Nuclear fission · Year 13
In Year 13 you learn: energy released by fission in MeV and joules; fissions per second in a reactor.
In fission, a heavy nucleus such as uranium-235 absorbs a slow neutron, becomes unstable and splits into two smaller nuclei, releasing energy and two or three more neutrons. Those neutrons can split more nuclei in a chain reaction. A nuclear reactor keeps the chain reaction steady and uses the energy to make steam for its generators.
Key facts
- E = mc² (c = 3.0 × 10⁸ m/s); 1 u of mass ≡ 931.5 MeV; 1 MeV = 1.6 × 10⁻¹³ J.
- One fission of uranium-235 releases about 200 MeV, which is 3.2 × 10⁻¹¹ J.
Worked examples
Example 1
One uranium-235 fission releases about 200 MeV. Using 1 MeV = 1.6 × 10⁻¹³ J, how much energy is that, in units of 10⁻¹¹ J?
Answer: 3.2 × 10⁻¹¹ J
Example 2
A 1,000 MW reactor gets 3.2 × 10⁻¹¹ J from each fission. How many fissions happen every second?
Answer: 3.1 × 10¹⁹
Example 3
In the reaction ²³⁵U + ¹n → ¹⁴¹Ba + ⁹²Kr + 3¹n, the mass lost is 0.18616 u. How much energy is released, in MeV, to 3 significant figures? (1 u = 931.5 MeV)
Answer: 173 MeV
Example 4
A reactor produces 500 MW of thermal power. Each fission releases 3.2 × 10⁻¹¹ J. How many fissions happen each second, in units of 10¹⁹, to 3 significant figures?
Answer: 1.56 × 10¹⁹