled so that each fission event induces exactly one subsequent fission.
Control rods, made of neutron-absorbing material, are used to regulate the number of neutrons available to induce fission. Inserting the control rods slows down the fission process, while removing them increases it. Fully inserting the control rods stops the fission process entirely.

The reactor vessel contains the uranium fuel core and water. In a PWR, the water is kept at high pressure (approximately $2,000$ psi) and high temperature (around $600^{circ}F$ ).
The high pressure prevents the water from boiling, ensuring that the reactor remains filled with water.
A pressurizer, containing a mixture of air and steam, is used to maintain the desired pressure. Operators can adjust the pressure by adding or removing air.
The pressurizer includes a safety valve that automatically opens if the pressure exceeds safe limits. A malfunction of this valve was a key factor in the Three Mile Island (TMI) accident.

Hot water from the reactor flows to a steam generator, a large tank containing pipes through which the reactor water flows.
The heat from the reactor water is transferred to the water in the tank, which is at a lower pressure. This causes the tank water to boil and produce steam.
The steam then drives a turbine, which in turn drives a generator to produce electricity.

A condenser is located on the exit side of the turbine to maximize the temperature difference between the entering and exiting steam, as dictated by the second law of thermodynamics.
The condenser cools the steam via contact with external piping and releases waste heat to the environment, often a lake, river, or, as in the case of TMI, the atmosphere via cooling towers.
The water vapor released from cooling towers is simply water vapor and does not contain radioactivity.
A PWR system consists of three separate water loops:
- The first loop transfers heat from the reactor to the steam generator.
- The second loop transfers heat from the steam generator to the turbine.
- The third loop transfers heat from the turbine to the environment via the condenser.

Nuclear power plants typically operate at an efficiency of 35-40%, meaning that 35-40% of the heat generated in the reactor is converted into electricity. The rest is released as waste heat to the environment.
Fossil fuel plants typically have efficiencies of 40-45%.
A condensate pump returns water from the turbine to the steam generator for reheating.
Ideally, none of the water loops should contain radioactivity. The uranium fuel is encased in zirconium cylinders (fuel pellets) designed to prevent leaks.
However, some fuel pellets may leak, releasing fission products into the reactor water loop.
To mitigate this, a purification system continuously removes most of the radioactivity from the reactor water.

Coolant pumps circulate water within the reactor to maintain cooling.
Feed pumps circulate water in the turbine system, crucial for removing heat from the reactor water.
Emergency feed pumps serve as backups to the feed pumps in case of failure.