21.4 Transmutation and Nuclear Energy

Nuclear Reactions and Energy

Creation of Heavy Elements

All elements heavier than uranium on the periodic table don't exist naturally. They're called transuranic elements, and scientists create them by bombarding heavy nuclei with neutrons in nuclear reactors or particle accelerators. Plutonium (Pu), neptunium (Np), americium (Am), and curium (Cm) are all examples.

Here's how plutonium-239 is produced from uranium-238:

1. Uranium-238 captures a neutron, becoming uranium-239
2. Uranium-239 undergoes beta decay to become neptunium-239
3. Neptunium-239 undergoes another beta decay to become plutonium-239

$^{238}_{92}U + ^1_0n \rightarrow ^{239}_{92}U \xrightarrow{\beta^-} ^{239}_{93}Np \xrightarrow{\beta^-} ^{239}_{94}Pu$

Americium-241 follows a similar pattern. Plutonium-239 captures successive neutrons to become plutonium-241, which then undergoes beta decay:

$^{239}_{94}Pu + ^1_0n \rightarrow ^{240}_{94}Pu + ^1_0n \rightarrow ^{241}_{94}Pu \xrightarrow{\beta^-} ^{241}_{95}Am$

These transuranic elements are produced as specific isotopes, meaning atoms of the same element with different numbers of neutrons. The isotope matters because it determines the atom's stability and how it decays.

Fission vs Fusion Reactions

Nuclear fission splits a heavy nucleus into two lighter nuclei, releasing energy. This is the reaction used in nuclear power plants and atomic bombs. The energy comes from converting a small amount of mass into energy, described by $E = mc^2$.

A classic example is uranium-235 fission:

$^{235}_{92}U + ^1_0n \rightarrow ^{141}_{56}Ba + ^{92}_{36}Kr + 3^1_0n + \text{Energy}$

Notice that fission produces three neutrons. Those neutrons can go on to split other uranium nuclei, which is what makes a chain reaction possible.

Nuclear fusion combines two light nuclei into a heavier one. Fusion releases even more energy per nucleon than fission. This is the process that powers the Sun and all other stars.

The most studied fusion reaction combines deuterium and tritium (both hydrogen isotopes):

$^2_1H + ^3_1H \rightarrow ^4_2He + ^1_0n + \text{Energy}$

Fusion has the potential to provide clean, abundant energy, but it requires extreme temperatures (over 100 million K) to force positively charged nuclei close enough to overcome their electrostatic repulsion. That's the main reason we haven't built practical fusion power plants yet.

Both fission and fusion release nuclear binding energy. This is the energy that holds a nucleus together. When nuclei rearrange into more tightly bound products, the "leftover" binding energy gets released.

Critical Mass in Chain Reactions

For a fission chain reaction to sustain itself, you need enough fissile material (like uranium-235 or plutonium-239) in one place. The minimum amount required is called the critical mass.

Here's how to think about it:

• Subcritical mass: Too many neutrons escape or get absorbed without causing fission. The chain reaction fizzles out.
• Critical mass: The number of neutrons produced by fission exactly balances the number lost to absorption and escape. The reaction sustains itself at a steady rate.
• Supercritical mass: More neutrons are produced than lost, so the reaction grows exponentially. This is what happens in a nuclear weapon.

In a reactor, control rods made of neutron-absorbing materials like boron or cadmium are inserted or withdrawn to keep the reaction right at critical. Pull the rods out a bit, and the reaction speeds up. Push them in, and it slows down.

Components of Nuclear Reactors

Fission reactors have five main components:

1. Fuel: Enriched uranium (with a higher percentage of U-235 than found in nature) or plutonium
2. Moderator: Slows neutrons down so they're more likely to cause fission. Common moderators are water and graphite.
3. Control rods: Absorb neutrons to regulate the chain reaction (boron, cadmium)
4. Coolant: Carries heat away from the reactor core to generate steam for electricity. Water and liquid sodium are common coolants.
5. Containment structure: A thick barrier that prevents radioactive materials from escaping into the environment

Fusion reactors are still experimental and face major engineering challenges:

• Fuel: Light elements, typically deuterium and tritium
• Confinement: The fuel must be heated into a plasma at temperatures exceeding 100 million K. Two main approaches exist: magnetic confinement (tokamaks use powerful magnets to contain the plasma) and inertial confinement (lasers compress fuel pellets)
• Challenges: Keeping the plasma stable long enough for sustained fusion, and managing neutron bombardment that damages reactor walls over time

Nuclear Processes and Waste Management

Nuclear power plants generate electricity through controlled fission, but they also produce nuclear waste as a byproduct. This waste contains radioactive materials that emit harmful radiation as unstable nuclei undergo radioactive decay, the spontaneous emission of particles or energy.

Some fission products remain dangerously radioactive for thousands of years, which is why nuclear waste requires careful handling and secure long-term storage. Spent fuel is typically stored first in cooling pools at the reactor site, then transferred to dry cask storage. Finding permanent geological repositories for high-level waste remains one of the biggest challenges facing the nuclear power industry.