Thermonuclear Reactions - an Introduction

The ‘Punch Line’ First In the Sun, hydrogen nuclei are being fused (merged) together to form helium. [details will follow]

Hydrogen Helium Hydrogen Helium

In the Sun, the Atoms are Ionized! So we can ignore the electrons! Let’s focus on the nuclei. 

Is it Like Building Blocks? (simply ‘link them together’?)

Consider the Masses [don’t memorize these!] Mass of 1 proton (p) = x kg Mass of 1 neutron (n) = x kg Expected mass of Helium (2 p + 2 n) = x kg = x kg Four protons all by themselves = x kg

A Puzzle! The actual mass of a Helium nucleus is x kg where we were expecting x kg! The difference is 0.75% The difference is 0.75%

“Binding Energy” When we ‘bind together’ small nuclei to form heavier ones, almost 1% of the total mass vanishes -- or rather, it is released as energy! (Remember E = m c 2 !)

The ‘Binding Energy’ Curve to be explored in detail later

Two Crucial Features 1. The curve has a peak, with the highest point corresponding to the element Iron. It drops off on both sides of that. 2. The curve is very steep on the left (the low-mass elements), but quite shallow on the right (the more massive ones).

‘Binding Energy’ Nature can extract energy from nuclei in two ways: Break very large nuclei into smaller pieces. This is fission! This is relatively easy and spontaneous. Example: U  Ba + Kr Break very large nuclei into smaller pieces. This is fission! This is relatively easy and spontaneous. Example: U  Ba + Kror Merge smaller nuclei to form bigger ones: fusion! This requires very high temperatures (as found in stars). Merge smaller nuclei to form bigger ones: fusion! This requires very high temperatures (as found in stars).

Fission vs Fusion

Can We Control Either of These On Earth? For Fission reactors: YES (we do it in the Bruce and Pickering reactors) One problem: this produces dirty (toxic, or long-lived radioactive) waste products. One problem: this produces dirty (toxic, or long-lived radioactive) waste products. On the positive side: no greenhouse gases; almost unlimited fuel supply. On the positive side: no greenhouse gases; almost unlimited fuel supply.

Let’s Consider Fission Reactors NOTE: This is NOT how the stars work, but it is very useful to know about. This is NOT how the stars work, but it is very useful to know about. Let us spend a few minutes on this topic, therefore.

Simple in Principle

In a Reactor, Control is the Key An incoming neutron strikes a nucleus, which it spontaneously breaks apart, yielding some energy (according to the binding energy curve!), and smaller lumps (nuclei) …plus a few more neutrons, which can spark further events – and so on.

Think About Mousetraps!

Watch the Simulations

Critical Mass Leads to Runaway Reactions (Bombs!)

Atomic Bombs (Fission) No need for high temperatures No need for high temperatures Need only a ‘critical mass’ of material in close proximity Need only a ‘critical mass’ of material in close proximity Have to keep it together long enough to release most of the energy (it’s blowing up!) Have to keep it together long enough to release most of the energy (it’s blowing up!) Simulation:

Fat Man and Little Boy

A Frighteningly Small Quantity A Frighteningly Small Quantity Watch youtube at

From the web…

“I am become Death, the shatterer of worlds.”

In Full… If the radiance of a thousand suns If the radiance of a thousand suns Were to burst into the sky, Were to burst into the sky, That would be like That would be like The splendour of the Mighty One. The splendour of the Mighty One. I am become Death, the shatterer of worlds. I am become Death, the shatterer of worlds. From the Bhagavad-Gita (quoted by Robert Oppenheimer after the successful test of the first atomic bomb in New Mexico, in 1945)

Back to Fusion: Why “Thermonuclear”?

Can We Control Fusion? Problems: how do we contain a gas at tens of millions of degrees? Problems: how do we contain a gas at tens of millions of degrees? Positive side: clean; virtually unlimited fuel supply. Positive side: clean; virtually unlimited fuel supply.

Magnetic Confinement in ITER See this at

Laser Confinement See this at

Uncontrolled Reactions

Hydrogen Bombs Need a ‘fuse’ to make them hot Need a ‘fuse’ to make them hot Release much greater amounts of energy Release much greater amounts of energy than atomic bombs (the binding energy curve again!)

From the Web the ‘fuse’ is an atomic (fission) bomb!

The Sun is Not a Bomb! Although fusion reactions are happening in the hot central core, they do not make the sun explode! Gravity holds it together. The actual rate of reactions and the yield of energy are small: just enough to maintain the equilibrium.