Transmutation and Nuclear Energy

Core Concepts & Principles

Nuclear Transmutation

Nuclear transmutation changes one nuclide into another, either through decay or an induced transmutation reaction. Bombarding a target with neutrons or other nuclei can change its proton and neutron counts. Charged projectiles must overcome electrostatic repulsion; particle accelerators use electric fields to accelerate them and magnetic fields to guide or focus their paths. Neutrons have no charge and are not accelerated this way.

Transuranium elements have $Z>92$. Most are produced artificially through particle bombardment or neutron capture followed by decay; traces of some, including plutonium, also occur naturally.

Fission and Chain Reactions

Fission splits a heavy nucleus into lighter fragments, usually releasing neutrons and a large amount of energy. One possible uranium-235 fission pathway is:

$$^{235}_{92}\text{U} + ^1_0\text{n}\rightarrow ^{141}_{56}\text{Ba}+^{92}_{36}\text{Kr}+3,^1_0\text{n}$$

Fission can produce different fragment pairs. Released neutrons can trigger more fissions, forming a chain reaction. Fissionable means capable of fission; fissile more specifically includes material that fissions with slow neutrons, such as U-235. U-238 requires fast neutrons for fission.

• Subcritical mass: Too few effective fission neutrons reach other nuclei; the chain reaction dies out.
• Critical mass: Enough material, under specified conditions, to maintain a steady chain reaction: on average, each fission causes one more.
• Supercritical mass: On average, each fission causes more than one further fission, so the rate grows.

Criticality depends on isotope composition, geometry, neutron leakage, and surrounding materials as well as mass. Joining subcritical pieces can make a system supercritical; uncontrolled rapid energy release is the basis of fission weapons.

Nuclear Fission Reactors

A nuclear reactor sustains a controlled chain reaction. Its main components are:

• Nuclear fuel: Fissile material, often uranium enriched in U-235, arranged in fuel rods.
• Nuclear moderator: Water, heavy water, or graphite slows neutrons so they can cause fission efficiently.
• Control rods: Neutron absorbers such as boron or cadmium. Lowering rods reduces the reaction rate; raising them increases it.
• Reactor coolant: Carries heat away from the core. Heat produces steam, directly or through a heat exchanger; steam drives a turbine and generator.
• Containment system and shielding: Vessels, steel, and concrete contain pressure and radioactive material and reduce radiation exposure.

Spent fuel remains radioactive and needs cooling, shielding, and long-term management. Loss of cooling or containment can cause severe accidents, as illustrated by Three Mile Island, Chernobyl, and Fukushima.

Fusion

Fusion combines light nuclei into heavier ones. Suitable reactions release energy because the products are more tightly bound. For example:

$$^2_1\text{H}+^3_1\text{H}\rightarrow ^4_2\text{He}+^1_0\text{n}+17.6\text{ MeV}$$

Fusion powers stars. Positively charged nuclei repel one another, so very high temperatures are needed to help overcome this Coulomb barrier. A fusion reactor must confine hot plasma long enough for useful reactions. Research approaches include magnetic confinement (tokamaks) and inertial confinement (intense laser pulses). Maintaining confinement and producing useful net energy are major challenges.

Problem-Solving Routines & Methods

Practice & Concept Checks

Key Terms & Vocabulary