Do modern nuclear weapons use fission, fusion, or both? Explain.

Imagine the nucleus of an atom as a tightly packed cluster of protons and neutrons. In nuclear fission, this nucleus is split into smaller parts, which often results in the production of free neutrons and photons in the form of gamma rays. This splitting releases a tremendous amount of energy in accordance with Einstein's equation, \(E = mc^2\), which demonstrates the conversion of mass into energy.

For instance, when an atom of Uranium-235 undergoes fission after absorbing a neutron, it creates a highly energized, unstable state. This instability leads to the atom splitting into two smaller atoms, known as fission fragments, as well as additional neutrons and energy. These newly released neutrons can then trigger fission in other atoms, creating a chain reaction that can result in a massive release of energy – the principle behind both nuclear reactors and atomic bombs.

Given the right circumstances, such as within a nuclear weapon, the chain reaction is uncontrolled and rapid, leading to an explosion. The efficiency of this chain reaction is a measure of how much of the material can undergo fission before the assembly disassembles under the released energy, which in turn determines the yield of the weapon.

To fathom the concept of nuclear fusion, picture the nuclei of two light atoms colliding and merging to form a heavier nucleus. This process, which powers our sun and the stars, is contingent upon extreme conditions, namely, high temperatures and pressures.

Under such conditions, atomic nuclei overcome their natural repulsion (due to being positively charged) and come sufficiently close for the strong nuclear force to bind them together. The resulting nucleus has a slightly smaller mass compared to the sum of the original masses. The 'missing' mass is released as energy, again in line with \(E = mc^2\). This is why fusion is a potent source of energy, frequently more powerful than fission.

For example, in the core of the sun, hydrogen atoms fuse to form helium, emitting vast amounts of energy in the process. Achieving the conditions necessary for fusion on Earth requires sophisticated technology, and is the subject of ongoing research for its potential as a clean and virtually limitless energy source. In the context of nuclear weapons, however, the challenge is not to sustain the reaction, but to cause it to occur forcefully in a split second, leading to an intensely powerful explosion.

Going beyond the singular processes of fission or fusion, thermonuclear bombs combine these nuclear reactions to unleash devastating power. As mentioned in the solution to the exercise, these weapons use a fission reaction to begin with. The conventional nuclear bomb serves as a trigger, subjecting the thermonuclear fuel to the high-pressure, high-temperature environment it needs to undergo fusion.

A thermonuclear weapon typically encompasses a primary fission bomb and a secondary fusion stage. When the primary bomb detonates, it prompts fission and creates the extreme conditions necessary for fusion in the secondary stage, typically involving isotopes of hydrogen such as deuterium and tritium. The result is a massive explosion that can be tens or hundreds of times more powerful than a fission-only bomb.

This blend of reactions in thermonuclear weapons is the reason they are often referred to as hydrogen bombs or H-bombs. They represent one of the most advanced and destructive applications of nuclear chemistry and physics. While they offer insights into the potentials of nuclear energy, they also pose severe ethical and security challenges in our modern world.