After a fuel rod in a fission reactor reaches the end of its life cycle (typically 3 years), most of the energy that it produces comes from the fissioning of plutonium- \(239 .\) How can this be?

A fission reactor, often known as a nuclear reactor, is a complex system designed to manage the nuclear fission process. At its heart, a reactor contains fuel rods filled with fissile material, typically uranium-235 (²³⁵U). In the reactor, neutrons collide with the uranium atoms, instigating fission, which releases a substantial amount of energy in the form of heat. This heat is used to produce steam, which then drives turbines to generate electricity. The reactor is designed to control the rate of fission, ensuring that the reaction can be sustained safely over time.

• Control Rods: Integral to the reactor, these rods absorb excess neutrons and are used to regulate the rate of the reaction.
• Coolant: A substance, often water, that transfers heat away from the reactor core to prevent overheating.
• Moderator: A material that slows down neutrons so they can effectively induce fission upon collision with fissile uranium atoms.

The design of fission reactors encompasses various safety systems to prevent accidents, and they operate within a containment structure that offers an additional layer of protection.

Plutonium-239 (²³⁹Pu) is a man-made isotope that is produced in nuclear reactors from uranium-238 (²³⁸U) through neutron absorption and subsequent beta decay. Similar to uranium-235, plutonium-239 is fissile, meaning it can sustain a nuclear fission chain reaction. During fission, a plutonium-239 nucleus absorbs a neutron and splits into smaller atoms, releasing energy, more neutrons, and gamma radiation. The released neutrons can then continue to induce fission in other ²³⁹Pu nuclei, contributing to a self-perpetuating chain reaction. This property of plutonium-239 is utilized in both nuclear reactors and nuclear weapons.

• High Neutron Yield: Plutonium-239 produces more neutrons per fission event compared to uranium-235, which is beneficial for maintaining the chain reaction.
• Hazardous: Being highly radioactive and toxic, plutonium-239 requires careful handling and stringent safety protocols.

The use of plutonium-239 is an integral aspect of nuclear power and has implications for nuclear proliferation and radioactive waste management.

Uranium decay is a natural process by which unstable isotopes of uranium transform into other elements over time. Uranium-238, the most abundant isotope, decays through alpha emission, which is not part of the nuclear fission process in reactors. However, uranium-238 is relevant in reactors because it can absorb neutrons to ultimately become plutonium-239, another valuable fuel source.In a reactor setting, uranium-235, another isotope, undergoes a different type of decay known as fission. Unlike natural decay, fission is a nuclear reaction triggered by neutron absorption, leading to the nucleus splitting apart. The decay of uranium in reactors is carefully manipulated to maximize energy production while maintaining safety.

• Half-Life: Uranium isotopes have extremely long half-lives, extending over billions of years, which makes them a persistent source of radiation.
• Decay Series: Uranium decay ultimately leads to the formation of stable lead isotopes, but it involves a chain of radioactive decay steps, each with its unique byproduct and half-life.

The intricate process of uranium decay is harnessed in fission reactors to provide a steady supply of energy for electricity generation.

Neutron absorption is a fundamental phenomenon in the fission process occurring in nuclear reactors. It describes the capture of free neutrons by atomic nuclei. The outcome of neutron absorption varies depending on the type of nucleus it encounters:

• If a neutron is absorbed by a fissile nucleus like uranium-235 or plutonium-239, it can cause the nucleus to become unstable and fission, thereby releasing energy.
• If a neutron is captured by a fertile nucleus like uranium-238, it undergoes a series of beta decays to become a fissile isotope, such as plutonium-239.

The absorption of neutrons is also influenced by their energy level. Slow-moving, or thermal, neutrons are more likely to be absorbed and cause fission in fissile materials.
Good neutron economy is essential for a sustained chain reaction in a fission reactor. Materials known as moderators are used to slow down the speed of neutrons, increasing the likelihood of absorption and subsequent fission events. The ability of a substance to absorb neutrons is characterized by its neutron cross-section, a critical factor in reactor design.

A chain reaction in nuclear reactions is the self-sustaining sequence of fission events that occur when neutrons released by one fission process trigger additional fissions in nearby fissile atoms. For a chain reaction to be maintained, at least one of the emitted neutrons from each fission event must cause another fission. This balance is crucial:

• If too few neutrons cause subsequent fissions, the reaction will die out.
• If too many neutrons lead to fissions, the reaction can increase exponentially, potentially leading to a runaway reaction or explosion, which must be controlled in power generation contexts.

Fission reactors are designed to precisely control the chain reaction, usually by inserting or withdrawing control rods and by adjusting the concentration of fissile material. The introduction of control rods (which absorb neutrons) temporarily disrupts the reaction until desired equilibrium is restored. The role of the moderator, to slow down the neutrons, also contributes to maintaining a stable chain reaction by facilitating further fission events.
In essence, the chain reaction is the core mechanism that allows nuclear reactors to produce continuous energy and is an example of how harnessing the fundamental principles of physics can lead to significant technological advancements.