Preview
IB DP Physics — Unit E: Nuclear and Quantum Physics | SL & HL | E.4 Fission
📋 What You Will Learn
- Observe and describe the physical process of nuclear fission using real-world examples from nuclear reactors and weapons
- Define all key quantities: binding energy per nucleon, mass defect, critical mass, and chain reaction
- Derive the energy released in a fission reaction using Einstein's mass–energy equivalence \( E = mc^2 \)
- Explain the conditions required for a sustained chain reaction and the role of moderators, control rods, and coolants
- Analyse binding energy per nucleon curves and use them to predict whether fission releases energy
- Evaluate the advantages and disadvantages of nuclear power as an energy source, including safety, waste, and sustainability considerations
🌍 Real-World Context On 26 April 1986, reactor number four at the Chernobyl Nuclear Power Plant in Ukraine underwent an uncontrolled nuclear chain reaction. The resulting explosion and fire released radioactive material across Europe and remains one of the most significant nuclear accidents in history. Yet today, over 400 nuclear reactors worldwide generate approximately 10% of global electricity — safely, reliably, and with near-zero carbon emissions during operation. The same physical process that caused catastrophe at Chernobyl is, when carefully controlled, one of humanity's most powerful tools for generating clean energy. Understanding exactly how that process works — and what makes it so difficult to control — is the subject of this topic.
🔍 Guiding Question How does the splitting of a single atomic nucleus release enough energy to power a city, and what physical principles must be understood and engineered to harness that energy safely?
Physical Setup: What Is Nuclear Fission?
Imagine a large, unstable nucleus — uranium-235, for example — being struck by a slow-moving neutron. Rather than simply bouncing off, the neutron is absorbed, and the resulting compound nucleus becomes so unstable that it splits into two smaller nuclei, releasing two or three neutrons and a tremendous amount of energy. This is nuclear fission. Unlike chemical reactions, which involve electrons rearranging between atoms, fission involves the nucleus itself breaking apart — and the energy scales involved are millions of times larger.
The key observation is that the products of fission — the two daughter nuclei and the released neutrons — have a combined mass that is slightly less than the original nucleus plus the incoming neutron. This missing mass, called the mass defect, has been converted directly into energy according to Einstein's famous relation. To understand why this happens, we need to look at how nuclear binding energy changes as nuclei become smaller.
Notice from the diagram that a single fission event produces two or three free neutrons. These neutrons can go on to trigger further fission events in neighbouring uranium nuclei — and those events produce yet more neutrons. This is the basis of the chain reaction, which we will examine in detail shortly. The critical question is: can we control how fast this chain reaction proceeds?
Core Concepts and Definitions
Before we can calculate the energy released in fission, we need to build up a set of precise concepts. The starting point is understanding why nuclei are held together at all, and why splitting a large nucleus releases energy rather than requiring it.
📖 Key Definition — Nuclear Binding Energy The nuclear binding energy of a nucleus is the energy that must be supplied to completely separate all of its nucleons (protons and neutrons) into free, isolated particles. Equivalently, it is the energy released when free nucleons come together to form the nucleus. It is always a positive quantity.
The binding energy arises because the strong nuclear force holds nucleons together, and energy must be put in to overcome this force and pull them apart. A nucleus with a high binding energy is more tightly bound and therefore more stable. The binding energy \( E_B \) is related to the mass defect \( \Delta m \) by Einstein's mass–energy equivalence: \( E_B = \Delta m \cdot c^2 \), where \( c = 3.00 \times 10^8 \text{ m s}^{-1} \) is